Method to resolve interferometric ambiguities

Details Method to resolve interferometric ambiguities Method to resolve interferometric ambiguities

2001-02-12

2002-07-16

Tarcza, Thomas H. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Directive

Beacon or receiver

C342S156000

Reexamination Certificate

active

06421008

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to phase interferometric antenna systems that measure the direction of the received signal and, more specifically, to methods for resolving phase ambiguities in phase interferometers that provide direction finding capabilities over a wide field of view and bandwidth.
2. Description of the Related Art
Interferometric systems measure the direction of arrival of signals received by the antenna elements comprising the interferometer. The interferometer antenna system consists of an array of antenna elements separated by known distances. These distances are commonly referred to as baselines. In operation, the phase difference of the signals received by the antenna elements is measured and used to establish the signal arrival direction. For example, if the antenna elements are located on a planar surface and if the signal arrives normal to the surface, then signal outputs of each antenna element are in phase, and the relative phase difference between the elements is ideally zero. If the signal arrives obliquely to the plane, the phase differences between the elements vary dependent on the signal frequency, the baseline values and signal direction. The arrival direction of the signal is then derived based on the phase difference values between the antenna elements comprising the interferometer.
A problem arises, however, because the phase between the elements can only be measured over a 360° range. When the baseline dimension exceeds one half of the wavelength of the incident signal, the phase difference between antenna elements can span more than 360 degrees. Consequently, more than one possible signal arrival direction can be obtained, and these multiple arrival directions are commonly referred to as ambiguities. What is needed is a way to determine the proper ambiguity so that the direction of the signal can be uniquely and correctly determined.
In practice, the design of an interferometric system must satisfy three requirements. First, the antenna elements must have sufficient sensitivity to detect the signals of interest. Second, the angular accuracy depends on the maximum baseline dimension and the accuracy with which the phase difference values can be measured. Third, the design must be able to detect and locate signals over a required field of view and a required frequency range.
These three requirements have some apparent conflicts. The antenna element size must be sufficiently large to receive an adequate signal level. The physical size of the element derived from the sensitivity requirement limits the separation between elements because the elements cannot physically overlap. Thus, a lower bound exists on element spacing. When the antenna element size exceeds one half wavelength, ambiguities exist even when the elements are touching. The overall baseline dimension is determined from the angular accuracy requirement, required field of view and frequency of operation, and the phase measurement accuracy. Increasing the baseline dimension, i.e., the overall separation between elements, increases the angular accuracy of the solution and the number of phase ambiguities but reduces the spacing between ambiguities. The ability to meet the field of view requirement results from having antenna elements that achieve the required sensitivity over the field of view; but the broad antenna coverage conflicts with high antenna gain levels that may be needed for sensitivity requirements. Thus, the design of practical interferometers requires examination of these requirements and resolution of the conflicts between them.
Interferometers are configured for a variety of applications, and consequently have differing approaches to the problem posed by the ambiguities. Interferometers with narrow field of view requirements and relatively high gain levels (to meet sensitivity requirements) typically use aperture antennas that can also provide monopulse-processing capabilities. Monopulse processing uses two types of antenna patterns produced in the same aperture, a sum beam and a difference beam. The sum beam provides the signal reception, and the combined processing of the sum and difference beams provides a coarse estimate of the signal direction. The coarse estimate of the signal arrival direction is obtained from the ratio of the signals received by the difference and sum beams and its sign. To first order, the ratio of the difference signal and the sum signal linearly increases as the signal arrival direction moves away from the antenna's axis. The sign of this ratio changes depending on which side of the antenna's axis corresponds to the signal arrival direction. For example, the sign can be positive for signals arriving to the left of the antenna's axis and negative for signals arriving to the right of the antenna's axis. Thus, by measuring the ratio of the difference and sum signals and the sign of this ratio, a coarse estimate of the signal arrival direction can be determined.
When the above method is applied to narrow field of view interferometers, the interferometric elements have a monopulse processing capability. The coarse estimate of the signal arrival direction derived from the monopulse processing of the interferometric element is used to resolve the correct signal arrival direction from the possible interferometric ambiguities. The overall accuracy of the signal arrival direction is achieved from the interferometric processing and is significantly better than the accuracy provided by the monopulse processing of the interferometric elements. Thus, the overall angular accuracy of the system depends on the interferometric measurement, and the monopulse processing identifies the correct signal arrival direction from the possible ambiguities of the interferometer.
In practice, if the baseline of the interferometer is about five times the diameter of the elements, the ambiguities can be resolved with high confidence. The overall angular accuracy of such an interferometer is about ten times better than the monopulse estimate provided by the interferometric antenna element. One advantage of this method is that the ambiguity resolution is not frequency dependent so that the system can be used to locate signals over a very wide bandwidth. Frequency increases also improve the accuracy of the monopulse estimate of signal arrival, offsetting the reduction in the angular separation between phase ambiguities. Similarly, frequency decreases reduce the accuracy of the monopulse estimate of the signal arrival, offsetting the increase in the angular separation between phase ambiguities. Thus, the ability to resolve phase ambiguities by monopulse processing persists over a very broad bandwidth. Similar frequency independent operation is desired for interferometers for wide field of view applications.
Another method to resolve interferometric ambiguities is commonly used by the radio astronomy community. In this application, the antenna elements are widely separated and have a very large number of ambiguities. However, the required sensitivity for this application requires very long time periods to integrate the signal for detection. During this long integration period, the earth rotates, and this rotation of the earth provides the ambiguity resolution. This method cannot be used without motion between the interferometer and the signal and when the signal arrival direction must be determined in a timely fashion.
Yet another method applies to the case where a wide field of view is required. In this case, the interferometric elements have a broad pattern to meet the field of view requirements and are small and relatively inexpensive. A common approach to ambiguity resolution uses multiple elements within the overall baseline. These additional elements and their smaller baseline values have a reduced number of ambiguities. The minimum spacing between elements is limited to the size of the elements and this baseline has the minimum number of ambiguities. In fact, if this separation is less than

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