GNSS-based Precise Orbit Determination for a Highly Eccentric Orbit in the STE-QUEST Mission

G. Hechenblaikner, J-J. Floch, F. Soualle, M-P. Hess

Abstract:	Space-Time-Explorer-Quantum Equivalence Principle Space Test (STE-QUEST) is a fundamental science mission which is currently under consideration for launch in 2022/2024 within the framework of the ESA Cosmic Vision program. Dedicated mission assessment and definition studies have been performed to establish and consolidate the mission and system design. Complementary instrument studies were performed by consortia of collaborating universities and research institutes to mature the design of the scientific instruments. We report on preliminary results of the industrial study team which is headed by EADS Astrium under contract with the European Space Agency (ESA), where particular emphasis is given to GNSS-based precise orbit determination and its impact on the time-and-frequency measurements performed during the mission. The primary science objectives of STE-QUEST are to test and possibly find violations of the major cornerstones of Einstein’s Equivalence Principle by experimentally addressing them from two sides: On the one hand, the gravitational red-shifts experienced by a space-clock (instrument 1) in the gravity field of either sun or earth are compared to theoretical predictions at an unprecedented accuracy in the search of possible discrepancies. On the other hand, the principle of the universality of free fall -or, equivalently the Weak Equivalence Principle WEP- is tested in the quantum regime by measuring the differential acceleration between two atomic species, where any deviation from zero points to an inequality between inertial and gravitational mass. To support these measurements the scientific payload comprises two dedicated instruments: a.) PHARAO NG is an atomic clock (based on PHARAO of the ACES mission) with a fractional frequency inaccuracy of 1E-16 and a similar level of instability (after sufficiently long integration times) which supports the red-shift measurements. b.) An atom interferometer based on two isotopes of Rubidium performs the differential acceleration measurements for WEP tests at a level of the Eoetvoes parameter of 1E-15 when integrated throughout the mission. In addition to the two instruments, the payload also comprises equipment for microwave links (S-band, Ka-band) and optical links (Laser communication terminals) between the spacecraft and the three ground-terminals which are distributed over three different continents. The fractional frequency instability of the links is brought down to the level 1E-18 after integration on the order of hours (optical link) to days (microwave link). This ensures that link performance does not compromise highly accurate time and frequency comparisons between space and ground clock or between two ground clocks. A particularly important feature of STE-QUEST is the highly eccentric orbit where spacecraft altitudes range from 700 km at perigee to 50000 km at apogee in order to maximize the gravity gradients and therefore the measurable frequency shifts experienced by the space-clock. This represents a major challenge for another crucial payload component, the GNSS equipment used for precise orbit determination (POD). In order to compare the measured gravitational red-shifts to theoretical predictions it is not only important to measure frequency offsets accurately but also to know the location and velocity of the space-clock within the gravitational potential to very good accuracy. High accuracy PVT measurements based on the space-qualified GNSS receiver are indispensable for this purpose and therefore play a key role in the mission. In this paper we discuss how the requirements on POD can be derived from the underlying mission objectives and from the base-lined mission orbit and architecture. We then discuss the problem of GNSS signal reception for the STE-QUEST HEO orbit where for certain sections of the orbit spacecraft altitudes are well above those of GNSS satellites at 20200 km. At these altitudes only signals from GNSS satellites behind earth and therefore only the signal side-lobes can be tracked, as GNSS satellites broadcast their signals in the direction of earth. Whilst this scheme requires the accommodation of a nadir pointing antenna onto the spacecraft, we also discuss the pros and cons of accommodating an additional zenith pointing antenna for tracking around perigee. In the next step the GNSS signal availability for different antenna accommodation schemes (nadir, zenith, nadir + zenith) and different GNSS constellations (GPS, GPS + Galileo) is evaluated during several periods of the baseline orbit, where the characteristic gain and radiation patterns of GPS and Galileo transmitter antennas and a typical receiver antenna on the S/C are used. In addition to the visibilities, the GDOP is calculated over the orbit and the ensuing PVT performance is derived. These results are then used to simulate precise orbit determination based on a reduced dynamical model for the highly eccentric orbit of STE-QUEST, where uncertainties in solar radiation pressure, drag-forces, and other perturbations as well as spacecraft orbit maneuvers must be accounted for. We also point out that the inclusion of a GNSS receiver on-board the satellite allows investigating and probing the so-called “fly-by” anomaly, an as yet unexplained acceleration observed for several spacecraft on a gravitational assist around earth.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3347 - 3356
Cite this article:	Hechenblaikner, G., Floch, J-J., Soualle, F., Hess, M-P., "GNSS-based Precise Orbit Determination for a Highly Eccentric Orbit in the STE-QUEST Mission," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3347-3356.
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