| WO/1998/010307 | LOCATION OF A MOBILE STATION |
| WO/2022/182893 | SYSTEMS AND METHODS FOR TRACKING LOCATION |
HJELMSTAD JENS FREDRIK (NO)
| US4638320A | 1987-01-20 | |||
| GB2303266A | 1997-02-12 | |||
| EP1118870A2 | 2001-07-25 |
| 1. | A method for identifying side lobe signals in a directionfinder system, including comparing the phases of a signal from an emitter source 5 received with a first and a second directive search antenna (10, 10'), comparing the amplitude of the signal received with a guard antenna (13) and the amplitude of the signal received with one of the search antennas (10, 10' ) , said guard antenna o (13) having a broad or omnidirectional antenna diagram, determining a direction of arrival for the signal from the emitter source based on the phase and amplitude information, characteri zed in : selecting a signal from the emitter source for further s processing in the directionfinder system if the direction of arrival lies within a main lobe (3) of the search antennas (10, 10'), and identifying the signal as a side lobe signal if the direction of arrival lies outside said main lobe (3) . |
| 2. | o. |
| 3. | A method as claimed in claim 1, characterized in canceling said side lobe signal. |
| 4. | An arrangement for identifying side lobe signals in a directionfinder system, including a first directive search antenna (10) receiving a signal 5 from an emitter source, a second directive search antenna (10') receiving the signal from the emitter source, a first phase channel (11) processing the signal received by said first search antenna (10), 0 a second phase channel (H' ) processing the signal received by said second search antenna (10'), said first and second phase channels (11, 11') being identical, a guard antenna (13) receiving the signal from said emitter source, said guard antenna (13) having a broad or omni 5 directional antenna diagram, a first amplitude channel (14) processing the signal received by the guard antenna (13) , a second amplitude channel (14') processing the signal received by one of said search antennas (10, 10' ) , said first and second amplitude channels (14, 14') being identical, a phase comparison unit (12) comparing the phases of signals output from said first and second phase channels (H, 11'), an amplitude comparison unit (15) comparing the amplitudes of signals output from said first and second amplitude channels (14, 14'), a processing unit (16) determining a direction of arrival for the signal from the emitter source based on the phase and amplitude information from said phase and amplitude comparing units (12, 15), characteri zed in : said processing unit (16) being arranged to select a signal from the emitter source for further processing if the direction of arrival lies within a main lobe (3) of the search antennas (10, 10' ) , and identify the signals as a side lobe signal if the direction of arrival lies outside said main lobe (3) . |
| 5. | An arrangement as claimed in claim 3, characterized in that said arrangement being adapted to cancel said side lobe signal. |
Field of the invention
The present invention relates to direction-finder systems for identifying and localising radio frequency emitters, and in particular a method for identifying signals received in the minor lobes of an antenna system in such a system.
Technical background
Radio frequency emitters (radars, satellite uplink stations, cell-phone base stations, relay links) can be detected, analysed, and geo-referenced from a remote observation platform. This is achieved using a sensor with an antenna system for detecting the radiation, connected to a receiver and processing system. These systems can be deployed from satellites, aircraft, UAVs, ships vehicles or mounted in masts.
Typical solutions employ radio receiver systems operating in the frequency bands 1 through 12 GHz. These systems employ multiple receiving antennas and multiple receivers to derive a course direction to the emitters.
The direction to an emitter source is found by comparing the phase of signals received in two or more antennas. The signals are also analysed in order to identify the emitter source. However, an antenna diagram is not perfect, and consists of a main lobe with several minor lobes at the sides of the main lobe. Prior art direction-finder systems can not distinguish between a small signal received in the main lobe, or a large signal received in one of the minor lobes. Thus, pulses received in the side lobes will create ambiguities in the detections. Since there is no means of determining the correct position, these pulses will be regarded by the processing system to be within the observation area. In the process of identifying the emitter source, the received pulses may be recognized to be the same as one seen previously. One might think that such pulses could be used to improve parameter estimates, but beware: the signal to noise ratio will be drastically reduced, so the result might more probably be a reduction in estimate quality.
Another source of ambiguity in direction of arrival lies in the phase comparison itself. It is well known that that the phase angle determined when comparing the signals from two receiving antennas will repeat itself at even intervals. This means that a given phase angle will not be conclusive as regards the direction to the emitter. To solve this ambiguity, prior art systems have included an omni¬ directional guard antenna in addition to the directive search antennas. The amplitude of the signal received by the guard antenna is compared with the signal amplitude from one of the search antennas in order to decide which angular direction in space that corresponds with a given phase angle.
Summary of the invention
It is an object of the present invention to provide a method and arrangement for finding the direction to an emitter source which effectively identify and possibly cancels all signals received in the minor lobes of the directive search antennas of a direction-finder system.
This is achieved in a method and arrangement as claimed in the appended patent claims. The core idea of the invention is to utilize direction information obtained with a guard antenna to actively identify signals received in minor antenna lobes.
