FIELD The present invention relates to passive landing of an aircraft (e.g. fixed-wing aircraft or rotorcraft) usin at least one passive visio sensor. In particular, the invention relates to a tracker to track the extents of a landing site (e.g. an airfield runway, ship deck or helipad) and provide track data associated with the extents as navigation data for a navigation system of the aircraft.

BACKGROUND

Unmanned aerial vehicles (UAVs) rely on considerable ground infrastructure or emissions from the landing si te to ensure they are able to successfully operate and return to a _. runway from which the vehicle has taken off. Whilst a UAV flight computer may perform tasks for flying the aircraft, hunian operators are typically required to plan and undertake flight missions and control ground infrastructure is needed to recover and return an aircraft when issues arise. For example, for fixed-wing aircraft that are required to land at an airfield, there is infrastructure at the airfield to support the landing, and for ship-based landings, the ship provides active emissions which can make it susceptible, to detection. Failure of a local GNSS augmentation system normally renders the aircraft unable to land or poses a significant risk to the airframe. Circumstances can also arise where the aircraft cannot be returned t its designated airfield, and the UAV must be discarded at considerable cost. Whenever a UAV crashes, there is the added risk of loss of human life.

The requirement for takeoff and landing from a specific runway further limits the operational range of a UAV. Complex no fly areas also pose a difficulty as flying through no fly areas can result in catastrophic collisions, such as with elevated terrain. For example, some operational UAVs have a fully automatic takeoff and landing capability, but the landing phase is usually carried out using a combination of pre- surveyed runways- with known landing waypoints, an accurate height sensor for determining height above ground, and a differential GPS or GNSS augmentation system. These requirements can severely limit the use of modern UAV technology. There are several examples of unplanned mission events that can lead to a UAV operator needing to land the aircraft as soon as possible, such as: engine performance problems; extreme weather conditions; bird strike or attack damage; and flight control problems related to malfunctioning hardware. In current systems, these situations can easily lead to the complete loss of the aircraft, The operator must either attempt a recovery to a mission plan alternate .runway, or in the worst case, undertake a controlled ditching. Most modern UAV control system allow multiple alternate landing sites to be specified as part of the mission plan. However, the problem with these alternate landing sites is that they require the same level of a priori knowledge (i.e., accurate survey) and support infrastructure as the primary recovery site. Therefore, this generally limits the number and location of the alternate landin sites. This is due to the amount of time and manpower required to setup, maintain, and Secure the sites. The combined cost/effort and low probability of use detracts from the willingness to establish alternates. As mission requirements become more complex, it may not; be possible for the aircraft to reach one of its alternate landing runways, and controlled ditching may result in considerable loss and damage.

Vision sensors that rely upon visual servoing have been used where guidance is achieved by using direct feedback for individual features in an image. This is problematic because there is no direct wa t identify fault in the image processing or camera, it is not robust to occlusions, and it makes it difficult to combine different types of imagery together such as electro-optical (EO) and infrared (IR) images in the feedback path. In addition, it does not provide an easy way to provide an independent assessment on the accuracy of the alignment with the landin site.

Even in a manned aircraft there may be situations where normal navigation aids, such as ILS or GPS, are no available at a landing site or assistance is needed to navigate the aircraft and successfully land on a site. For example, assistance ma be required for an obscured runway or a moving runway, such as on a aircraft carrier.

It is desired to address the above or at least provide a useful alternative, and preferably provide a system that is able to land a vehicle passively on a confirmed landing site, that may be moving, without receiving transmitted emissions associated with the site,

SUMMARY

At least one embodiment of the present invention provides landing site tracker of an aircraft, including:

a track filter configured to process feature data representing the location of features of a candidate landing site, initialise and maintain tracks of the features as a track of the candidate site, compare geometry constraints for landing site with the track to validate the candidate site as the landing site, and convert the track into navigation data, representing the position of the landing site, for a navigation system, of the aircraft.

At least one embodiment of the present invention provides a landing site tracking process performed by an aircraft, including:

processing feature data representing the locatio of features of a candidate l ndin site;

initialising and maintaining tracks of the features as a track of the candidate site; cheeking geometry constraints for landing site with the track to validate the candidate site a the landing site; and

coupling the track, as navigation data representin the position of the landing site, into a navigation system of the aircraft.

At least one embodiment of the present invention provides a tracker to track the extents of a landing site and provide track data associated with the extents as navigation dat for a. navigation system of the aircraft. DESCRIPTTON OF THE DRA WINGS

Preferred embodiments of the present invention are hereinafter described, by wa of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a subsystem decomposition of preferred, embodiments of a landing system of an aircraft;

Figure 2 is an architecture diagram of an embodiment of a flight control computer for the aircraft;

Figure 3 is a block diagram of components of the control computer;

Figure 4 is schematic diagram of the relationship between components of the control computer;

Figure 5 is a flowchart of an autonomous recovery process of the landing system;

Figure 6 is an example ERSA airfield data for West Sale Aerodrome- Figure 7 is a diagram for computation of time of flight using great-circle geometry; Figure 8 is a example of ERSA airfield reference points;

Figure 9 is a diagram of survey route waypoint geometry;

Figure 10 is a flowchart of a. survey route generation process;

Figure 11 is a diagram of standard runway markings used to classify the runway;

Figure 12 is a diagram of runway threshold marking geometry;

Figure 13 is a pinhole camera model used to covert pixel measurements into bearing/elevation measurements in measurement frame;

Figure 14 is a diagram of coordinate frames used for tracking;

Figure 15 is a diagram of runway geometr corner definitions;

Figure 16 is a diagram of the relationship between adjustable erosswind, C, and downwind, D, circui template parameters; and

Figure .17 is a diagra of dynamic waypoints used during landing.

DETAILED DESCRIPTION A landing system 10, as shown in Figure 1, of an aircraf (or a flight, vehicle) provides an autonomous recovery (AR) system 50, 60. 70 for use on unmanned aerial vehicles (UAVs). The landing system 10 includes the following subsystems:

1) A Flight Control Computer (FCC) 1.00 for managing flight vehicle health and status, performing waypoint following, primary navigation, and stability augmentation. The navigation system used by the FCC 10Q uses differential GPS, and monitors the health of an ASN system 20.

2) An Ail-Source Navigation (ASN) system. 20 for providing a navigation system for use by the FCC 100. The ASN 20 is tightly linked to the autonomous recovery (AR) system 50. 60, 70 of the aircraft in that runway tracking initialised b the AR system is ultimately performed inside the AS 20 once the initial track is verified. As described below, establishing independent tracks or tracking of the landing site, confirming or verifying the position of the site relative to vehicle using the tracks, and then coupling or fusing the tracking to th navigation for subsequent processing by the ASN 20 is particularly advantageous for landing, particularly on moving site. The ASN is also described in Williams, P., and Crump, , Αϋ-so rce navigation for enhancing UAV operations in GPS-denied environments. Proceedings of the 28 ^th International Congres of the Aeronautical Sciences, Brisbane, September 2012 ("the ASN paper") herein incorporated by reference.

3) A Gimbaled Electro-Optical (GEO) camera system 30 fo capturing and time- stamping images obtained using a camera 34, pointing a camera turret 32 in the desired direction, and controlling the camera zoom.

