Pascal Bonnefond

Absolute Calibration of Jason-1 and TOPEX/Poseidon Altimeters in Corsica Special Issue: Jason-1 Calibration/Validation

The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.

Absolute Calibration of Jason-1 and Jason-2 Altimeters in Corsica during the Formation Flight Phase

The Corsica region of the western Mediterranean, including sites at Ajaccio-Aspretto, Cape Senetosa, and Capraia (Italy), hosts a dedicated, long-term experiment to support absolute calibration of space-borne radar altimeters. The objective of the experiment is to continuously monitor the bias and drift of the altimeter measurement systems. In addition, with complementary data from a local weather station, we have derived GPS-based wet tropospheric path delay measurements that are compared with those from the on-board Microwave Radiometers at the overflight times. In this article, we focus on the analysis of data from the Formation Flight Phase of Jason-1 and Jason-2.

In situ Absolute Calibration and Validation: A Link from Coastal to Open-Ocean Altimetry

The determination of global and regional mean sea level variations with accuracies better than 1 mm/year is an important yet challenging problem, the resolution of which is central to the current debate on climate change and its impact on the environment. To address this, highly accurate time series from both satellite altimetry and tide gauges are needed. In both cases, the desired accuracy represents a significant challenge for the geodetic community. From the perspective of space borne altimetry, systematic errors from the orbit, reference frame and altimeter systems are all important limiting factors and must be minimized in order to derive data products of greatest geophysical value. Indeed, the objective for the overall accuracy of future altimeter systems is 1-cm (RMS) along with a stability of 1 mm/year. From the terrestrial perspective, estimating the vertical velocity of tide gauge sites to sufficient accuracy is also one of the most important and challenging problems in modern geodesy. Essential to reaching these goals in the measurement of mean sea level variation are ultra-precise validation and calibration techniques, including in situ absolute calibration experiments. Most of the present calibration experiments are on or near the coast, reinforcing the need for developing such techniques to unify the altimetric error budget for both open-ocean and local (coastal) conditions.

French transportable laser ranging station: scientific objectives, technical features, and performance

The French Transportable Laser Ranging Station (FTLRS) is a highly mobile satellite laser ranging (SLR) system unit that weighs 300 kg and is housed in eight containers. This telemetry laser station is dedicated to the tracking of geodetic satellites equipped with retroreflectors. There are fascinating uses in the geosciences for such a system: in tectonics, oceanography, terrestrial reference frames, and precise positioning. The idea is to use a very small 13-cm-diameter telescope installed on a motorized mount and derived from a geodetic motorized theodolite of high precision. The laser is also compact, and the use of an avalanche photodiode makes detection possible at a single photoelectron level. On-site installation of this new SLR system is fast, and the systems routine operation is quite automated. It started its operational phase in late 1996. At present, it can track satellites at altitudes of as much as 3000 km and is designed to range to the Laser Geodynamic Earth Orientation Satellite (LAGEOS) at 6000 km in a further step. The standard error of individual measurements during the first observation campaign in Corsica is estimated to be of the order of 2-3 cm. Since then, significant improvements have been implemented. A technical description of the FTLRS is given, and the main results of the Corsica campaign are summarized.

Offshore Absolute Calibration of Space-Borne Radar Altimeters

Absolute calibration of sea level measurements collected from space-borne radar altimeters is usually performed with respect to collocated sea level in situ records from tide gauges or GPS buoys (Ménard et al. 1994; Haines et al. 1996; Bonnefond et al. 2003; Haines et al. 2003; Schum et al. 2003; Watson et al. 2003; Watson et al. 2004). Such a method allows regular and long-term control of altimetric systems with independent records. However, this approach is based on a single, geographically dependent point. In order to obtain more significant and accurate bias and drift estimates, there is a strong interest in multiplying the number of calibration opportunities. This article describes a method, called the “offshore method” that was developed to extend the single-point approach to a wider regional scale. The principle is to compare altimeter and tide gauge sea level data not only at the point of closest approach of an overflying pass, but also at distant points along adjacent satellite passes. However, connecting sea level satellite measurements with more distant in situ data requires a more accurate determination of the geoid and mean ocean dynamic topography slopes, and also of the ocean dynamical changes. In this demonstration experiment, 10 years of averaged TOPEX/Poseidon mean sea level profiles are used to precisely determine the geoid and the mean ocean circulation slope. The Mog2d barotropic ocean model (Carerre et Lyard 2003) is used to improve our estimate of the ocean dynamics term. The method is first validated with Jason-1 data, off Corsica, where the dedicated calibration site of Senetosa provides independent reference data. The method is then applied to TOPEX/Poseidon on its new orbit and to Geosat Follow On. The results demonstrate that it is feasible to make altimeter calibrations a few tens to hundreds of kilometers away from a dedicated site, as long as accurate mean sea level altimeter profiles can be used to ensure the connection with reference tide gauges.

