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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.

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.

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1. A method comprising: providing a plurality of radiolocation transmitters for a desired coverage area;locking the plurality of radiolocation transmitters to a common timing reference;transmitting a radiolocation signal, using a hybrid spread spectrum waveform that combines a direct sequence modula

1. A method comprising: providing a plurality of radiolocation transmitters for a desired coverage area;locking the plurality of radiolocation transmitters to a common timing reference;transmitting a radiolocation signal, using a hybrid spread spectrum waveform that combines a direct sequence modulation with a frequency hopping, a time hopping, or a time gating protocol, from each of the plurality of radiolocation transmitters; andsequentially turning off or reducing output power for a first radiolocation transmitter of the plurality of radiolocation transmitters in an inverse pulse time sequence that turns off or reduces the output power of the first radiolocation transmitter for a minority portion of a total data-cycle period and applies a standard output power for a majority portion of the total data-cycle period. 2. The method of claim 1, where the radiolocation signal comprises: location information that indicates a geographic position of one of the plurality of radiolocation transmitters that sent the radiolocation signal; andtime information that indicates a time when the radiolocation signal was sent from the one of the plurality of radiolocation transmitters. 3. The method of claim 2, further comprising calculating a position of a radiolocation receiver based on the location information and the time information of the radiolocation signal received at the radiolocation receiver. 4. The method of claim 1, where the act of transmitting the radiolocation signal comprises transmitting the radiolocation signal in an 80-120 kHz frequency range. 5. The method of claim 4, where the act of transmitting the radiolocation signal in the 80-120 kHz frequency range comprises splitting the radiolocation signal into an 80-90 kHz band and a 110-120 kHz band. 6. The method of claim 4, where the act of transmitting the radiolocation signal in the 80-120 kHz frequency range comprises transmitting the radiolocation signal in a 90-110 kHz band. 7. The method of claim 1, where the act of transmitting the radiolocation signal comprises transmitting the radiolocation signal in a frequency band with a center frequency around 3.3 MHz. 8. The method of claim 1, where the act of transmitting the radiolocation signal comprises transmitting the radiolocation signal in a frequency band with a center frequency around 2.45 GHz. 9. The method of claim 1, where the act of transmitting the radiolocation signal comprises: transmitting a first radiolocation signal from one of the plurality of radiolocation transmitters in a first frequency range; andtransmitting a second radiolocation signal from the one of the plurality of radiolocation transmitters in a second frequency range that is different than the first frequency range. 10. The method of claim 1, where the common timing reference comprises a Global Positioning System timing signal; a highly stable local oscillator; signals from WWVB, LORAN-C, or OMEGA; signals transmitted from GOES satellites; signals transmitted from CDMA cell-phone base stations; VLF, LF, or HF standard-frequency broadcast signals; standard AM broadcasting signals; international shortwave broadcasting signals; or analog and digital television broadcasting signals. 11. The method of claim 1, where act of transmitting comprises transmitting the radiolocation signal using a code-division multiple-access scheme. 12. The method of claim 1, where the act of transmitting the radiolocation signal comprises setting a ratio between chipping time and a carrier period of the radiolocation signal to avoid integer carrier-cycle ambiguities. 13. The method of claim 1, where the radiolocation signal comprises a first signal component at a first frequency and a second signal component at a second frequency, and the act of transmitting the radiolocation signal comprises setting a ratio between a chipping time and a difference-frequency period of the first and the second signal components to avoid integer carrier-cycle ambiguities. 14. The method of claim 1, where the act of transmitting the radiolocation signal comprises transmitting the radiolocation signal with a radiolocation signal format that rejects power-line related noise. 15. A method comprising: providing a plurality of radiolocation transmitters for a desired coverage area;locking the plurality of radiolocation transmitters to a common timing reference;transmitting a radiolocation signal, using a spread-spectrum waveform, from each of the plurality of radiolocation transmitters;turning off or reducing an output power of a first radiolocation transmitter of the plurality of radiolocation transmitters during a first time period;resuming standard output power for the first radiolocation transmitter during a second time period;turning off or reducing an output power of a second radiolocation transmitter of the plurality of radiolocation transmitters during the second time period; andsequentially turning off or reducing output power for the first radiolocation transmitter in an inverse pulse time sequence that turns off or reduces the output power of the first radiolocation transmitter for a minority portion of a total data-cycle period and applies the standard output power for a majority portion of the total data-cycle period;where during the first time period, the first radiolocation transmitter is turned off or has its output power reduced while multiple other radiolocation transmitters of the plurality of radiolocation transmitters transmit at a standard output power to allow a radiolocation receiver to acquire a fix with the multiple other radiolocation transmitters. 16. The method of claim 15, where during the second time period, the second radiolocation transmitter is turned off or has its output power reduced while the first radiolocation transmitter and one or more other radiolocation transmitters of the plurality of radiolocation transmitters transmit at a standard output power to allow a radiolocation receiver to acquire a fix with the first radiolocation transmitter and the one or more other radiolocation transmitters. 