Robust low-frequency spread-spectrum navigation system
Methods and apparatus are described for a navigation system. A process includes providing a plurality of transmitters distributed throughout a desired coverage area; locking the plurality of transmitters to a common timing reference; transmitting a signal from each of the plurality of transmitters. An apparatus includes a plurality of transmitters distributed throughout a desired coverage area; wherein each of the plurality of transmitters comprises a packet generator; and wherein the plurality of transmitters are locked to a common timing reference.
1. A method, comprising:
receiving a ground-wave radiolocation signal at an antenna of a radiolocation receiver;
receiving a sky-wave radiolocation signal at the antenna of the receiver;
correlating the ground-wave radiolocation signal with the sky-wave radiolocation signal to determine a three-dimensional location of the receiver based on radiolocation data received from the ground-wave radiolocation signal and radiolocation data received from the sky-wave radiolocation signal; and
computing a height of the receiver from a differential delay figure between the sky-wave radiolocation signal and the ground-wave radiolocation signal.
2. The method of claim 1 , where the ground-wave radiolocation signal and the sky-wave radiolocation signal comprise spread spectrum signals.
3. The method of claim 1 , further comprising:
converting coordinates received from transmitters to an earth-centered format;
calculating chord distances;
converting the chord distances to great-circle distances for ground-wave radiolocation signals;
correcting for environmental propagation factors; and
processing a positioning algorithm to determine the three-dimensional receiver location and a system time.
4. The method of claim 1 , further comprising:
calculating a differential propagation delay between the ground-wave radiolocation signal and the sky-wave radiolocation signal; and
transmitting a correction factor associated with the differential propagation delay to one or more other radiolocation receivers or transmitters to improve positioning or timing accuracy.
5. A method, comprising:
receiving a ground-wave or a sky-wave spread spectrum radiolocation signal at a receiver;
producing a correlation waveform with a series of narrow doublet pulses that correspond to transitions of a pseudorandom code associated with the spread spectrum radiolocation signal;
applying the series of narrow doublet pulses to the spread spectrum radiolocation signal to obtain radiolocation information from the spread spectrum radiolocation signal; and
determining a location of the receiver based on the radiolocation information.
6. The method of claim 5 , where the narrow doublet pulses are shorter than a full chip period, and where the act of applying the series of narrow doublet pulses comprises applying the series of narrow doublet pulses to the spread spectrum radiolocation signal by a spread spectrum correlator of the receiver.
7. The method of claim 5 , where the act of producing the correlation waveform comprises selecting a length for the narrow doublet pulses to reject signals arriving at times other than at a time of a desired signal.
8. The method of claim 5 , where the correlation waveform includes a doublet pulse that transitions from off to high, then from high to low, then from low to off when the pseudorandom code transitions from high to low; and
where the correlation waveform includes a doublet pulse that transitions from off to low, then from low to high, then from high to off when the pseudorandom code transitions from low to high.
9. The method of claim 5 , where the act of applying the series of narrow doublet pulses comprises:
sampling the spread spectrum radiolocation signal during a time period that corresponds to a first doublet pulse of the series of narrow doublet pulses;
sampling the spread spectrum radiolocation signal during a time period that corresponds to a second doublet pulse of the series of narrow doublet pulses; and
rejecting signals arriving at the receiver at a time between the first doublet pulse and the second doublet pulse.
10. A method, comprising:
receiving a plurality of spread spectrum radiolocation signals at a receiver;
determining a location of the receiver based on receipt of the plurality of spread spectrum radiolocation signals;
rotating a directional antenna of the receiver to find a null in a directional pattern relative to a spread spectrum radiolocation signal of the plurality of spread spectrum radiolocation signals; and
calculating a bearing of the receiver based on a position of the directional antenna when the null is detected.
11. The method of claim 10 , where the act of calculating the bearing comprises calculating a first bearing between the receiver and a first radiolocation transmitter based on a position of the directional pattern of the directional antenna when a first null in the directional pattern is detected, the method further comprising:
calculating a second bearing between the receiver and a second radiolocation transmitter based on a position of the directional pattern of the directional antenna when a second null in the directional pattern is detected;
calculating a third bearing between the receiver and a third radiolocation transmitter based on a position of the directional pattern of the directional antenna when a third null in the directional pattern is detected; and
determining an orientation of the receiver based on the first bearing, the second bearing, and the third bearing.
12. The method of claim 10 , where the act of rotating the directional antenna of the receiver comprises rotating the directional pattern of the directional antenna by a mechanical rotation or an electronic rotation.