**9. GLONASS modernization**

The design of the GLONASS satellite has been improved several times, resulting in three satellite generations: the original GLONASS (started in 1982), GLONASS-M (started in 2003), and GLONASS-K (started in 2011). There are two types of GLONASS spacecraft in the constellation: the GLONASS-M satellite and GLONASS-K satellite. A brief description of each type is as follows.

#### **9.1. First generation (GLONASS)**

The first generation of GLONASS satellites (Uragan) was launched in 1982. Each satellite weighed approximately 1250 kg and was equipped with a basic propulsion system enabling it to relocate within its orbit [26]. Initially, GLONASS's main role was to control the formulation of the navigation signal and obtain the satellite ephemeris and almanac data. This generation is no longer in use.


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

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

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 satel-

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

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

The design of the GLONASS satellite has been improved several times, resulting in three satellite generations: the original GLONASS (started in 1982), GLONASS-M (started in 2003), and GLONASS-K (started in 2011). There are two types of GLONASS spacecraft in the constellation: the GLONASS-M satellite and GLONASS-K satellite. A brief description of each

The first generation of GLONASS satellites (Uragan) was launched in 1982. Each satellite weighed approximately 1250 kg and was equipped with a basic propulsion system enabling it to relocate within its orbit [26]. Initially, GLONASS's main role was to control the formulation of the navigation signal and obtain the satellite ephemeris and almanac data. This generation

from these signals, and calculate a position, velocity, and time (PVT) solution.

lites twice a day, while this is done once a day by the GPS [25].

Master Control Station and its monitoring stations [22].

sections.

**8.2. GLONASS control segment**

130 Multifunctional Operation and Application of GPS

**8.3. GLONASS user segment**

**9. GLONASS modernization**

**9.1. First generation (GLONASS)**

type is as follows.

is no longer in use.

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

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

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

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 over the years.
