GLONASS General Introduction

GLONASS
Title	GLONASS General Introduction
Author(s)	GMV
Level	Basic
Year of Publication	2011

GLONASS is a space-based global navigation satellite system (GNSS) that provides reliable positioning, navigation, and timing services to users on a continuous worldwide basis freely available to all. GLONASS receivers compute their position in the GLONASS Reference System using satellite technology and based on triangulation principles. It is an alternative and complementary to other GNSS systems such as the United States' Global Positioning System (GPS), the Chinese COMPASS navigation system or the planned GALILEO positioning system of the European Union (EU).

Introduction

The first Soviet navigation spacecraft “Cyclone” was launched into orbit in 1967. This was the beginning of the first Soviet low orbit navigation system, called “Cicada”. It was composed of four satellites placed in circular orbits at an altitude of 1000 km and an inclination of 83 deg and could provide positioning data within the limits of several hundred meters. Nevertheless the requirements to space navigation were constantly increasing and low-orbit systems could not comply with the requirements of all potential users.

Flight tests of high altitude (20000 km) satellite navigation system, called GLONASS were started in 12 October 1982 with the launch of the Kosmos-1413, Kosmos-1414, and Kosmos-1415. After the dissolution of the Soviet Union in 1991, the system was continued by the Russian Federation which formally declared the system operational in 1993 and brought to its optimal status of 24 operational satellites in 1995.

Following completion, the system fell into disrepair with the collapse of the Russian economy and the reduction in funding for space industry . In the early 2000s, under Vladimir Putin's presidency, the restoration of the system was made a top government priority and funding was substantially increased. In May 2007 Russian President Vladimir Putin signed a decree on the GLONASS navigation system to provide the service free for customers: "Access to civilian navigation signals of global navigation satellite system GLONASS is provided to Russian and foreign consumers free of charge and without limitations".

GLONASS Signal Structure

GLONASS Signal Spectrum

Each GLONASS system Space Vehicle (SV) "GLONASS" and "GLONASS-M" transmits navigational radiosignals on fundamental frequencies in two frequency sub-bands (L1 ~ 1,6 GHz, L2 ~ 1,25 GHz).

It is important to remark that GLONASS relies on the Frequency Division Multiple Access (FDMA) technique instead of the CDMA one used by other GNSS systems such GPS or GALILEO. Each satellite transmits navigation signals on its own carrier frequency, so that two GLONASS satellites may transmit navigation signals on the same carrier frequency if they are located in antipodal slots of a single orbital plane.

The frequency of transmission of each GLONASS satellite can be derived from the channel number k by applying the following expressions:

• [math]\displaystyle{ f_{k1} =f_{01} + k\Delta f_1 }[/math], where [math]\displaystyle{ f_{01} =1602 MHz }[/math] and [math]\displaystyle{ \Delta f_1=562.5 KHz }[/math]
• [math]\displaystyle{ f_{k2} =f_{02} + k\Delta f_2 }[/math], where [math]\displaystyle{ f_{02} =1246 MHz }[/math] and [math]\displaystyle{ \Delta f_2=437.5 KHz }[/math]

The modernization of GLONASS will add a new third frequency G3 in the ARNS band for the GLONASS-K satellites. This signal will provide a third civil C/A2 and military P2 codes, being especially suitable for Safety-Of-Life applications.

GLONASS Reference Frame

Accurate and well-defined Time References and Coordinate Frames are essential in GNSS, where positions are computed from signal travel time measurements and provided as a set of coordinates (please refer to GNSS positioning). GLONASS time (GLONASST) is generated by the GLONASS Central Synchroniser (CS). Unlike GPS, the GLONASS time scale is adjusted for periodic leap seconds and the difference between the UTC (Coordinated Universal Time) and GLONASST should not exceed 1 millisecond plus three hours.

As specified in the GLONASS Interface Control Document, the GLONASS broadcast ephemeris describes a position of transmitting antenna phase center of given satellite in the PZ-90.02 Earth-Centered Earth-Fixed reference frame defined as follows:

• The ORIGIN is located at the center of the Earth's body.
• The Z-axis is directed to the Conventional Terrestrial Pole as recommended by the International Earth Rotation Service (IERS).
• The X-axis is directed to the point of intersection of the Earth's equatorial plane and the zero meridian established by the Bureau International de l'Heure (BIH).
• The Y-axis completes the coordinate system to the right-handed on.

