On-board Altimeter Research Articles

Assessment of Sentinel-6MF low resolution numerical retracker over ocean: Continuity on reference orbit and improvements

On April 7th, 2022, after 16 months of tandem flight with its predecessor, Jason-3, the Copernicus Sentinel-6 Michael Freilich (MF) satellite became the regional reference mission in the sea level climate record. Its on-board altimeter, POSeidon-4, is the first radar altimeter allowing a simultaneous and continuous acquisition in Low Resolution (LR) as well as in High Resolution (HR) mode. In March 2023, a new version of Sentinel-6MF ground segment (processing baseline F08) brought major improvements for the LR mode, with the implementation of a numerical retracker in addition to the historical Maximum Likelihood Estimator-4 (MLE4) retracker. The present work covers the full assessment of this new LR numerical retracker over open ocean, spanning from the retracker’s outputs to their contribution to the Global Mean Sea Level. Improvements with respect to MLE4 appeared mainly in terms of sea-state related effects, leading to a 60 % reduction of the Sentinel-6MF/Jason-3 Sea Surface Height Anomaly bias correlated to Significant Wave Height. Such result improves an already very good continuity between the two missions. The agreement between Jason-3 and Sentinel-6MF over the tandem phase is also precisely assessed. The small remaining discrepancies are attributed to different components of the system, such as the orbit, the radiometer wet troposphere correction, C-band processing or an MLE4-based empirical adjustment. Another important feature of the Sentinel-6MF numerical retracker is the use of the in-flight Point Target Response to mitigate instrumental changes and thus improve long term stability. The Global Mean Sea Level analysis presented in this paper shows no significant trend difference with respect to Jason-3, once the radiometer wet troposphere correction impact is removed.

Data-Driven Calibration of SWOT’s Systematic Errors: First In-Flight Assessment

The SWOT satellite, carrying the KaRIN first wide-swath onboard altimeter, was launched in December 2022, and has now delivered more than a year of surface water elevation data over the ocean and inland lakes/rivers. These data are affected by systematic errors which constitute the dominant part of the error budget at scales larger than a few thousands of kilometers. Some strategies for their estimation and calibration were explored during the pre-launch studies with performance estimations. Now, based on the real data, we propose in this study to assess the systematic error budget with statistical methods relying on spectral and co-spectral analysis. From this assessment, suggesting very low error levels (below requirements), we propose the implementation of the calibration algorithms at Level-2 and Level-3 with a few minor adjustments justified by the error spectra. The calibrated products are then validated with usual CalVal metrics.

Terrain-aided Navigation for an Underwater Glider

A terrain-aided navigation method for an underwater glider is proposed that is suitable for use in ice-covered regions or areas with heavy ship traffic where the glider may not be able to surface for GPS location updates. The algorithm is based on a jittered bootstrap algorithm, which is a type of particle filter that makes use of the vehicle's dead-reckoned navigation solution, onboard altimeter, and a local digital elevation model (DEM). An evaluation is performed through postprocessing offline location estimates from field trials that took place in Holyrood Arm, Newfoundland, overlapping a previously collected DEM. During the postprocessing of these trials, the number of particles, jittering variance, and DEM grid cell size were varied, showing that convergence is maintained for 1,000 particles, a jittering variance of 15 , and a range of DEM grid cell sizes from the base size of 2 m up to 100 m. Using nominal values, the algorithm is shown to maintain bounded error location estimates with root-mean-square (RMS) errors of 33 and 50 m in two sets of trials. These errors are contrasted with dead-reckoned errors of 900 m and 5.5 km in those same trials. Online open-loop field trials were performed for which RMS errors of 76 and 32 m- were obtained during 2-h-long trials. The dead-reckoned error for these same trials was 190 and 90 m, respectively. The online open-loop trials validate the filter despite the large dead-reckoned errors, single-beam altitude measurements, and short test duration.

Model Rocket Workshop: A Project-Based Learning Experience for Engineering Students

A Project-Based Learning (PBL) experience for undergraduate students of aerospace engineering is described in this paper. The experience allows the students to build a model rocket using materials which can be easily obtained. They also compute all the relevant quantities to design and characterize the rocket and they test the robustness of their design. They furthermore launch the rocket with the corresponding payload and verify the flight parameters using an on-board altimeter. Finally, they also compare the flight parameters with the theoretically expected values. Using this simple scheme the students are later introduced in the simulation of complex flows, using standard techniques. We find that our students get rapidly involved in the project, allowing them to acquire several practical abilities, besides developing an accurate knowledge of the physics of rockets and of fluid dynamics.

Tides on Europa, and the thickness of Europa's icy shell

It has been shown previously that measurements of tides on Jupiter's moon Europa can be used to determine whether there is a liquid ocean beneath this moon's icy outer shell. In this paper we examine the further possibility of constraining the thickness of the icy shell in the case where a liquid ocean exists, by combining measurements of tidal gravity obtained from tracking an orbiting spacecraft with measurements of vertical tidal surface displacements obtained from a precise onboard altimeter. By simulating a 1‐month Europa mapping mission we demonstrate that this combination of tidal measurements would provide a much better estimate of ice thickness than could be obtained using either tracking or altimeter measurements alone. The thickness value inferred from the combined data would also require an estimate of the shear modulus of Europa's icy shell. This introduces an additional uncertainty in the thickness estimate that is approximately proportional to the uncertainty in the inverse of the shear modulus.

Statistical Evaluation of the Jason-1 Operational Sensor Data RecordSpecial Issue: Jason-1 Calibration/Validation

The Jason-1 satellite altimeter mission represents a first step towards operational oceanography from satellite altimeter missions. An operational data product, the Operational Sensor Data Record (OSDR), provides measurements from the on-board altimeter and radiometer within 3–5 h of real time. This data product is a wind and wave product that is aimed towards near-real–time meteorological applications. A higher accuracy and more detailed data product, the Interim Geophysical Data Record (IGDR), that is better suited to detailed scientific studies of ocean topography, is available no sooner than 2–3 days from real time. The measurements reported on the OSDR primarily differ from those on the IGDR in that the OSDR reports measurements derived from on-board processing of the altimeter waveforms, while ground retracking of the waveforms is performed for the IGDR. The altimeter-derived measurements on the OSDR are validated through a statistical evaluation of the differences between data on the OSDR and IGDR. In doing so, the impact of ground retracking of the altimeter waveforms is also illustrated.

The solar tides and the Sun‐synchronism of satellite altimetry

Several days separate the consecutive overflights of a fixed geographical point by a geodesic satellite along the same track so that the semi‐diurnal and diurnal oceanic tides as measured by an on‐board altimeter will appear as low frequency oscillations of the ocean surface. This was an argument against the sun‐synchronous satellite altimeters for the study of (luni‐) solar tides, these constituents being rejected under the 2 cy/year frequency or even in the steady sea surface. However, this simplified scheme should be drastically modified by the adjunction of supplementary informations supplied by other data (e.g. from tide gauges), hydrodynamical models or statistics. Here we only show that using the a priori spatial statistical properties of the tides in a total inversion of altimeter data, the sun‐synchronous satellites may contribute to map the (luni‐) solar tides. In particular the accuracy of a tidal inverse solution is little dependent on its apparent frequency and on the proximity of the other aliased tides.