Design of Local User Terminals for Search and Rescue Systems with MEO Satellites Pau Closas G´ omez (1) * , Carles Fern´ andez Prades (1) , Juan A. Fern´ andez Rubio (1) , Gonzalo Seco Granados (2) , Igor Stojkovic (2) (1) Department of Signal Theory and Communications, Universitat Polit` ecnica de Catalunya UPC Jordi Girona 1–3, Edifici D5.Campus Nord UPC. 08034 Barcelona, Spain E-mail: {carlos, closas, juan}@gps.tsc.upc.es (2) European Space Agency ESA/ESTEC Keplerlaan 1, 2200 AG Noordwijk, The Netherlands E-mail: {gonzalo.seco.granados, igor.stojkovic}@esa.int Abstract The extension of the Cospas-Sarsat Search and Rescue system to medium earth orbit satellites, as a payload in Galileo satellites, requires a performance analysis in terms of positioning accuracy and power budget. This paper presents an analysis of signal structure, both for the present and a recently proposed 406 MHz distress beacons. Considering these signals, time of arrival and frequency of arrival techniques has been tested to determine signal power requirements for parameter estimation. 1 Introduction Cospas-Sarsat is a satellite system designed to provide distress alert and location data to assist search and rescue (SAR) operations, using spacecraft and ground facilities to detect and locate the signals of distress beacons operating on 406 MHz or 121.5 MHz. This work deals with the design of ground receivers, called Local User Terminals (LUTs), in case of the extension of the Cospas-Sarsat system to Medium Earth Orbit (MEO) satellites, in particular to the Galileo satellites forming the SAR/Galileo system, which will complement the current Low Earth Orbit (LEO) and Geostationary Earth Orbit (GEO) constellations, considering the existing digital 406 MHz beacon and a recently proposed new signal structure. 2 Distress beacons signals structures In [1] complete specifications for Cospas–Sarsat 406 MHz actual distress beacons are available and [5] details the proposed new beacon specification, therefore this section is intended to be a brief summary, commenting only the points of interest for this work. Both types of beacon allow two types of messages: short and long. The existing beacon modulates a 406 MHz carrier at 400 bps ±1 percent and the proposed will modulate the same carrier at 400 sps ±1 percent, both have similar formats for the message structure with 3 differentiated parts, exposed in Fig. 1 and depicted in Fig. 2. The message is filtered with a signal mask. This filtering is performed to avoid in–band spurious emissions or, at least, reduce them to acceptable levels. The existing beacon, comprising the carrier and the whole distress * Work partially supported by the Spanish government under grants FPU-AP2000-3893 and TIC2001-2356-C02-01 and by the European Space Agency under ESTEC Contract no. 17713/03/NL/LvH/jd 1