Fast Delivery Products Research Articles

Online Cafeteria E

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DORIS/Jason-2: Better than 10 cm on-board orbits available for Near-Real-Time Altimetry

DIODE (DORIS Immediate Orbit on-board Determination) is a real-time on-board orbit determination software, embedded in the DORIS receiver. The purpose of this paper is to focus on DIODE performances. After a description of the recent DORIS evolutions, we detail how compliance with specifications are verified during extensive ground tests before the launch, then during the in-flight commissioning phase just after the launch, and how well they are met in the routine phase and today. Future improvements are also discussed for Jason-2 as well as for the next missions. The complete DORIS ground validation using DORIS simulator and new DORIS test equipments has shown prior to the Jason-2 flight that every functional requirement was fulfilled, and also that better than 10 cm real-time DIODE orbits would be achieved on-board Jason-2. The first year of Jason-2 confirmed this, and after correction of a slowly evolving polar motion error at the end of the commissioning phase, the DIODE on-board orbits are indeed better than the 10 cm specification: in the beginning of the routine phase, the discrepancy was already 7.7 cm Root-Mean-Square (RMS) in the radial component as compared to the final Precise Orbit Ephemerides (POE) orbit. Since the first day of Jason-2 cycle 1, the real-time DIODE orbits have been delivered in the altimetry fast delivery products. Their accuracy and their 100% availability make them a key input to fairly precise Near-Real-Time Altimetry processing. Time-tagging is at the microsecond level. In parallel, a few corrections (quaternion problem) and improvements have been gathered in an enhanced version of DIODE, which is already implemented and validated. With this new version, a 5 cm radial accuracy is achieved during ground validation over more than Jason-2 first year (cycles 1–43, from July 12th, 2008 to September 11th, 2009). The Seattle Ocean Surface Topography Science Team Meeting (OSTST) has recommended an upload of this v4.02 version on-board Jason-2 in order to take benefit from more accurate real-time orbits. For the future, perhaps the most important point of this work is that a 9 mm consistency is observed on-ground between simulated and adjusted orbits, proving that the DORIS measurement is very precisely and properly modelled in the DIODE navigation software. This implies that improvement of DIODE accuracy is still possible and should be driven by enhancement of the physical models: forces and perturbations of the satellite movement, Radio/Frequency phenomena perturbing measurements. A 2-cm accuracy is possible with future versions, if analysis and model improvements continue to progress.

On the Quality of Real-Time Altimeter Gridded Fields: Comparison with In Situ Data

Abstract The timeliness of satellite altimeter measurements has a significant effect on their value for operational oceanography. In this paper, an Observing System Experiment (OSE) approach is used to assess the quality of real-time altimeter products, a key issue for robust monitoring and forecasting of the ocean state. In addition, the effect of two improved geophysical corrections and the number of missions that are combined in the altimeter products are also analyzed. The improved tidal and atmospheric corrections have a significant effect in coastal areas (0–100 km from the shore), and a comparison with tide gauge observations shows a slightly better agreement with the gridded delayed-time sea level anomalies (SLAs) with two altimeters [Jason-1 and European Remote Sensing Satellite-2 (ERS-2)/Envisat] using the new geophysical corrections (mean square differences in percent of tide gauge variance of 35.3%) than those with four missions [Jason-1, ERS/Envisat, Ocean Topography Experiment (TOPEX)/Poseidoninterlaced, and Geosat Follow-On] but using the old corrections (36.7%). In the deep ocean, however, the correction improvements have little influence. The performance of fast delivery products versus delayed-time data is compared using independent in situ data (tide gauge and drifter data). It clearly highlights the degradation of real-time SLA maps versus the delayed-time SLA maps: four altimeters are needed in real time to get the similar quality performance as two altimeters in delayed time (sea level error misfit around 36%, and zonal and meridional velocity estimation errors of 27% and 33%, respectively). This study proves that the continuous improvement of geophysical corrections is very important, and that it is essential to stay above a minimum threshold of four available altimetric missions to capture the main space and time oceanic scales in fast delivery products.

