Satellite Properties

SP3 Propagator

The SP3 propagator reads SP3 files of type 'a' and 'c' and enables you to use two or more files in sequence. These files are used to provide precise GPS orbits from the National Geodetic Survey (NGS).

Analysis time period

The satellite's analysis time period and step size are controlled by the selected SP3 file. Therefore, the Start, Stop, and Step Size fields are read only and cannot be edited.

Managing SP3 files

Click Add to browse to an SP3 file and add it to the satellite's state definition. Once you have added at least one SP3 file, a table with valid satellite identifiers appears to the right. Click an entry in that table to select an identifier for the satellite.

You can add two or more files to a single satellite object and, if there are no gaps between the files, the whole ephemeris will be propagated in sequence. The table automatically sorts the file entries by their epochs.

To delete an entry from the table, click it to select it and then click Remove. To refresh a file that has been changed, click its entry in the table and then click Reload.

Properties

The properties that you can specify for the SP3 propagator are described in the following table:

Property Description
Interpolation Order An interpolation order of 1 specifies linear interpolation and is appropriate for closely spaced data or data with significant jitter. You can use higher interpolation orders to yield more accurate interpolation of the position and velocity of the object when the data is smooth and continuous.
Interpolation Method By default, the SP3 propagator uses the standard Lagrange interpolation scheme, interpolating position and velocity separately. Some files may be configured to enable you to select the standard Hermitian interpolation scheme, which uses the position and velocity ephemeris to interpolate position and velocity together (i.e., using a polynomial and its derivative). Use the shortcut menu to select the desired method.
Interpolate across file boundaries If you select this option and the ephemeris for a satellite uses two or more SP3 files, then interpolation samples are allowed to span different SP3 files. If you do not select this option, then interpolation samples are restricted to be in the same SP3 file as the requested time.
Extrapolate 1 step beyond last data point If you select this option, STK will calculate an additional ephemeris step beyond the last data point provided by the SP3 file(s) assigned to the satellite.