4) An Automatic Path Generation (APGJ system 60 for generating routes (waypoints) for maneuvering the vehicle through no-fly regions, and generating return to base (RTB ) waypoints. This subsystem 60 is also described in Williams, P., and Crump, M„ Auto-routing system for L½Vs in complex flight areas. Proceedings of the 28* International Congress of the Aeronautical Sciences, Brisbane, September 2012 ("the Routing paper") herein incorporated by reference.

5) An Autonomous Recovery Controller (ARC) system 50 fo controlling the health and status of an autonomous recovery process, runway track initialization and health monitoring, and real-time waypoint updates. The ARC 50 controls independent tracking of the landin site until the site is verified and then transforms the track for insertion, coupling or fusing into the navigation system (e,g, the ASN 20) used by the aircraft. The runway track can include four comer points (extents) of the runway and associated constraints.

6) A Feature Detection Controller (FDC) system 70 for performing image processing, detecting, classifying, and providing corner and edge data from images for the ARC 50. The FDC is also described in Graves, K„ Visual detection and classification o runways in aerial imagery, Proceedings of the 28t International Congress of the Aeronautical Sciences, Brisbane. September 2012,

7) A Gimbalecl Camera 34, and a camera turret 32 provided by Rubicon Systems Design Ltd to control the position of the camera 34.

The ASN. APG, ARC and FDC subsystems 20, 50, 60, 70 are housed on a Kontron CP30S board produced by Kontron AG, which includes an Intel Core -2 Duo processor. One core of the processor is dedicated to running the ASN system 20, and the second core is dedicated to running the AR system 50, 60, 70. In addition, inputs and outputs from all processes are logged on a solid state computer memory of the board. The GEO subsystem 30 is housed and runs on a Kontron CP307 board provided by Kontron AG, and manages control, of the turret 32 and logging of all raw imagery obtained by the camer 34. The subsystems 20, 30, 50, 60, 0 may use Linux operating system running a real time kernel and the processes executed by the sub-systems can be implemented and controlled using C computer program code wrapped in C++ with appropriate data message handling computer program code, and all code is stored in computer readable memory of the CP308 and CP307 control boards. The code carl also be replaced, at least in part, by dedicated hardware circuits, such as field programmable gate arrays (FPGAs) or application specific integrated circuits (ASICs), to increase the speed of the processes.

The Flight Control Computer The flight control computer (FCC) 100, a shown in Figures 2 and 3, accepts and processes input sensor data from sensors 250 on board the vehicle. The FCC 100 also generates and issues command data for an actuator control unit (ACU) 252 to. control various actuators, on ^' board the vehicle in order to control movement of the vehicle according to a validated flight or mission plan. The ACU 252 also provides response data, in relation to the actuators and the parts of the vehicle that the actuators control, back to the computer 100 for it to process as sensor data. The computer 100 includes Navigation, Waypoint Management and Guidance components 206, 208 and 210 to control a vehicle during phases of the flight plan. The computer 100, as shown in Figure 2, includes a single board CPU card 120, with a Power PC and input/output interfaces (such as R.S232, Ethemel and PCI), and an I/O card 140 with flash memory 160, a GPS receiver 180 and UART ports, The computer 100 also houses an incrtial measurements unit (IMU) 190 and the GPS receiver (e.g. a Novate! OEM VI) 180 connects directly to antennas on the vehicle for a global positioning system, which may be a differential or augmented GPS.

The FCC 100 controls, coordinates and monitors the following sensors 250 and actuators on the vehicle:

a air data sensor (ADS) comprising air pressure transducers, an accurate height sensor (AHS), e.g. provided by a ground directed laser or sonar,

(iii) a weight on wheels sensor (WoW),

(i.v) a transponder, which handles communication with a ground vehicle controller (GVC),

the electrical power system (EPS),

primary flight controls, such as controls for surfaces (e.g. ailerons, rudder. elevators, air brakes), brakes and throttle,

propulsion system, including

(a) an engine turbo control unit (TCU),

(b) an engine management system (EMS),

(c) an engine kill switch,

Ul) carburettor heater.

(e) engine fan.

(0 oil fan iviii) fuel system,

(ix) environmental control system (ECS) comprising aircraft temperature sensor, airflow valves and fans,

( ) Pi tot Probe head ng.

(xi.) external lighting, and

(xii.) icing detectors.

The actuators of (v) t (xi) are controlled by actuator data sent by the FCC 1.00 to at least one actuator control unit (ACU) or processor 252 connected to the actuators.

The FCC 100 stores and executes an embedded real time operating system. (RTOS), such as Integrity- 178B by Green Hills Software Inc. The RTOS 304 handles memory access by the CPU 120, resource availability, I/O access, and partitioning of the embedded software components (CSCs) of the computer by allocating at least one virtual addres space to each CSC.

The FCC 100 includes a computer system configuration item (CSCI) 302, as shown in Figure 4, comprising the computer software components (CSCs) and the operating system 304 on which the components run. The CSCs are stored on the flash memory 160 and may comprise embedded C++ or C computer program code. The CSCs include the following components:

(a) Health Monitor 202;

(b) System Management 204 (flight critical and non-flight critical);

(c) Navigation 206;

(d) Waypoint Management 208;

(e) Guidance 210;

(f) Stability Augmentation 212;

(¾ ^' ) Data Loading /Instrumentation 214; and

(h) System Interface 216 (flight critical and non-flight critical).

The Health Monitor CSC 202 is connected to each of the components comprising the CSCI 302 so (hat the components can send messages to the Health Monitor 202 when they successfully complete processing.

The System Interface CSC 216 provides low level hardwar interfacing and abstracts data into, a format useable by the other CSC's.

The Navigation CSC 206 uses a combination of IMU data and GPS data and continuously calculates (he aircraft's current position (latitude/longitude/height), velocity, acceleration and attitude. The Navigation CSC also tracks IMU bias errors and detects and isolates IMU and GPS errors. The data generated by the Navigation CSC represents WGS-84 (round earth) coordinates .

Whilst the FCC 100 can rely entirely upon the navigation solution provided by the ASN system 20, the navigation CSC 206 can be used, as desired, to validate the navigation data generated by the ASN 20. .

The Waypoint Management (WPM) CSC 208 is primarily responsible within the FCC for generating a set of 4 waypoints t send to the Guidance CSC 210 that determine the intended path, of the vehicle through 3D space. The WPM CSC 208 also

(a) Supplies event or status data t the System Management CSC 204 to indicate the occurrence of certain situations associated with the vehicle.

(b) Checks the validity of received flight or mission plans

(c) Manages interactions with an airborne Mission System (MS) 254 of the vehicle. The MS sends route requests to the WP 208 based the waypoints and the current active mission plan.

The Guidance CSC 210 generates vehicle attitude demand dat (representing roll, pitch and yaw rates) to follow a defined three dimensional path specified by the four waypoints. The attitude rate demands are provided to the Stability Augmentation CSC 212. The four waypoints used to generate these demands are received from the Waypoint Management CSC 208. The Guidance CSC 210 autonomously guides the vehicle in all phases of moveme t.

