[teqc] helpful tip of week 1993

Lou Estey lou at unavco.org
Thu Mar 22 08:04:51 MDT 2018

This week's tip: GLONASS: slot and frequency channel numbers

Users who start collecting and trying to use GLONASS data sometimes get lost,
even though they might be very familiar with GPS data.  Let's quickly review
one of the main differences for GLONASS legacy signals.

Except for GLONASS, the signals from GPS and other GNSS systems are entirely
CDMA: code-division multiple access.  This is a data transfer method by which
several "channels" of information can be sent simultaneously over the same
carrier wave, i.e. the different channels are all using the same carrier wave
frequency.  For example, GPS signals L1C/A, L1P(Y), the military L1M, and the
upcoming L1C on future Block III SVs all use a carrier frequency (as observed
on or near Earth) of exactly 1575.42 MHz (not including effects like doppler
shifting).  The different "channels" of the same signal are distinguished by
means of a pseudo-random noise/number (PRN) bit pattern.  In GPS 32 different
patterns are assigned the numerical IDs of 1-32 and, hence, the PRNs of the
different GPS SVs.  The same idea applies to the other non-GLONASS GNSS.

Legacy GLONASS, however, uses FDMA -- frequency-division multiple access -- to
distinguish the different "channels" of information.  Thus, in GLONASS, there
are usually a dozen different carrier frequencies being used for the same "signal",
e.g. one of these being what I'll call G1SA, which is analogous to GPS L1C/A.
The GLONASS SV identifier for these signals is based on a physical "slot" location.
There are three orbital planes for GLONASS, with 8 nominally "in operation" slot
locations (*) per orbit, for a total of 24 slots, numbered 1-24, for the nominally
"in operation" constellation.  (See, e.g. https://www.glonass-iac.ru/en/GLONASS/ (**))
These slot numbers are what are used to uniquely identify each source SV for GLONASS
observables.  The exact carrier frequency for each FDMA signal is separately
controlled by a frequency channel number (fcn) (or 'RF chnl' number in the table
in the above link).  The fcn is used to specify the carrier frequency of
a given slot number, i.e. the G1 and G2 carrier frequencies are:

   G1 = 1602.00 MHz + fcn * 9/16 MHz
   G1 = 1246.00 MHz + fcn * 7/16 MHz

Note that each slot number is unique for the "in operation" GLONASS SVs,
whereas a frequency channel number is usually used twice in the same
orbital plane, the two SVs on opposite sides of the orbital plane typically
using the same fcn.

Another detail you can't see in a static table like the above is that
the Russian ground control for GLONASS sometimes changes the frequency
channel number for two or more SVs.  So the slot-to-fcn mapping is not
static over time due to those changes.

Also, the ranges of fcn values have changed over time, these being:

    0 to 24 from 1982 to 1998,
    0 to 13 from 1998 to 2005,
   -7 to  6 from 2005 to present.

For non-GLONASS GNSS systems we know the carrier frequencies by definition and
these frequencies are fixed.   Therefore one knows what frequency to use to
convert the phase of a given signal from units of cycles (as needed in RINEX) to
units of distance (as needed in data processing, like teqc's qc).

For legacy GLONASS, the carrier frequencies are specified by the fcn of the
SV, and the fcn of a specific SV might change over time.

The points that we arrive at for GLONASS are that:

- the GLONASS slot number is used to identify the SV's observables (like phase),

- a slot-to-fcn mapping is needed in order to convert GLONASS G1 and G2 phase
observable back and forth between units of cycles to units of distance.

Thus, if you need to qc GLONASS data, you absolutely need the slot-to-fcn
mapping of the constellation when reading phase data in cycles -- as it
is represented in RINEX.  However, because of this exact need back when
the GLONASS RINEX nav message was defined, the fcn for each navigation
epoch is part of what is recorded.  Using a GLONASS RINEX file, then, is
a simple way to inform teqc about the slot-to-fcn mapping, as is needed
during qc.

On occasion, I've had to use my own slot-to-fcn mapping and not rely on
what might be in a GLONASS RINEX nav file, or the GLONASS RINEX nav file
might not have been available.  In this case, one can create a simple
ASCII file with pairs of numbers, the SV slot number followed by the
corresponding frequency channel number, for all pertinent GLONASS SVs.
Then use teqc's '-glonass_s2fcn', followed by the name of this file,
as needed, e.g. during qc.

Happy teqc-ing!


* nominally "in operation": Occasionally there might be one or more GLONASS
SVs in slots 1-24 which are temporarily in maintenance, undergoing slot
repositioning, and so on, and if so, the "health" and/or "operability" bits
of the navigation messages of any such SV should have indicate this.