Brief description of the drawings
The invention will now be described in detail in reference to the appended drawings, in which:
Fig. 1 is a general overview of a direction-finder system in which the present invention may be implemented,
Fig. 2 shows the architecture of a receiving system for use in the direction-finder system in Fig. 1,
Fig. 3 shows two channels in a direction-finder system, with the resulting antenna diagram, for illustrating the problem solved by the present invention,
Fig. 4 shows a corresponding system with an additional guard antenna, as used in the present invention.
Fig. 5 shows the direction ambiguity intervals for an antenna system including an antenna pair with center displacement of 0.16 - 0.5 m.
Fig. 6 shows an alternative use of the invention.
Detailed description of the invention
Initially, we will give an overview of the direction-finder system as a background for the present invention. As shown in Fig. 1, the direction-finder system includes an antenna unit 1 at left receiving RF signals from a number of emitter sources. The signals are delivered to an RF unit 2, center, where they are amplified, transposed down to baseband and demodulated. The demodulated signals are delivered to a processing unit 3 for processing and analysis.
Briefly, the antenna arrangement includes four antenna panels mounted in a 2X2 relationship, as well as an omni- directional guard antenna that may be mounted at the center of the antenna unit.
Fig. 2 shows the receiver and processor section in a direction-finder system according to the invention. The receiver section comprises four phase channels and one guard channel. The figure shows the main components included in the receiver chains, i.e. amplifier 21, mixer 22 and bandpass filter 23. The signals from the receiver section are entered into the following processing unit, which includes a digitizer with two stacked A/D converters 24.
Fig. 3 is a schematic illustration showing an antenna 1 connected to a receiving system 2. In the receiving system, signals received by the antenna 1 are processed and analyzed. The antenna diagram includes a main lobe 3 and a number of minor lobes 4, 5, 6, 7. By careful design of the antenna 1 (or rather an antenna array) , the size and number of side lobes may be minimized, in some degree. However, no antenna system is perfect, and most antennas will have side lobes in the antenna diagram. The side lobes creates a problem for the signal processing as the system can not distinguish between a small pulse 8 appearing in the main lobe 3 and a large pulse 9 received in one of the minor lobes, here lobe 4. The lower gain in the side lobe will make the pulse look identical, from the receiver's point of view.
Fig. 4 is an illustration of a direction finder system with directive search antennas 10 and 10' . The signals from the search antennas 10, 10' are received, detected and amplified in receiving systems 11 and 11' . The receiving systems 11, 11' are phase linear, and the signals will be output with phase angles φ and φ' . The phase angles are compared in phase comparator 12 to determine a phase difference Δφ. This phase difference is indicative of the direction of arrival of the incoming signal. The direction of arrival is determined by a processor unit 16, and is indicated with an arrow in the figure. However, there are several directions of arrival that may satisfy a given phase difference Δφ. In Fig. 4, this is illustrated with the three angular directions 17a - 17c. There are three corresponding directions of arrival 17a' -17c' on the other side of the midline 18, but the system is able to distinguish between signals coming in from the left or right side of the midline, so these directions poses no problem. The angle interval is determined by the wavelength and phase center displacement. For the geometries of the antenna system mentioned above, with possible phase-center displacements of 0.16 - 0.5m, ambiguity intervals of 13.3 degrees to 4.7 degrees are obtained, as shown in Fig. 5. In order to detect which of the directions 17a - 17c that is the correct one, an additional guard antenna 13 has been added to the system. This antenna is oinni-directional, or has a much broader antenna pattern than the search antennas 10, 10' . The signal from the guard antenna 13, and the signal from one of the search antennas are fed to identical receivers 14, 14', and the amplitude in the two channels compared in comparing circuit 15. The output from the comparing circuit 15 is used by the processor unit 16 to select the correct direction of arrival from the three alternative directions 17a - 17c. This technique is basically known from direction-finder systems according to the Adcock system. However, according to the present invention the processing unit is using this information to cancel all signals arriving from all directions other than covered by the main lobe 3. The receiving channels may include means for sorting the received signals into amplitude groups. This "multiple thresholding" may be obtained by using a gain controlled amplifier, or an array of comparators. The lowest comparator level should be above the levels of the side lobes. By this means, the received signals may quickly be sorted into which part of the main lobe they arrive in.
Fig. 6 illustrates another application of the invention. A number of antennas 61a - βln are placed side by side in a circle to cover parts of or the entire horizon. An omni¬ directional guard antenna 62 is placed in the center of the circle. The direction to any incoming signal may then fast . be determined. This system may be used in cellular phone systems. By placing the antenna array at a base station, the position of a mobile station within the cell may be determined, and the transmitted energy directed towards said mobile station. Such information may of course be used for other purposes as well.
The arrangements and methods of the present invention have been described in a two-dimensional setting. However, the invention may as well be expanded to cover all three dimensions, by using a 3-D stack of antennas pointing in all directions.