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GNSS Reflectometry aboard the International Space Station GEROS-ISS: Numerical Simulation of Expected Observation Coverage

Constellation of the GEROS-ISS experiment.
Lupe

Vera Leister (Sucessful finalization September 2015)

GEROS-ISS stands for GNSS REflectometry, Radio Occultation and Scatterometry onboard the International Space Station. It is a scientific experiment, proposed to the European Space Agency (ESA) in 2011 to be installed aboard the International Space Station ISS. GEROS-ISS is currently in phase A of realisation, the launch is foreseen for 2019.

The main focus of GEROS is the dedicated use of signals from Global Navigation Satellite Systems (GNSS), reflected from water, ice and land surfaces, and also refracted by the atmosphere for Earth system observation. The main mission goals are the determination of the mesoscale sea surface height and the derivation of wind velocities over ocean surfaces. Secondary mission goals are land surface characterisation and radio occultation for global atmosphere sounding.

This study is part of the GEROS-ISS mission preparation and focuses on the geometrical simulation and visualisation of the locations of the expected GNSS reflectometry measurements. The global observation coverage with respect to the different GNSS constellations (GPS, GLONASS, Galileo and BeiDou) are compared and analysed for various time intervals. In this study, 32 GPS satellites, 27 Galileo satellites, 24 GLONASS satellites and 24 BeiDou satellites are considered. For orbit propagation within the MATLAB simulation Broadcast Ephemeris (BRC) data for the GPS orbits and Two Line Element (TLE) data for the GLONASS, Galileo and BeiDou orbits as well as for the ISS orbit was used. The reflection points were computed, applying the spherical mirror equation [1], for a time period of one day and one week respectively. For the coverage, a bin size of 1x1 and a sampling rate of 12 seconds is defined. The various Global Navigation Satellite Systems provide a similar reflection coverage structure with some variation in latitude distribution due to the diverse satellite constellations. The number of satellites has an obvious impact on the reflection density. For the simulation period of one day for GPS (125,755 specular points), GLONASS (95,784), Galileo (107,216) and BeiDou (98,207), a fragmentary reflection coverage occurs. The coverage structure is aligned to the ground track structure of the ISS and bound to tropical and mid-latitudes through the ISS orbital inclination at 51:6. An equator-symmetric coverage with an accumulation of specular points along the minimum and maximum latitude of the ISS occurs. The coverage is fairly consistent in dependency on longitude. The combination of all 107 GNSS (426,962) satellites leads to a realtively dense coverage between 50N and 50S with a number of 114 blank bins after one day.

After a simulation period of one week the individual systems provide a dense coverage between 50N-50S with less than three blank bins. The combination of all 107 GNSS satellites leads to 2,998,141 specular points with a gapless coverage between 60N-60S. The observation density is approximatly 60 observations per sampling. The mean revisit time depends on the latitude and ranges between one and 10 hours.

For the GPS system, reflection events for a month-long period were computed and the coverage structure shows consistency with an increase in the overall number of reflection points, 3,886,564. A specific visibility mask for the GNSS reflectometry antenna aboard the ISS, taking into account the realistic ISS geometry, has been implemented and the impact on the potential reflection coverage is evaluated in detail. The resulting field of views, near-nadir and grazing, and the influence of the limitation in the port-side field of view through the ASIM payload are studied separately. Furthermore all limitations are combined and the coverage loss assessed. The overall number decreases by about 70%. A coverage structure, with a non-symmetric latitude distribution and a higher density at the minimum latitude of the ISS, 51:6S, and a cutoff at 52N, evolves. The observation density results in approximatly 10 observations per sampling and mean revisit time varies from one to 30 hours.

References:

[1] Martin-Neira, M. (1993). ‘A Passive Reflectometry and Interferometry System (PARIS): Application to Ocean Altimetry’. In: ESA Journal vol. 17, pp. 331–355.

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