** In the GLONASS table at the IAC, https://www.glonass-iac.ru/en/GLONASS/
you can find the GLONASS number of each physical SV in the '# GC' column.

Louis H. Estey, Ph.D.              office:  [+001] 303-381-7456
UNAVCO, 6350 Nautilus Drive           FAX:  [+001] 303-381-7451
Boulder, CO  80301-5554            e-mail:  lou  unavco.org

"If the universe is the answer, what is the question?"
                                                -- Leon Lederman

Past helpful tips:

week 1894: using teqc config files - http://postal.unavco.org/pipermail/teqc/2016/002067.html
week 1895: qc of high-rate data - http://postal.unavco.org/pipermail/teqc/2016/002071.html
week 1896: UNIX/Linux shells for Windows - http://postal.unavco.org/pipermail/teqc/2016/002072.html
week 1897: '-' vs. '+' teqc options - http://postal.unavco.org/pipermail/teqc/2016/002076.html
week 1898: auto-identification of formats - http://postal.unavco.org/pipermail/teqc/2016/002092.html
week 1899: auto-identification vs. format flags - http://postal.unavco.org/pipermail/teqc/2016/002096.html
week 1900: square brackets in options - http://postal.unavco.org/pipermail/teqc/2016/002105.html
week 1901: using option '+mds' - http://postal.unavco.org/pipermail/teqc/2016/002108.html
week 1902: qc results w/ problematic nav messages - http://postal.unavco.org/pipermail/teqc/2016/002113.html
week 1903: '-no_orb[it]' and '-no_pos[ition]' options - http://postal.unavco.org/pipermail/teqc/2016/002115.html
week 1904: '-week' option - http://postal.unavco.org/pipermail/teqc/2016/002117.html
week 1905: using '+bcf' for XYZ/geodetic conversion - http://postal.unavco.org/pipermail/teqc/2016/002126.html
week 1906: the '+v[erify]' option - http://postal.unavco.org/pipermail/teqc/2016/002128.html
week 1907: '+C2', '+L5', "+L6', '+L7', '+L8', '+all' options - http://postal.unavco.org/pipermail/teqc/2016/002130.html
week 1908: getting RINEX doppler and L2 - http://postal.unavco.org/pipermail/teqc/2016/002131.html
week 1909: using paths w/ file names - http://postal.unavco.org/pipermail/teqc/2016/002132.html
week 1910: the (un)importance of file names - http://postal.unavco.org/pipermail/teqc/2016/002133.html
week 1911: notices, warnings, and errors - http://postal.unavco.org/pipermail/teqc/2016/002134.html
week 1912: the '-max_rx_SVs' option - http://postal.unavco.org/pipermail/teqc/2016/002137.html
week 1913: the end of '++igs' and '+igs' - http://postal.unavco.org/pipermail/teqc/2016/002140.html
week 1914: splicing together RINEX files - http://postal.unavco.org/pipermail/teqc/2016/002144.html
week 1915: using '-O.int' and '-O.dec' - http://postal.unavco.org/pipermail/teqc/2016/002145.html
week 1916: '+doy' option - http://postal.unavco.org/pipermail/teqc/2016/002146.html
week 1917: '-tbin' and '-ast' options - http://postal.unavco.org/pipermail/teqc/2016/002152.html
week 1918: mp12 RMS before/after Oct 2013 - http://postal.unavco.org/pipermail/teqc/2016/002158.html
week 1919: the global windowing options - http://postal.unavco.org/pipermail/teqc/2016/002159.html
week 1920: '-M.dec' and '-N.dec' options - http://postal.unavco.org/pipermail/teqc/2016/002163.html
week 1921: combining time filtering options - http://postal.unavco.org/pipermail/teqc/2016/002176.html
week 1922: helping me (or someone else on the list) help you - http://postal.unavco.org/pipermail/teqc/2016/002187.html
week 1923: the "build" line - http://postal.unavco.org/pipermail/teqc/2016/002190.html
week 1924: the qc '-w[idth]' option - http://postal.unavco.org/pipermail/teqc/2016/002193.html
week 1925: qc with explicit time windowing - http://postal.unavco.org/pipermail/teqc/2016/002194.html
week 1926: the '+rx_state' option - http://postal.unavco.org/pipermail/teqc/2016/002200.html
week 1927: the '-O.sum' option - http://postal.unavco.org/pipermail/teqc/2016/002204.html
week 1928: the '+meta' and '+mds' options - http://postal.unavco.org/pipermail/teqc/2016/002206.html
