The next images, I’m comparing the data using my ephemeris in the upper side and Horizons ephemeris in the lower side. Remember that Horizons dont give data for planetary probes, and are using the data of the planet that is orbiting the planet.
Data for Mars Express
As you can apreciate, in the upper side, the range calculated using SPICE and the mission kernels is smaller than the range of the MARS calculated by Horizons. Then the MEX space probe not is eclipsed by the planet and then detectatble for our stations in the earth! Check the difference of 60 KHz in the predicted frecuency and doppler too.
Data for Mars Reconnaissance Orbiter
Now the inverse case, the MRO probe is more away from the earth, then the probe is eclipsed by the planet.
Another issue observed is a small position difference from JPL/Horizons to my ephemerris using SPICE. It happend, because JPL/Horizons uses TDB as time reference and I’m using UTC. Then, exists a difference of few secconds produced by the leap seconds.
As I was write in my last post, one of the challenges of the amateur-DSN stations is get the ephemeris of the space probes to get the position and deldot for the doppler. JPL Horizons helps, but dont give info about the planetary space probes. How solve it? The solution is use the same data and tools that are using the mission control 😉 Every mission, have a dataset called kernel that contains info about position, instruments and clock onboard, mission operations, etc… this data is public and is controlled by NASA / NAIF and can be obtenied from here.
JPL / NAIF website
The Nasa NAIF, proces a toolkit called SPICE to use this data. This toolkit is available in a lot of programming languages and have a good documentation. Using a small piece of code from Achim Vollhardt DH2VA that he was send-me in 2006 and after some years googling a lot, finally I build my own planetary probes ephemeris generator.
Spice tookit code
In the next post the results…