Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Utility Patent
1999-09-21
2001-01-02
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S444000, C342S445000
Utility Patent
active
06169519
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a direction finding antenna system. More specifically, it relates to a receiver system in a Traffic/Alert Collision Avoidance System (TCAS) which is particularly suited for determining the bearing of a target aircraft relative to a protected aircraft and methods of operation.
The TCAS equipment located aboard a protected aircraft periodically transmits interrogation signals which are received by transponders located aboard other aircraft, hereinafter referred to as target aircraft, in the vicinity of the protected aircraft. Mode S altitude reporting transponders are preferred so that TCAS capabilities can be fully exploited. In reply to the interrogation signals, the target aircraft's transponder transmits a response signal. The TCAS equipment aboard the protected aircraft determines the range of the target aircraft in accordance with the round trip time between transmission of the interrogation signal and receipt of the response signal. Relative bearing to the target aircraft is determined from differences in the time to different elements in the TCAS antenna. In addition, if the target aircraft is equipped with a Mode C or Mode S transponder, the protected aircraft can determine the altitude of the target aircraft from the content of the response signal.
Information in the response signal, as well as information derived from the response signal, is used by the TCAS equipment to determine whether there is a threat of collision between the protected aircraft and the target aircraft. The response from each target aircraft is processed individually to determine the degree of threat and then, if necessary, an appropriate advisory is issued by the TCAS equipment to the pilots of the protected aircraft to minimize the degree of threat.
Relative bearing to the target aircraft may be determined from the response signal by a multi-element antenna array, for example, by a four-element antenna array and an associated receiver system. Typically, a TCAS antenna consists of an array of four vertically polarized elements. The elements are preferably spaced equally about the circumference of the antenna array, that is geometrically at 90° intervals. A first pair of elements, therefore, is aligned on an axis which is perpendicular to an axis on which the second pair of elements are aligned. Adjacent elements are those which are circumferentially spaced apart by 90° geometrically. Opposite elements are those which are circumferentially spaced apart by 180° geometrically.
Conventionally, the relative bearing to the target aircraft is determined by measuring the phase difference of the response signal between opposite element pairs. If both pairs of opposite elements are used, for example, then the phase difference between one opposite element pair is K sin (&bgr;) and the phase difference between the other opposite element pair is K cos (&bgr;), where K is the maximum phase excursion of the response signal between the elements in the respective opposite pair, and &bgr; is the angle of the target signal incidence with respect to the axes connecting antenna elements within an element pair. The actual relative bearing is then calculated as follows:
&bgr;=tan
−1
(K sin (&bgr;)/K cos (&bgr;))
Such a system functions properly as long as K is less than 180° electrical degree in space at the operating frequency. K is related to element spacing. When K is greater than or equal to 180°, an ambiguity exists as phase detectors in the receive system are unable to properly distinguish phase differences. In such cases, for example, the phase detector cannot differentiate between measured phase differences of 180° and −180°. Under these circumstances, the relative bearing to the intruder aircraft cannot be determined with certainty. This ambiguity conflicts with a desire to utilize an antenna which has the greatest phase excursion between opposite elements (i.e. large element spacing) in order to maximize the signal to noise ratio of the system.
Also, in a phase only measurement system, the phase error of the measurement means cannot be ignored. For example, some of the receive system induced phase errors cannot be corrected by factory calibration of system equipment since phase variations occur in the system components with varying temperature, age and other variables.
Several methods of addressing these phase errors have been proposed in the past. U.S. Pat. No. 5,122,808 issued on Jun. 18, 1992, to Constantinos S. Kyriakos entitled “PHASE ONLY BEARING MEASUREMENT WITH AMBIGUITY CORRECTION IN A COLLISION AVOIDANCE SYSTEM”, is one of such methods.
While this approach has enjoyed considerable use in recent years, it has several drawbacks. One drawback with this approach is its inability to determine the bearing of an intruding aircraft without receipt of at least two or more transmissions from the intruding aircraft.
In prior TCAS bearing implementations using phase measuring techniques, two or more transmissions were often necessary before a bearing value could be computed. Between the transmissions, hardware reconfigurations were required that could introduce errors that could result in errors in relative bearing calculations.
When more than one transmission is required to obtain the data needed to calculate the relative bearing, target data points in one transmission must be paired up with corresponding target data points, target for target. Any error in pairing could result in considerable error in bearing calculations. One method of reducing the probability of incorrect pairing is to implement digital filtering that operates on several transmissions, but this further extends the time delay before valid relative bearing can be determined.
Within a TCAS system, interrogations often occur at the rate of 1 per second. In prior inventions, relative bearing determination is often made after two or more interrogations. Thus, bearing update is often after a delay on the order of a few seconds. Each message transmission from a transponder unit is typically 64 uSec or 120 uSec in length, and is made up of several pulses that are typically 0.5 uSec each in duration.
Consequently, there exists a need for improved receiver systems for measuring the bearing of an intruder aircraft.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide for an improvement in bearing determinations of an intruding aircraft.
It is a feature of the present invention to include a four-element antenna array where each antenna element in the array has associated with it, its own receiver.
It is an advantage of the present invention to provide for bearing determination without the need for “swap switches” which have been common in prior art TCAS receiver systems.
It is another object of the present invention to provide for bearing determinations of intruding aircraft with the reception of a single transmission from the intruding aircraft.
It is another feature of the present invention to include receivers dedicated to the antenna elements where each receiver is capable of reception and demodulation of both in-phase (I) and quadrature (Q) components of transmissions incident thereon from an intruding aircraft.
It is another advantage of the present invention to eliminate the need for receiving multiple transmissions from an intruding aircraft before a bearing calculation can be made.
It is an objective to make all four I and all four Q measurements at the same time to avoid errors that can be introduced when only two I and two Q measurements are made at one time and the other two I and two Q measurements are taken at another time, possibly with hardware reconfigurations that can introduce errors.
The present invention is a method and apparatus for determining the relative bearing of an intruding aircraft which is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features and achieve the already articulated advantages. The present invention is carried on in a “multiple reception-less” sys
Holecek Charles L.
Kyriakos Constantinos S.
Wahab Sami R.
Eppele Kyle
Jensen Nathan O.
O'Shaughnessy J. P.
Phan Dao L.
Rockwell Collins, Inc.
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