GPS Navigation Message

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.

In order to restrict civilian users access to full system accuracy, the following protections were introduced:

• S/A or Selective Availability: intentional satellite clock degradation (process-δ) and ephemeris manipulation (process-ε). The effect on horizontal po- sitioning implies going from about 10 m (S/A=off) to 100 m (S/A=on) (2σ-error). The process δ acts directly over satellite clock fundamental frequency, which has a direct impact on pseudoranges to be calculated by user’s receivers. The process ε consists in truncating information related to the orbits. USA President Bill Clinton ordered the cessation of GPS Selective Availability on May 1st, 2000.
• A/S or Anti-Spoofing: it consists in P code encryption by combining it with a secret W code, resulting in the Y code, which is modulated over the two carriers L1 and L2. The purpose is to protect military receivers from an adversary transmitting a fault copy of GPS signal to mislead the receiver, and to deny the access of non authorised users to the precise ranging code P in L1 and L2 frequencies, being solely C/A code available over L1.

The signal structure is summarised in the following figure 1 and table 1: [[File:Signal.png|none|thumb|400px|alt=signal]|Figure 1: GPS signal structure (source: G. Seeber, pp 218).]

GPS signal modernisation

The GPS signal modernisation will introduce the additional Link 5 (L5) fre- quency and several new ranging codes on the different carrier frequencies. They are referred to as the civil signals L2C, L5C and L1C and the military code M. All of them are right-hand circularly polarised. The modernisation of the GPS system began in 2005 with the launch of the first IIR-M satellite. This satellite supported the new military M-signal and the second civil signal L2C. This new L2C civil signal is specifically designed to meet commercial needs, allowing the development of low-cost dual frequency civil GPS receivers. It has a lower tracking threshold and better correlation properties than the L1-C/A signal due to the longer codes, which leads to faster signal acquisition, enhanced reliability, and greater operating range. The L2C code is composed of two ranging codes multiplexed in time: The L2 Civil Moderate (L2CM) code and the L2 Civil Long (L2CL) code. L2C is broadcast at a higher effective power level than the original L1-C/A signal. This fact, together with its powerful cross-correlation properties, facilitates tracking with large signal level variations from satellite-to-satellite3, making it easier the reception under trees and even indoors. This signal will also be interoperable with the Quasi-Zenith Satellite System (QZSS) under development by Japan. At April 2009 there were seven GPS satellites broadcasting L2C (PRN01, 07, 12, 15, 17, 29, 31)4. The military M-code signals are designed to use the edges of the band with only minor signal overlap with the preexisting C/A and P(Y) signals (see figure 2). This military M-code is modulated into L1 and L2 carriers using the Binary Offset carrier (BOC) scheme. It has been designed for autonomous acquisition, so that a receiver will be able to acquire the M code signal without access to C/A code or P(Y) code signals. The modernisation continues with the launch of the Block IIF satellites (the first one was launched on May 28th 2010) that include, for the first time, the third civil signal on L5 band (i.e., within the highly protected Aeronautical Radio Navigation Service (ARNS) band). This new L5C signal has a new modulation type and it was designed to meet Safety of Life (SoL) requirements, mainly for civil aviation. There are two signal components: The In-phase component (I5 or L5-I) with data and ranging code, both modulated via BPSK onto the carrier, and the Quadrature component (Q5 or L5-Q), with no data, but also having a ranging code BPSK modulated onto the carrier. This signal has an improved code/carrier tracking loop and its high power and signal design provide robustness against interference. Moreover, its higher chipping rate than the C/A-code (see table 2) provides superior multipath performance. The next step will go on with the Block-III satellites, which will provide the fourth civil signal on L1 band (L1C). This signal is designed to enable inter- operability between GPS and international satellite navigation systems (such as Galileo5). The Multiplexed Binary Offset Carrier (MBOC) modulation is used to improve mobile reception in cities and other challenging environments. L1C is composed by L1CD data channel and L1CP pilot channel. This signal will broadcast at the same frequency as the original L1-C/A signal, which will be retained for backwards compatibility. Figure 2 shows a layout of the different GPS signals and ranging codes for the different modernisation phases.

Figure 2: GPS modernised signal spectra.