The Stability Augmentation (SA) CSC 212 converts vehicle angular rate demands into control surface demands and allows any manual rate demands that may be received by the GVC to control the vehicle during ground operations when necessary. The SA CSC 212 also consolidates and converts air data sensor readings into air speed and pressure altitude for the rest of the components.

The Infrastructure CSC is a common software component used across a number of the CSCs. It handle functions, such as message generation and decoding. It) layer interfacing, time management functions, and serial communications and protocols such UDP.

The System Management CSC 204 is responsible for managing a number of functions of the FCC, including internal and external communications, and establishing and executing state machine of the FCC CSCI 302 that establishes one or a number of stales for each of the phases of movement of the vehicle. The states each correspond to a specific contained operation of the vehicle and transitions betwee states are managed carefully to avoid damage or crashin of the vehicle. The state machine controls operation of the CSCI together with the operations that it instructs the vehicle to perform. The stales and their corresponding phases are described below in Table 1.

Table 1 The System Management Component 204 determines the existing state and effects the changes between the states based on conditions for applicable transitions for each state, and based on data provided by the CSCs, such a Guidance, Navigation and Stability Augmentation and Waypoint Management which depend on the current state, and represent the current status of the vehicle, The status data provided by the CSCs affecting the states is in turn dependent on the sensor data received by the FCC 100.

Landing System Process

The autonomous recovery process 500, as shown in Figure 5, executed by the landing system 10 includes:

1) The AR system 50- 60, 70 is triggered (step 502) by the FCC 100. This can be because the FC 100 determines that the state of the vehicle is unfit for its intended purpose, or remotely via an operator command.

2) The closest airfield is selected (504) from a runway database, taking into account current wind conditi m .

3) A runway survey route is generated (506) based on runway feature data of the selected airfield in the runway database. The survey route is used to fly the vehicle on a route that gives the vehicle stron likelihood of being able to locate the desired runway. Once the vehicle is in the runway vicinity. The survey route takes into account any no-fly area enforced, during the mission.

4) A route is generated (508) to take the vehicle from its current position to the vicinity of the airfield. This route takes into account any no-fly area enforced during the mission.

5) In the vicinity of the airfield, the girii baled camera 34 is controlled so as to detect and image the likely runway candidate whilst the vehicle flies the survey route (510).

6 Images of the runwa candidate are scanned for key runway features, and classified as being of the candidate runway if it has the required features ( 12).

7) The camera is controlled to locate the corners of the runway piano keys (514). The piano keys are gee-located using a tracking process of a tracker implemented with an unseented Kalman fillet.

8) If the tracked runway has features corresponding to the features for the runway in the runway database, the runway track, which can comprise four constrained corner points, is transformed into a set of runway coordinates (centre position, length, width and heading) and inserted into the ASN 20 as a Simultaneous Localisation and Mapping (SLAM) feature set to provide a coupled navigation-tracking solution (516).

9) A SBtum-to-base (RTB) waypoint set: is generated (518) to enable the aircraft to perform inbound, circuit, approach, and landing. The RTB set takes into account any no-fly areas enforced during the mission, as well as the prevailing wind conditions, to determine the landing direction.

10;) The aircraft executes the RTB (520) and augments its height during landing using the height senso and its lateral track the runway edge data obtained into the run y track that has been fused or coupled into the navigation filter as runway navigation coordinates. The landing waypoint are dynamically updated to null cross-track errors relative to the estimated runway centerlirte.

The autonomous recovery process 500 executes a passive landing process for the vehicle that creates a map of features that have been detected i the images, and couples navigation with tracking to give a robust method for landing even on objects that are moving.

Removal of any independence of feature detection/tracking and the navigation loop is significant. It is the ability to directl couple these processes together that enables highly accurate relative positioning without the need for ground augmentation systems. The system .10 has the ability to (i) distinguish the target landing area, and (ii) detect features from the target landing area for relative positioning. By using an image processing system that provide information about the landing area (In the camera frame, and by knowledge of camera calibration, and the aircraft body frame), the system 10 is able to derive key relative information about the landing area. For generality, the landing area can be modeled as a 6-DOF object (3 components -of position/velocity, 3 components of attitude) with landing site data for verification purposes such as the geometry of the landing area. In the case of an airfield, t e runway has paiticular features that can be used to establish a runway track with high confidence (threshold markings, landing markings, touch down markings, runway centerline). As described below, because of the static nature of an airfield runway, the track states can be transformed into six navigation, coordinate states being 3 positions, runway direction, and length and width. In the case of a helipad on a ship deck, there are similar features on the shi deck, such as helipad markings, that allow for the landing area to be detected and tracked. The primary difference between a shi deck and an airfield is the additional dynamic states and prediction used in the navigation/tracking processes. For example, the monitored states of the landing site are 3 positions, 3 velocities, 3 attitudes, 3 attitude rates, and helipad or landing deck geometry. The fact that the landing area is attached to ship (with characteristic motion) is used to constrain the predictive element of the navigation/tracking processes. Because the tracking and navigation processes are coupled together, the resulting relative positioning algorithms are extremely robust to navigation errors or even faults in aiding sources such as GPS multipath interference, as described below.

Runway Database The AR system 50, 60, 70 stores database of runway and airfield data similar to that provided by Jeppesen NavData ^{1 M} Services,, and provides runway characteristics or feature data on runways of airfields. In one embodimen the En-Route Supplement Australia (ERSA) data (see Joo. S., Ippolito, C, Al-Ali, , and Yeh, Y.-H., Vision aided inertia! navigation with measurement delay for fixed-wing unmanned aerial vehicle landing. Proceedings of the 2008 IEEE Aerospace Conference, March 2008. pp.1 -9.) has been used, and this provides dat about all airfields in Australia. An example of the key airfield data provided from ERS A is provided below in Table 2 and shown in Fig. 6. WEST SALE

AVFAX CODE 3059 ELE 93

VIC l TG +10 Y WSL

S 38 05.5 E 146 57.9 VAR 12 DEG E REG

A OPR. Wellington Shite Council, PO Box 506, Sale, VIC, 3850 . Ph 03 514 3333, FAX.

5142 3499, ARC ⁾ 5149 233 ^' ?; 0407 835 419,

HANDLING SE VICES AND FACILITIES

Aero Refuellers 24 HR JET A l . AVGS by tanker daylight HR only. Limited service weekends. Phone 0458 411 599.

