Background of the invention The purpose of so-called air borne early warning (AEW) radar systems is to overview a large air space in order to detect, track and classify objects. Operating at high levels, these radar platforms have the advantage that they overcome the shadow cast by the curvature of the earth in relation to ground systems more efficiently and hence widely exceed the surveillance volume of the latter systems.

One known system denoted AWACS (Airborne Early Warning and Control System) comprises a rotating substantially plane phase array panel mounted in a disc shaped radome on the back of an aircraft. The phase array antenna comprises electrically ele- vation beam steering and has a narrow azimuth beamwidth.

It is believed that the above system may pursue many different flight patterns for moni- toring a given airspace. For instance, a circular track may be envisioned in order to maintain overview of an area of certain interest.

Phased array systems are widely known, confer for instance US5412414.

Other similar AEW systems (Hawkeye), which has a narrow beam with regard to hori- zontal, utilise a flight track with so-called flat turns, i. e. turns with no or very little aircraft roll in order to maintain the radome at a substantial horizontal alignment. However, only certain aeroplane types, predominantly propeller planes, are able to perform this ma- noeuvre, which moreover may be perceived as unpleasant by the craft.

Another known system named ERIEYETM, comprises a sleek phase array antenna sys- tem fixedly mounted on the back of an aircraft. The system comprises two opposing plane antenna panels scanning horizontally two, approximately 120° wide windows, on each side of the aircraft with a relative narrow beamwidth with regard to horizontal. The

windows may be sweeped in an arbitrary fashion, and with a certain weighting being given to certain sectors or individual identified objects of special interest, that is, some sectors and objects may be illuminated more frequent than average. In order to com- pensate for the systems blind angles with regard to fore and after, a slalom flight track may advantageously be used.

The above system may utilise a so-called racetrack shaped flight path, i. e. linear motion with two narrow 180° turns, each with a duration of approximately 60 seconds, for sur- veying a given air space of interest. During turn, monitoring can not be achieved be- cause of the roll of the aeroplane and the radar emissions are turned off. For the identi- fied objects, the system continuously estimates an area corresponding to the respective objects possible future locations taking into account their possible manoeuvres. The system utilises this information to facilitate re-identification of objects after signal inter- ruption. This mechanism is used when echoes are lost such as after aircraft turn.

An algorithm for generating optimised flight paths with regard to radar coverage for plat- forms having angle dependent ranges is described in patent application W099/32900.

US3984837 shows an aircraft borne low drag radome, which is kept horizontal during varying flight attitudes. The antenna element is rotating and the compensation is made for roll and pitch by making use of the gyro system of the aircraft.

Summary of the invention It is a first object of the present invention to set forth a method for enhancing the effec- tive radar coverage during aircraft turns.

This object has been accomplished by the antenna system according to claim 1 and the method according to claim 8.

It is a secondary object to set forth an antenna system attaining aerodynamic effects being independent of radar operation.

This object has been accomplished by the subject matter set forth in claim 2.

It is a third object to set forth an antenna system, which provides effective cooling.

This object has been achieved by claim 5.

It is a fourth object to set forth an antenna system, which imposes minimum radome ef- fects to the radar system and minimal aerodynamic drag.

This object has been accomplished by claim 6.

Further advantages will appear from the following detailed description of the invention.

Brief description of the drawings Fig. 1 shows a cross section of a first preferred embodiment of the invention, Fig. 2 shows a fore cross-section of the first preferred embodiment of the invention, Fig. 3 shows an aft cross section of the first preferred embodiment of the invention, Fig. 4 shows a second preferred embodiment of the invention, Fig. 5 shows an aft cross section of the second preferred embodiment of the inven- tion, Fig. 6 shows a third preferred embodiment of the invention, Fig. 7 is a schematic representation of one example of the invention in use Fig. 7.1-3 are side views of fig. 7 in positions 1', 2'and 3', and Fig. 8 shows an exemplary window mask according to the invention.

Detailed description of preferred embodiments of the invention In figures 1-3, a first preferred embodiment of the invention has been shown. The radar system 1 is adapted to be mounted on the back of an aircraft (not shown) by means of a fixture 6 of rods.

As is shown on fig. 1, which is a cross section of plane A A of fig. 2, the fixture is carry- ing a radome 2, which houses and protects the radar components. The radome 2 is on a middle portion substantially cylindrical with a rotational symmetric wall cross section. A cradle 17, comprising two opposing antenna panels 18, are pivotally mounted in relation to the radome on an axis which is intended to be substantially parallel with the aircraft's operational direction of flight. As shown in fig. 2, one section of the cradle 17 in the fore of the radome has outer portions that are provided with self aligned first roller bearings 13. Aft comer portions of the cradle, as shown in fig. 3, are provided with second roller bearings 14. The above first and second roller bearings 13,14 roll in respective first and second peripheral bearing races 15,16, which are provided on the inside radome wall.

The fore roller bearings are selfadjusting and adapted to take up axial and radial forces, while the aft roller bearing is only taking up radial forces.

The radome 2 is of a composite material, which is widely temperature stable, strong and light.

The panels of the radar system are phased array panels 18, which are connected to a plurality of logically controlled T/R modules 19. The T/R modules are connected to distri- bution nets 20, which connect to an exciter receiver module (not shown), which allow electrically controlled horizontal scans within a certain angular area. The phased array panels according to the invention are either not able to perform vertical scans or are only able to perform vertical scans within a limited angular area.

