Communications: radio wave antennas – Antennas – With aircraft
Reexamination Certificate
2002-08-09
2004-10-19
Phan, Tho (Department: 2821)
Communications: radio wave antennas
Antennas
With aircraft
C342S169000, C342S170000
Reexamination Certificate
active
06806837
ABSTRACT:
FIELD OF INVENTION
This invention relates to direction finding and more particularly to a system for calibrating an array of direction finding antennas on an aircraft.
BACKGROUND OF THE INVENTION
Typically and for many years, surveillance aircraft have been provided with an array of for instance sixteen to thirty-two loop and or monopole-type antennas dispersed about the surface of the aircraft to be able to get the bearing line from this aircraft to a source of electromagnetic radiation. This source can be from for instance transmitters used by enemy troops, transmission sources associated with weapons and ordinance, or can be radiation from any type of communications device.
In the past, surveillance aircraft with such an array of direction finding antennas have been calibrated by establishing a calibration antenna on the ground and flying at some distance from this antenna so that the depression angle between the aircraft and the antenna is close to 0°. By depression angle is meant the angle down from the horizontal of a bearing line between the plane and a radiation source on the ground. The calibration of the antenna array involved the flying of an aircraft in a horizontal circular or banana pattern such that the aircraft was in essence turned 360° in azimuth, with measurements made of the response of the antennas at 1° or 20° azimuth increments and for all of the frequencies of interest. This provided a data set so that actual measurements from the aircraft could be correlated with the calibrated data set in order to arrive at a precise bearing line from the source of the electromagnetic radiation to the aircraft. In one example, the desired accuracy was 5°.
As was usually the case, these surveillance aircraft operated a fairly large distance away from enemy territory for safety reasons. Thus, the signals coming from enemy radios or transmitters would come in at a relatively shallow depression angle.
However, with the use of unmanned aerial vehicles, or UAV'S, due to the fact that they are unmanned, they can be flown directly over enemy territory unlike the manned surveillance aircraft used previously. The reason for using unmanned aircraft is to limit the exposure of airmen to hostile fire. However, the use of such UAV's requires that the antenna arrays on the UAV's be calibrated for all depression angles including the relatively deep depression 80°-90° angles that exist as the UAV flies directly over a surveilled area.
The problem of utilizing a full-scale airplane and flying it over a calibrating antenna is that it is very difficult for a plane to maintain a constant depression angle relative to the calibrating antenna when flying the aircraft in a circle. The reason is that it is not possible to spin the aircraft 360° on its own axes above the ground in order to get calibration data for all azimuths. Rather the plane can only execute a relatively large circle or oval. If the plane is close to the calibration antenna, the depression angle at the nearest point on the circle varies greatly from the depression angle at the far point of the circle. Thus, it is exceeding difficult to maintain a constant depression angle for a 360° azimuth sweep when flying a full-scale aircraft. This is due to the dynamics of flight which prohibit tight turns.
In short, when trying to calibrate a DF antenna array at a constant deep depression angle, one cannot do it by flying a plane.
SUMMARY OF THE INVENTION
Noting that there is a difficulty of rotating an aircraft 360° while maintaining a predetermined depression angle for calibration purposes, in the subject invention, an electrically similar scale model of the aircraft is provided with antennas at the same positions as they are on the full-scale aircraft. An optimization technique adjusts the response of the antennas on the model to the expected outputs of the antennas on the full-scale platform. This scale model is located on, the ground at a calibration range and is supported by a gantry which rotates the model over a number of depression angles and also swings the model over the full 360° azimuth range that is required. Measurements are then taken from the model at a wide variety of depression angles, one of which is identical to the shallow depression angle of the full-scale aircraft executing maneuvers at a distance from the calibration antenna. The depression angle measurements from the full-scale aircraft are made at quite some distance from the calibration antenna so that, for instance, a nearly constant depression angle in the range of −2° to −5° can be obtained. The plane is flown in a pattern that will establish the response of the antennas in a 360° azmuth sweep for 1° increments and for all of the frequencies of interest. This provides a data set for the full-scale platform and the particular antenna array, which is then used as a base line to be able to correlate the results of the model with the full-scale aircraft.
Data collected from the model at this shallow depression angle for the indicated frequencies, and at 1° azimuth increments when processed with live data from the aircraft at this shallow depression angle results in a set of complex weights which are used to account for differences between the full-scale and model antenna responses.
Once having model data for this shallow depression angle, data is then taken from them model at the other desired depression angles. This data is corrected by the weights derived from the model and the full-scale aircraft at the above-mentioned shallow depression angle. It is thus a finding of the subject invention that weights generated for the single shallow depression angle done in this fashion can be used to adjust and correct the model airborne array data recorded for all depression angles.
The model therefore provides virtually all of the data that is to be used in the full-scale aircraft. The result is that the full-scale aircraft will be provided with a data set or array manifold that permits accurate direction finding when the aircraft is flying at stand off or stand-in ranges from electromagnetic sources.
Thus, in the present invention, live data need only be taken at one depression angle, which data is then compared with data at a number of different depression angles taken from the model. With the advent of airborne vehicles that fly directly over hostile territory for detecting the direction of RF sources, a method for calibrating the antennas on the vehicles is provided so as to correctly determine the direction of the source of electromagnetic radiation, especially at deep depression angles associated with such flights. In order to accomplish this, all that is required is to obtain a set of data from a given relatively shallow depression angle in a flight test and then provide a model of the aircraft with antennas appropriately located. A weighting system is then devised to be able to weight the outputs of the various antennas on the model such that a data set or array manifold is available at the aircraft to correct the output of the airborne antenna array. When a direction finding algorithm is applied, the accuracy of the direction finding result will be within specified accuracy requirements.
Note that a complex optimization technique is used to generate complex weights that are then used to adjust the data collected from the model to account for the differences between the full-scale and the model antennas arrays. The result is an easily obtained deep depression calibration database.
In summary, a system for calibrating airborne direction finding antenna arrays eliminates the problem of trying to maintain a constant depression angle when flying an airplane directly over a calibration source antenna to collect deep depression angle data. The deep depression angle data necessary for calibration is provided by data from a scale model of the aircraft having a direction-finding array which simulates the actual direction-finding array on the aircraft. In order to collect deep depression angle data, the model is pivoted through 360° while mai
Paul Norman D.
Saucier Norman E.
BAE Systems Information and Electronic Systems Integration Inc.
Long Daniel J.
Phan Tho
Tendler Robert K.
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