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 satellite constellations with enough satellites 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

The GLONASS space segment consists of 24 operational satellites, distributed over three orbital planes. Currently, 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 first orbital plane has slot numbers 1…8, the second orbital plane - slots 9…16, and the third orbital plane - slots 17…24.

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 GPSsatellites, the same GPSsatellite is visible at the same point in the sky every (sidereal) day.

Besides its atomic clock 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.

GPS Satellite Description

GPS satellite at San Diego Museum

The design of GPS satellites has evolved with time. Each generation of satellites with similar characteristics is called a block. A brief description of the different GPS blocks follows:

• Block I, Navigation Development Satellites: Eleven satellites of this kind were launched between 1978 and 1985. The Selective Availability (S/A) was not implemented. They weighed about 845Kg and had a planned average life of 4.5 years, although some of them lasted up to 10. They were capable of giving positioning service for 3 or 4 days without any contact with the control centre.
• Block II and IIA, Operational Satellites: They consist of 28 satellites in total that were launched from 1989 on and many are still operating. They weigh about 1 500 Kg and have a planned average life of 7.5 years. Since 1990, an improved version was used, Block IIA (advanced), with capability of mutual communication. They are able to supply positioning service for 180 days with no contact with the control segment. However, under normal operating mode, they communicate daily.
• Block IIR, Replacement Operational Satellites: From 1997, these satellites are being used as spares for Block II. Block IIR is formed by a set of 20 satellites, although it could be increased by 6 more. They weigh about 2 000Kg and have a planned average lifespan of 10 years. These satellites have the capability to autonomously determine their orbits and generate their own navigation message. They are able to measure distances between them and transmit observations to other satellites or to the control segment. A satellite of this category, completely developed, must be capable of operating about half a year without any support from Control Segment and no degradation in ephemeris accuracy. It is expected that some of them could be equipped with Hydrogen masers.
• Block IIR-M, Modernized Satellites: They include a new military signal and the more robust civil signal L2C. There will be eight satellites in the Block IIR-M series. The first Block IIR-M satellite was launched on September 26, 2005 .
• Block IIF, Follow-on Operational Satellites: The first satellite (SVN62) was launched on May 28th 2010 . These satellites will include the third civil signal on the L5 band. Their theoretical average life is about 15 years, and they will have inertial navigation systems.
• Block III: The new generation of GPS satellites will introduce significant enhancements in navigation capabilities, by improving interoperability and jam resistance. They will provide the fourth civil signal on L1 band (L1C). The first launch is expected as of 2016.

Finally, the following table gathers the main characteristics of a GPS Satellite: