ERS-1 ALTIMETER FAST DELIVERY DATA QUALITY FLAGGING OVER LAND SURFACES

Over land ice and land, satellite altimeters provide valuable topograp hic information, in spite of having been designed primarily to operate over the ocean. There is a need, however, for careful data quality as sessment and screening as erroneous elevation measurements can be incl uded within the telemetered data, especially when the range tracker en counters complex echoes, or rapidly varying topography. The ERS-1 Fast Delivery (FD) data product provides an excellent source of near real- time data, which has already been used for ice sheet mapping. This is a reduced data set consisting of onboard parameters generated once per second. In this paper we show that by applying thresholds to two para meters, significant amounts of poorly tracked data can be eliminated. The effectiveness of this filtering technique is demonstrated by a com parison of filtered and unfiltered altimeter data with a digital eleva tion model. This filtering technique is applied to the first 35 day re peat cycle of FD data obtained over land to produce the first map of g lobal topography from ERS-1.