GLONASS
The Global Navigation Satellite System, universally recognized by its acronym GLONASS, is a Russian satellite navigation system operating as part of a radionavigation-satellite service. It provides an alternative to Global Positioning System and is the second navigational system in operation with global coverage and of comparable precision.
Satellite navigation devices supporting both GPS and GLONASS have more satellites available, meaning positions can be fixed more quickly and accurately, especially in built-up areas where buildings may obscure the view to some satellites. Owing to its higher orbital inclination, GLONASS supplementation of GPS systems also improves positioning in high latitudes.
Development of GLONASS began in the Soviet Union in 1976. Beginning on 12 October 1982, numerous rocket launches added satellites to the system until the completion of the constellation in 1995. In 2001, after a decline in capacity during the late 1990s, the restoration of the system was made a government priority, and funding increased substantially. GLONASS is the most expensive program of Roscosmos, consuming a third of its budget in 2010.
By 2010, GLONASS had achieved full coverage of Russia's territory. In October 2011, the full orbital constellation of 24 satellites was restored, enabling full global coverage. The GLONASS satellites' designs have undergone several upgrades, with the latest version, GLONASS-K2, launched in 2023.
System description
GLONASS is a global navigation satellite system, providing real time position and velocity determination for military and civilian users. The satellites are located in middle circular orbit at altitude with a 64.8° inclination and an orbital period of 11 hours and 16 minutes. GLONASS's orbit makes it especially suited for usage in high latitudes, where getting a GPS signal can be problematic.The constellation operates in three orbital planes, with eight evenly spaced satellites on each. A fully operational constellation with global coverage consists of 24 satellites, while 18 satellites are necessary for covering the territory of Russia. To get a position fix the receiver must be in the range of at least four satellites.
Signal
FDMA
GLONASS satellites transmit two types of signals: open standard-precision signal L1OF/L2OF, and obfuscated high-precision signal L1SF/L2SF.The signals use similar DSSS encoding and binary phase-shift keying modulation as in GPS signals. All GLONASS satellites transmit the same code as their standard-precision signal; however each transmits on a different frequency using a 15-channel frequency-division multiple access 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. Signals are transmitted in a 38° cone, using right-hand circular polarization, at an EIRP between 25 and 27 dBW. Note that the 24-satellite constellation is accommodated with only 15 channels by using identical frequency channels to support antipodal satellite pairs, as these satellites are never both in view of an Earth-based user at the same time.
The L2 band signals use the same FDMA as the L1 band signals, but transmit straddling 1246 MHz with the center frequency 1246 MHz + n × 0.4375 MHz, where n spans the same range as for L1. In the original GLONASS design, only obfuscated high-precision signal was broadcast in the L2 band, but starting with GLONASS-M, an additional civil reference signal L2OF is broadcast with an identical standard-precision code to the L1OF signal.
The open standard-precision signal is generated with modulo-2 addition of 511 kbit/s pseudo-random ranging code, 50 bit/s navigation message, and an auxiliary 100 Hz meander sequence, all generated using a single time/frequency oscillator. The pseudo-random code is generated with a 9-stage shift register operating with a period of 1 milliseconds.
The navigational message is modulated at 50 bits per second. The superframe of the open signal is 7500 bits long and consists of 5 frames of 30 seconds, taking 150 seconds to transmit the continuous message. Each frame is 1500 bits long and consists of 15 strings of 100 bits, with 85 bits for data and check-sum bits, and 15 bits for time mark. Strings 1-4 provide immediate data for the transmitting satellite, and are repeated every frame; the data include ephemeris, clock and frequency offsets, and satellite status. Strings 5-15 provide non-immediate data for each satellite in the constellation, with frames I-IV each describing five satellites, and frame V describing remaining four satellites.
The ephemerides are updated every 30 minutes using data from the Ground Control segment; they use Earth Centred Earth Fixed Cartesian coordinates in position and velocity, and include lunisolar acceleration parameters. The almanac uses modified orbital elements and is updated daily.
The more accurate high-precision signal is available for authorized users, such as the Russian military, yet unlike the United States P code, which is modulated by an encrypting W code, the GLONASS restricted-use codes are broadcast in the clear using only security through obscurity. The details of the high-precision signal have not been disclosed. The modulation of the data bits on the L2SF code has recently changed from unmodulated to 250 bit/s burst at random intervals. The L1SF code is modulated by the navigation data at 50 bit/s without a Manchester meander code.
The high-precision signal is broadcast in phase quadrature with the standard-precision signal, effectively sharing the same carrier wave, but with a ten-times-higher bandwidth than the open signal. The message format of the high-precision signal remains unpublished, although attempts at reverse-engineering indicate that the superframe is composed of 72 frames, each containing 5 strings of 100 bits and taking 10 seconds to transmit, with total length of 36 000 bits or 720 seconds for the whole navigational message. The additional data are seemingly allocated to critical Lunisolar acceleration parameters and clock correction terms.
Accuracy
At peak efficiency, the standard-precision signal offers horizontal positioning accuracy within 5–10 metres, vertical positioning within, a velocity vector measuring within, and timing within 200 nanoseconds, all based on measurements from four first-generation satellites simultaneously; newer satellites such as GLONASS-M improve on this.GLONASS uses a coordinate datum named "PZ-90", in which the precise location of the North Pole is given as an average of its position from 1990 to 1995. This is in contrast to the GPS's coordinate datum, WGS 84, which uses the location of the North Pole in 1984. As of 17 September 2007, the PZ-90 datum has been updated to version PZ-90.02 which differ from WGS 84 by less than in any given direction. Since 31 December 2013, version PZ-90.11 is being broadcast, which is aligned to the International Terrestrial Reference System and Frame 2008 at epoch 2011.0 at the centimetre level, but ideally a conversion to ITRF2008 should be done.