PASSENGER FACILITIES

PT / ^' TX / LG / WC

SURFACE MOVEMENT GUIDANCE

Fixed distance &■ touchdown markings no AVBL,

METEOROLOGICAL INFORMATION PROVIDED

1. TAF CAT D.

2. East Sale W IS - .125.4 or Phone 03 146 7226

PHYSICAL CHARACTERISTICS

05/23 044 16c Grassed grey silt clay. WD 30 RWS 90

09/27 087 50a PCN 12 /F7B /6O (87PS1) (Ύ. W1D 30 R WS 150

14/32 133 23c Grassed grey slit clay. WID 3 RWS 90

Table 2 reference point 602 (latitude = -38,05.5, longitude = 146,57.9), height above mean sea-level (93 feet), magnetic field offset (+12 deg), number of railways (3), runway surface characteristics (2 grass, 1 bitumen), runway lengths (1527, 699, 500 m) and width (30 tn), and runway magnetic heading (44, 87, and 133 deg). The airfield reference point 602 gives the approximate location of the airfield to the nearest tenth of a minute in latitude, and longitude (±0.0083 deg). This equates to an accuracy of approximately 100 m horizontally. Furthermore, the reference point in general does not lie on any of the runways and cannot be used by itself t land the aircraft. It is suitable as a reference point for pilots to obtain a visual of the airfield and land. The landing system 10 performs a similar airfield and runway recognition and plans a landing/approach path. For the UAV to identify and perform an autonomous landing on the desired runway, an accurate navigation solution is used. In low-cost UAVs, a GPS-aided Inertial Navigation System (INS) system is used. Yet, GPS is heavily relied upon due to the poor performance of low-cost inertial measurement units. GPS has very good long term stability, but can drift in the short term due to variations in satellite constellation and ionospheric delays. The amount of drift is variable, but could be on the order of 20 m. This is one of the main reasons why differential GPS is used fo automatic landing of UAVs. Differential GPS allows accuracies of the navigation solution on the order of approximately 1-2 m. The autonomous recover (AR) system is assumed to have and CA operate with no differential GPS available, but may use at least one functional GPS antenna. A GPS antenna is not required if the Simultaneous Localisation and Mapping (SLAM) capability of the Ail-Source Navigation system 20 is used, as described in the ASN paper.

The AR system 50, 60, 70 uses image processing to extract information about a candidate airfield. Virtu ally all UAVs are equipped with gimbaled cameras as part of their mission system, and in an emergency situation, the camera system can be re-tasked to enable a safe landin of the UAV. Other sensors such as LIDAR, although very useful for helping to characterize the runway, canno always be assumed to be available. Additional sensors are not required to be installed on the UAV to enable the autonomous recovery system 50, 60, 70 to work. Only image processing is used, and electro-optical (EG) sensing is used during daylight hours and other imaging, such as infrared (1R) imaging is used in identifying the runway during night operations.

Airfield Selection

When the AR proces is triggered (502), the current vehicle state and wind estimate are obtained from the FCC 100. The latitude and longitude of the vehicle is used to initiate a search of the runway database to locate potential or candidate landing sites. The distances to the nearest airfields are computed by the ARC 50 using the distance on the great circle. The vehicle airspeed and wind estimate are used to estimate the time of flight to each airfield assumin a principally direc flight path. The shortest flight time is used by the AR 50 to seiect the destination or candidate airfield. In practice, the closet airfield tends to be selected, but accounting for the prevailing wind conditions allows the AR system to optimize the UAV's recovery.

Figure 7 shows the geometry of the problem of finding the time of flight using the great- circle. The starting position is denoted as ρ in Barth-Centered-Earth-Fixed (ECEF) coordinates. The final positio is denoted f _> also in ECEF coordinates. The enclosing angle Is given by

A coordinate frame with an jt-axis is aligned with the direction of p _s , so a normal vector is given by n -p _s x p _f ^P _f x p ₁ 1| , and a bi-normal vector g ven by h = n x The time of flight is computed using a discrete integration approximation as follows: i = T Ai _i (2)

( 1

where Wi is the local estimated win

represents a planar rotation matrix in the p_ -b plane that effectively rotates the vector p to p . . The rotation is around the +n -axis. For short distances, the above may be simplified by computing the time of flight using in a local North-East-Down (NED) frame and ignoring the effect of spherical geometry. When selecting potential airfields, the type of runway and runway lengths is also taken into account For example, one embodiment of the AR system 50, 60, 70 requires a runway with standard runway markings, i.e., a bitumen runway, so the runway can be positively confirmed by the AR system as being a runway. The minimum landing distance required by the aircraft is also used to isolate runways that are not useable. Once an airfield is selected, the ERSA data is converted into a runway feature data format to be further used b the AR system. This includes conversion of the airfield reference height to the WGS.84 standard using EGM96 (Earth Gravitational Model 1.996) (see Lemoine, E.G., enyon, S.C., Factor, J, „ Trimmer, R.G., Pavlis, N.K., Chin , D.S„ Cox, CM, Klesko, S.M., Luthcfce, S.B., Torrencc, M.H., Wang, Y.M., Williamson, R.G., Pavlis, EC, Rapp, R.H., and Olson. T.R., The Development of the Joint NASA GSFC and NIMA Geopoi ntial Madid EGM96, NASA TP- 1998-206861), and conversion of the runway heading from magnetic to true, Generation Of Survey Route

The survey route i generated and used to provide the UAV with the maximum opportunity to identify and classify the candidate runway. The system 10 also needs to deal with no-fly areas around the targe runway when determining and flying the urvey route. The APG system 60 executes a survey route generation process that iterates until it generates a suitable survey route. The data used by the APG 60 includes the ERSA reference point runway length, and runway heading. Maximum opportunity is afforded by flying the vehicle parallel to the ERSA runway heading (which is given to ±1 deg). The desired survey route i a rectangular shaped flight path with side leg approximately 3 runway lengths £ long, as shown in Figure 9. Tlie width W of the rectangle is dictated by the turn radius R of the flight vehicle. The center of the survey route is specified a the ERSA reference point. This point is guaranteed to be on the airfield, but is not guaranteed to lie on a runway. For example. Figure 8 shows three differen airfields and their respective reference points 802, 804 and 806. If there are no no-fly zones around -the airfield, then the survey route generation process is completely quickly, but generally iteration is required to select the combination of survey route center point, side length, width, and rotation that -fits, within the available flight area. The survey route 900 consists of 4 waypoints 902, 904, 06 and 908, as shown in Figure 9. The waypoints are defined relative to the center of the rectangle in an NED coordinate frame. The side length L varies from. 0 to 2, _v . and the width w varies from 0 to H> „ ,

In the worst case, the side length and width are zero, giving a circular flight path with minimum turn radius R . An iterative survey route generation process 1000, as shown in Figure 10, is used to determine the survey route is a follows ^*

1 ) While a valid survey route solution docs not exist (i.e., path does not yet lie- completely within flyable regions), do the following:

(a) Vary w from ·¼·> , to 0, where w is equal to the runway length (1004). (b) Vary the lateral position of the center of the route (perpendicular to runway direction) in step 1006.

(c) Vary I from L .. . to 0, where L _¥ is three times the runway length (1008).

(d) Vary the longitudinal position of the center of the survey route (parallel to ranway direction) in ste 1010.

2) When a valid survey route is found (1002), the waypoints are stored (1012) together with the center of the route for late use.

Route To Airfield

The aircraft needs to be routed from its current position and heading, to the generated Survey route. The route to the airfield must take- into account any no-fly regions, such as those that may be active during a mission. The route to the airfield is constructed by the APG system 60 using the route generation process discussed in the Routing paper. The initial position and velocity are taken from the ASN system 20, and the destination point used is the center of the generated survey route. To ensure that the route is able to be generated to the desired destination, a new node is inserted into the route generation process. The route t the destination is then constructed. In practice, it is sometimes possible that a route cannot be constructed due to the UAV's proximity to flight extents at the time the AR process 500 is initiated. The ARC 50 of the AR system monitors the route generation process and if no valid route is returned, it restarts the process. The AR system will attempt to route the vehicle to the destination point until, it is successful.

