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A source for satellite coordinates is required. Most commonly, one would use a precise ephemeris. run_survey can recognize the SP1and SP3 ASCII formats, and a binary format used interally in NOAA for orbit processing (the latter will be excluded from this discussion). The SP3 format is the distribution format for the IGS. A standard naming convention for the ephemerides is used: CCCWWWWD.sp3 where
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.
Tools are provided to convert between year / doy and week / dow.
The U.S. Coast Guard distributes precise ephemerides in ECF18 binary format but provides programs which convert from ECF18 to SP3.
In some circumstances, verifying data in the field for example, one may wish to proceed without a precise ephemeris. run_survey can be used to process data with a broadcast navigation message as the source of satellite coordinates. Later, a precise ephemeris can be swapped in with minimal reprocessing.
At the end of the last tutorial, the current directory was ~/tutorial/96_360.
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. The ephemerides are ASCII and, at the CDDIS, stored in an uncompressed form and in a different directory than the data.
This task could have been done while connected to the CDDIS in the previous tutorial. It has been separated to better distinguish the different file types required for processing.
The directory ~/tutorial/96_360 should now contain five files:
brdc3600.96n.Z chl13600.96o gode3600.96o.Z igs08853.sp3 sol13600.96o.Z.
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February 1, 1997
Steve Hilla