CDMA
Since 2008, new CDMA signals are being researched for use with GLONASS.The interface control documents for GLONASS CDMA signals was published in August 2016.
According to GLONASS developers, there will be three open and two restricted CDMA signals. The open signal L3OC is centered at 1202.025 MHz and uses BPSK modulation for both data and pilot channels; the ranging code transmits at 10.23 million chips per second, modulated onto the carrier frequency using QPSK with in-phase data and quadrature pilot. The data is error-coded with 5-bit Barker code and the pilot with 10-bit Neuman-Hoffman code.
Open L1OC and restricted L1SC signals are centered at 1600.995 MHz, and open L2OC and restricted L2SC signals are centered at 1248.06 MHz, overlapping with GLONASS FDMA signals. Open signals L1OC and L2OC use time-division multiplexing to transmit pilot and data signals, with BPSK modulation for data and BOC modulation for pilot; wide-band restricted signals L1SC and L2SC use BOC modulation for both data and pilot, transmitted in quadrature phase to the open signals; this places peak signal strength away from the center frequency of narrow-band open signals.
Binary phase-shift keying is used by standard GPS and GLONASS signals. Binary offset carrier is the modulation used by Galileo, modernized GPS, and BeiDou-2.
The navigational message of CDMA signals is transmitted as a sequence of text strings. The message has variable size - each pseudo-frame usually includes six strings and contains ephemerides for the current satellite and part of the almanac for three satellites. To transmit the full almanac for all current 24 satellites, a superframe of 8 pseudo-frames is required. In the future, the superframe will be expanded to 10 pseudo-frames of data to cover full 30 satellites.
The message can also contain Earth's rotation parameters, ionosphere models, long-term orbit parameters for GLONASS satellites, and COSPAS-SARSAT messages. The system time marker is transmitted with each string; UTC leap second correction is achieved by shortening or lengthening the final string of the day by one second, with abnormal strings being discarded by the receiver.
The strings have a version tag to facilitate forward compatibility: future upgrades to the message format will not break older equipment, which will continue to work by ignoring new data, but up-to-date equipment will be able to use additional information from newer satellites.
The navigational message of the L3OC signal is transmitted at 100 bit/s, with each string of symbols taking 3 seconds. A pseudo-frame of 6 strings takes 18 seconds to transmit. A superframe of 8 pseudo-frames is 14,400 bits long and takes 144 seconds to transmit the full almanac.
The navigational message of the L1OC signal is transmitted at 100 bit/s. The string is 250 bits long and takes 2.5 seconds to transmit. A pseudo-frame is 1500 bits long, and a superframe is 12,000 bits or 120 seconds.
L2OC signal does not transmit any navigational message, only the pseudo-range codes:
Glonass-K1 test satellite launched in 2011 introduced L3OC signal. Glonass-M satellites produced since 2014 will also transmit L3OC signal for testing purposes.
Enhanced Glonass-K1 and Glonass-K2 satellites, to be launched from 2023, will feature a full suite of modernized CDMA signals in the existing L1 and L2 bands, which includes L1SC, L1OC, L2SC, and L2OC, as well as the L3OC signal. Glonass-K2 series should gradually replace existing satellites starting from 2023, when Glonass-M launches will cease.
Glonass-KM satellites will be launched by 2025. Additional open signals are being studied for these satellites, based on frequencies and formats used by existing GPS, Galileo, and Beidou/COMPASS signals:
- open signal L1OCM using BOC modulation centered at 1575.42 MHz, similar to modernized GPS signal L1C, Galileo signal E1, and Beidou/COMPASS signal B1C;
- open signal L5OCM using BPSK modulation centered at 1176.45 MHz, similar to the GPS "Safety of Life", Galileo signal E5a, and Beidou/COMPASS signal B2a;
- open signal L3OCM using BPSK modulation centered at 1207.14 MHz, similar to Galileo signal E5b and Beidou/COMPASS signal B2b.
With the introduction of CDMA signals, the constellation will be expanded to 30 active satellites by 2025; this may require eventual deprecation of FDMA signals. The new satellites will be deployed into three additional planes, bringing the total to six planes from the current three—aided by System for Differential Correction and Monitoring, which is a GNSS augmentation system based on a network of ground-based control stations and communication satellites Luch 5A and Luch 5B. GLONASS-KM satellites will also use new L3SVI open signal to broadcast Precise Point Positioning to deliver GLONASS High Accuracy Services.
Six additional Glonass-V satellites, using Tundra orbit in three orbital planes, will be launched starting in 2025; this regional high-orbit segment will offer increased regional availability and 25% improvement in precision over Eastern Hemisphere, similar to Japanese QZSS system and Beidou-1. The new satellites will form two ground traces with inclination of 64.8°, eccentricity of 0.072, period of 23.9 hours, and ascending node longitude of 60° and 120°. Glonass-V vehicles are based on Glonass-K platform and will broadcast new CDMA signals only. Previously Molniya orbit, geosynchronous orbit, or inclined orbit were also under consideration for the regional segment.
Roscosmos also plans to launch up to 240 small size satellites on the low Earth orbit to improve signal availability and interference; LEO satellites will have a limited lifespan of 5 years to allow a faster pace of replenishment.