En Route to the Moon using GNSS Signals

Paper number

IAC-07-D1.4.02

Author

Dr. Giovanni B. Palmerini, University of Rome "La Sapienza", Italy

Coauthor

Mr. Marco Sabatini, University of Rome "La Sapienza", Italy

Coauthor

Dr. Giorgio Perrotta, Italy

Year

2007

Abstract

In recent years a number of missions have managed to track and acquire signals from GPS satellites at altitudes higher than the GPS constellation itself. The mission Equator-S investigated the possibility of using GPS navigation signals in GTO orbits, while AMSAT OSCAR-40 spacecraft tracked GPS signals even at its apogee, at an altitude of about 59000 km. These experiments show that not only the spill-over from the main lobe, but also signals from the second and the third lobes have sufficient signal to noise ratio to be tracked.

In the present paper the possibility to exploit GPS signals for spacecraft flying at even higher altitudes is analysed. In particular, the European Student Moon Orbiter (ESMO) is taken as a sample mission for the appealing possibility of autonomy and accuracy in the kinematic state determination. ESMO is likely to use electric propulsion for the Earth-Moon transfer, travelling on a spiral trajectory which will require about 8 months for completing the manoeuvre. This would allow for a complete mapping of the GPS signals versus radius and off-bore sight angle, gathering real data on the signal to noise ratio achievable.

Simulations carried out show that the mission profile can be divided, according to current GPS constellation visibility, in three different phases. In the first phase of the mission (GTO-like), the probe is able to track four GPS satellites almost at any time, with rare failures only at apogee. When both apogee and perigee are above the GPS constellation, periods of data outages can be detected, and their frequency and duration increase as the radius grows. When the probe reaches the moon altitude, simultaneous tracking of four GPS satellites should not be considered feasible.

The inclusion of the GALILEO satellites (probably operative in the ESMO epoch) is proposed for increasing the rate of position determinations. Considering traditional receiving antenna patterns and tracking thresholds, the probe manages to determine its position at least once or twice a day even in the last phase of the transfer. 

Furthermore, a great improvement can be expected when implementing software receivers, which can lower by far the tracking threshold, thus increasing the number of visible satellites. A real-time position determination (a possible limit for software receivers) is not actually an issue for this kind of missions, where an accurate dynamic modelling of the orbit is available and the state can be therefore easily propagated from epoch to epoch.

Abstract document

IAC-07-D1.4.02.pdf

Manuscript document

IAC-07-D1.4.02.pdf (🔒 authorized access only).

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