Description and validation of the OMI very fast delivery products

The Dutch‐Finnish Ozone Monitoring Instrument (OMI) is on board the NASA's Earth Observing System (EOS) Aura satellite. The Aura satellite has a so‐called direct broadcast (DB) capability to broadcast the measurements to ground stations at the same time as the measurements are being stored in the spacecraft's memory for later transmission to Earth. The Finnish Meteorological Institute's very fast delivery (VFD) processing system utilizes this direct broadcast to produces maps of total ozone and ultraviolet radiation over Europe within 15 min after the satellite overpass Sodankylä ground station in northern Finland. The VFD products include maps of total ozone, ultraviolet index, and ultraviolet daily dose. The aim of this service is to provide up‐to‐date information on the ozone and ultraviolet situation for the general public and snapshots of the current situation for scientists. The accuracy of the VFD products compares well with standard off‐line OMI ozone products as well as ground‐based Brewer measurements. After the VFD update in May 2006, the differences with Brewer measurements are −1.0 ± 7.2 DU for Sodankylä and −0.6 ± 8.0 DU for Jokioinen. Differences to OMDOAO3 and OMTO3 products are 2.1 ± 3.3 DU and 1.6 ± 6.6 DU for Sodankylä and 1.7 ± 2.0 DU and 0.0 ± 6.1 DU for Jokioinen, respectively. Between the VFD and Brewer measured UV daily dose products the relative error is generally less than 30%, which compares well to the global off‐line OMUVB product.

The wave forecast system of the “Deutscher Wetterdienst” and the “Bundesamt für Seeschiffahrt und hydrographie”: A verification using ERS-1 altimeter and scatterometer data

A new numerical wave forecast system, implemented on the computer of the “Deutscher Wetterdienst” (DWD), has provided operational wave forecasts since June 1992. The system consists of three modules, a deep water wave model for the North Atlantic and two shallow water wave models for the North European shelf area and the Baltic. The North Atlantic wave model is forced by the wind fields of the global atmospheric model T106, while the shallow water wave models are driven by wind forecasts of the “European model” of the DWD. A limited verification of the wave forecasts is based on comparisons with local buoy measurements. A thorough validation of the wave model results follows using “Fast Delivery Products” of the ERS-1 satellite for the one week period from 4th to 10th of January 1993. The ERS-1 products include windspeed and-direction derived by the scatterometer plus windspeed and significant waveheight recorded by the radar altimeter. In general the comparisons show a fairly good agreement between the altimeter waveheights and the wave model results. However, there are considerable deviations between the scatterometer wind directions and those computed by the atmospheric DWD-models in some cases, particularly in rather data sparse regions of the ocean. The measured windspeeds recorded by scatterometer and altimeter are usually smaller than the windspeeds computed by the atmospheric models. In conclusion, the comparisons between satellite and model data demonstrate that the new forecast system works properly and provides reliable data.

The use of ERS-1 products in operational meteorology

The Meteorological Office processes ERS-1 fast-delivery products from the scatterometer, the altimeter, and the Along-Track Scanning Radiometer. Wind vectors, wind speed, wave height, and sea-surface temperature data are all validated against the operational atmospheric, wave, and sea-surface temperature analyses. Summary statistics from these validations are presented. The ERS-1 data appear to have smaller errors than the ship data currently used. Surface winds from the scatterometer should both improve the atmospheric analyses and forecasts, and improve the fluxes from the atmospheric analyses used to drive wave and ocean models. The wave data have made it worthwhile for the first time to develop a wave assimilation, rather than deducing the wave field solely from the history of atmospheric forcing. Results from preliminary parallel tests measuring the impact of these data sources are presented. Further work developing the model assimilation schemes is needed, before they give the full improvement hoped for.

ERS-1 Synthetic Aperture Radar Fast Delivery Processor

The ESA ground station facility at Kiruna, Sweden will be responsible for the reception of ERS-1 data and its processing into fast delivery products. Due to its high latitude, Kiruna can receive data from 10 of 14 passes per day, making its processing throughput requirements among the highest in the ERS-1 network. This paper discusses the SAR processor system at Kiruna responsible for part of this product generation. A description of the ground station environment is presented as the context in which the processor must operate. The processor requirements are then discussed followed by descriptions of the hardware and software components.