When a route to the airfield i successfully generated, a transfer route is also generated by the APG 60 that connects the route to the airfield and the runway survey route. This process is also iterative, and attempts to connect to a point along each edge of the survey route, and begins with the last waypoint on the airfield route.

Once the routes are all generated they are provided by the ARC 50 to the WPM 208 of the FCC .100 to fly the aircraft to the airfield and follow the survey route.

Airfield Detection And Tracking

Once the aircraft i following the survey route, the GEO system 30 commands the turret 32 to point the camera 34 so as to achieve a particular sequence of events. In the first phase, the camera 34 is commanded to a wide field- f-view (FOV), with the camera pointed toward the airfield reference point. In this phase, the FDC 70 attempts to locate the most likel feature in the. image to be the candidate runway. The runway edges are projected into a local navigation frame used by the ASN 20. The approximate edges of the runway are then used to provide an estimate of the runway centreline. The camera 34 is slewed to move along the estimated centreline. The FDC 70 analyses the imagery in an attempt to verify that the edges detected in the wide FOV images in fact correspond to a runway. This is done by looking for ke features that are present on runways, such a runway ^• threshold markings (piano keys), runway designation markings, touchdown zone markings, - .2.1 - and aiming point markings. These markings are standard on runways, as shown in Figure 11. By using all of these features, the FDC 70 is able to confirm an actual specific railway, flight deck or helipad is within view of the aircraft, as opposed to simply confirming a possible site to land.

Once the runway is confirmed to be the desired runway, the FDC 70 alternately points the turret 32 towards the threshold markings at each end of the runway. This is designed to detect the correct number of markings for the specific runway-width. The layout of the piano keys is standard and is function of runway width, as shown in Figure 12 and explained in Table 3 below. The corners of the threshold markings shown in Figure 12 are detected as pixel coordinates and are converted into bearing/elevation measurements for the corners before being passed from the FDC 70 to the ARC 50.

The corners of the threshold markings are not the corner of the actual runway, so the edge i offset by an amount given by d - w- 1 2 - Na , where N is the number of piano keys, w is the runway width, and is the width of the piano keys, as shown in Table 3. This is used to compare the estimated width of the runway based on the piano keys, and the ERSA width in the runway database. It is also used in runway edge fusion, described later.

Table 3

The FDC 70 computes the pixel coordinate of the piano keys in the image plane. The pixel coordinates are corrected for lens distortion and converted into an equivalent set of bearing φ and elevation ό measurements in the camera sensor frame. A pinhole camera model is assumed which relates measurements in the sensor frame to the image frame («, v) , as shown in Fi ure 13,

where f and are the camera focal lengths, and u ₀ and v ₀ are pixel coordinate data

The bearing and elevation measurements are derived from the pixel information according to

φ - tan ^_i [u -u j f„ ] (7)

Distortion effects are also accounted for before usin the raw pixel coordinates. The uncertainty of a measurement is specified in the image plane, and must be converted into an equivalent uncertainty in bearing/elevation. The uncertainty in bearing/elevation takes int account the fact that the intrinsic camer parameters involved in the computation given in Eqs. (7) and (8) are not known precisely. The uncertaint is computed via where t _j J, ν - x - [u _: ,vf „ σ _χ is the uncertainty in the pixel plane coordinates _* and _ν is the uncertainty in the camer parameters.

Runway Track Initialization

Once the PD 70 detects runwa comers, the ARC 50 converts the bearing/elevation measurements into a representation of the candidate runway that can be used for landing. The gimbaled camera system 30 does not provide a range with the data captured, and bearing elevation only tracker i used by the ARC 50 to properly determine the position of the runway in the navigatio frame used by the ASN 20. The runway track initialization is performed hy the AR system independent of and without direct: coupling to the ASN 20 as false measurements or false tracka can corrupt the ASN 20 which could be detrimental to the success of the landing system 10. Instead, enough confidence is gained in the runway track before it is directly coupled to the AS 20. An unscented Kalraan filter (UKF) is used to gain that confidence and handle the nonlinearities and constraints present in the geometry of the landing site.

Tracking State One tracking approach is to use the four corners provided by the FDC 70 as independent points in the navigation frame. The points could be initialized for tracking using a range- parameterized bank of Extended Kalman filters (E F ^' s) as discussed in Peeeh, N., Bearings-only tracking using a set of range- rameteri ed extended alman filters, IEEE Proc. Control Theory Appi., Vol., 142, No. 1, pp.73-80, 1995, or using a. single inverse depth filter as discussed in Civera, J., Davison, A. J., and Montiel, J.M.M., Inverse depth paramelaHzation for monocular SLAM, IEEE Transactions on Robotics, Vol. 24, No. 5, pp.932-945, 2008. The problem with using independent points is that it does not account for the correlation of errors inherent in the tracking process, nor any geometric constraints present. The tracking filter should also account for the fact that the geometry of the four corner points provided by the FDC 70 represents a runway. One option is t represent the runway using the minimal number of coordinates (i.e., runway center, length, width, and heading), however a difficulty with treating the runway as runway initially is that all four points are not or may not be visible in one image frame. This makes it difficult to be able to initialize tracking of a finite shaped object with measurements of only one or two comers.

The ARC 50 addresses the limitations stated above, by using a combined strategy. The FDC 70 does not provide single corner points, and operates to detect a full set of piano keys in an image. Accordingl for each image update, two points are obtained. The errors in these two measurements are correlated by virtue of the fact that the navigation/timing errors are identical. Thi fact is exploited in the representation of the state of the runway. Each comer point is initialized usin an unseen ted Ka!man filler using an inverse depth representation of the state. The inverse depth representatio uses six states to represent a 3D point i space. The states are the camer position (three coordinates) at: the first measurement the bearing and elevation at first measurement, and th inverse depth at the first measurement. These six states allo the position of the corner to be computed in the navigation frame. An optimization ca be used as two corner points are always provided and hence, only one camera position is required fo each end of the runway. Thus, the ARC 50 represents the runway using a total of 18 states (two camera position each represented by coordinate x, y, ¾, and 4 set of inverse depth (j/d), bearing, and elevation) for the four corners of the runway.

Tracking is performed in the ECEF frame, which as discussed below i also the navigation frame. The camera positions used in the state vector are the position in the ECEF frame at the first measurements. The bearing and elevation are the measurements made in the sensor frame of the camera 34 at first observation, rather than an equivalent bearing and elevation in the ECEF or local MED frame. The reason for maintaining the bearing/elevation in the measurement frame of the camera 34 is to avoid singularities in any later computation which can arise if bearing/elevation is transformed to a frame other than the one used to make the measurement.

The advantage of the state representation of the candidate runway is that it allows each end of the runway to be initialized independently. Geometric constraint are also exploited by enforcing a set of constraints on the geometry after each runway end has been initialized. Each end of the runway is therefore concatenated into a single state vector rather than two separate state vectors, and a constraint fusion process is performed as discussed below.

