Apparatus and method for determining signal quality in a geolocation system
In a geolocation system for determining a geolocation of a target emitter, a method for determining the geolocation. The method comprises receiving a signal transmitted from the target emitter at each one of a plurality of sensors; determining whether signals received at n sensors, from among the plurality of sensors, satisfy one or more threshold values related to a condition of the received signals; if signals received at n sensors satisfy the threshold value, commanding m of the n sensors to transmit the signal received thereat or information related to the signal received thereat to a processor; at the processor, determining time difference estimates for the m received signals and determining the geolocation of the target emitter from the time difference estimates.
1. In a geolocation system for determining a geolocation of a target emitter, a method for determining the geolocation comprising:
receiving a signal transmitted from the target emitter at each one of a plurality of sensors;
determining whether signals received at n sensors, from among the plurality of sensors, have a signal parameter that satisfies an associated threshold values, wherein the signal parameter comprises one or more of a signal amplitude, a signal bandwidth, a signal-to-noise ratio and a stare period, the associated threshold value selected to minimize a lower bound of a variance estimate according to a Cramer Rao equation;
if signals received at n sensors satisfy the threshold value, commanding m of the n sensors to transmit one or both of the signal received thereat or information related to the signal received thereat to a processor;
at the processor, determining time difference estimates for the m received signals; and
determining the geolocation of the target emitter from the time difference estimates.
2. The method of claim 1 wherein the geolocation system comprises one of a direction-finding system, a time difference of arrival system, an angle of arrival system and a frequency difference of arrival system.
3. The method of claim 1 wherein the step of determining time difference estimates further comprises determining pairwise time difference estimates using the m received signals.
4. The method of claim 1 wherein the step of determining time difference estimates further comprises:
determining a signal magnitude over a frequency range for each of the m received signals;
determining signal magnitudes that are below a magnitude threshold and determining a frequency or a frequency interval associated with any signal magnitudes below the threshold;
setting a value of determined signal magnitudes to zero or near zero to create a modified signal; and
determining pairwise time difference estimates using the modified signals.
5. The method of claim 1 wherein the step of receiving further comprises time-stamping the signal received at each one of the plurality of sensors.
6. The method of claim 1 wherein a value of n is determined to achieve a predetermined variance for the time difference estimate.
7. The method of claim 1 wherein a value of n is determined responsive to a lower bound for a variance of the time difference estimate based on a Cramer-Rao equation.
8. The method of claim 1 wherein a value of n equals three.
9. The method of claim 1 wherein n sensors comprise all of the plurality of sensors.
10. The method of claim 1 wherein the m sensors comprise the n sensors receiving a signal that satisfies the one or more threshold values.
11. The method of claim 1 wherein the m sensors comprise all of the plurality of sensors.
12. The method of claim 1 wherein the signal bandwidth comprises 15 kHz, the stare interval comprises 0.5 seconds and the signal-to-noise ratio comprises 15 dB.
13. The method of claim 1 further comprising affixing a time stamp to the signal received at each one of the plurality of sensors and including the time stamp in the information related to the signal received at the sensor to the processor.
14. In a geolocation system for determining a geolocation of a target emitter, a method for determining the geolocation comprising:
receiving a signal transmitted from the target emitter at each one of a plurality of sensors;
determining whether signals received at n sensors, from among the plurality of sensors, satisfy one or more threshold values related to a condition of the received signals;
if signals received at n sensors satisfy the threshold value, commanding m of the n sensors to transmit one or both of the signal received thereat or information related to the signal received thereat to a processor;
at the processor, determining time difference estimates for the m received signals;
determining the geolocation of the target emitter from the time difference estimates; and
wherein the step of determining whether n of the received signals satisfy one or more threshold values further comprises, transmitting data packets from each of the plurality of sensors to the processor, each data packet representing one of the received signals;
at the processor receiving the data packets and determining a Fourier transform of each data packet; and
if the Fourier transform indicates that n of the received signals satisfy one or more threshold values related to the condition of the received signal, then commanding m of the n sensors to transmit the signal received thereat or information related to the signal received thereat to the processor.
15. The method of claim 14 wherein the step of transmitting data packets executes periodically.
16. The method of claim 14 wherein the step of transmitting data packets further comprises transmitting about 10 packets per second.