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A receiver determines a roll rate of a spinning projectile that receives a signal from a satellite or terrestrial transmitter at a directional antenna fixed to the projectile. The receiver tracks a code phase and a rate of change thereof of the received signal relative to a local model signal corres

A receiver determines a roll rate of a spinning projectile that receives a signal from a satellite or terrestrial transmitter at a directional antenna fixed to the projectile. The receiver tracks a code phase and a rate of change thereof of the received signal relative to a local model signal corresponding to the transmitter. A roll correlator of the receiver correlates the received signal with the model signal based on (i) the tracked code phase and rate, and (ii) a roll-correlator integration time assumed to produce a predetermined number N of correlation samples per revolution of the antenna, to produce sequential correlation samples for each of successive revolutions of the antenna. The receiver determines a time difference between successive periods of observed signal reception based on the correlation samples, and determines a roll rate based on the time difference.

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1. A method, comprising: at a directional antenna fixed to a rotating projectile, receiving a signal from a transmitter that is spaced apart from the rotating projectile;tracking a code phase and a code rate of the received signal relative to a local model signal corresponding to the transmitter;cor

1. A method, comprising: at a directional antenna fixed to a rotating projectile, receiving a signal from a transmitter that is spaced apart from the rotating projectile;tracking a code phase and a code rate of the received signal relative to a local model signal corresponding to the transmitter;correlating the received signal with the model signal based on the tracked code phase and the tracked code rate to produce sequential correlation samples per revolution of the directional antenna, such that each of successive revolutions of the directional antenna produces sequential correlation samples;determining, for each revolution, a period of observed signal reception corresponding to when the directional antenna comes into view of the transmitter, the determining including: determining first correlation samples having amplitudes above a threshold that indicate a presence of the received signal and second correlation samples having amplitudes below the threshold that indicate an absence of the received signal;determining a start time and an end time corresponding respectively to a first correlation sample having an amplitude above the threshold and a last correlation sample having an amplitude above the threshold among the first correlation samples; anddetermining a difference between the start time and the end time as the period of observed signal reception;determining a time difference between successive periods of observed signal reception; anddetermining a roll rate based on the time difference. 2. The method of claim 1, wherein the determining the time difference includes: determining a first midpoint time of a first of the successive periods of observed signal reception that falls within a first of the successive revolutions of the directional antenna;determining a second midpoint time of a second of the successive periods of observed signal reception that falls within a second of the successive revolutions of the directional antenna; anddetermining the time difference as a time difference between the first midpoint time and the second midpoint time. 3. The method of claim 2, wherein the determining the first midpoint time includes: determining the first midpoint time as a time equidistant between the start time and the end time. 4. The method of claim 2, further comprising determining a roll angle of the directional antenna relative to a line-of-sight to the transmitter corresponding to a given one of the sequential correlation samples in the first of the successive revolutions, wherein the roll angle is based on a number of correlation samples between the given correlation sample and a correlation sample corresponding to the first midpoint time. 5. The method of claim 4, wherein the determining the roll angle includes determining the roll angle as a ratio of the number of correlation samples between the given correlation sample and the correlation sample corresponding to the first midpoint time and 360°. 6. The method of claim 1, wherein: the correlating includes correlating the received signal also based on a roll-correlator integration time that is set to produce a predetermined N of the sequential correlation samples per revolution of the directional antenna; andif the correlating does not produce the predetermined number N of correlation samples per revolution, adjusting the roll-correlator integration time based on the roll rate so that the correlating will produce the predetermined number N of correlation samples per revolution. 7. The method of claim 1, wherein the correlating includes correlating the received signal also based on a roll-correlator integration time set to produce a predetermined N of the sequential correlation samples per revolution of the directional antenna, the method further comprising: determining a roll-correlation start time for each correlation sample based on the tracked code phase and the tracked code rate,wherein the correlating includes correlating the received energy over the roll-correlator integration time beginning with the roll-correlation start time, to produce each correlation sample. 8. The method of claim 1, wherein: the received signal is a spread spectrum signal; andthe tracking includes correlating the received signal with the model spreading code for the received signal based on a track-correlator integration time that is at least an order of magnitude greater than a roll-correlator integration time. 9. The method of claim 1, further including: at the directional antenna, receiving a second signal from a second transmitter spaced apart from the rotating projectile;tracking a second code phase and a second code rate of the second received signal relative to a second local model signal corresponding to the second transmitter;correlating the second received signal based on the second tracked code phase and the second tracked code rate to produce second sequential correlation samples per revolution of the directional antenna, such that each of the successive revolutions of the directional antenna produces second sequential correlation samples;determining a second period of observed signal reception corresponding to when the directional antenna comes into view of the second transmitter in each revolution based on the second sequential correlation samples for that revolution;determining a second time difference between successive second periods of observed signal reception; anddetermining a second roll rate based on the second time difference; andaveraging the determined roll rate and the determined second roll rate, to produce an averaged roll rate. 10. The method of claim 1, wherein the receiving includes receiving the signal from a satellite-based transmitter. 