Coordinate Frames

In order to compute the camera position in the ECEF frame, the AR systems 50, 60, 70 use the coordinate frames shown in Figure 14. The Earth- Centered- Earth -Fixed (ECEF) frame (X ,.Y ,Z) is the reference frame used for navigation of the aircraft. The local navigation frame (N ,B,D) is an intermediate frame used for the definition of platform Euler angles. The NED frame has its origin on the WGS 4 ellipsoid. The IMU/hody frame >¾, ,¾) is aligned with axis of the bod of the vehicle 1400 and has its origin at the IM U 1 0. The installation frame (x^y - _t z _{ ) has its origin at a fixed point on the camera mount. This allows some of the camera extrinsics to be calibrated independently of the mounting o the airframe 1402. The gimbal frame (x _? ,.v ,, z , ) has i ts origin at the center of the gimbal axes

^• of the turret 32. Finally, the measurement frame (.½, y _m> x _m ) has its origin at the focal point of the camera 34. The position of the camera in the ECEF frame is given by

Pi = Pt +€£ ^< «<ρ» + C (p _i ' _; + (10) where ρζ is the position of the aircraft ΓΜ0 1 0 in the ECEF frame, C * is the direction cosine matrix representing the rotatio from the ED frame to the ECEF frame, C is the direction cosine matrix representing the rotation from the body to the NED frame, p is the position of the installation origin in the body frame, C f is the direction cosine matrix representing the rotation from the installation from to the body frame, p' is the position of the gimbal origin in the installation frame, C ' is the direction cosine matrix representing the rotation from the gimbal frame to the installation frame, and p _n is the origin of the measurement frame in the gimbal frame.

The direction cosine matrix representing the rotation from the measurement frame to the ECEF frame is given by

c* = c'czae 'c* ( l

Where C i the direction cosine matrix representing the rotation .front the measurement frame to the gimbal frame. First Observed ion

The FDC 70 provides measurement data a soeiated with a set of comer IDs, As mentioned previously, each end of the candidate runway is initialized with measurements of the two extreme piano key corners for that end. The unit line of sight for feature k in the measurement frame is given by

c s^ ₍ .cost¾

im sin<¾cos (12) - sin

The unit line of sight of the same feature in the ECEF and NED frames are given respecti vely by

(13)

The initial inverse depth of the feature is estimated using the ERS A height o the runway (expressed as height above WGS84 ellipsoid) and the current navigation height above ellipsoid. The inverse depth is given by

In equation (15), k is the unit vector along the x-axis in the NED frame (D-axis), The dot product is used to obtain the component of Line of sight along the vertical axis.

The uncertainty of the inverse depth is set equivalent to the depth estimate, i.e., the corner can in theory lie anywhere between the ground plane and the aircraft. The initial co-variance for the two comers is thus given by

(16)

where P , is the uncertainly in the camera position, P _Sf is the uncertainty in the bearing and elevation measurement fo feature k, and P _¾ is the uncertainty in the inverse depth for feature fc, and the. function .blkdiag gives the block diagonal of the component matrices, i.e. blkdiag (Pi, P ₂ ) = [Pi, 0; 0, Pzl The position of the feature in the ECEF frame can be estimated from the state of the tracking filter of the ARC 50. The initial CI from the first measurement, it is stored and the ECEF position is given by

where I™ is calculated using the filter estimated bearing and elevation, not the initial measured one, and p ^* is the filter estimated initial camera positi n. The bearing/elevation and inverse depth of each feature are assumed to be uneorrelated when initialized. The inverse depth is in fact correlated due to the fact that the ERSA height and navigation heights are used for both corners. However, the initial uncertainty in the estimates is such that the effects of neglecting the cross-correlation is small. The error correlation is built-up by the filter during subsequent measurement updates.

For the purposes, of assessing the accuracy of the corner estimates, the covariance of the filter state is translated into a physically meaningful covariance, i.e., the covariance o the corner in the ECEF frame. This can be done b usin the Jacobian of Eq. (17),

A similarity transformation is used to obtain the covariance in the ECEF frame

Obs tva tlon Fusion

The tracker of the ARC 50 uses an unscented Kalman filler (UKF) (as described in Juliex, S.J. _* and Uhlmann, J.K., A new extension of the Kalman filter to nonlinear systems. Proceedings of SPIE, Vol. 3, No. 1 , pp.182-193, 1997) to perform observation fusions. The UKF allows higher order terras ^' to be retained in the measurement update, and allows for nonlinear propagation of uncertain terms directly through the measurement equations without the need to perform tedious Jacobian computations. For the UAV 1400, more accurate tracking. results were obtained compared to an E F implementation. There is no need to perform a propagation of the covariance matrix whe the runway is a static feature. Due to the random walk (variation of errors) in the navigation data provided by the ASN 20, a small amoun of process noise can be added to the covariance as a function of time to prevent premature convergence of the solution. This noise is treated as additive and does not need to be propagated using the U F.

The UKF updates the filter state and covariance for the four tracked features of the runway from the bearing and elevation measurements provided by the FDC 70. The state vector with the process and measurement noise as follows

where x _k ^* represents the filter state at discrete time L and w represents the measurement noise for the same discrete time. The first step in the filter (as discussed in Van der Merwe, R., and Wan, E.A., The square- root unseented -alman filter for state and parameter estimation, Proceedings of the 2001 IEEE international Conference on Aa/ iics _* Speech, and Signal Processing, Ma 2001, pp.3461-3464) is to compute the set of sigma points as follows

where x is the mean estimate of the state vector, S _k is the Cholesky form of the covariance matrix, and the parameter γ- is defined by

and —a"[L + k)—L is a scaling parameter, with the values of and k selected appropriately, and L is the dimension of the augmented state. The sigma points are then propagated through the nonlinear measurement equations as follows

The mean observation is obtained by 21

ft-- V w (24) i-0

Where

The output Cholesky covarianee is calculated using

Where

and qr{ } represents the QR decomposition of the matrix, and cholupdate} ] represents the Cholesky factor update. The cross x>r elation matrix is determined from

The gain for the Kalman update equations is computed from

(30)

The state estimate is updated with a measurement using

** = * ^" K + ^Κ ;0* - ) (31) and the covarianee is updated using

$ _k - hoh date{s _t K _k S. -l} (32)

The ARC 50 accounts for angle wrapping when computing the difference between the predicted bearing/elevatio and the measured ones in Eqs. (26), (27), (29), and (3.1 ). The state i augmented by measurement noise to account for the significant errors in the back projectio of the corner points into a predicted bearing and elevation for fusion. The errors that are nonlinearly propagated through the measurement prediction equations are: 1 ) navigation eule angles, 2) installation angles of the turret relative t the aircraft body, 3) navigation position uncertainty, and 4) the gimbal angle uncertainties. These errors augment the state with an additional 12 states, leading to an augmented state size of 30 for the tracker of the ARC 50.

Constraint Fusion

The fi al step of the runway initialization takes into account the geometric constraints of the candidate runway. The UKFs ability to deal with arbitrary measurement equations t perform a. fusion using 6 constraints is used and the constraints are formed with reference to the runway geometry show in Figure 15.

The constraints that are implemented are that the vectors- between corners 1501 to 1504 and 1501 to 1502 are orthogonaL 1501 to 1502 and 1502 to 1502 are orthogonaL 1503 to 1504 and 1502 t 1503 are orthogonaL and 1503 to 1504 and 1501 to 1.504 are orthogonal. The runway lengt vectors 1501 to 1502 and 1503 to 1504, as well as the width vectors 1502 to 1503 and 1501 t 1504, should have equal lengths. The vector are computed in the NED frame and omit the down component. Similar known geometry constraints can be employed for flight decks and helipads.

