From J. Fri Oct 22 16:30:38 1993
From: J. (J.)
Date: 22 OCT 1993 16:30:38
Subject: [IGSMAIL-353] Satellite Accuracy Exponents
Message-ID:
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IGS Electronic Mail 22-OCT-1993 16:30:38 Message Number 353
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>From: J. Feltens
Subject: Satellite Accuracy Exponents
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ANALYSIS CENTRE: ESA/ESOC
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INCLUSION OF "SATELLITE ACCURACY EXPONENTS" INTO THE HEADER OF OUR
DAILY ORBIT FILES
J.M.Dow, J.Feltens, T.J.Martin-Mur, C.Garcia-Martinez
Starting with the week 0718 we are including orbit sigma estimates in
the SP3 header format (lines 8-12) of our daily orbit files. The orbit
sigma estimates are represented by "satellite accuracy exponents" (see
NOAA Technical Report NOS 133 NGS 46). The orbit sigma estimates are
computed at ESOC as can be seen from the figure:
day|(i-1) day|(i)
12h 0h 12h 18h 0h 6h 12h
a) +-----------|-----------+-----:-----|-----:-----+
:<=========>:
:<=========>:
b) +-----:-----|-----:-----+-----------|-----------+
12h 18h 0h 6h 12h 0h 12h
day|(i) day|(i+1)
As already pointed out in earlier IGS-reports (e.g. report no. 224), we
are processing the GPS-data in 48-hour-fits, i.e. the orbit determina-
tion of a day (i) starts on day (i-1) at 12 hours noon and ends on
day (i+1) at 12 hours noon, resulting in 24-hour overlaps for sucessive
days. The figure shows the situation for the two successive days (i-1)
and (i).
For the computation of day (i)'s orbit sigma estimates, the overlap
extending from 18 hours on day (i-1) to 6 hours on day (i) will be con-
sidered. This time interval covers 12 hours and is marked by :<======>:
in the figure. Starting at 18 hours on day (i-1), every 15 minutes the
differences deltaX, deltaY and deltaZ between the earth fixed position
vector components from the orbital fits of day (i) and of day (i-1) are
computed and summed up into squared sums:
SX = sum(deltaX**2) every 15 minutes,
SY = sum(deltaY**2) every 15 minutes,
SZ = sum(deltaZ**2) every 15 minutes.
This is performed individually for every satellite until 6 hours on
day (i) are reached. The number N of epochs are counted. Then the orbit
sigma estimates OSIG are computed as follows (also individually for
every satellite):
sx = SX/(N-1)
sy = SY/(N-1)
sz = SZ/(N-1)
OSIG = sqrt((sx + sy + sz)/3)
And since the satellite accuracy exponents are defined as 2**OSIG in
the SP3 convention:
SAE = nearest integer(ln(OSIG)/ln(2))
These SAE (Satellite Accuracy Exponents) are the values that will appear
appear from now on in the headers of our daily orbit files.
The GPS group at ESOC