11. The method of claim 1, wherein the receiving includes receiving the signal from a terrestrial-based transmitter. 12. An apparatus, comprising: a directional antenna fixed to a rotating projectile and configured to receive a signal from a transmitter;a receiver configured to track a code phase and a code rate of the received signal relative to a local model signal corresponding to the transmitter, wherein the receiver includes: a roll correlator configured to correlate the received signal with the model signal based on the tracked code phase and the tracked code rate to produce sequential correlation samples per revolution of the directional antenna, such that each of successive revolutions of the directional antenna produces sequential correlation samples; anda roll feedback loop configured to determine, for each revolution, a period of observed signal reception corresponding to when the directional antenna comes into view of the transmitter, wherein the roll feedback loop is configured to: determine first correlation samples having amplitudes above a threshold that indicate a presence of the received signal and second correlation samples having amplitudes below the threshold that indicate an absence of the received signal;determine a start time and an end time corresponding respectively to a first correlation sample having an amplitude above the threshold and a last correlation sample having an amplitude above the threshold among the first correlation samples; anddetermine a difference between the start time and the end time as the period of observed signal reception;determine a time difference between successive periods of observed signal reception; anddetermine a roll rate based on the time difference. 13. The apparatus of claim 12, wherein the roll feedback loop is further configured to: determine a first midpoint time of a first of the successive periods of observed signal reception that falls within a first of the successive revolutions of the directional antenna;determine a second midpoint time of a second of the successive periods of observed signal reception that falls within a second of the successive revolutions of the directional antenna; anddetermine the time difference as the time difference between the first midpoint time and the second midpoint time. 14. The apparatus of claim 13, wherein the roll feedback loop is further configured to: determine the first midpoint time as a time equidistant between the start time and the end time. 15. The apparatus of claim 13, wherein the roll feedback loop is further configured to determine a roll angle of the antenna relative to a line-of-sight to the transmitter corresponding to a given one of the correlation samples in the first of the successive revolutions, wherein the roll angle is based on a number of correlation samples between the given correlation sample and a correlation sample corresponding to one of the midpoint times. 16. The apparatus of claim 12, wherein: the roll correlator is configured to correlate the received signal also based on a roll-correlator integration time that is set to produce a predetermined N of the sequential correlation samples per revolution of the directional antenna; andif the roll correlator does not produce the predetermined number N of correlation samples per revolution, the roll feedback loop is configured to adjust the roll-correlator integration time based on the roll rate so that the roll correlator will produce the predetermined number N of correlation samples per revolution. 17. The apparatus of claim 12, wherein: the roll correlator is configured to correlate the received signal also based on a roll-correlator integration time that is set to produce a predetermined N of the sequential correlation samples per revolution of the directional antenna;the roll feedback loop is configured to determine a roll-correlation start time for each correlation sample based on the tracked code phase and the tracked code rate; andthe roll correlator is configured to correlate the received signal over the roll-correlator integration time beginning with the roll-correlation start time, to produce each correlation sample. 18. A method, comprising: at a directional antenna fixed to a rotating projectile, receiving a Global Positioning System (GPS) signal from a GPS satellite spaced apart from the rotating projectile;tracking a code phase and a code rate of the received GPS signal relative to a local model signal corresponding to the GPS satellite, wherein the tracking includes correlating the GPS signal with the model signal based on a track integration time per correlation sample;correlating the received GPS signal with the model signal based on (i) the tracked code phase and the tracked code rate, and (ii) a roll-correlator integration time that is at least one order of magnitude less than the track integration time and set to produce a predetermined number N of sequential correlation samples per revolution of the antenna, such that each of successive revolutions of the directional antenna produces sequential correlation samples;determining, for each revolution, a period of observed signal reception corresponding to when the directional antenna comes into view of the transmitter, the determining including: determining first correlation samples having amplitudes above a threshold that indicate a presence of the received signal and second correlation samples having amplitudes below the threshold that indicate an absence of the received signal;determining a start time and an end time corresponding respectively to a first correlation sample having an amplitude above the threshold and a last correlation sample having an amplitude above the threshold among the first correlation samples; anddetermining a difference between the start time and the end time as the period of observed signal reception;determining a time difference between successive periods of observed signal reception; anddetermining a roll rate based on the time difference. 19. The method of claim 18, wherein: if the correlating does not produce the predetermined number N of correlation samples per revolution, adjusting the roll-correlator integration time based on the roll rate so that the correlating will produce the predetermined number N of correlation samples per revolution.