GNSS Radio Occultation from LEO: Occultation Management and Open-loop Tracking Aspects

This paper focuses on particular aspects of customised GPS receivers for radio occultation: occultation management and open-loop tracking. The two aspects are closely related because open-loop tracking requires a-priori geometric information about the occultation events. The simplest way to perform this task is to let the receiver to autonomously compute in quasi real-time all the needed parameters. Alenia Spazio-Laben is the designer and manufacturer of the ROSA instrument, a space-based, geodetic-quality radio occultation receiver commissioned by the Italian Space Agency (ASI), which features velocity and anti-velocity antennas in addition to the navigation one [1]. ROSA is an evolution of the LAGRANGE dual-frequency navigation receiver that will fly on Radarsat-2, GOCE and COSMO satellites. Handling of the occultations seen by the velocity antenna (“rising” occultations) is the novel aspect with respect to currently flying occultation receivers (like those onboard the CHAMP, SAC-C and GRACE missions). Rising events require special attention in order to acquire the signal in the low troposphere as soon as possible and the special techniques described hereafter are implemented in ROSA. Open-loop tracking (raw-sampling) is nowadays seen as a reliable technique to have information about the atmospheric parameters in the low troposphere from occultation signals, where the closed-loop carrier phase tracking may be challenging because of the weakness and “blinking” nature of the signal. This is particularly true for rising occultations, where the signal suddenly exits from the Earth mask and the receiver must be internally instructed exactly “when”, “where” and “how” to search. To this aim a simple atmospheric model that provides the atmospheric excess doppler must be available on-board, in order to let the received signal to be centred as near as possible to the correct frequency of the incoming GPS signal. Relatively high sampling rate (i.e. up to 100 Hz) will then help the users to reconstruct the signal characteristic without loss of information [3]. When these customised algorithms/models are ported to a target (in our case the ADSP 21020) performance issues rise, mainly related to numerical inaccuracies introduced by the native 40-bit architecture of the DSP. Another important issue that may be a driver in the receiver design is the average number of floating point operations needed for the preparation of open-loop data related to a single occultation event, that translates in computational load and overhead for the target processor. This paper will present the ideas behind the occultation manager and its performances connected to the open-loop tracking (i.e. real-time, on-board predictions accuracy of LEO and GPS SV). In particular the required accuracy of LEO ephemerides “forward” predictions was analysed. Different methods were tried, but at the end it was found that using the Navigation Kalman Filter (NKF) state and covariance predictions, always available inside the receiver and obtained with minimum effort from navigation receiver operations, provide the required accuracy for the needs of occultation handling. The “absolute” performances of the excess-doppler prediction technique and of the selected on-board atmospheric model are presented in other papers and validated against CHAMP real data [8], [9]. In this paper numerical accuracies in the computation of open-loop data using available real-time information are compared to an ideal double precision architecture. To this aim simulations were run on an ADSP 21020 PC simulator and the results were compared with Matlab truth results. Analyses were carried out in order to assess the sensitivity of computed parameters like the geometric impact parameter, tangent height and excess doppler to numerical and navigation errors.