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Description
A source for satellite coordinates is required. Most commonly, one would use a
precise ephemeris. run_survey can use a nav file, an SP1 or SP3 ASCII format
file, or a GAMIT binary format file used internally in NOAA for orbit processing
(the latter will be excluded from this discussion). The U.S. Coast Guard
distributes precise ephemerides in ECF18 binary format but provides programs
which convert from ECF18 to SP3. The SP3
format is the distribution standard for the IGS. A standard naming
convention for the ephemerides is used: CCCWWWWD.sp3
where
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CCC |
is a three letter ID for the center which generated
the file. |
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WWWW |
is the four digit GPS week number. |
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D |
is the single digit day-of-week (dow):
0 = Sunday, 1 = Monday, ... |
Occasionally, an extension of .eph will be used but this is largely avoided
because of the possible confusion when selecting a file of the desired format.
In some circumstances, verifying data in the field for example, one may wish
to proceed without a precise ephemeris. As mentioned, run_survey can be used to
process data with a broadcast navigation file as the source of satellite
coordinates. Later, a precise ephemeris can be swapped in with minimal
reprocessing.
Tutorial
At the end of the last tutorial, the current directory was
"~/tutorial/01_033/".
We will use an IGS precise ephemeris. These IGS products are stored at the
IGS global data centers so we'll access the CDDIS again. Tools are available as
part of the software distribution to convert between year-doy and week-dow.
Scripts are available to facilitate downloading ephemerides.
Convert year-doy to GPS week
gpswk 01 033
1099
Convert year / doy to dow
dow 01 033
5
Download a precise ephemeris from the CDDIS data center. The ephemerides are
stored in a different directory than the data so the steps will be similar but
not identical to downloading data.
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