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Downloading a Precies GPS Ephemeris
A source for satellite coordinates is required in all GPS data processing.
In most projects involving only shorter baselines or having lower accuracy
requirements, a broadcast navigation file is adequate. However, when longer
baselines are used or any time higher accuracies are required, one should
use a precise ephemeris. Typically, this precise ephemeris will be in the
SP3
format.
The SP3
format is the distribution standard for the IGS and a 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.
Tutorial
The IGS products are stored at the
IGS global data centers. The date arbitrarily selected for these tutorials
is February 2, 2001. GPos provides tools to convert between year-doy
and week-dow, and those tools will be used in this example.
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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