REAL TIME ALTIMETRY FROM ERS-2 John Lillibridge NOAA Laboratory for Satellite Altimetry, 1305 East-West Highway, Silver Spring, MD 20910, USA phone: (301) 713-2857, fax: (301) 713-4475 e-mail: johnl@bigbird.grdl.noaa.gov Robert Leben Colorado Center for Astrodynamics Research, University of Colorado, Boulder, CO 80309, USA phone: (303) 492-4113 e-mail: leben@orbit.colorado.edu Femke Vossepoel Delft Institute for Earth-Oriented Space Research, Delft University of Technology, Kluyverweg 1, NL 2629 HS, Delft, The Netherlands phone: +31 15 2781483, fax: +31 15 2783444 e-mail: F.Vossepoel@lr.tudelft.nl ABSTRACT ERS-1, and has continued since May, 1996 with ERS-2. These "Real-Time Geophysical Data Records" (RGDRs) Since November 1995, NOAA has been generating are based on ESA's Fast-Delivery "URA" product,which "Real-time Geophysical Data Records" (RGDRs) for is received at NOAA within 6 hours of satellite ERS-1 and ERS-2. ESA's Fast-Delivery altimeter data acquisition. Since the URA data contain only crude arrive at NOAA within 6 hours of acquisition, are satellite state vector information, the most important combined with JGM-3 orbits produced by the Delft step in RGDR production is the addition of orbital Univ. of Technology, and are enhanced with several information from ephemerides computed by the Delft environmental corrections. The operationally computed Institute for Earth-Oriented Space Research. In order to orbits are generated with a 2-3 day lag. To create produce RGDRs on a daily basis (within 12 hours of RGDRs within 12 hours of acquisition, we use a satellite acquisition) it is necessary to utilize a predicted predicted orbit extension to each orbital solution. The extension to the computed orbits. After applying the RGDRs contain orbit errors of ~50 cm, so are most predicted orbit, NOAA enhances the RGDR with useful for short-arc mesoscale studies. Results from two improved geophysical corrections and makes the data such applications are presented: monitoring of the Gulf available to select real-time users. Stream region and the Gulf of Mexico Loop Current. If the timeliness of data production is relaxed to three A refined data set, the "Interim Geophysical Data days, it is possible to use the highest precision part of Records" (IGDRs), can be computed within 3 days the Delft JGM-3 orbits, rather than the predicted phase. using the most precise part of the Delft orbits. These In this case one obtains an "Interim GDR" (IGDR), data have orbit errors of ~10 cm, and are suitable for another standard NOAA ERS product. Converting an large-scale interannual monitoring. The IGDR data will RGDR into an IGDR simply entails replacing the soon be assimilated into a coupled ocean/atmosphere predicted orbit with the best part of the computed orbit, model running at NOAA, as is currently being done and adjusting the height data accordingly. with near real-time TOPEX altimetry. In this paper we discuss the preparation of the NOAA Keywords: operational altimetry, mesoscale data sets and illustrate a few current uses. The RGDR oceanography, western boundary currents. data have relatively large orbit errors, so are most suitable for mesoscale studies with explicit orbit error 1. INTRODUCTION removal. We show examples from the Gulf of Mexico and Gulf Stream regions. The IGDR data are sufficiently One of the great advantages of working with altimetry accurate to monitor sea level on monthly time scales data from the ERS satellites is the timeliness with which without removing orbit error, as shown in comparisons the Fast-Delivery data are received. Beginning in with tide gauges. The IGDR data will soon be November, 1995 the NOAA Laboratory for Satellite assimilated into an operational ocean model at NOAA, Altimetry began generating near real-time products from along with near real-time TOPEX analyses. 1