GLONASS Services

Two services are available from GLONASS system:

• SPS: The Standard Positioning Service (or Standard Accuracy Signal service) is an open service, free of charge for worldwide users. The navigation signal was initially provided only in the frequency band G1, but from 2004 on the new GLONASS-M transmits also a second civil signal in G2.
• PPS: The Precise Positioning Service (or High Accuracy Signal service) is restricted to military and authorized users. Two navigation signals are provided in the two frequency bands G1 and G2.

GLONASS Architecture

GLONASS is comprised of three segments: a GLONASS Space Segment (SS), a GLONASS Ground Segment (CS), and a GLONASS User Segment (US).

According to the GLONASS Interface Control Document, the GLONASS Space Segment is composed of 24 satellites in three orbital planes whose ascending nodes are 120 deg apart. Eight satellites are equally spaced in each plane with argument of latitude displacement of 45 deg. The 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. The spacing of the satellites allows providing continuous and global coverage of the terrestrial surface and the near-earth space.

GLONASS Ground Segment includes the System Control Center and the network of the Command and Tracking Stations that are located throughout the territory of Russia. The control segment provides monitoring of GLONASS constellation status, correction to the orbital parameters and navigation data uploading.

Finally, GLONASS User Segment consists in the user receivers which compute coordinates, velocity and time from the GLONASS navigation signals.

GLONASS Performances

Integral availability of GLONASS navigation (PDOP≤6) during the 24 hours period (mask angle ≥5 deg) Date: 23.01.2012

Currently (January 2012), there are 31 GLONASS satellites in orbit, being 23 of them in operational status. Availability performance (calculated as the percentage of time during which the condition PDOP ≤ 6 is valid at mask angles ≥ 5 deg) obtained with the current constellation is shown in the right figure.

With regard to GLONASS accuracy, according to the Russian System of Differentional Correction and Monitoring (SDCM), as of 23 January 2012, GLONASS horizontal precision is in the order of 4-7 m whereas the vertical error is in the order of 10-15 meters:

Station	GLONASS Error of navigation def. (p=0,95)			Mean number of NSV
	latitude (m)	longitude (m)	altitude (m)
Bellinsgauzen	4.80	5.23	11.44	8
Gelendzhik	5.60	6.28	14.08	8
Irkutsk	6.35	6.39	10.52	8
Kamchatka	5.73	5.25	12.72	8

Analyzing the accuracy obtained with GPS in the same stations it can be derived that GLONASS is slightly less accurate than GPS. In the same way, the mean number of GLONASS satellites in view is lower than GPS:

Station	GPS Error of navigation def. (p=0,95)			Mean number of NSV
	latitude (m)	longitude (m)	altitude (m)
Bellinsgauzen	2.74	2.70	8.09	11
Gelendzhik	2.46	2.09	5.62	10
Irkutsk	2.32	2.02	5.16	10
Kamchatka	3.13	2.05	6.10	10

GLONASS Future and Evolutions

Glonass K satellite at the CeBIT 2011 Expo in Germany

GLONASS modernization began with the launch of second generation of satellites, known as GLONASS-M, in 2003. Currently (January 2012), the number of satellites in the constellation is 31, 23 operational, 3 spares, one in commissioning phase, one in flight tests phase and three in maintenance. The following generation of satellites, GLONASS-K, has a service life of 10 years and includes, for the first time, code-division-multiple-access (CDMA) signals in addition to the legacy FDMA signals. GLONASS-K satellites will be developed in two different versions:

• GLONASS-K1 will broadcast a CDMA signal 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 tracking by several receiver companies.
• GLONASS-K2 to be launched in 2013, will include six new signal, namely three CDMA signals, an encrypted signal at 1242 MHz (L2), and two signals at 1575.42 MHz (L1). The CDMA signals will be placed in the vicinity of the current FDMA signals.

The evolution of the GLONASS-K satellite block, GLONASS-KM, is under study nowadays. It is being considered to add a L5 signal at 1179.45 MHz and to revisit the current constellation deployment. GLONASS-KM would be launched after 2015.