GLONASS General Introduction

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

GLONASS(Russian: ГЛОНАСС, abbreviation of ГЛОбальная НАвигационная Спутниковая Система; tr.: GLObal'naya NAvigatsionnaya Sputnikovaya Sistema; "GLObal NAvigation Satellite System" in English) is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces. It is an alternative and complementary to the United States' Global Positioning System (GPS), the Chinese Compass navigation system, and the planned Galileo positioning system of the European Union (EU).

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 1000 km and an inclination of 83 ° 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. In 1993, the system consisting of 12 satellites, was formally declared operational and in December 1995, the constellation was finally brought to its optimal status of 24 operational satellites.

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

Each GLONASS system SVs "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 in contrast to CDMA employed by all the 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 : [EXPRESIONNNN]

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 [IMAGENNNN]

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. GLONASS time (GLONASST) is generated on a base of GLONASS Central Synchronizer (CS) time. Due to the leap second correction there is no integer-second difference between GLONASS time and UTC (SU). However, there is constant three-hour difference between these time scales due to GLONASS control segment specific features

• TГЛ = TUTC (SU) + 03 hour 00 minutes:

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 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 (U.S.) .

GLONASS Space Segment is composed of 24 satellites in three orbital planes whose ascending nodes are 120 apart. 8 satellites are equally spaced in each plane with argument of latitude displacement 45. The orbital planes have 15 -argument of latitude displacement relative to each other. The satellites operate in circular 19100-km orbits at an inclination 64.8, 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. The 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 of receives and processors receiving and processing the GLONASS navigation signals, and allows user to calculate the coordinates, velocity and time

GLONASS Performances

The levels of performance that the user can expect from GPS are specified in the Standard Positioning Service Performance Standard, and the Precise Positioning Standard. However, the values provided by these documents are very conservative, being the actual performances usually better than these official values. Moreover, the performance obtained with GPS depends strongly on the mode of operation. For instance, a stand-alone receiver that uses only the signals received from the satellites, the levels of performance are:

• C/A-code receivers ~ 5 -10 m.
• P/Y-code receivers ~ 2 -9 m

In case of using GPS in a differential mode, the performances that can be expected are:

• C/A-code DGPS receivers ~0.7 -3 m.
• P/Y-code DGPS receivers ~0.5 -2.0 m.

At peak efficiency, the SP signal offers horizontal positioning accuracy within 5–10 meters, vertical positioning within 15 meters, a velocity vector measuring within 10 cm/s, and timing within 200 ns, all based on measurements from four first-generation satellites simultaneously;[34] newer satellites such as GLONASS-M improve on this. The more accurate HP signal is available for authorized users, such as the Russian Military, yet unlike the US P(Y) code which is modulated by an encrypting W code, the GLONASS P codes are broadcast in the clear using only 'security through obscurity'. Use of this signal bears risk however as the modulation (and therefore the tracking strategy) of the data bits on the L2P code has recently changed from unmodulated to 250bps burst at random intervals. The GLONASS L1P code is modulated at 50bps without a manchester meander code, and while it carries the same orbital elements as the CA code, it allocates more bits to critical Luni-Solar acceleration parameters and clock correction terms.

GLONASS Future and Evolutions

Aimed at improving the performance for civilian users, the GPS modernization will introduce the following signals:

• L2C (1227.6 MHz: It enables the development of dual-frequency civil GPS receivers to correct the ionospheric group delay. This signal is available since 2005, with the launch of the first IIR-M satellite.
• L5 (1176.45 MHz): It will be compatible with other GNSS systems and will transmit at a higher power than current civil GPS signals, and have a wider bandwidth. This signal is available since the launch of the Block IIF satellites (May 28th 2010).
• L1C (1575.42 MHz): Designed for interoperability with Galileo, it will be backward compatible with the current civil signal on L1

Moreover, in order to improve the anti-jamming and secure access of the military GPS signals, a new military signal (M-code) will be transmitted in L1 and L2 frequencies. Regarding the Ground Segment, the new Operational Control Segment (OCX) will replace the current GPS Operational Control System placed at Schriever Air Force Base. The OCX will maintain backwards compatibility with the Block IIR and IIR-M constellation satellites, providing command and control of the new GPS IIF and GPS III families of satellites, and enabling new modernized civil signal capabilities.

GPS Modernization