[IGSMAIL-2240] Turbo Rogue L2 tracking problems in mid-latitude stations.

[Rene]

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IGS Electronic Mail      Thu Apr 15 22:04:22 PDT 1999      Message Number 2240
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Author: Rene Warnant
Subject: Turbo Rogue L2 tracking problems in mid-latitude stations.

Recently, several igsmails concerning Turbo Rogue L2 tracking problems
have been posted (igsmails 2071, 2075, 2190) : these mails have reported

important data loss on L2 in stations located in the vicinity of the
magnetic equator where the ionospheric Total Electron Content is
becoming very high with the increasing solar activity.

The origin of the problem is explained in igsmail 2190 : when the
receiver is working at a 30s sampling rate under AS, tracking on L2
stops when the difference P2-P1 (or more exactly P2-C/A) exceeds 12 m.
In addition, when 8 m < P2-P1 < 12 m, the difference P2-P1 is quantized.

In fact,

P2-P1 = 0.105 (TEC/cos(Z)) + hardware biases + noise

where   TEC is the vertical Total Electron Content in TEC Units (TECU)
             Z is the satellite zenith angle

in most of the cases, the receiver and satellite hardware biases do not
change much with time; for this reason, we can consider that P2-P1 is a
measure of the TEC. This explains why the L2 tracking problems have been

detected in stations where where the TEC is very high.

Our station of Brussels (Belgium, Europe, lat 51° N) is a mid-latitude
station where the ionosphere is "quiet" when compared to equatorial
stations. Since the second part of the year 1998, the TEC observed at
Brussels is increasing. The maximum value we have observed (in November
98) is 50 TECU and we regularly observe values between 30- 40 TECU. When

I read igsmail 2190, I computed if these TEC values could give P2-P1 >12

m. THE ANSWER IS YES !!! I give you some examples:

if TEC = 30 TECU ===> P2-P1 >12 m for elevation angles below about 15°
if TEC = 40 TECU ===> P2-P1 >12 m for elevation angles below about 20°
if TEC = 50 TECU ===> P2-P1 >12 m for elevation angles below about 25°

This has been confirmed when having a look to our data files ! I have
observed an important data loss at "low" elevation. This means that the
L2 tracking problem is a reality for every IGS site working with a Turbo

Rogue receiver.

The solar maximum is foreseen for the begining of year 2000; it is clear

that we can expect to reach values of 60 - 70 TECU and higher even in
mid-latitude "quiet" stations as it has been the case during the
previous solar cycle.

A solution to this problem consists in observing at 1 sec sampling rate.

Unfortunately, in this case the measurement noise of all observables
increases significantly. This is due to the fact that when observing
with a sampling rate of 10 s or more, the receiver makes an internal
smoothing of the data; it is not the case with a sampling rate lower
than 10 s. This increase of the measurement noise can give some problems

with processing softwares.

IGS station managers should be aware of these problems and have a look
to their data.

Rene Warnant.


--

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  Dr. Rene WARNANT                  Tel: +32-2-373 02 51
  Royal Observatory of Belgium       Fax: +32-2-374 98 22
  Avenue Circulaire, 3
  B-1180 BRUSSELS
  Belgium

  e-mail: R.Warnant at oma.be
  http://www.oma.be/KSB-ORB/D1/GPS/main.html

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[Mailed From: Rene Warnant <R.Warnant at mailserv.oma.be>]