**4. GPS signal characteristics**

GPS satellites produce a central L-band frequency of 10.23 MHz using very stable clocks. Satellites then multiply this frequency by 154 and 120 to generate two carrier frequencies at L1 = 1575.42 MHz and L2 = 1227.60 MHz [13]. GPS signals consist of a carrier signal with frequency L1 or L2, a unique code assigned to each satellite, and a data message conveying information about satellite position, velocity, and clock bias. The two carrier frequencies are modulated by a combination of the data message and the unique code to carry required information to the user. The L1 frequency is modulated by two ranging code signals: the coarse/ acquisition code (C/A) and the precise (P) code [2].

Each satellite has a unique C/A PRN code, and all these PRN codes are nearly orthogonal to each other, enabling a GPS receiver to differentiate among the satellites even though the satellites are broadcasting on the same two carrier frequencies, L1 and L2 [14]. Each C/A code repeats every millisecond and has a length of 1023 bit. The duration of each chip in a C/A code is about 1 ms, and the code rate is 1.023 MHz (or megachips/second (Mcps)) with a wavelength of about 300 m. The duration of the P code is about 7 days, and it modulates both L1 and L2. Used only by the military, this code has a rate of 10.23—10 times than that of a C/A code. The P code wavelength is about 30 m, making it much shorter and consequently much more precise than the C/A code [2].

The last key part of the GPS signal is the navigation message. It takes 12.5 min to receive the entire message, which is downloaded at a rate of 50 bit/s [6]. Its most important parts are the ephemeris, almanac data, and satellite clock bias parameters.

To prepare the GPS signal for transmission by the satellite, first, an XOR operation is applied to combine the binary navigation message with the code. If the message bit and the code chip are the same, the result is 0; if they are different, the result is 1. Second, the combined signal is merged with the carrier using binary phase shift keying (BPSK) modulation: a "0" bit leaves the carrier signal intact, whereas a "1" bit causes the signal to be multiplied by −1 and shifts the carrier by 180°. **Figure 4** illustrates this process.

As mentioned above, the PRN code patterns are nearly orthogonal, an important property that makes the satellite identification process much easier [2]. Two codes are orthogonal when the sum of their term products shifted arbitrarily against each other is nearly zero. The cross correlation function for satellites m and n, with PRN codes *C*(*k*) and *C*(l) , is expressed as

$$\sum\_{1}^{1023} \mathbf{C}^{(k)}(\mathbf{i}) \cdot \mathbf{C}^{(0)}(\mathbf{i} + \mathbf{n}) \approx \ 0,\\\\frac{1}{k} \not\sim \mathbf{l} \tag{1}$$

This orthogonality makes the cross satellite interference small [14].

Another important property of PRN codes is that each PRN pattern is almost uncorrelated with itself:

$$\sum\_{1}^{1023} \text{C}^{(0)}(\text{i}) \cdot \text{C}^{(0)}(\text{i} + \text{n}) \approx 0,\\\\frac{1}{}\text{n} \mid \text{n} \mid \ge 1\tag{2}$$

**Figure 4.** GPS signal structure [15].

**4. GPS signal characteristics**

124 Multifunctional Operation and Application of GPS

antennas, and 16 monitoring sites [11].

more precise than the C/A code [2].

acquisition code (C/A) and the precise (P) code [2].

ephemeris, almanac data, and satellite clock bias parameters.

GPS satellites produce a central L-band frequency of 10.23 MHz using very stable clocks. Satellites then multiply this frequency by 154 and 120 to generate two carrier frequencies at L1 = 1575.42 MHz and L2 = 1227.60 MHz [13]. GPS signals consist of a carrier signal with frequency L1 or L2, a unique code assigned to each satellite, and a data message conveying information about satellite position, velocity, and clock bias. The two carrier frequencies are modulated by a combination of the data message and the unique code to carry required information to the user. The L1 frequency is modulated by two ranging code signals: the coarse/

**Figure 3.** The locations of the GPS Master Control Station, an alternate Master Control Station, 12 command and control

Each satellite has a unique C/A PRN code, and all these PRN codes are nearly orthogonal to each other, enabling a GPS receiver to differentiate among the satellites even though the satellites are broadcasting on the same two carrier frequencies, L1 and L2 [14]. Each C/A code repeats every millisecond and has a length of 1023 bit. The duration of each chip in a C/A code is about 1 ms, and the code rate is 1.023 MHz (or megachips/second (Mcps)) with a wavelength of about 300 m. The duration of the P code is about 7 days, and it modulates both L1 and L2. Used only by the military, this code has a rate of 10.23—10 times than that of a C/A code. The P code wavelength is about 30 m, making it much shorter and consequently much

The last key part of the GPS signal is the navigation message. It takes 12.5 min to receive the entire message, which is downloaded at a rate of 50 bit/s [6]. Its most important parts are the Autocorrelation of a PRN pattern is nearly zero for any shift |n| ≥ 1. When n is zero, however, the function reaches a peak. Using this feature, the receiver compares the PRN code on the received signal against a locally generated replica of the same code to identify which satellite has generated the corresponding signal.

After the antenna and LNA comes the RF front end. This unit generates a clean sampled signal for the signal-processing block [12]. Indeed, the front-end pre-filters amplify, downcon-

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Filtering is crucial for several reasons: it rejects out-of-band signals, reduces noise in the received signal, and lessens the impact of aliasing. Wide bandwidth signals can provide high-resolution measurements in the time domain but demand higher sampling rates, causing the receiver to consume much more power [18]. A filter can mitigate this by allowing narrower band signals. Down-conversion is the process performed by the front end to lower the RF signal frequency to either an intermediate frequency or directly to baseband [3]. This is necessary to facilitate the sampling and filtering processes. The down-conversion is often done using a mixer which multiplies the received signal by a locally generated replica and, then, filters the output signal to remove double-frequency terms [1], as depicted in **Figure 6**. The filtering and downconversion of the signal frequencies are typically achieved in multiple, consecutive, stages due to the difficulty in implementing a stable band-pass filter with a high central frequency. The last stage in the processing of the signal inside the RF front end is the conversion of the analogue signal to a digital signal. The band-pass sampling completes both discretization and

GPS receivers make their measurements using the estimates of the signal TOA and received carrier phase and frequency. A single local reference oscillator (see **Figure 4**) forms all frequency references in the receiver [19]. Because the oscillator is critical to receiver performance, particular attention needs to be given to its size, power consumption, stability (both short and long terms), and its temperature and vibration sensitivity [3]. In some cases, GPS receivers have multiple frequency references for down-conversion. In these instances, each mixer requires a precise reference frequency. The process of producing reference frequencies in the receiver from the local oscillator is called frequency synthesis, which uses a combination of

**Figure 4** illustrates that the final stage of a GPS receiver is the navigation processor. This unit receives the conditioned signal (the output of the front end). This filtered and downconverted signal should contain all the necessary information carried by the signal when it

vert, and digitize the received signal.

down-conversion of the signal [12].

integer and fractional frequency multiplications [20].

**Figure 6.** Block diagram of two-cascaded-stage down-conversion.