The constraint fusion is implemented with the UKF as a perfect measurement update by settin the measure covariance in the UKF to zero. The constraints are applied as pseudo- observations due to the complexity of the constraints and their relationship to the state variables (see Julier, S.J., and La Viola, J, J., On Kalman filtering with nonlinea equality constraints, IEEE Transactions on Signal Processing, Vol. 55, No. 6, pp.2774-2784, 2007). Runway Initialization Validation

Once the ARC 50 establishes the runway track, i.e., all four comers have been initialized, it is compared with the known ERSA runway characteristics. The runway track produced by the tracker of the ARC 50 needs to pass a series of checks in order for the landing system 10 to allow the vehicle to land o the runway. The checks performed are:

1) Runway length edges are in agreement with each other, and within a tolerance of the ERSA run way length

2) Runwa width edges are in agreement with each other, and within a tolerance of the ERSA runway width (accountin for piano key offset from the edge)

3) Runway alignment is within a tolerance of the ERS supplied heading

4) Runway centre uncertainty is less than a tolerance in the North, East, and Down directions

5) A movin average of a number of th last absolute corner corrections is less than a tolerance for all 4 corners. For each filter update a change in the filter state, being a representation of the four ^' corners, _, is computed. A projected corner position before and after a filter update is used to also generate a change of position for the corner points and this is also stored. The check is accordingly passed when the positions of the corners do not change significantly.

Once all of the cheeks pass, the runway track ha been confirmed by the ARC 50 as a track of an actual runway that is part of the ERSA database, the track is inserted into the navigation filter provided by the ASN 20 to provide a tightly-coupled fusion with the navigation state of the aircraft.

Coupled Navigation Runway Tracking Fusion

The runway track is inserted into the na vigation fi lter of the ASN 20 using a minimal state representation. The 18 state filter used to initialise and update the runway track is converted into a 6 state representation with the states defined by; 1 ) North, East and Down position of the runway relative to the ERS A reference point, 2) Runway length, 3) Runway width, and 4) Runwa heading. For a runway that is not fixed, such as a flight deck on an aircraft carrier, other states can be represented and defined b other degrees of freedom (DQF) of movement. For example, states may be defined by the roll, yaw and pitch (attitude) of the .runway or the velocity and rate of change of measurements of the runway relative to the aircraft A runway, flight deck or helipad can he represented by 3 position states (e.g. x, y, z), 3 velocities (representing the rates of change of each position state) 3 attitude states (roll, yaw and pitch), 3 attitude states (representi g the rates of change of each attitude state) and stales representing the geometr of the runway, flight deck or helipad.

For the confirmed runway track, subsequent corner measurements are fused directly into the navigation filter, or in other words combined with or generated in combination with the navigation data generated by the navigation filter. The fusions are performed by predicting the bearin and ele tion for each corner. Consider the position of corner k in the runway frame

where s ₍ . and & _w represent the signs (+1 , -1) of the particular corner in the runway frame,

C " represents the direction cosine matrix relating the runway frame to the navigation frame, L is the runway lengt state, and W is the runway width state. The relative position of the corner in the measurement frame is obtained from

P , = c? (pi - P: )

The predicted bearing and elevation is then obtained by solving for the bearing and elevation in Eq. (12). By fusing these position measurements of the. corners into the navigation performed by the ASN 20 and transforming them to the navigation frame, the position of the runway relative to the aircraft at that point in time is set

The advantage of coupling the runwa tracking to the navigation solution provided by the ASN 20 is that the relative navigation solution .remains consistent with the uncertainties in the two solutions. Jumps in the navigation solution caused by changes in GPS constellation are taken into account through the erOss-correlation terms in the navigation covariance. This makes the runway track, once it validated or confirmed, much more robust than if it is tracked independently.

Tracking the runway for landing i important during the approach and landing phase as it is important to null any cross-track error so that the aircraft lands on the runway. This is provided by using runway edges detected during the approach. On transition to approach, the turret 32 is pointed alon the estimated runway heading direction in the navigation frame. The FDC 70 detect the runway edges and passe them to the ASN subsystem 20. The runway track state (represented by the 4 corners of the extreme runway threshold markings) is the related to the physical edges of the runway in the measurement frame. Considering the corners marked by 1501 and 1502 in Figure 15 as I and 2, b utilizing Eq. (33), _. but adjusting the width term to account for the edge offset, we obtain the vector between comers 1 and 2 in the measurement frame i obtained as

2 = C?(p* - O (35) The FDC 70 detects the edges in pixel coordinates and is able to compute a gradien and intercept of the edge in pixel coordinates. For generality, and referring to Eq. (6), set of nondimensional measurement frame coordinate is defined as

V in = * V x !n . - z = z t x ) (36)

The FDC 70 computes the slope and intercept in terms of nondimensional pixels by subtracting the principal point coordinates and scaling by the focal length. The measured nondi ensional slope and intercept of a runway edge is predicted by projectin the relative corners 1 and 2 into the measurement frame and scaling the y and z components by the x- component according to Eq. (36). The slope and intercept are computed from the two comer points, and it does not matter whether the two corner points are actually visible in the frame for thi computation. The runway edge is then used as a measurement to update the filter state b using the measured slope and intercept of the edge and a .lacobian transformation. Return- To-Base ( ^" RTB) Generation

Once the landing system 10 has con-firmed that the candidate runway is the desired and valid one- to land on. the APG 60 generates a full RTB waypoint set, which actually is a. land on candidate runway waypoint set. The RTB waypoint set generated for the FCC 100 includes an inbound tree, circuit, approach, landing, and abort circuit. Ail of these sequences are subject to a series of validity checks by the FCC 100 before the RTB set is acti ated and flown. The inbound tree that is generated is a set of waypoints, as shown in Figure 16, to allow the aircraft to enter into circuit from any flight extent. The FCC traverses the tree from a foot node and determines the shortest path through the tree to generate the waypoint sequence for inbound. The AR system generates the tree onboard as a .function, of the runway location and heading. Because the FCC performs the same checks on the dynamically generated inbound tree as for a static one generated for a mission plan, the AR system uses an inbound waypoint in every flight extent. Also, for every inbound waypoint, the APG 60 needs to generate a flyabie sequence from it to the parent or initial inbound point. The parent, inbound point is connected to two additional waypoint in a straight line that ensures the aircraft enters into circuit in consistent and reliable manner. This is important when operating in the vicinity of other aircraft. These two waypoints act as a constrain on the final aircraft heading at the end of the inbound tree.

The inbound tree is generated from a graph of nodes created using the process described in the Routing paper. A set of root inbound nodes are inserted into the graph based on a template constructed in a coordinate frame relative to a generic runway. These root inbound node are adjusted a a function of the circuit waypoints, described below. The nodes are rotated into the navigation frame based on the estimated runway heading. A complete inbound tree is found by using the modified Dijkstra's algorithm discussed in the Routing paper, where the goal is to find a waypoint set to take the vehicle from an initial point to a destination point. For the tree construction, the goal is to connect all nodes to the root. B using the modified form of Dijkstra's algorithm, a connected tree is autornatically determined since it inherently determines the connections between each node and the destination.

