Satellite Properties

SP3 Propagator

The SP3 propagator reads .sp3 files of type 'a' and 'c' and allows you to use multiple 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 on an entry in that table to select the desired identifier for the satellite.

You can add multiple 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 on it to select it and then click Remove. To refresh a file that has been changed, click on its entry in the table and then click Reload.


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. Higher interpolation orders can be used to yield more accurate interpolation of the position and velocity of the object when the data is smooth and continuous. Typical interpolation orders for smooth satellite ephemerides are 5-7. The use of higher interpolation orders with jittery data or with data which has large and sudden variations in time spacing can result in undesirable interpolation artifacts which can introduce high frequency oscillatory behavior into the interpolated trajectory.

The recommended range of values is 1-7.
Interpolation Method By default, the SP3 propagator uses the standard Lagrange interpolation scheme, interpolating position and velocity separately. Some files may be configured to allow 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 drop-down menu to select the desired method.
Interpolate across file boundaries If this option is selected and multiple SP3 files have been assigned to the satellite, STK will interpolate ephemeris steps according to the user-defined Step Size between the end of one SP3 file and the beginning of the next; otherwise, the step between two files will simply be the time between their last and first points.
Extrapolate 1 step beyond last data point If this option is selected, STK will calculate an additional ephemeris step beyond the last data point provided by the SP3 file(s) assigned to the satellite.