[teqc] helpful tip of week 1895

Lou Estey lou at unavco.org
Wed May 4 08:52:00 MDT 2016

This week's tip is about using teqc to quality check (qc) "high-rate" data.

The original UNAVCO QC algorithms were developed by Chris Rocken, Jim Johnson,
and a few others in the early '90s back when the usual sample interval
for GPS observables was 30 seconds.  Those algorithms still form the core
of teqc's qc and certain default parameters are still "tuned" to a sample
interval of order 10 or more seconds.  For example, these parameters are fine
for the now normal sample interval of 15 seconds used in the PBO, COCONet,
TLALOCNet, and other networks.  However, when you start to collect and qc
GNSS data with a sample interval of 5 seconds or less, especially 1 second
or less, you need to start making a couple of parameter adjustments.

One of these involves the multipath '-mp_win' parameter, which has a default
value of 50.  And what does this mean?  To get an estimate of the multipath
rms about the multipath mean, one needs some idea of what the mean is.  To do
this, as was done in the original QC program, teqc uses a default moving average
window of N points, N = 50.  At 30 second sampling this amounts to the last
25 minutes of data that can be combined into a multipath combination (i.e.
any single multipath combination requires a code pseudorange value and two
different phase values all at the same epoch).  The time variation of
multipath typically has cycles that are on the order of several minutes in
length, so that the default 50-point 25-minute window for 30-second data
averages over a number of multipath cycles.  It also works well enough for
15-second data.

For intervals much smaller than that, though, especially when one gets down
to intervals of 1 second or less, you need to increase that multipath moving
average window length.  This is done with '-mp_win' option.  The "rule-of-thumb"
that we use here at UNAVCO is to use a window size that is 900 divided by the
sample interval in seconds.  So for 1-Hz data, you would want '-mp_win 900';
for 5-Hz data, '-mp_win 4500'; for 10-Hz data, '-mp_win 9000'; etc.   Since 1996
when this part of teqc was written, this window size has an upper limit of N =
65536 points, which means this adjustment will work as is to 50-Hz data (well,
up to 73-Hz data), so for intervals less than 0.02 seconds, just use '-mp_win 65536'
which should be fine for intervals down to 0.01 seconds, i.e. 100-Hz data.
(I guess I'll have to redefine the storage of this parameter in teqc when
users start collecting and qc-ing 200-Hz or even higher rate data.)

Is that all?  Not quite.  For high-rate data you will probably also need to
change the parameters for detecting ionospheric slips.  Going back again to
the early QC parameters, the slip detection threshold for time-rate of change
of the GPS L1 - L2 combination for the ionosphere was set to be 400 cm/minute,
i.e. the default '-iod_jump' option value.  Again, this works pretty well for
intervals greater than about 1 second, but for intervals around 1 second or
less, you start to see the ionospheric scintillations -- and, in fact, special
receivers are developed for ionospheric scintillation monitoring which are run
at, say, 10-Hz to perhaps as high as 50-Hz.  At these rates, the "smoothed"
time rate of change threshold of 400 cm/minute becomes far too sensitive (i.e.
normal scintillations will incorrectly be detected as "slips"), so you want
to set 'iod_jump' to something huge, say, '-iod_jump 1e38'.  But, you still
want to detect ionospheric slips and this is now done by introducing a realistic
jump size, in centimeters, using the '-ion_jump' option.  A good value to
start with is 8 centimeters.  So:

interval > 1 second:  '-iod_jump 400 -ion_jump 1e38'  (teqc's defaults)

interval <= 1 second: '-iod_jump 1e39 -ion_jump 8' ... although you might
want to play around with the '-ion_jump' value a bit

These are the two major issues I've run into when qc-ing high-rate data, based
largely on GPS and some GLONASS data.  There could be other issues as users
start to qc more complex GNSS high-rate datasets, so this is probably not
the end of the story ...


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
    WWW:  http://www.unavco.org   http://jules.unavco.org

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

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