Συνεισφορές στην βαθμονόμηση δορυφορικής αλτιμετρίας με αναμεταδότες μικροκυμάτων και γωνιακούς ανακλαστήρες στο πλαίσιο των θεμελιωδών μετρήσεων αναφοράς - Contributions to Satellite Altimetry Calibration with Microwave Transponders and Corner Reflectors in the context of Fiducial Reference Measurements.
Επταμελής Εξεταστική Επιτροπή
1. Στέλιος Μερτίκας, Καθηγητής (επιβλέπων), Πολυτεχνείο Κρήτης,
2. Διονύσιος Χριστόπουλος , Καθηγητής, Πολυτεχνείο Κρήτης,
3. Παναγιώτης Παρτσινέβελος , Καθηγητής, Πολυτεχνείο Κρήτης,
4. Εμμανουήλ Βαρουχάκης, Επίκουρος Καθηγητής, Πολυτεχνείο Κρήτης,
5. Dr. Josh Willis, Jet Propulsion Lab, NASA, 4800 Oak Grove Drive M/S 300-323, Pasadena, CA, USA,
6. Dr. Constantin Mavrocordatos, European Space Agency, ESTEC, Directorate of Earth Observation Programmes, Keplerlaan 1, PO Box 299, 2200AG Noordwijk, The Netherlands,
7. Dr. Craig Donlon, Head, Earth Surfaces and Interior Section, European Space Agency/ESTEC, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands.
Satellite altimetry plays a key role in monitoring sea level and providing fundamental observations for assessing Earth’s climate change. The accuracy and reliability of altimetry products are ensured through dedicated calibration. In the last decade, the developments in instrumentation and algorithms increased the accuracy of satellite altimetry measurements. On the contrary, the fundamental methodology of range calibration, and thus the bias uncertainty, remained practically unchanged. To fill the gap, this work advances the validity and accuracy of altimetry calibration while following Fiducial Reference Measurements standards. It increases the calibration validity by adding the Ocean and Land Colour Instrument on board Sentinel-3A & B and the MP-3000A ground radiometer for determining its most variable (spatially and temporally) parameter, the wet delay correction. Moreover, this work improves the calibration accuracy by mitigating its major error sources. These are the transponder’s internal delay and the calibration processing approximations with an uncertainty of 3 cm. The internal delay estimation is enhanced by combining active (transponder) and passive (corner reflectors) point targets. This tandem operation of diverse targets, allows to estimate in situ the internal delay by determining the additional path of the transponder’s echo compared to this of the corner reflector. Thus, the transponder’s internal delay is checked at every pass with an uncertainty of about 1 cm. The proposed technique preserves the transponder as the main reference target because of its higher signal to noise ratio. Additionally, a new technique for altimetry calibration is proposed called Altimeter Differential Corner Reflector (ADCR). The differential bias of ADCR quantifies the quality of altimetry measurements without the error of atmospheric and geophysical corrections. This is achieved by co-locating corner reflectors in order to experience identical effects. The differential bias originates from the comparison of corner reflectors range difference (estimated using altimetry measurements) against their known distance. As far as the processing approximations, this work detected a procedure of conventional processing that degrades the bias accuracy. Specifically, the common reference of measured and geometric range was conventionally realized by adding a constant offset to the measured range. The offset equals to the distance between the altimeters phase center (APC) and the satellite center of gravity (CoG) along the z-axis. This practice ignores the attitude effects that alter the relative position of APC and CoG and the difference along x and y axes that affects the datation bias. A comprehensive methodology is proposed in which the APC and CoG three-dimensional position and its variations with the satellite’s attitude is incorporated into the calibration. Jason-3 is used as the demonstration case. The revised calibration correction 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 attitude corrections on datation bias corresponds to about 30% of its total value per cycle. Finally, the mean bias difference of Jason-3 ascending and descending orbits over the GVD1 transponder is improved by 12%.