17. The method of claim 15, where during the first time period, the output power of the first radiolocation transmitter is reduced by at least 40 dB from a standard output power of the first radiolocation transmitter. 18. The method of claim 15, further comprising turning off or reducing output power of a third radiolocation transmitter of the plurality of radiolocation transmitters during the first time period or the second time period. 19. The method of claim 15, where each of the plurality of radiolocation transmitters are switched off or placed in a reduced output power state for about five to about twenty-five percent of a total data-cycle period. 20. A method comprising: providing a plurality of radiolocation transmitters for a desired coverage area;locking the plurality of radiolocation transmitters to a common timing reference;transmitting a ground wave radiolocation signal, using a spread-spectrum waveform, from each of the plurality of radiolocation transmitters; andsequentially turning off or reducing output power for a first radiolocation transmitter of the plurality of radiolocation transmitters in an inverse pulse time sequence that turns off or reduces the output power of the first radiolocation transmitter for a minority portion of a total data-cycle period and applies a standard output power for a majority portion of the total data-cycle period. 21. The method of claim 20, where the spread-spectrum waveform comprises a direct sequence spread-spectrum waveform. 22. The method of claim 20, where the spread-spectrum waveform comprises a hybrid spread spectrum waveform that combines a direct sequence modulation with a frequency hopping, a time hopping, or a time gating protocol. 23. The method of claim 20, where the radiolocation signal comprises: location information that indicates a geographic position of one of the plurality of radiolocation transmitters that sent the radiolocation signal; andtime information that indicates a time when the radiolocation signal was sent from the one of the plurality of radiolocation transmitters. 24. The method of claim 23, further comprising calculating a position of a radiolocation receiver based on the location information and the time information of the radiolocation signal received at the radiolocation receiver. 25. The method of claim 23, where the radiolocation signal further comprises differential position correction data. 26. The method of claim 23, where the radiolocation signal further comprises alert data. 27. The method of claim 20, where the act of transmitting the radiolocation signal comprises transmitting the radiolocation signal in an 80-120 kHz frequency range. 28. The method of claim 27, where the act of transmitting the radiolocation signal in the 80-120 kHz frequency range comprises splitting the radiolocation signal into an 80-90 kHz band and a 110-120 kHz band. 29. The method of claim 27, where the act of transmitting the radiolocation signal in the 80-120 kHz frequency range comprises transmitting the radiolocation signal in a 90-110 kHz band. 30. The method of claim 20, where the act of transmitting the radiolocation signal comprises transmitting the radiolocation signal in a frequency band with a center frequency around 3.3 MHz. 31. A method comprising: providing a plurality of radiolocation transmitters for a desired coverage area;locking the plurality of radiolocation transmitters to a common timing reference;transmitting a radiolocation signal, using a frequency hopping protocol and a spread-spectrum waveform, from each of the plurality of radiolocation transmitters; andsequentially turning off or reducing output power for a first radiolocation transmitter of the plurality of radiolocation transmitters in an inverse pulse time sequence that turns off or reduces the output power of the first radiolocation transmitter for a minority portion of a total data-cycle period and applies a standard output power for a majority portion of the total data-cycle period. 32. The method of claim 31, where the radiolocation signal comprises location information and system time information. 33. The method of claim 32, where the location information of the radiolocation signal comprises information that indicates a geographic position of one of the plurality of radiolocation transmitters that sent the radiolocation signal. 34. The method of claim 32, where the system time information of the radiolocation signal comprises information that indicates a time when the radiolocation signal was sent from one of the plurality of radiolocation transmitters. 35. The method of claim 32, further comprising calculating a position of a radiolocation receiver based on the location information and the system time information of the radiolocation signal received at the radiolocation receiver. 36. The method of claim 1, where the act of transmitting the radiolocation signal comprises transmitting the radiolocation signal using a constant-envelope hybrid spread spectrum waveform. 37. The method of claim 1, where the act of transmitting the radiolocation signal comprises transmitting the hybrid spread spectrum waveform with the direct sequence modulation and the time hopping protocol. 38. The method of claim 20, where the act of transmitting the ground wave radiolocation signal comprises transmitting an earth-curvature-following ground wave radiolocation signal from a vertically polarized antenna. 39. The method of claim 31, where the act of transmitting the radiolocation signal comprises transmitting a constant-envelope spread-spectrum waveform. 40. A method comprising: providing a plurality of radiolocation transmitters for a desired coverage area;locking the plurality of radiolocation transmitters to a common timing reference; andtransmitting a radiolocation signal, using a hybrid spread spectrum waveform that combines a direct sequence modulation with a frequency hopping, a time hopping, or a time gating protocol, from each of the plurality of radiolocation transmitters;where the act of transmitting the radiolocation signal comprises splitting the radiolocation signal into an 80-90 kHz band and a 110-120 kHz band. 41. A method comprising: providing a plurality of radiolocation transmitters for a desired coverage area;locking the plurality of radiolocation transmitters to a common timing reference; andtransmitting a ground wave radiolocation signal, using a spread-spectrum waveform, from each of the plurality of radiolocation transmitters;where the act of transmitting the radiolocation signal comprises splitting the radiolocation signal into an 80-90 kHz band and a 110-120 kHz band.