Satellite Tracking

There is a project to use the VLBA to provide positional data to help navigate interplanetary spacecraft. For this project, the VLBA must be able to point at the spacecraft so the ability to do so has been added to SCHED  as of March 2004. The spacecraft positions are obtained with the help of spice files that are typically from JPL. The NAIF software package from JPL is called to read the spice files and calculate positions. The NAIF software significantly increases the size of a SCHED  distribution, and the satellite tracking capability is unlikely to be needed outside the AOC. Therefore the tracking capability is not included in the default SCHED  distribution.

To use the tracking capability, a Satellite Initialization section needs to be included in the main input file. That section contains a group of inputs for each satellite. There are four input parameters in each group:

Note that the satellite routines also set the velocity for the satellite for use with DOPPLER. The satellite frequencies can be specified with their rest frequencies in a LINEINIT section.

1. SATNAME: The name of the satellite. This is only used internally in SCHED. It is the name that should be used as the SOURCE in the scan inputs. This name is not sent to the NAIF software.

2. SATNUM: The number, used in the spice files, for the satellite (or other celestial body, for that matter). This number is assigned by JPL. You need to know this number but I'm not really sure how you get it. This number is sent to the NAIF software to tell it which satellite to process. For satellites, these numbers are negative. The are positive for planets etc.

3. KERFILE: A spice kernel file that gives information such as leap seconds. It is likely to have the extension .tls.

4. SATFILE: The spice file for the satellite. It is likely to have the extension .bsp.

When groups have been given for all satellites, give a line that contains the word ENDSAT and a slash.

If the above section is provided and one of the satellites is a source in the schedule, SCHED  will call the NAIF software every scan to get updated positions and rates. It will also calculate an approximate parallax correction for each station. This can amount to several arcseconds, and the calculation is believed to be good to an arcsecond or better.

For a satellite (or any moving source, for that matter), SCHED  plotting can help you see where the object is going. In the RD (RA/Dec) plots, a line will be plotted for each scan. A likely use for this capability would be to obtain the transmission schedule for a satellite over some days or weeks, make a schedule with a scan for each period that it is transmitting, then make the RD plot and show the calibrators. This will help identify times when the satellite is both on and near a likely phase reference source.

There is a SCHED  example, egsat.com that demonstrates the use of the satellite capability. Interested users are recommended to start with that example.