week 1930: more on '-O.sum' - http://postal.unavco.org/pipermail/teqc/2017/002207.html
week 1931: the '-O.s[ystem]' option - http://postal.unavco.org/pipermail/teqc/2017/002208.html
week 1932: leap seconds - http://postal.unavco.org/pipermail/teqc/2017/002215.html
week 1936: what you can (and shouldn't) do in a RINEX obs file - http://postal.unavco.org/pipermail/teqc/2017/002229.html
week 1938: the '+psp' option - http://postal.unavco.org/pipermail/teqc/2017/002231.html
week 1939: the '+diag' option - http://postal.unavco.org/pipermail/teqc/2017/002235.html
week 1951: '-n_<system>' and SV filtering options - http://postal.unavco.org/pipermail/teqc/2017/002277.html
week 1953: more with '+diag' option - http://postal.unavco.org/pipermail/teqc/2017/002287.html
week 1954: using '+diag' output to split raw files - http://postal.unavco.org/pipermail/teqc/2017/002290.html
week 1955: current qc notation - http://postal.unavco.org/pipermail/teqc/2017/002302.html
week 1956: the '+qcq' option - http://postal.unavco.org/pipermail/teqc/2017/002304.html
week 1957: using Trimble formats - http://postal.unavco.org/pipermail/teqc/2017/002305.html
week 1958: ToC != ToE messages - http://postal.unavco.org/pipermail/teqc/2017/002310.html
week 1959: receivers vs. formats - http://postal.unavco.org/pipermail/teqc/2017/002311.html
week 1960: when the '-week' option is very wrong to use - https://postal.unavco.org/pipermail/teqc/2017/002314.html
week 1961: "less" is usually best - https://postal.unavco.org/pipermail/teqc/2017/002315.html
week 1962: using GPS L2C with teqc - https://postal.unavco.org/pipermail/teqc/2017/002316.html
week 1964: the '+eds' options - https://postal.unavco.org/pipermail/teqc/2017/002317.html
week 1965: handling RINEX comment lines - https://postal.unavco.org/pipermail/teqc/2017/002324.html
week 1966: the '+dUTC_p' options - https://postal.unavco.org/pipermail/teqc/2017/002331.html
week 1967: the strange position from '+meta' - https://postal.unavco.org/pipermail/teqc/2017/002355.html
week 1972: what shows up as metadata in RINEX headers - https://postal.unavco.org/pipermail/teqc/2017/002362.html
week 1973: GPS L2C navigation messages - https://postal.unavco.org/pipermail/teqc/2017/002363.html
week 1974: the '+ion_p' options - https://postal.unavco.org/pipermail/teqc/2017/002370.html
week 1975: the '+event' options - https://postal.unavco.org/pipermail/teqc/2017/002372.html
week 1976: options '+smtt' (default) vs. '-smtt' - https://postal.unavco.org/pipermail/teqc/2017/002374.html
week 1977: the reported interval with '+meta' for a RINEX obs file - https://postal.unavco.org/pipermail/teqc/2017/002377.html
week 1978: the '-N.dUTC' options - https://postal.unavco.org/pipermail/teqc/2017/002378.html
week 1979: the various qc elevation angles - https://postal.unavco.org/pipermail/teqc/2017/002383.html
week 1980: avoid in RINEX: Transit data - https://postal.unavco.org/pipermail/teqc/2017/002385.html
week 1981: avoid in RINEX: epoch flag = 6 - https://postal.unavco.org/pipermail/teqc/2017/002389.html
week 1982: avoid in RINEX: RCV CLOCK OFFS APPL = 1 - https://postal.unavco.org/pipermail/teqc/2018/002392.html
week 1983: don't count on in RINEX: receiver clock offset per epoch - https://postal.unavco.org/pipermail/teqc/2018/002393.html
week 1984: requirements for multiple target files/stdin - https://postal.unavco.org/pipermail/teqc/2018/002410.html
week 1985: default output for various input - https://postal.unavco.org/pipermail/teqc/2018/002412.html
week 1986: the '+latency' options - https://postal.unavco.org/pipermail/teqc/2018/002419.html
week 1987: the 'O.px and 'O.pg' options - https://postal.unavco.org/pipermail/teqc/2018/002422.html
week 1988: the '+relax' option - https://postal.unavco.org/pipermail/teqc/2018/002423.html
week 1992: the '+x_tilt' options - https://postal.unavco.org/pipermail/teqc/2018/002452.html

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