GNSS Satellite (GIOVE-A)


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Sunday, 24 August 2014

Groundtrack of the Galileo Satellites

Purely out of interest I have made a plot of the ground track of the orbits in which the last two Galileo Satellites ended up in. The results is shown in the figure below.
The figures shows the ground track of Galileo-101 (E11), one of the older Galileo satellites, and Galileo-201, one of the two new Galileo satellites, over a period of 24 hours for August 23, 2014. A couple of interesting features can be seen from the plot. Firstly the difference in inclination, 56 vs 49 degrees, is clearly visible as the maximum latitude of the Galileo-201 satellite is clearly smaller. Secondly, the orbital revolution period is clearly significantly shorter. The normal Galileo satellites have an orbital period of around 13 hours. This means that in 24 hours, the period used for the plot, they will complete less then two orbital revolutions. The new satellite clearly completes two full revolutions within the 24 hour interval. This nicely shows that the new two satellites will not fit into the Galileo constellation in their current orbit. They will move through the constellation in a very wild way.

What I find a bit scary is that with the Perigee of these orbits at merely 20000 km these three objects, the two Galileo satellites and the Fregat, are actually moving through the orbital planes of the GPS and GLONASS constellations. This means to sooner or later they will be on a "collision" course with one of those satellites. Fortunately the satellites seems to be in good order so they can be steered but not sure about the Fregat. So GPS and GLONASS may have to do collision avoidance manoeuvres because of this mishap.

But not all is bad. GNSS scientist have actually concluded a couple of times that different orbits for the different GNSS satellites would be advantegeous for their high accuracy investigations. So at least some of the GNSS scientist might be happy with this situation!

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Saturday, 23 August 2014

Galileo Orbit Anomaly

It seems that the Galileo satellites, numbers 5 and 6, launched yesterday (August 22, 2014) have not reached their intended orbit. The intended orbit would have had an altitude of around 23'000 km whereas it is said that the lowest point of the achieved orbit is only 13'000 km, i.e. 10'000 km below the intended orbit. Also the intended inclination of the orbit was not reached with 47 deg versus the planned 55 degrees.
The launch seemed to go picture perfect, I watched it life at the European Space Operations Centre in Darmstadt, Germany. So my guess is that the Fregat upper stage did not perform as expected. I think that the 10'000 km altitude may be corrected but the inclination error may be harder to correct. I would have to do some calcuations but if I remember correctly an inclination manoeuvre is more "expensive" (in terms of fuel consumption) than the altitude correction. So I fear that these satellites are more or less "lost" in terms of being part of the final Galileo constellation. But, as these are brand new satellites, they will serve a very much needed purpose of testing and validating these new birds! So not all is lost. In any case this is unfortunately an other big blow for the Galileo project. But this is basically just a small technical "hick-up" which does happen in the Space Industry. I am sure the Galileo project will survive this and continue on the path it has set out of brining us a very much needed European Satellite Navigation System.

For more information here some of the links on this topic (but they do not have too much information yet).
ESA Navigation
Spaceflight Now

Small add-on. Just looked up the TLE (two line elements) from NORAD (celestrak) and computed the orbits myself. Based on the last set of TLE's the semi-major axis is 26'196 km, the eccentricity 0.23, and the inclination 49.76 degrees. This gives a Perigee (closest distance from Earth) of 20'170 km which is indeed about 10'000 km too low. But it would also mean that the Apogee (furthest distance from Earth) is around 32'000 km. That would be about 3'000 km too much. So seems that the Fregat had enough power but the orbit injection was miscalculated and/or misfired... Not good!

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