**5. GPS receiver architecture**

**Figure 5** shows the high-level architecture of a GPS receiver. GPS receivers are made up of the antenna, RF front end, local oscillator, and navigation processor. The first element of the receiver architecture is the antenna, which must be able to receive right-hand circularly polarized (RHCP) signals because this is the type of signal transmitted by GPS satellites [1]. Also important is the antenna gain pattern, which indicates how well the antenna performs at various center frequencies, polarizations, and elevation angles.

The preamplifier is the first active component that comes after the antenna. It is often housed in the same enclosure as the antenna element. Because the antenna can receive multiple frequency bands, typically, there is one preamplifier per band; nonetheless, a single preamplifier may cover multiple bands. The main function of the preamplifier is to amplify the signal at the antenna's output [3]. Preamplifiers generally have three components: (1) a preselector filter that removes out-of-band interference and limits the noise bandwidth, (2) burnout protection that prevents possible high-power interference with the electronic components of the receiver, and (3) a low-noise amplifier (LNA). GPS signals are typically very weak, around −160 dBw or 10–6 W; thus, an LNA amplifies the signals by 20 to 35 dB to increase them to levels suitable for processing [17].

**Figure 5.** High-level architecture of GPS receivers [16].

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, downconvert, and digitize the received signal.

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 satel-

**Figure 5** shows the high-level architecture of a GPS receiver. GPS receivers are made up of the antenna, RF front end, local oscillator, and navigation processor. The first element of the receiver architecture is the antenna, which must be able to receive right-hand circularly polarized (RHCP) signals because this is the type of signal transmitted by GPS satellites [1]. Also important is the antenna gain pattern, which indicates how well the antenna performs at vari-

The preamplifier is the first active component that comes after the antenna. It is often housed in the same enclosure as the antenna element. Because the antenna can receive multiple frequency bands, typically, there is one preamplifier per band; nonetheless, a single preamplifier may cover multiple bands. The main function of the preamplifier is to amplify the signal at the antenna's output [3]. Preamplifiers generally have three components: (1) a preselector filter that removes out-of-band interference and limits the noise bandwidth, (2) burnout protection that prevents possible high-power interference with the electronic components of the receiver, and (3) a low-noise amplifier (LNA). GPS signals are typically very weak, around −160 dBw or 10–6 W; thus, an LNA amplifies the signals by 20 to 35 dB to increase them to

lite has generated the corresponding signal.

ous center frequencies, polarizations, and elevation angles.

**5. GPS receiver architecture**

126 Multifunctional Operation and Application of GPS

levels suitable for processing [17].

**Figure 5.** High-level architecture of GPS receivers [16].

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 down-conversion of the signal [12].

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 integer and fractional frequency multiplications [20].

**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

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

was received by the antenna. At the navigation-processing stage, the receiver extracts the measurements for pseudorange and rate-of-change of pseudorange to all satellites in view, and using these, it estimates the PVT solution for the antenna.

Despite their high level of accuracy, satellite clocks still drift slightly from GPS time. For affordability reasons and size, receiver clocks are usually much cheaper; consequently, they drift from GPS time rapidly. This drift translates into significant range errors in receiver

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Once it departs the satellite antenna, the GPS signal needs to travel thousands of kilometers to reach to the receiver antenna and then the receiver circuitry. The first and longer part of this trip is by space where the signal maintains its characteristics. However, when the signal enters the atmosphere, this medium causes some unwanted effects. The two primary layers of the atmosphere, namely, ionosphere and troposphere, respectively, will add delays to the

Once it nears the receiver antenna, the signal usually experiences reflections and echoes, i.e., it often bounces off objects near the receiver causing it to hit the antenna from different directions—a phenomenon known as multipath. Multipath is one of the major sources of errors, which harms GPS signals [6]. All aforementioned disturbances are a result of the nature of the signal or the propagation medium and are considered unintentional. Intentional signal degradation or replacement is, in many cases, a more problematic source of GPS errors. One major type of intentional errors is signal jamming. Signal jamming is deliberate interference caused by broadcasts of radio frequency (RF) signals around the receiver neighborhood with

Like GPS, GLONASS offers three-dimensional positioning and navigation services for both civilian and military users. In this system too, users determine their position and velocity using pseudorange and carrier phase measurements. Both systems use time-of-arrival (TOA) ranging to determine user position and velocity [21]. The GLONASS includes three components: a constellation of satellites (equivalent to the GPS space segment), ground control stations (also equivalent to the GPS control segment), and user's equipment (as well, equivalent to the GPS user segment) [22]. The ground segment consists of a master control station (MCS).

The full GLONASS constellation consists of 24 satellites [21]. According to [23], 26 functional GLONASS-M satellites are in orbit, and 22 of them are in service, with four more having reserve status. With the launches of several GLONASS-M satellites and the GLONASS-K sat-

GLONASS satellites circle the earth in three orbital planes evenly spaced by 120°. Each plane has eight satellites that are separated by an argument of latitude of 45°, and those satellites have a target inclination of 64.8°—considerably higher than that of GPS satellites. GLONASS

signal transit time and, hence, cause some errors in the measurements.

the aim of preventing the tracking of true GNSS signals.

The user segment consists of all the military and civilian receivers.

ellites, a full constellation of 24 satellites is now available.

**8. Overview of GLONASS**

**8.1. GLONASS space segment**

measurements.

The navigation process usually happens in two stages: first, the pseudorange and pseudorange rates to each satellite are estimated; second, the user's position, velocity, and time information are estimated using these measurements. Signal processing at this level can be, in turn, divided into the following stages [12]:

