ALTITUDE ACCURACY IMPROVEMENT BY USING A NEW RADAR ALTIMETER SIMULATOR FOR ENVISAT DATA B. Osmanoglu (1) , M. Kartal (1) , S. Wdowinski (2) , T. H. Dixon (2) (1) Istanbul Technical University, Maslak Campus, Electrical and Communications Engineering Department, Istanbul, 34396, Turkey (2) Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA 1. ABSTRACT Radar Altimeters (RA) are designed to measure the topography of the ocean surface. As a result, early generations of RAs tended to lose “lock” over land (i.e., failed to provide useful height information because the tracking loop algorithm was not designed for rapid height changes) and hence have not provided useful information, except over large lakes. The new ENVISAT’s RA (RA-2) can extract height information over land using a Model Free Tracker. However, the RA-2 resolution of 390 m along track and 19 km across track, optimum for ocean monitoring, does not provide enough resolution for many land applications. In this work, we develop a RA simulator, which provides additional information on the radar’s waveform scattered over land. The simulator will serve to design a new retracker, which considers a priori knowledge of the echo waveform characteristics and previous measurement values along the satellite track. The two new features of our simulator are: (1) observation of echo waveform with respect to different surface characteristics, and (2) modification of the footprint analysis from a single mean value to equi-height annuluses (rings), and crescents, which we obtain by evaluating the coherency between along track measurements. Our current study area is the Everglades wetlands (south Florida), which is a wide, flat and smooth region, similar to the ocean surface. Preliminary results of our simulator show that it is possible to differentiate topography changes over land in time domain analysis. Further development of the simulator will include implementation of chirp modulation to the transmitted signal and transformation of the backscattered signal to the frequency domain in order to simulate the observed ENVISAT waveforms. 2. CURRENT STATUS OF RADAR ALTIMETER SYSTEMS Up until ERS-1, radar altimeters tended to lose lock over land. Envisat is able to perform improved data recovery over non-ocean surfaces due to its innovative tracking algorithm called Model Free Tracker (MFT). It is possible to track water level changes all over the world, provided that the surface of water is large enough. Over large body of waters echo waveforms are very similar to ocean waveforms. On the other hand, there is no model of echo shape for land surfaces due to their complex reflectivity pattern. Expert systems are being developed in order to analyze these complex reflectivity patterns and they are found to be helpful on areas with low or moderate slope changes. [1] Fig. 1. Relations between footprint area & reflected power a. Maximum Footprint size has not been reached. b. As a result of spherical pulse surface, footprint has a donut like form. c. Reflected power decreases proportional to footprint size.