**8. Overview of GLONASS**

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,

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,

• Signal acquisition: This involves detection of the signals from satellites in view and provides a rough estimation of the code delay and the Doppler frequency of the incoming

• Signal tracking: This is a recursive estimation process that continuously updates estimates

• Signal monitoring: This is simultaneous with tracking and involves estimation of several

• Navigation message extraction: This process, too, happens in parallel to signal tracking.

• Measurement generation: Uses the tracking parameters to estimate ranges and range rate

• PVT solution: Uses the range and range rate of change estimates to compute the desired

While tracking a satellite signal, a GPS receiver monitors three parameters: pseudoranges, carrier phase, and Doppler [7, 11]. A pseudorange is calculated by measuring the signal transit time from a satellite to the receiver and is described as "pseudo" ranges because these measurements are corrupted by satellite and receiver clock biases [6]. Carrier phase measurements track the difference between the carrier phases for the received and a locally generated replica of the signal. The Doppler measurement reflects the rate of change of the

GPS signals and measurements are prone to many disturbance factors commonly known as GPs errors. The first error source is due to the drift of both the satellite and receiver clocks.

The navigation message extraction includes satellite ephemerides' decoding.

). The receiver uses signal monitoring

and using these, it estimates the PVT solution for the antenna.

parameters, including the carrier-to-noise ratio (C/N<sup>0</sup>

to decide when loss of lock of signal occurs, for example.

divided into the following stages [12]:

128 Multifunctional Operation and Application of GPS

of time-varying signal parameters.

of change for all visible satellites.

navigational solution.

**6. GPS measurements**

carrier phase [12].

**7. GPS errors**

signal from each satellite.

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 user segment consists of all the military and civilian receivers.

#### **8.1. GLONASS space segment**

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

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 orbits are highly circular with eccentricities smaller than those of GPS and closer to zero [24]. GLONASS satellites have a radius of 25,510 km, which gives an altitude of 19,130 km [22]. Compared to GPS, GLONASS has a shorter orbital period (11 h 15 min 40 s) due to its lower altitude. A comparison of the main differences between GLONASS and GPS is given in later sections.

#### **8.2. GLONASS control segment**

A key task of the GLONASS control station is to synchronize the satellite clocks with GLONASS time and calculate the time offset between GLONASS time and UTC [3]. It also uploads clock corrections, predicted ephemeris, and almanac data to GLONASS satellites. Moreover, this segment monitors the status of the current GLONASS constellation and corrects the orbital parameters accordingly. GLONASS uploads its navigation data to the satellites twice a day, while this is done once a day by the GPS [25].

**9.2. Second generation (GLONASS-M)**

**Table 2.** Roadmap of GLONASS modernization.

**9.3. Third generation (GLONASS-K)**

**10. GLONASS signal characteristics**

over the years.

signals, and increased the stability of those signals [27].

GLONASS-M, the modernized version of the former constellation, was launched in 2003, boasting a longer design lifespan of about 7 years and a civil modulation to its L2 frequency band. These changes improved navigation performance, provided updated navigation radio

**Satellite series Launch Current status Clock error (s)** GLONASS 1982 Out of service 5 × 10−13 GLONASS-M 2003 In service 1 × 10−13 GLONASS-K1 2011 In service 5 × 10−14 GLONASS-K2 2013 Design phase 1 × 10−14

GNSSs, Signals, and Receivers

131

http://dx.doi.org/10.5772/intechopen.74677

Significant improvements came in 2011 with the launch of the third generation, GLONASS-K. Among these changes was the increase of its satellites' lifespan to a decade and the reduction of their weight by half [22]. The accuracy was improved as well, with each satellite transmitting five navigation signals instead of two. These new satellites were intended to transmit four military signals on the L1 and L2 carriers and one civilian signal on the L3 frequency. The GLONASS-K satellites broadcast other signals; two of them are compatible with GPS/ Galileo navigational signals. Adding the CDMA signals improved compatibility and enabled interoperability with services provided by other GNSSs, which paved the way for the production of receivers usable with all GNSSs [23]. **Table 2** shows how the system was upgraded

The GLONASS Interface Control Document (ICD) held by the Russian Institute of Space Device Engineering provides the detailed information about the structure of the GLONASS radio signals [22]. In contrast to GPS, GLONASS uses frequency division multiple access (FDMA) for signal modulation. This technique uses the same pseudorandom noise (PRN) code for all satellites to produce a spread spectrum signal. GPS, on the other hand, uses codedivision multiple access (CDMA) to identify each individual satellite. FDMA provides better interference rejection for narrow-band interference signals compared to CDMA. In CDMA a single source of narrow-band interference source can disrupt all GPS satellite signals simultaneously, such interference only affects one FDMA GLONASS signal at a time. A shortcoming

GLONASS's ground control segment has two main parts: the system control center (SCC), located in Moscow, and a network of command tracking stations (CTS), located throughout the former Soviet Union (SU). The roles of the SCC and CTS are similar to those of the GPS Master Control Station and its monitoring stations [22].

#### **8.3. GLONASS user segment**

Like that for GPS, the GLONASS user segment contains the end user receiver equipment, which tracks and receives satellite signals. Similar to GPS receivers, these also process signals transmitted by the seen satellites, estimate pseudorange and rate of change of pseudorange from these signals, and calculate a position, velocity, and time (PVT) solution.
