The global navigation satellite system glonass and segments

Space Segment- When fully deployed the space segment consists of 24 satellites in three orbit planes. The planes have a nominal inclination of 64.8 degrees and are separated by 120 degrees in longitude. 8 satellites are evenly distributed in each plane. The radius of the circular orbits is 25.510 kilometers. The orbital period is

8/17 of a sidereal day or approximately 11 hour and 16 minutes. GLONASS satellites complete “exactly” 17 orbital revolutions in eight sidereal days. After eight sidereal days a particular satellite will thus reappear at the same position in the sky for an observer on the Earth’s surface. Because each orbital plane contains eight equally spaced satellites, an observer on the Earth will see one of these satellites at the same position in the sky at the same sidereal time each day. The constellation of 24 satellites guarantees that at least five satellites are seen simultaneously from 99 percent of the Earth’s surface.
Satellites carry atomic clocks and the payload needed for handling (receiving, processing and transmitting) navigation data. They also have reflectors to allow laser ranging from dedicated ground stations.
GLONASS satellites are divided into blocks. Each block is a set of satellites usually launched within certain time interval. Below there is a brief description of the different blocks:

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  Prototypes (Generation zero):The first prototypes of GLONASS (Uragan) satellites were sent into orbit in October 1982, being up to 18 spacecrafts launched between 1982 and 1985.These first GLONASS satellites are referred to as Block I vehicles and, although being designed to last only one year, many of them had an actual lifetime up to more than 14 months
  
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  First generation:The first true GLONASS satellites were launched between 1985 and 1990. They are divided into different block vehicles (Block IIa, IIb, and IIv), being the design lifetimes the main difference between blocks. These satellites are all 3-Axis stabilized spacecrafts with satellite mass of about 1 250 Kg, being equipped with a basic propulsion system to allow relocation within the constellation.
  
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  Second generation: GLONASS-M (or Uragan-M) is the second generation of satellites, where -M indicates modernized or modified. They were developed from 1990 on, with the first one sent into orbit in 2003. A remarkable feature of these satellites is the addition of a second civil signal on G2 band, which allows the civil users to cancel out the ionospheric refraction.
  
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  Third generation: The following generation of satellites, GLONASS-K, has a service life of 10 years and a reduced weight of only 750Kg (allowing their launch in pairs from Plesetsk Cosmodrome on Soyuz-2-1a). This new generation of satellites includes, for the first time, code-division-multiple-access (CDMA) signals accompanying the legacy frequency-division-multiple-access signals⁵.