Minimization of Error Source

The performance of practical geo-location systems can be improved in several ways.

DF Techniques. The performance of DF systems can vary widely, depending on the implementation and choice of deployment sites:

1. System design choices and trade-offs need to be considered carefully. Antenna arrays with large baselines tend to have performance advantages, but they are generally undesirable for tactical applications. Conversely, attempts to cover a large frequency range with a single antenna array involve significant challenges;

2. Gain and phase mismatches contributed by the receiver hardware and the cables between the antenna and receiver can be corrected by measuring the errors and subtracting them from future measurements. The errors can be measured by using a suitable signal source and radio frequency switch to apply a calibration signal at the point where the cables connect to the antenna. Measurements obtained at suitably chosen test frequencies can be used to construct a calibration table containing the amplitude and phase-correction factors required at each of the test frequencies;

Systematic errors contributed by the antenna can be corrected using a calibration table to provide correction values to be subtracted from the measurements. A one-dimensional calibration table can be constructed by carrying out controlled tests using signals transmitted from a fixed angle at frequencies spaced through the frequency range covered by the system and measuring the discrepancy between the actual and observed angles.

Because the errors generally must be angle dependent, the use of a two-dimensional calibration table is desirable. This table can be constructed by the repeating the procedure for angles distributed around the full 360° interval. Interpolation can be used to generate calibration values for intermediate frequencies and angles.

3. The choice of sites for the deployment of DF systems is critical. Ideally, the site should be free of features that contribute to multipath propagation, and line-of- sight propagation should be possible over the area of interest. In these respects, the elevation of the antenna is an important factor. Another consideration is that the sites should be selected to provide favorable sensor-target geometries for geo-location via triangulation.

4. Geo-location performance improves as the number of sites from which DF information is available increases.

TDOA and FDOA Techniques. The performance of TDOA and FDOA geo-location systems is sensitive to system-implementation choices, the nature of the signals of interest, and various aspects of the system deployment:

1. If the system operation is dependent on the relaying of signals received at the sensor sites to a common site for processing, the system must be able to perform this function without significantly degrading the quality of the signals.

2. Provisions must be made to account for the delays contributed by the relaying of the signals observed at the sensor sites to a common site; these delays must be removed or accounted for.

3. The performance of TDOA estimation processing depends on the signal-to-noise ratio and the presence of suitable information contained in the signal modulation. Narrowband signals may require higher signal-to-noise ratios and/or longer observation times to achieve the desired accuracy;

Frequency shifts that result from relative motions of the receivers and transmitter affect TDOA measurement processing. If, for scenarios of interest, they are sufficiently important, then provisions must be made in the TDOA estimation processing to remove them. If FDOA information is used for geo-location, then the most favorable results will be obtained when the sensors move rapidly, because this action increases the relative frequency shifts, and a given error in frequency measurement becomes less significant. Also, uncertainties contributed by the movement of the signal source are reduced.

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