The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Quality Assessment and Unique Characteristics of AltiKa Data

The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. The mission ended its nominal phase after 3 years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to provide a state of the art of the achievements of the SARAL/AltiKa mission in terms of quality assessment and unique characteristics of AltiKa data. It shows that the AltiKa data have similar accuracy at the centimeter level in term of absolute water level whatever the method (from local to global) and the type of water surfaces (ocean and lakes). It shows also that beyond the fact that AltiKa data quality meets the expectations and initial mission requirements, the unique characteristics of the altimeter and the Ka-band offer unique contributions in fields that were previously not fully foreseen.

SARAL/AltiKa Absolute Calibration from the Multi-Mission Corsica Facilities

The geodetic Corsica site was set up in 1998 in order to perform altimeter calibration of the TOPEX/Poseidon (T/P) mission and subsequently, Jason-1 and OSTM/Jason-2. The scope of the site was widened in 2005 in order to undertake the calibration of the Envisat mission and most recently of SARAL/AltiKa. Here we present the first results from the latter mission using both indirect and direct calibration/validation approaches. The indirect approach utilizes a coastal tide gauge and, as a consequence, the altimeter derived sea surface height (SSH) needs to be corrected for the geoid slope. The direct approach utilizes a novel GPS-based system deployed offshore under the satellite ground track that permits a direct comparison with the altimeter derived SSH. The advantages and disadvantages of both systems (GPS-based and tide gauges) and methods (direct or indirect) will be described and discussed. Our results for O/IGD-R data show a very good consistency for these three kinds of products: their derived absolute SSH biases are consistent within 17 mm and their associated standard deviation ranges from 31 to 35 mm. The AltiKa absolute SSH bias derived from GPS-zodiac measurement using the direct method is −54 ±10 mm based on the first 13 cycles.

The Role of Laser Ranging for Calibrating Jason-1: The Corsica Tracking Campaign

The French Transportable Laser Ranging System (FTLRS), a highly transportable Satellite Laser Ranging (SLR) instrument, was set up in Corsica (from January to September 2002) for participating to the JASON-1 altimeter verification phase. In addition to the tracking of oceanographic satellite missions and in order to perform an accurate positioning, the FTLRS also acquired laser ranging data on geodetic satellites, STARLETTE and STELLA essentially. The paper describes the analysis strategy mainly based on the use of a short-arc orbit technique to compute accurate 1 cm local orbits, and then the geocentric positioning (2–3 mm relative to GPS). Finally, we established the JASON-1 absolute calibration value, based on 9 SLR short-arcs (between cycles 1 and 26), at 108.2 ± 8.7 mm; the 10-day repeatability is of 26.1 mm showing that a great accuracy has been reached.

Multi-Satellite Altimeter Validation along the French Atlantic Coast in the Southern Bay of Biscay from ERS-2 to SARAL

Monitoring changes in coastal sea levels is necessary given the impacts of climate change. Information on the sea level and its changes are important parameters in connection to climate change processes. In this study, radar altimetry data from successive satellite missions, European Remote Sensing-2 (ERS-2), Jason-1, Envisat, Jason-2, and Satellite with ARgos and ALtiKa (SARAL), were used to measure sea surface heights (SSH). Altimetry-derived SSH was validated for the southern Bay of Biscay, using records from seven tide gauges located along the French Atlantic coast. More detailed comparisons were performed at La Rochelle, as this was the only tide gauge whose records covered the entire observation period for the different radar altimetry missions. The results of the comparison between the altimetry-based and in-situ SSH, recorded from zero to five kilometers away from the coast, had root mean square errors (RMSE) ranging from 0.08 m to 0.21 m, 0.17 m to 0.34 m, 0.1 m to 0.29 m, 0.18 m to 0.9 m, and 0.22 m to 0.89 m for SARAL, Jason-2, Jason-1, ENVISAT, and ERS-2, respectively. Comparing the missions on the same orbit, ENVISAT had better results than ERS-2, which can be accounted for by the improvements in the sensor mode of operation, whereas the better results obtained using SARAL are related to the first-time use of the Ka-band for an altimetry sensor. For Jason-1 and Jason-2, improvements were found in the ocean retracking algorithm (MLE-4 against MLE-3), and also in the bi-frequency ionosphere and radiometer wet troposphere corrections. Close to the shore, the use of model-based ionosphere (GIM) and wet troposphere (ECMWF) corrections, as applied to land surfaces, reduced the error on the SSH estimates.

Continuous monitoring of the Jason-1 and TOPEX/Poseidon ocean altimetry missions from dedicated calibration sites

We present calibration results from Jason-1 and TOPEX/Poseidon overflights of the three dedicated verification sites: (1) a California offshore oil platform (Harvest); (2) the Mediterranean island of Corsica (Cape Senetosa), and (3) the Bass Strait off the coast of Tasmania. The high-accuracy of the Jason-1 measurement system is evident in the results from the dedicated calibration experiments. These experiments do indicate, however, that the Jason-1 sea-surface-height (SSH) measurements are biased high by approximately 12-15 cm. We discuss the implications of geographically correlated errors on the determination of the SSH bias.