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GPS Navigation Message: Difference between revisions
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The original GPS satellites transmit signals centred on two radio frequencies in the L band, referred to as Link 1 (L1) and Link 2 (L2)1. They are right- hand circularly polarised and their frequencies are derived from a fundamental frequency f0 = 10.23 MHz, generated by on board atomic clocks. | The original GPS satellites transmit signals centred on two radio frequencies in the L band, referred to as Link 1 (L1) and Link 2 (L2)1. They are right- hand circularly polarised and their frequencies are derived from a fundamental frequency f0 = 10.23 MHz, generated by on board atomic clocks. | ||
Two services are available from GPS system: | |||
SPS: The Standard Positioning Service is an open service, free of charge for worldwide users. This is a single frequency service in the frequency band L1. | |||
PPS: The Precise Positioning Service is restricted by cryptographic techniques to military and authorised users. Two navigation signals are provided in two different frequency2 bands L1 and L2. | |||
The GPS uses the Code Division Multiple Access (CDMA) technique to send different signals on the same radio-frequency, and the modulation method used is the Binary Shift Phase Keying (BPSK). | |||
The following types of PRN codes and messages are modulated over the two carriers (see figure 1): | |||
* Coarse/Acquisition code [C/A(t)]: it is also known as civilian code. This sequence contains 1023 bits and is repeated every millisecond (i.e, achipping-rate of 1.023 M bps). Thence, the duration of each C/A-code chip is 1μs, which means a chip-width or wavelength of 293.1m. This code is modulated only on L1. The C/A-code defines the Standard Posi- tioning Service (SPS). | |||
* Precision code [P(t)]: it is reserved for military use and authorised civilian users. The sequence is repeated every 266 days (38 weeks) and a weekly portion of this code is assigned to every satellite, called PRN sequence. Its chipping-rate is 10 M bps, which leads to a wavelength of 29.31 m. It is modulated over both carriers L1 and L2. The P-code defines the Precise Positioning Service (PPS). | |||
* Navigation message [D(t)]: it is modulated over both carriers at 50 bps, re- porting information about ephemeris and satellite clock drifts, ionospheric model coefficients, constellation status, etc. | |||
Revision as of 14:46, 19 January 2011
The original GPS satellites transmit signals centred on two radio frequencies in the L band, referred to as Link 1 (L1) and Link 2 (L2)1. They are right- hand circularly polarised and their frequencies are derived from a fundamental frequency f0 = 10.23 MHz, generated by on board atomic clocks.
Two services are available from GPS system: SPS: The Standard Positioning Service is an open service, free of charge for worldwide users. This is a single frequency service in the frequency band L1. PPS: The Precise Positioning Service is restricted by cryptographic techniques to military and authorised users. Two navigation signals are provided in two different frequency2 bands L1 and L2. The GPS uses the Code Division Multiple Access (CDMA) technique to send different signals on the same radio-frequency, and the modulation method used is the Binary Shift Phase Keying (BPSK). The following types of PRN codes and messages are modulated over the two carriers (see figure 1):
- Coarse/Acquisition code [C/A(t)]: it is also known as civilian code. This sequence contains 1023 bits and is repeated every millisecond (i.e, achipping-rate of 1.023 M bps). Thence, the duration of each C/A-code chip is 1μs, which means a chip-width or wavelength of 293.1m. This code is modulated only on L1. The C/A-code defines the Standard Posi- tioning Service (SPS).
- Precision code [P(t)]: it is reserved for military use and authorised civilian users. The sequence is repeated every 266 days (38 weeks) and a weekly portion of this code is assigned to every satellite, called PRN sequence. Its chipping-rate is 10 M bps, which leads to a wavelength of 29.31 m. It is modulated over both carriers L1 and L2. The P-code defines the Precise Positioning Service (PPS).
- Navigation message [D(t)]: it is modulated over both carriers at 50 bps, re- porting information about ephemeris and satellite clock drifts, ionospheric model coefficients, constellation status, etc.