GLONASS General Introduction

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

The GLONASS is a space-based global navigation satellite system (GNSS) that provides reliable positioning, navigation, and timing services to civilian and military users on a continuous worldwide basis. GPS receivers compute their position in the GPS Reference System using satellite technology and based on triangulation principles (please refer to GNSS positioning). GPS was originally developed for the U.S. military, but very early in the experimental phase of GPS the incident with the Korean Air Lines Flight 007 lead the US Government to decide to make GPS use free for civilian purposes. Deployment of the GPS system began on 22 February 1978 with the launch of the first Block I Navstar GPS satellite. Initial Operating Capability was declared in December 1993 with 24 operational GPS satellites in orbit. Full Operational Capability was declared in June of 1995. GPS is maintained by the United States government and is freely accessible by anyone with a GPS receiver. The Department of Defense is responsible for operating the system, but it also receives national-level attention and guidance through the National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT).

GPS block IIR-M satellite

RELLENAR CON EL ICD DE GLONASS !!!

GLONASS Signal Structure

GPS satellites transmit right-hand circularly polarized signals to the earth at two frequencies, designated L1 and L2. The main GPS carrier signal L1, at 1575.42MHz, is modulated by two codes: the coarse/acquisition (C/A) code also known as civilian code and the precision/secure (P/Y) code, reserved by cryptographic techniques to military and authorized civilian users. The GPS L2 signal, centered at 1227.6 MHz, only contains the precise code and it was established to provide a second frequency for ionospheric group delay correction.

The GPS modernization program began in 2005 with the launch of the first IIR-M satellite. Since that moment on, two new signals are transmitted: L2C for civilian users and a new military signal (M code) in L1 and L2 to provide better jamming resistance than the Y code.

Moreover, a new radio frequency link (L5 at 1176.45 MHz) for civilian users has been included. This signal, available since the launch of the Block IIF satellites (May 28th 2010) will be compatible with other GNSS systems.

GLONASS satellites transmit two types of signal: a standard precision (SP) signal and an obfuscated high precision (HP) signal.

The signals use similar DSSS encoding and binary phase-shift keying (BPSK) modulation as in GPS signals. All GLONASS satellites transmit the same code as their SP signal, however each transmits on a different frequency using a 15-channel frequency division multiple access (FDMA) technique spanning either side from 1602.0 MHz, known as the L1 band. The center frequency is 1602 MHz + n × 0.5625 MHz, where n is a satellite's frequency channel number (n=−7,−6,−5,...0,...,6, previously n==−7,...0,...,13). Signals are transmitted in a 38° cone, using right-hand circular polarization, at an EIRP between 25 to 27 dBW (316 to 500 watts). Note that the 24 satellite constellation is accommodated with only 15 channels by using identical frequency channels to support antipodal (opposite side of planet in orbit) satellite pairs, as these satellites will never be in view of an earth based user at the same time.

The HP signal is broadcast in phase quadrature with the SP signal, effectively sharing the same carrier wave as the SP signal, but with a ten times higher bandwidth than the SP signal.

The L2 signals use the same FDMA as the L1 band signals, but transmit straddling 1246 MHz with the center frequency determined by the equation 1246 MHz + n×0.4375 MHz, where n spans the same range as for L1.[33] Other details of the HP signal have not been disclosed.

Since 2008, new CDMA signals are being researched for use with GLONASS.

Two latest Glonass-K1 satellites to be launched in 2010-2011 will introduce an additional SP CDMA signal for testing purposes, located in the L3 band at 1202.025 MHz.

Glonass-K2 satellites, to be launched in 2013-2015, 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; subsequent Glonass-KM satellites to be launched after 2015 will feature an open signal in the L5 band at 1176.45 MHz and even more CDMA signals on existing frequencies.[35

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. GPS uses the World Geodetic System WGS-84, developed by The US Defence Department, which is a unified terrestrial reference system for position and vector referencing. Indeed, the GPS broadcast ephemeris are linked to the position of the satellite antenna phase centre in the WGS-84 reference frame. Thus, the user receiver coordinates will be expressed in the same ECEF frame. GPS System Time (GPST) is defined by the GPS Ground Segment on the basis of a set of atomic clocks aboard the satellites and in the Monitor Stations. It is not adjusted for leap seconds and it is synchronized with the UTC (USNO) at nanosecond level. The origin epoch of GPS time is 0h UTC (midnight) of January 5th to 6th of 1980.

GLONASS uses a coordinate datum named "PZ-90" (Earth Parameters 1990 – Parametry Zemli 1990), in which the precise location of the North Pole is given as an average of its position from 1900 to 1905. This is in contrast to the GPS's coordinate datum, WGS 84, which uses the location of the North Pole in 1984. As of September 17, 2007 the PZ-90 datum has been updated to differ from WGS 84 by less than 40 cm (16 in) in any given direction.

GLONASS Services

GPS provides two levels of service, Standard Positioning Service and the Precise Positioning Service: The Standard Positioning Service (SPS), is a positioning and timing service provided on GPS L1 frequency and available to all GPS users. The L1 frequency contains a coarse acquisition (C/A) code and a navigation data message. The Precise Positioning Service (PPS), is a highly accurate military positioning, velocity and timing service broadcasted at the GPS L1 and L2 frequencies. Both frequencies contain a precision (P) code ranging signal with a navigation data message that is reserved for authorized use by the use of cryptography.

GLONASS satellites transmit two types of signal: a standard precision (SP) signal and an obfuscated high precision (HP) signal.

The HP signal is broadcast in phase quadrature with the SP signal, effectively sharing the same carrier wave as the SP signal, but with a ten times higher bandwidth than the SP signal.

Currently, an additional civil reference signal is broadcast in the L2 band with an identical SP code to the L1 band signal. This is available from all satellites in the current constellation, except satellite number 795 which is the last of the inferior original GLONASS design, and one partially inoperable GLONASS-M satellite which is broadcasting only in the L1 band. (see www.glonass-ianc.rsa.ru for daily updates on constellation status.)

GLONASS Architecture

GPS is comprised of three segments: a GPS Space Segment, a GPS Ground Segment, and a GPS User Segment. The main functions of the GPS Space Segment are to transmit radio-navigation signals, and to store and retransmit the navigation message sent by the GPS Ground Segment. The GPS Ground Segment is composed of a master control station, a network of monitor stations and 4 ground antennas which upload the clock and orbit errors, as well as the navigation data message to the GPS satellites. Finally, the GPS User Segment consists on the millions of receivers performing the navigation, timing or other related functions.

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

References

Small introduction

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GPS block IIR-M satellite

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