PACKET ISOLATION between Terrestrial Users and Satellite Users The critical issue on sharing 144.39 with a Geostationary APRSat for full USA coverage is how well the high power mobile uplinks will be able to capture the satellite receiver over the noise from all the thousands of normal users. The GEO orbit places the satellite at 60 degrees above Miami, 47 deg above Kansas and 33 deg above most Canadians. Comparing these angles with typical APRS antenna patterns we can arrrive at the following table. This table is a guess of what power restrictions might need to be placed on APRS fixed stations to make this work. ANTENNA MIAMI KANSAS VANCOUVER -------------- ------ ------- ---------- dipole or whip 50 25 12 3 dBd gain 5/8 300 65 20 6 dBd gain 300 110 300 9 dBd gain 300 300 300 This is not bad! This table is based on limiting EIRP in the direction of the APRSat to less than 12 watts EIRP. THis is to allow the APRSat mobiles with 250W EIRP to be able to hit the satellite with 13 dB of margin over all other stations. These figures can be substantially improved by 8 dB if we make all high power mobiles use horizontal polarization and all ground stations use vertical (as they already do). Then if we can make the SATELLITE attitude controlled so that it listens only horizontally, then we pick up another 8dB of isolation. It could be more, but this takes into account the +/- 25 degree rotation of "vertical" from New ENgland to California as seen from orbit. But going to a stabalized satellite is a multiple order of magnitude change in spacecraft design! As you can seem, all digipeaters with gain antennas would not be affected at all. With a 6 dBd vertical there is a 30 degree null making it an ideal antenna for operation in the northern USA and Canada. THrowing in coax loss of a dB or so, most typical stations would be able to operate normally with typical rigs. But the norhtern states would need to encourage gain antennas, low power, or 6 dB nulls in the direction of the satellite.