Modulation signals for a satellite navigation system

1. A method of controlling a power spectrum of a navigation transmission signal in a navigation system, the navigation transmission signal comprising a carrier signal, a ranging code, a data signal, and at least one stepped subcarrier modulation signal, the method comprising the step of modulating the carrier signal by the at least one stepped subcarrier modulation signal, the data signal, and the ranging code to generate the navigation transmission ranging signal; wherein the at least one stepped subcarrier modulation signal comprises a number, m, of amplitude levels, where m>2, the amplitude levels being arranged to constrain the power spectrum of the navigation transmission signal in a predetermined manner.

2. The method as claimed in claim 1 , wherein in is selected from at least one of 3, 4, 5, 6, 7, 8 or 9.

3. The method as claimed in claim 1 , wherein at least one of the at least one stepped subcarrier modulation signal approximates or is derived from a predetermined basis waveform.

4. The method as claimed in claim 3 in which the basis waveform is at least one of a sine wave, cosine wave, triangular waveform.

5. The method as claimed in claim 3 wherein the basis waveform is selected according to desired power distribution characteristics of the navigation transmission signal.

6. The method as claimed in claim 1 , wherein the at least one stepped subcarrier modulation signal comprises at least two mutually orthogonal stepped subcarrier modulation signals.

7. The method as claimed in claim 6 , wherein the at least two stepped subcarrier modulation signals comprises a pair of subcarriers having a predetermined phase relationship.

8. The method as claimed in claim wherein the at least one stepped subcarrier modulation signal comprises an in-phase subcarrier and a quadrature phase subcarrier.

9. The method as claimed in claim 8 , further comprising determining from said number, m, of amplitude levels respective multiple amplitudes of the in-phase and quadrature phase subcarriers to maintain a substantially constant navigation transmission signal envelope.

10. The method as claimed in claim 1 , further comprising the steps of deriving from said number, m, of amplitude levels the amplitudes associated with the at least one stepped subcarrier modulation signal from a plurality of phase states.

11. The method as claimed in claim 10 , in which the phase states are equally angularly distributed around a unit circle.

12. The method as claimed in claim 1 , wherein durations of the amplitudes of said number, m, of amplitude levels of the at least one stepped subcarrier modulation signal are substantially equal.

13. The method as claimed in claim 1 , wherein the durations of at least a pair of amplitudes of said number, m, of amplitude levels of the at least one stepped subcarrier modulation signal are different.

14. The method as claimed in claim 12 , wherein the durations are quantized according to an associated clock signal.

15. The method of claim 1 , wherein the at least one subcarrier modulation signal comprises at least a pair of stepped subcarrier modulation signals resolved according to mutually orthogonal axes.

16. The method of claim 15 , wherein each subcarrier modulation signal of the pair of subcarrier modulation signals has a respective ranging signal.

17. The method of claim 15 , wherein dwell times of at least some of the amplitudes of the at least one stepped subcarrier modulation signal are unequal.

18. The method of claim 17 , wherein a first group of the amplitudes have a first dwell time and a second group of the amplitudes have a second dwell time.

19. The method as claimed in claim 15 , wherein the dwell tunes are quantized according to a clock.