title: Contributions to satellite altimetry calibration with microwave transponders and corner reflectors in the context of fiducial reference measurements

title: Συνεισφορές στην βαθμονόμηση δορυφορικής αλτιμετρίας με αναμεταδότες μικροκυμάτων και γωνιακούς ανακλαστήρες στο πλαίσιο των θεμελιωδών μετρήσεων αναφοράς
Creators: name: Κοκολακης Κωνσταντινος
Publisher: name: Technical University of Crete
Year: 2023

description: This dissertation improves satellite altimetry calibration by strengthening existing procedures, integrating active and passive targets, proposing new calibration methodologies and revisiting calibration processing. 
    Satellite altimetry provides measurements for assessing Earth’s climate by monitoring oceans and inland waters. The primary measurement of altimetry is the range between the altimeter and the surface of the Earth. Accuracy and reliability of altimetric range are ensured through dedicated calibration. Over the last decades the developments in instrumentation and algorithms increased the accuracy of altimetric measurements. For example, TOPEX/Poseidon (launched in 1992) measured ocean topography to an accuracy of 4 cm while Sentinel-3A (launched in 2016) reached an accuracy of 2 cm. On the other hand, external calibration, the only means of controlling altimeters accuracy post-launch, remained practically unchanged.
    To cover this gap, this work improves the reliability and accuracy of calibration by mitigating its largest sources of uncertainty. These have been determined following Fiducial Reference Measurements (FRM) principles and are the wet delay, transponder’s internal delay, geophysical corrections and calibration processing approximations. The FRM have been established by the European Space Agency to standardise the bias estimation. The dissertation is separated into four sections, each one presenting a research objective and the corresponding contribution of this work.
    The first objective has been to increase the confidence in the estimation of the most variable parameter in altimetry calibration, the wet delay. This was achieved by validating and implementing two independent techniques for estimating wet delays, i.e., the Ocean and Land Colour Instrument on-board Sentinel-3 and the MP-3000A ground radiometer. The implementation of these techniques additionally to the conventional GNSS methods, offers redundant and independent estimation of wet delay correction for range calibration.
    The second objective has been to evaluate alternative methods of point target calibration, and assess their advantages with respect to currently used methods. This would mitigate potentially the largest error of calibration using a transponder, the transponder’s internal delay. After a systematic assessment of several calibration techniques, the integration of active (transponder) and passive (corner reflectors) point targets at the same calibration network is proposed. The main reference target of the proposed calibration technique is the transponder because of its higher signal to noise ratio. The tandem operation of diverse targets allows to monitor the largest uncertainty of transponder calibration (i.e., internal delay) by comparing its echo with this of corner reflectors.
    The third objective has been to design new techniques, in order to mitigate the uncertainty of calibration, accounting for the increased accuracy of modern missions. This dissertation proposes a new technique for altimetry calibration called Altimeter Differential Corner Reflector (ADCR). The ADCR offers for the first time a bias free of atmospheric, geophysical and orbital errors. This elimination of calibration errors is achieved by co-locating corner reflectors to experience identical effects. A differential bias is thus estimated, which originates from the comparison of corner reflectors range difference (estimated using altimetry measurements) against their known distance.
    The last objective, has been to revisit conventional calibration processing to reduce approximations that could degrade bias reliability. For this objective, the approximation of applying a constant offset to perform the common reference of the measured and geometrical ranges is examined. A comprehensive methodology is proposed for accurately referencing the measured and geometric ranges at the same satellite point by incorporating satellite attitude information into calibration. The revised calibration correction on Jason-3, varies from −2 mm to 1 mm for the range bias and from −110 μs to 110 μs for the datation bias. The magnitude of corrections on datation bias corresponds to about 30% of its average value. The mean bias difference of Jason-3 ascending and descending orbits over the GVD1 transponder is improved by 12%.
    To sum up, this work removes the influence of systematic effects both in the ground infrastructure (i.e., internal delay knowledge, atmospheric and geophysical corrections) and on the satellite that depend on both physical characteristics (e.g., internal geometric structure) and attitude realization of each satellite. To ensure that calibration procedures are aligned with the requirements of future satellite missions and FRM standards, the methods proposed in this dissertation should be considered in every current and future Cal/Val infrastructure. The potential impact of this work is to reach sub-cm accuracy in the calibration of satellite altimeters.