Recursos y activos específicos

The current GPS satellite constellation is being modernized at L2 (1227.6) and L5 (1176.45 MHz) frequencies to overcome the limitations of the legacy GPS L1 C/A signal.

The civilian signal on L2 (L2C) is targeted at the existing community of dual frequency users and future L2-only single frequency users (Fontana et al 2001). The L5 signal is mainly intended for safety-of-life applications including aviation (Tran & Hegarty 2003). The most distinguishable change incorporated in the signal structure of these modernized signals, as compared to the legacy L1 C/A signal, is the inclusion of a dataless channel (pilot channel) in addition to the navigation data channel. This assists in weak signal tracking. The available transmission power is equally shared between the data and pilot channels. This sharing results in a 3 dB lower signal power on each individual channel. Still, the ability to use a PLL to track the pilot channel provides a gain of 6 dB in tracking threshold, which enables L2 signal tracking at a 3 dB lower strength than the legacy L1 C/A signal (Kaplan 2006). Since September 2005, the constellation is being modernized with IIR-M satellites capable of transmitting the L2C signal. The L2C signal will also be transmitted on future Block IIF and III satellites along with the other modernized signals. The availability of live L2C signals facilitates the option to evaluate the advantages gained by the presence of both the data and pilot signals from each SV.

In this thesis, the L2C signal is used as a tool to validate and analyse the algorithms proposed to use either the data or pilot channel independently (or together) for carrier tracking and C/N0 estimation. The following sections give a brief introduction to

the L2C signal structure and the receiver design changes required. For notational clarification, the term “channel” is often used in the literature to refer to a module that tracks the signal from a particular SV. Owing to the difficulty in extending this terminology for modernized signals due to the presence of a data and pilot signal from every SV, the term “data channel” herein refers to the channel in the receiver that tracks

the data signal and similarly the term “pilot channel” refers to the channel tracking the pilot signal.

2.4.1 L2C Signal Structure

The civilian signal on the L2 band is transmitted in phase quadrature to the military signal P(Y) on the same carrier frequency (Fontana et al 2001). The L2C signal carries two codes, the civil-moderate (CM) and civil-long (CL) codes. The CM code for each SV is 20 ms in length with a chipping rate of 511.5 Kbps (10230 chips). The CL code is 75 times longer than the CM code (767250 chips) with the same chipping rate (period = 1.5 s). The CM code is modulated by the navigation data whereas the CL code is not. Thus the former serves as the data channel and the latter as the pilot channel. Since both the data and pilot channels must be transmitted on a single carrier component, they are time-multiplexed on transmission, as shown in Figure 2-3. Thus the effective chipping rate of the time multiplexed code is 1.023 MHz, which is the same as the C/A code chipping rate. The CM code is aligned with the data bit boundaries, thus eliminating the need for a bit synchronization algorithm once signal acquisition is achieved.

The CM and CL codes for all the SVs are generated using a 27 bit linear feedback shift register (LFSR) with fixed taps. The generator polynomial is given as

G(x) = 1 + x3_{+ x}4_{+ x}5_{+ x}6_{+ x}9_{+ x}11_{+ x}13 _{+ x}16_{+ x}19_{+ x}21_{+ x}24_{+ x}27 _(2.9)

When allowed to run continuously, the LFSR described by Eq. (2.9) generates a m-sequence with a period of 227-1 chips. However for CM and CL code generation, the LFSR is short cycled to 10230 and 762750 chips, respectively. Distinct initial states of the LFSR generate different subsets of the original long-length m-sequence. The initial and final states of the LFSR corresponding to each SV are given in IS-GPS-200-D (2006). All the subset codes are chosen to be perfectly balanced, i.e. they have equal numbers of 1s and 0s (Fontana et al 2001). There are exactly 75 periods of CM code within each CL code and the CL code is synchronized with the Z-Count (1.5 s).

The CM code is modulated with the civil navigation (CNAV) data. The CNAV data differs from the original navigation data (NAV) carried by the L1 C/A signal, as it includes new parameters for SV ephemerides that improve the accuracy of the satellite position determination and also has a flexible structure for frames (Mongrédien 2008). The CNAV data bit rate is originally 25 bps. It is coded by a rate ½ convolutional encoder which yields a 50 sps symbol stream. Convolutional coding helps to reduce the bit error rate during data bit extraction in the receiver. Although the CNAV modulation on the CM code is the intended design for the data channel, any of the following combinations can be broadcasted on the data channel during the initial phases of the IIR- M satellites (IS-GPS-200-D 2006):

i. NAV data as on L1 C/A at 50 bps modulated on CM code

iii. NAV data modulated on C/A code iv. C/A code without data modulations v. CM code without data modulations vi. CNAV at 50 bps modulated on CM code 2.4.2 Limitations of L2C signal

As compared to the legacy L1 C/A signal, the following are considered the limitations or short comings of using a single frequency L2C receiver:

i. The minimum received signal power of the L2C signals is 1.5 dB lower than that of the L1 C/A signals (IS-GPS-200-D 2006). However, the addition of the pilot channel and the longer length codes compensates for the difference in signal power.

ii. The error introduced by the ionosphere is inversely proportional to the square of the carrier frequency. The L2 carrier is transmitted approximately 347 MHz lower than the L1 carrier frequency and thus has 65% more ionospheric refraction error (Fontana et al 2001). A L1/L2 dual frequency receiver can effectively remove the ionospheric error by up to 99% of the total delay (Skone 2005).

Although a single frequency L2C receiver can be considered a robust alternative to the L1 C/A only receiver, due to the complementary signal properties of L2C, a dual frequency receiver using both civilian signals (L1 C/A and L2C) will be advantageous compared to a receiver using individual signals (Gernot et al 2008, Gernot et al 2007). Since the focus of this thesis is on evaluating the advantages of the pilot channel

availability in addition to the data channel for signal tracking, dual frequency processing is not considered in this thesis.