Editing GLONASS (section)

==== FDMA ====
[[File:Glonass-receiver.jpg|thumb|290px|A combined GLONASS/GPS receiver, ruggedised for the Russian military, 2003]]
[[File:GLONASS GPS Personal Radio Beacon.jpg|thumb|290px|A combined GLONASS/GPS Personal Radio Beacon]]

GLONASS satellites transmit two types of signals: open standard-precision signal L1OF/L2OF, and [[obfuscated]] high-precision signal L1SF/L2SF.

The signals use similar [[Direct-sequence spread spectrum|DSSS]] encoding and [[Phase-shift keying|binary phase-shift keying]] (BPSK) 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]] (FDMA) technique spanning either side from 1602.0 [[Hertz|MHz]], known as the L1 band. The center frequency is 1602&nbsp;MHz + ''n'' × 0.5625&nbsp;MHz, where ''n'' is a satellite's frequency channel number (''n''=−6,...,0,...,6, previously ''n''=0,...,13). Signals are transmitted in a 38° cone, using right-hand [[circular polarization]], at an [[Effective radiated power|EIRP]] between 25 and 27 [[Decibel watt|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 point|antipodal]] (opposite side of planet in orbit) 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&nbsp;MHz with the center frequency 1246&nbsp;MHz + ''n'' × 0.4375&nbsp;MHz, where ''n'' spans the same range as for L1.<ref name="glonass_xmtr">GLONASS transmitter specs</ref> 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 [[XOR gate|modulo-2 addition]] (XOR) of 511&nbsp;kbit/s pseudo-random ranging code, 50 bit/s navigation message, and an auxiliary 100&nbsp;Hz [[Meander (mathematics)|meander]] sequence ([[Manchester code]]), 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 [[millisecond]]s.

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 (2.5 minutes) to transmit the continuous message. Each frame is 1500 bits long and consists of 15 strings of 100 bits (2 seconds for each string), with 85 bits (1.7 seconds) for data and check-sum bits, and 15 bits (0.3 seconds) 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 (i.e. [[almanac]]) 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 [[ECEF|Earth Centred Earth Fixed]] (ECEF) Cartesian coordinates in position and velocity, and include lunisolar acceleration parameters. The almanac uses modified [[orbital elements]] (Keplerian 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(Y) 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 (and therefore the tracking strategy) 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 code|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 (12 minutes) for the whole navigational message. The additional data are seemingly allocated to critical [[Lunisolar calendar|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 {{cvt|15|m}}, a velocity vector measuring within {{cvt|100|mm/s}}, and timing within 200 [[nanosecond]]s, all based on measurements from four first-generation satellites simultaneously;<ref name="miller_2000">"A Review of GLONASS" Miller, 2000</ref> newer satellites such as GLONASS-M improve on this.

GLONASS uses a coordinate [[Geodetic datum|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 1990 to 1995. This is in contrast to the GPS's coordinate datum, [[World Geodetic System|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 {{cvt|400|mm}} 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.<ref name=icg8>[http://www.oosa.unvienna.org/pdf/icg/2013/icg-8/wgD/D3_3_2.pdf National Reference Systems of the Russian Federation used in GLONASS.] {{Webarchive|url=https://web.archive.org/web/20140714182933/http://www.oosa.unvienna.org/pdf/icg/2013/icg-8/wgD/D3_3_2.pdf |date=14 July 2014 }} V. Vdovin and M. Vinogradova (TSNIImash), 8th ICG meeting, Dubai, November 2013</ref><ref>{{cite web|url=http://www.glonass-iac.ru/en/content/news/?ELEMENT_ID=721|title=The transition to using the terrestrial geocentric coordinate system "Parametry Zemli 1990" (PZ-90.11) in operating the GLObal NAvigation Satellite System (GLONASS) has been implemented|website=glonass-iac.ru|access-date=2 September 2015|archive-date=7 September 2015|archive-url=https://web.archive.org/web/20150907014714/https://www.glonass-iac.ru/en/content/news/?ELEMENT_ID=721|url-status=dead}}</ref>