# Performance Limits of GNSS Code-based Precise Positioning: GPS, Galileo & Meta-Signals
## 4 GPS Signals  
Nowadays, most of the GNSS applications are based on the signals transmitted at the L1 frequency band. Four GPS signals are transmitted at this L1 band: the C/A signal, the precise P(Y) signal, the modern military M signal, and the new L1C civil signal. In addition, other GNSS signals of interest are broadcasted at lower bands, namely GPS L5 and GPS L2 signals. In this section, we provide a brief overview of the GPS civil signals allocated in the L1 and L5 bands.  
* _GPS L1 C/A Signal_: The GPS L1 coarse/acquisition (C/A) signal is composed of three components: (i) the NAV message \(d(t)\)[22] with a data rate of 50 bits per second. Each bit lasts 20 ms; (ii) the PRN sequence of 1023 chips (i.e., from the Gold codes family [23]), repeated every 1 ms, meaning the system operates with a frequency rate of 1.023 Mchips/s; and (iii) the carrier frequency 1575.42 MHz is used to allocate the BPSK modulation to transmit the GPS L1 C/A signal in the L1 band (i.e., denoted BPSK(1)).
* _GPS L1C Signal_: GPS L1C is a modernized GPS civil signal designed to improve the performance of the legacy GPS L1 C/A signal. As for the Galileo E1 Open Service Signal [24], the Multiplexed BOC (MBOC) modulation is considered, namely a MBOC(6,1,1/11) [25] modulation was selected to transmit the GPS L1C signal. The MBOC(6,1,1/11) is defined in the PSD domain as a mix of a BOC(1,1) and BOC(6,1) modulations, where \(\frac{10}{11}\)th of the power is associated to the BOC(1,1) modulation and \(\frac{11}{11}\)th of the power is associated to the BOC(6,1). Note that the BOC modulation is generally denoted \(BOC(p,q)\), where \(p\) refers to the sub-carrier frequency \(f_{sc}=p\times 1.023\) MHz (i.e., \(sc(t)=sign\left[\sin\left(2\pi f_{sc}t\right)\right]\) with \(sign\) the sign function) and \(q\) to the ranging code frequency \(f_{c}=q\cdot 1.023\) MHz [26]. Then, the PSD of the MBOC modulation can be expressed as, \[G_{MBOC(6,1,1/11)}(f)=\frac{10}{11}G_{BOC(1,1)}(f)+\frac{1}{11}G_{BOC(6,1)}(f),\] (11) where \(G_{BOC(1,1)}(f)\) is the PSD of a low frequency BOC component and \(G_{BOC(6,1)}(f)\) is the PSD of a high frequency BOC component. This signal can be generated with two different architectures: (i) the Composite BOC (CBOC) modulation [24], which is used in the Galileo E1 open service signal; and (ii) the implementation used for the GPS L1C signal, i.e., a Time-Multiplexing BOC (TMBOC) modulation [25] used to transmit the pilot component of the GPS L1C signal and a BOC(1,1) modulation to transmit the corresponding data component. Notice that the TMBOCmodulation divides temporally the power of the BOC(1,1) and BOC(6,1) of the pilot component. Indeed, the pilot component time-series comprises 29/33 BOC(1,1) spreading symbols and 4/33 BOC(6,1) spreading symbols. Moreover, an unequal power distribution is used to transmit the data and pilot components of the GPS L1C signal, i.e., the data and pilot channels represent 25% and 75% of the total power, respectively. The L1C PRN sequences last 10 ms and the CNAV-2 message [27] is transmitted at 100 symbols per second.
* _GPS L5 Signal:_ A third GPS civil signal was proposed at the L5 frequency band (1176.45 MHz) [28], in order to support safety-of-life applications. This GPS L5 signal has a clock rate of 10.23 MHz that is 10 times the clock rate of the legacy GPS L1 C/A signal. Moreover, this signal has two different components: (i) a data component (in-phase) used to broadcast the CNAV L5 navigation message [29]; and (ii) a pilot component (quadrature). The GPS L5 PRN sequence has a length of 10230 chips and the the chip rate is the 10.23 Mchips/s, yielding to a PRN sequence period of 1 ms. Each chip is mapped into a BPSK(10) modulation. The L5 CNAV navigation message is transmitted at 100 symbols per second.