The details of such phased array antenna systems are known in the art and shall there- fore not be explained further. In the present embodiment, two plane panels have been shown, but it should be understood that also curved panels or segments of a circular ring would fall under the scope of the present invention.

The cradle 17 is moved via a gear ring 26 mounted on the cradle, which engages a drive shaft of a stepper motor 25 arranged in the tail of the antenna system.

Advantageously, the stepper motor is operated automatically in response to signals pro- vided by the gyro system of the aircraft (not shown), such that the panels 18 are held in an upright position-or at a constant desired angle to an upright position-independently of the roll of the aircraft.

Due to the power consumption of the plurality of T/R modules 19, effective cooling of the antenna system is required. An air intake 8 is arranged in the front end of the radome 2.

The rear side of the air intake is coupled to an intake duct bearing 10, which couples to an intake duct 7. The intake duct 7 is mounted to a lower portion of the front end of the cradle 17 and is adapted to swivel with the latter. An internal intake duct 9 and an inter- nal outlet duct 23 are arranged between the two opposing panels 18. Cooling air is dis- persed to/and from the plurality of electronic components between these internal ducts and in particular over the T/R modules 19. The internal outlet duct 23 is connected to an outlet duct 22, which then again is pivotally mounted to the outlet 21 over an outlet duct bearing 24.

Due to the symmetry of the mid radome section 4, being formed as symmetric ring, the reflective and dampening electromagnetic properties of the radome will not vary as the cradle swivels inside it. As can be seen, the distance between various points on the an- tenna panel 18 and the radome 2 differs in various directions, however these effects can be compensated for by suitable calibration.

Since the radome 2 is fixedly arranged with regard to the aircraft body, the structure is aerodynamically constant. This implicates that the radar system will affect the fly char- acteristics of the aircraft minimally. Moreover, the above structure will render airworthi- ness tests more economical.

Fig. 4 and 5 disclose a second preferred arrangement for suspending the cradle. In this embodiment, the cradle 17 is centrally mounted to a hub 28, which is mounted on a central pivotal mount 29 on a bridge portion 27. The bridge portion 27 is connected to the fixture or is a part of the fixture 6. As appears from fig. 4 and 5, which shows the aft bridge portion, the legs of the bridge portion is not obstruct the movement of the air duct.

The legs of the fore bridge portion (not shown) is arranged at 180 with respect to one another for allowing the movement of the air duct.

According to this embodiment, the radome 2 can be made lighter as it is de-coupled from the cradle 17. Consequently, the centre of gravity of the combined antenna struc- ture/aircraft structure may be lowered toward the aircraft's centre of gravity.

In Fig. 6, a third embodiment of the invention has been shown. A radome with a sub- stantially rectangular radome is centrally mounted on a lower pivotal mount 30 and is actuated for side to side movement over a hydraulic lever 31. The radome yields ho- mogenous reflecting/dampening properties over the entire antenna panel field, as the distance to the radome from the antenna panel is constant. Moreover, the radome pres- ents a minimised front area reducing aerodynamic draft and improving the maximum speed of the vehicle.

Fig. 7 shows a first preferred embodiment of operating the system in an exemplary air- monitoring situation. The radar platform 33 follows a race track path 34 as known in the art with brief 180° turns. As depicted in fig. 7.1, the aircraft is level when flying straight at location 1'and the aircraft is for instance scanning the entire surrounding air space while tracking objects in directions a, b, c and d. However, in situation 2', shown in fig. 7.2, a turn has been initiated and the aircraft rolls. Consequently, the antenna system is oper- ated to compensate for the roll by holding the antenna panels 18 in a given upright posi- tion. However, as appears from fig. 7.2, the right upwardly pointing wing shades for visi- bility to directions c and b. At the exit of the turn, shown in fig. 7.3, the wings does not obstruct any of the given directions of particular interest.

According to the invention, the radar system is operating according to an aircraft specific window mask 35 that is shown in fig. 8 by which a certain area of the scanning field is not illuminated. The window mask as a function of aircraft roll has been determined and is stored in a memory (not shown) in the system. When the aircraft rolls, a control unit (not shown) controls the illumination in correspondence to the window mask, such that no scans are made into the wing or other parts of the aircraft. Preferably, the window mask is calculated by using computer aided design tools for the aircraft/antenna sys- tem combination.

It should be understood that the invention not only is applicable for manned aircrafts but also for unmanned flying vehicles.

Hence, the signal interruptions or outages, which occur during aircraft turn, have been significantly reduced leading to enhanced monitoring and tracking capabilities.

Reference signs 1 antenna system 2 radome 3 radome fore section 4 radome mid section 5 radome aft section 6 radome fixture 7 intake duct 8 air intake 9 internal air intake 10 intake duct bearing 13 fist roller bearing 14 second roller bearing 15 first peripheral race 16 second peripheral race 17 cradle 18 antenna panel 19 T/R module 20 distribution net 21 air outlet 22 outlet duct 23 internal outlet duct 24 outlet duct bearing 25 stepper motor 26 gear ring 27 bridge portion 28 hub 29 central pivotal mount 30 lower pivotal mount 31 lever 32 race track flight path 33 aircraft 34 angular window 35 window mask