The circuit/abort circuit wayp.oin.ts are generated from a template with adjustable erosswind, C, and downwind, D, lengths, as shown in Figure 16. The template is rotated into the NED frame from the runway reference frame, and converted into latitude/iongitude/height. The cross ind, C, and downwind, D, lengths are adjusted so as to ensure the circuit waypoints all lie within flight extents. To allow' for maximum performance of the runwa edge detection, at least one ninway length is required between the turn onto final approach and the runway threshold.

Dynamic Landing Waypoint Control

Durin circuit, approach, and landing, the landing waypoints 1702, 1704, 1706 and 1708 shown in Figure 17 are updated at 100 Hz, based on the current best estimate of the runway position and orientation. This allow variations in the coupled navigation-runway track to be accounted for by the Guidance CSC on the FCC 100. This is inherently more robust than visual servoing since it does not close-the- oop directly on actual measurements of the runway 1710. For example, if the nose landing gear is blocking the view of the runway, then visual servoing fails, whereas the landing system 1,0 is still capable of performing a landing.

The four waypoints 1702, 1704, 1706 and 1706 adjusted dynamically are labeled approach, threshold, touchdown and rollout. All four waypoints are required to be in a straight line in the horizontal plane. The approach, threshold and touchdown waypoints are aligned along the glides lope, which can be fixed at 5 degrees.

Gimhaled Camera A gimbaled camera 32, 34 on the UAV allows the AR system 50, 60, 70 to control the direction and zoom level of the imagery it is analysing. The turret 32, such as a Rubicon mode! number AH1 25D, includes an electro-optical (EO) and infrared (JR.) camera 34, and is capable of performing full 360 pan and -5 to 95 degrees tilt. The EO camera may be a Sony FCB-EX408C which use the V1SCA binary communication protocol, which is transmitted over an RS-232 to the GEO subsystem 30. Turret control commands are transmitted by the GEO 30 to the Rubicon device 32 using an ASCII protocol, also over RS-232.

Gimbal Control The commands sent to the turret 32 are in the form of rate commands about the pan and tilt axes. These are used in a stabilization function on the turret wherein a stabilization mode gyroscopes in the turret and used to mitigate the effects of turbulence on the pointing direction of the camera 34. A velocity control loop is executed on the GEO subsystem 20. which is responsible for control of the turret 32, camera 34, and collecting and forwarding image data and associated meta-data to the FDC 70. The velocity control loop uses pan and tilt commands and closes the loop with measured pan and tilt values. The control loop is able to employ a predicti ve mechanism to provide for fine angular control.

High-level pointing commands are received by the GEO 30 from the ARC 50. The ARC 50 arbitrate to select from commands issued from a ground controller, the ASN system 20, the FDC 70, and the ARC 50 itself. In all cases, a ground controller has priority and can manually command the turret to move to a specified angle, angular rate, or point at selected latitude/longitude/height. During autonomous recovery, the turret 32 is commanded to point in variety of different modes. The turret can be made to "look at" selected points specified in different reference frame (camera frame, body frame, NED frame, ECEF frame). One mode is bounding box mode that adaptively changes the pointing position and zoom level to fit up to 8 points in the camera field-of-view. This mode is used to point at the desired ends of the runway using the best estimate of the piano keys, and takes int account the uncertainty in their position. A GEO control process of the GE 30 computes a line of sight and uses a Newton algorithm (a root-finding algorithm to solve for the zeros of a set of nonlinear equations) to iterativel calculate the required pan/tilt angles.

Zoom Control

Camera zoom is either controlled independently of the pointing command, or coupled to it. The zoom can be set. via a direct setting command as a ratio or rate, or can be specified as an equivalent field-of-view measured by its projection onto the ground plane (i.e., units are in meters). This type of control maintains an area in the image quite well by adjusting the zoom level as a function of the navigation position relative to the pointing location. The combined zoom mode uses up to 8 points in the ECEF frame to select a zoom setting such that all 8 points lie within the image.

Image And Gimbal Timestamp

In addition to running the turret control loop, the GEO subsystem 30 is responsible for capturing still images from the video feed and time stamping the data. The GEO subsystem obtains frequent, e,g. 100 Hz, navigation dat from the AS subsystem 20, and zoom and gimbal measurements from the camera and turret, respectively. These measurements are buffered and interpolated upon receipt of an image timestamp (titties tamps are in UTC Time). Enler angles are interpolated b using a rotation vector method.

Navigation data is time stamped accordin to the UTC Time of an IMU data packet used on a navigation data frame, which is kept in synchronization with GPS time from the GPS unit 180. Gimbal data is time stamped on transmission of a trigger pulse sent by the GEO 30 to the turret 32, The trigger is used by the turret to sample the gimbal angles, which is transmitted to the GEO after it receives a request for the triggered data. Zoom data is lime stamped by the GEO on transmission of the zoom request message. Images are time stamped upon receipt of the first: byte from a capture card of the GEO 30, and is intercepted on a device driver level. However _* this does not provide the time that the image was actually captured by the camera 34. A constant offset is applied that is determined by placing an LED light in front of the camera 34 in a dark room. A static map of pixel location versus pan position was obtained by manually moving the turret to various positions. The turret is then commanded to rotate at various angular rates while simultaneously capturing images. By extracting the position of the LED. and by using the inverse map of pan position, the image capture time can be estimated and compared with the time of arrival of the first image byte. For example, a constant offset of approximately 60 rfis between the image capture time and the arrival of the first byte on the GEO can be used.

Conclusion

The landing system 10 differs fundamentally from previous attempts to utilise vision based processes i landing systems. One difference arises from the way the syste treats the landing problem. Previous researchers have attempted to land an aircraft by its lateral position relative to information provided from an on-board camera. Unfortunately, thi type of approac alone is not onl impractical (it relies on somehow lining the aircraft up with the runway a priori), but also dangerous. Any obfuscation of the on-board camer during the final, landing phase is usually detrimental. Instead of treating the landing phase in isolation, the landing system 10 adopts a synergistic view of the entire landing sequence. The system 10 seeks out a candidate runwa based on a runway data on the aircraft (or obtained from elsewhere using communications on the aircraft), generates a route t the runway, precisel locates the runway on a generated surve route, tracks and validates the runway during vehicle flight and establishes a final approach and landing path.

It is particularly significant that the aircraft is able to use a camera system to obtain images of a candidate runway, and then process those images t detect features of the runway in order to confirm that a candidate landing site includes a valid runway, flight deck or helipad on which the aircraft could land. The images may be obtained from ineident radiation of the visual spectrum or infrared radiation; and the FDC is able to use multi- pectral images to detect the extents of the landing site, i.e. corners of a runway. Whils comparisons can be made with an onboard runway database, candidate sites and runways can be validated without comparison simply confirming that the detected features correspond to a runway on which the aircraft can land.

Coupling or fusing the runway track initialised and generated by the ARC 50 with the navigatio system 20 used by the aircraft also provides a considerable advantage in that the aircraft is able to virtually track the runwa along with the navigation data that is provided so as t effectively provide a virtual form of an instrument landing system (ILS) that does not rel upon any ground based infrastructure. This is particularly useful in both manned and unmanned aerial vehicles.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described.