GLONASS Space Segment

GLONASS
Title	GLONASS Space Segment
Author(s)	GMV
Level	Basic
Year of Publication	2011

The main functions of the Space Segment are to transmit radio-navigation signals, and to store and retransmit the navigation message sent by the Control Segment. These transmissions are controlled by highly stable atomic clocks on board the satellites.

Two main aspects define the space segment: One is the number of satellites in the constellation to ensure that the users will have, at least, 4 simultaneous satellites in view from any point at the earth surface at any time. The other aspect is the features of the satellites that occupy each orbital slot.

GLONASS Satellite Constellation

Glonass Constellation

The GLONASS space segment consists nominally of 24 operational satellites, distributed over three orbital planes. At the time of editing this article, the number of satellites in the constellation is 27, 23 operational, one in commissioning phase and three in maintenance.

The longitude of ascending node differs by 120 deg from plane to plane. Each plane comprises eight satellites, staggered by 45 deg in argument of latitude. The arguments of latitude of satellites in equivalent slots in two different orbital planes differ by 15 deg. Each satellite is identified by its slot number, which defines the orbital plane and its location within the plane.

The GPS space segment also consists of nominally 24 satellites, which are, however, distributed over six orbital planes, differing from plane to plane by 60 deg in longitude of the ascending node.

GLONASS satellites operate in circular orbits at an altitude of 19100-km, an inclination of 64.8 deg and each satellite completes the orbit in approximately 11 hours 15 minutes. This means that for a stationary observer the same satellite is visible at the same point in the sky every eight sidereal days. Since there are eight satellites in each orbital plane, each day a different satellite appears at the same point in the sky. With the 11 h 58 min orbital period for GPS satellites, the same GPS satellite is visible at the same point in the sky every (sidereal) day.

Besides its atomic clocks and the equipment for receiving, processing, storing and transmitting navigational data, GLONASS satellites carry an extensive propulsion system, enabling the satellite to keep its orbital position, to control its attitude and even to manoeuvre to a different orbital position. The attitude control system obtains its information from a number of different sensors, including an earth sensor and a magnetometer. Reflectors on the satellite body near the transmission antennae serve for purposes of laser ranging from ground stations.

Differences between GPS and GLONASS constellations are summarized in the following table:

	GLONASS	GPS
Number of satellites	24	30
Number of orbital planes	3	6
Orbital Inclination	64°8'	55°
Orbital altitude	19.140 Km	20.180 Km
Period of revolution	11h 15m	11h 58m
Launch site	Baikonur/Plesetsk	Cape Canaveral
Date of first launch	02/10/82	22/02/78
Satellites for launch	1/3	1
L1	1602+K*0,5625	1575.42
L2	1246+K*0,4375	1227,60
Clock Precision	1*10-13	1 sec/300.000 years
Datum	PZ-90.02	WGS-84

GLONASS Satellite Description

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:

• 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.

• 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. The lifetimes ranged from the 2-year design of Block IIb to 3-year of Block IIv, with many spacecraft exceeding this (up to 4.5 years).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. They have improved time and frequency standards over the previous spacecraft prototypes, with increased frequency stability.

• 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, and a total of twenty satellites launched up to March 2010. GLONASS-M satellites have a longer design lifetime of seven years as a result of propulsion system and clock stability (Cesium clocks) improvements. This is a big lifetime increase compared with the 2-3 years of previous first generation spacecraft, but it is still under the 10 years mean life of the GPS. These satellites have a mass of around 1 480 kg. Their size is of 2.4 m of diameter and 3.7 m in high, with dual solar arrays of 7.2 m. They also carry corner-cube laser reflectors for precise orbit determination and geodetic research. 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.

Glonass K satellite at the CeBIT 2011 Expo in Germany

• 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 signals. There will be two versions:
  • GLONASS-K1 will transmit a CDMA signal located in the L3 band. The first GLONASS-K1 satellite was successfully launched on February 26 2011 and the new L3 CDMA signal is already being tracked by several receiver companies.
  • GLONASS-K1 to be launched in 2013, will feature three additional CDMA signals near the original FDMA frequencies, one obfuscated signal located at 1242 MHz in the L2 band, as well as two signals at 1575.42 MHz in the L1 band.

A modernized GLONASS-K satellite, GLONASS-KM, for launch after 2015 may also transmit on the GPS L5 frequency at 1179.45 MHz. An alternative to the present three-plane, equally spaced satellite constellation, is also under consideration.

Finally, the following table gathers the main characteristics of the different GLONASS satellite blocks:

	Glonass	Glonass-M	Glonass-K
Status	out of service	in service	in service
First launch	1982	2003	2011
Lifetime (years)	3	7	10
Mass (Kg)	1250	1480	750
Output power (W)	1000	1600	1600
Number of civil signals	1	2	2
On board clocks stability	5x10-13	1x10-13	1x10-14