Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2000-01-19
2001-07-03
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S442000, C342S444000, C342S445000
Reexamination Certificate
active
06255991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an angle of arrival measurement system, and more particularly, to an angle of arrival interferometer measurement system for a received signal that has reduced complexity and cost over existing interferometer systems.
2. Discussion of the Related Art
Many situations exist where it is desirable to know the direction from which an RF signal is received. For example, it is desirable to use RF interrogation signals to determine the location of aircraft in flight for traffic separation purposes. An aircraft may transmit an RF beacon signal that is received by other aircraft in its vicinity. The aircraft that receives the beacon signal determines the direction and distance of the signal source to determine the location of the aircraft. The direction of the received signal can be used to map the location of the aircraft on a screen to determine its location relative to other aircraft. Signal direction or angle of arrival (AoA) estimations are also useful in certain military situations. U.S. patent application Ser. No. 09/404,387, filed Sep. 23, 1999, titled “An Interferometer Multi-Arm Antenna System for Accurate Angle of Arrival (AoA) Estimation” discusses providing AoA estimations in this type of environment.
Interferometer antenna systems and AoA estimation algorithms are used to make high precision measurements of the direction of arrival of RF signals. These interferometer systems typically use an antenna base line that includes a linear array of antenna elements separated by known distances. Receivers connected to the antenna elements measure the relative phases of the received RF signals. Digital processing algorithms then calculate the angle of arrival from the phase differences measured between the antenna elements. The spacing between the antenna elements is typically small integers or prime numbers related to half-wavelengths of the received signal to resolve ambiguities that arise as a function of frequency. Ambiguities occur as a result of the periodicity of the sine functions used in the algorithms to calculate the AoA estimates. The resulting ambiguities give several AoA estimation solutions of the direction of the received signal that have to be resolved to identify the actual direction. Guard antenna elements are oriented so that their beam patterns differ in such a way that by measuring the signal power received by them, algorithms can resolve the ambiguities that remain in the direction of arrival determined from the linear array.
FIG. 1
is a block diagram of a known antenna system
10
used to determine the direction of arrival of an RF signal. The antenna system
10
includes an interferometer antenna system
12
and a guard antenna system
14
. The interferometer system
12
includes a plurality of antenna elements
16
positioned at predetermined distances from each other, typically at a ratio that is a small integer. Each of the antenna elements
16
is connected to a separate receiver
18
that measures the phase of the received signals. In this example, five interferometer antenna elements
16
are used, however, other numbers of antenna elements could be provided in other applications. In a desirable configuration, the antenna elements
16
are arranged in a linear array, such as along the leading edge of an aircraft wing, but can be configured along any desirable structure.
The guard antenna system
14
includes four antenna elements
20
that are arranged 90° apart from each other to provide directional sensitivity. Because the linear array of antenna elements
16
are only able to determine the direction of the signal within 360°, it is necessary to further define the direction of the signal by sensing received power from the elements
20
. Each of the antenna elements
20
is connected to a separate receiver
22
that measures the received power. Each of the receivers
18
and
22
are connected to a controller
24
that processes the signals using known AoA algorithms to generate the best estimate of the direction of the received signal.
As is apparent from the discussion of the antenna system
10
above, a separate receiver is used for each separate antenna element
16
and
20
. Typically, these types of interferometer antenna systems employ five or six antenna elements to provide the desired signal direction calculation accuracy, without overwhelming cost and space drawbacks. It is possible to provide such an interferometer antenna system having only two antenna elements, but accuracy is sacrificed and the antenna elements have to be located close together, within a half-wavelength of the received signal.
Because receivers are typically complex, and add significant hardware to the overall antenna system, it is desirable to reduce the number of receivers and the complexity of the system. Also, for smaller aircraft, where space constraints are of a significant concern, it is also desirable to reduce the overall system hardware because of space limitations. Further, it is typically desirable in many situations to reduce the overall cost of the system. However, it is desirable to provide these things without reducing the accuracy of the overall system.
What is needed is an interferometer antenna system for determining signal direction that reduces the complexity and cost of the existing systems. It is therefore an object of the present invention to provide such an angle of arrival interferometer system.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, an interferometer antenna system is disclosed for measuring the angle of arrival of RF signals. The interferometer antenna system includes a plurality of interferometer antenna elements coupled to a first switching network. The first switching network is coupled to only two interferometer receivers. The first switching network selectively connects the plurality of interferometer antenna elements in pairs to the two interferometer receivers in a sequential manner, so that a phase difference between the received signal for a combination of several of pairs of the interferometer elements can be generated. Additionally, a plurality of guard antenna elements are coupled to a second switching network. A single guard receiver is coupled to the second switching network, where the second switching network selectively connects one of the guard antenna elements to the guard receiver in a sequential manner.
A controller employs a suitable AoA algorithm to generate a best estimate of the direction of the received signal based on the phase signals from the two interferometer receivers and the power signal from the guard receiver. In one embodiment, the controller generates predicted or expected phase differences based on a nominal direction of arrival, and subtracts the predicted phase differences from the actual measured phase differences to generate a nominal phase difference. The controller performs a linear transform operation to rotate the nominal phase differences and align phase trajectories with one coordinate axis and place the ambiguities on other n−1 axes. A threshold is provided around the nominal coordinate axis based on an acceptance versus rejection probability to eliminate those received signals outside of a predetermined threshold range.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims.
REFERENCES:
patent: 4481519 (1984-11-01), Margerum
patent: 5381150 (1995-01-01), Hawkins et al.
patent: 5457466 (1995-10-01), Rose
patent: 5574468 (1996-11-01), Rose
patent: 5608411 (1997-03-01), Rose
patent: 5936575 (1999-08-01), Azzarelli et al.
patent: 5999129 (1999-12-01), Rose
patent: 6011514 (2000-01-01), Frankovich et al.
Keller Robert W.
Phan Dao
TRW Inc.
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