NATweb MISSION PROFILE: NATsat is a real proof-of-concept satellite that we will try to put together in under 2 months to take advantage of a launch opportunity! NATsat CONCEPT: Given the unique nature of APRS (low duty cycle bursts shared by thousands of users), and the recent coordination of 144.39 North American Continent wide, provides a unique opportunity to operate a geostationary APRS satellite over the Western Hemisphere on 144.39 to provide low duty cycle APRS position/Status reporting for mobiles far from the existing APRS infrastructure. Today, 95% of the US amateur population is covered by the APRS network, BUT only 30% of the surface area of the USA is, or will probably ever be, covered. Thus, distant travelers, boats at sea, and stations in the rest of the hemisphere are presently unable to use the APRS system as a travelers safety reporting system when on distant trips. An APRSat payload at geostationary orbit would make such reporting possible over half of the globe! FREQUENCY CONSIDERATIONS: Two meters is the ideal uplink band (by 9 dB) but there never will be a 20 KHz AFSK AX.25 channel allocated in the very precious 145.8 amateur satellite spectrum for APRS satellite use. But the unique nature of APRS and the geometry of a Geostationary position over the western hemisphere makes dual use of 144.39 entirely possible! This paper shows how. UPLINK MOBILES: A 150W amplifier into a car roof Turnstyle antenna can give about 250W EIRP horizontally polarized towards the GEO APRSat. The number of such mobiles is limited by duty cycle. 1200 mobiles could report once every 20 minutes, or 3600 mobiles could report once every 3 hours (this could be the target loading for long distance travelers). This maximum sharing is possible by assigned slotting based on GPS time. This is trivial to do. It is built into most APRS TNCs already. Or initially, 20% of these totals could operate randomly... FREQUENCY SHARING: Here is how these mobiles share the frequency with the other thousands of APRS users already on the channel. First, all terrestrial antennas are vertical and have reduced radiated power upwards toward the satellite. Radiation from a dipole, a 3 and a 6 dB vertical are down by from 2 to 20 dB in the direction of the satellite depending on latitude from Miami to Vancouver. Also considering the Longitude variation from Maine to California results in an additional 8 dB of vertical versus horizontal polarization isolation. Thus it is entirely possible to provide over 18 dB of power differential between the APRSat mobiles and all terrestrial stations operating at 50W or less. This isolation, combined with the FM capture effect, will assure that the satellite will not even see the terrestrial masses. MOBILE INTERFERENCE: A further consideration is that the ranging high-power mobiles transmitting 250W EIRP UPwards will have ZERO impact on terrestrial APRS users. First because they are only transmitting the same packets like all other APRS mobiles on the channel, and secondly, being horizontally polarized, their actual vertical EIRP is similar to any other 25 watt APRS mobile! THus they are welcome on the frequency at their very low rates when they drive through an APRS network. DOWNLINK: It is even possible to downlink the digipeated packets on 144.39 as well (tho at a 50% reduction in satellite channel capacity). In fact, ANY terrestrially used amateur frequency can be used by the downlink, because the signal will be designed to be below the noise floor (for omni antenna users). Lets use 144.39 as a worst case example. Even though there are thousands of users and 500 digipeaters on the 144.39 channel, none of them can HEAR a 10 watt transmitter 36,000 km away. THus there is NO interference issue to existing networks. APRsat GROUND STATIONS: APRS is unique in the way it combines all packets heard everywhere in the world into a single common internet feed channel. THus the infrastructure for distributing APRS traffic no matter where its source into all other stations already exists. So all we need is a few APRSat donwlink stations to feed in the packets heard from space. This again is not a problem on the shared 144.39 freuqency! To any single ground station receiver, the channel is still 70% vacant. (The Optimum Aloha TDMA channel operates at less than 20% occupancy). Since only a few ground stations are needed to link the APRSat downlink into the internet, any local instantaneous QRM at one receive site would not be present at all others simultaneouslly. Thus with only 5 APRsat downlinks sites all feeding the APRServe internet system and experiencing say 30% channel QRM in their area, still gives a 0.3 to the 5th power chance of a lost packet. Or better than 99.8% probabiliy of SUCCESSFUL receipt! Thus 144.39 is a GOOD downlink in the USA and there are no frequency coordination issues. In other countries, or on other frequencies, the QRM from GEO orbit of a 10W transmitter is at -127 dBm or completely undetectible unless someone is looking up with a high gain antenna. TERRESTRIAL ANTENNA ANALYSIS: The statellite will be above 45 degrees to most APRS Station antennas in the USA ALL of the time. At that angle, radiation from a home station is down 5 dB from a vertical dipole omni or 11 dB from a 3 dBd gain antenna. Signals from 6 dBd APRS digipeaters would be down 16 dB or more. Plus there is an additional 8 dB vertical to horizontal polarization isolation (over the longitudes from New England to California). Thus, APRSat mobiles with 250W EIRP would capture the APRSat receiver by the following amounts: 18 dB above any 50 watt home station on a 3 dBi gain antenna 19 dB aobve any 50 watt home station on a 3 dBd gain antenna 23 dB above any 50 watt digipeater on a 6 dBd gain antenna This is well above the threshold of FM capture effect. Thus the mobiles have a perfect chance of being heard. Even though these figures are 3 to 5 dB worse for northern and Canadian stations where the angle of the satellite above the horizon is only 30 degrees, they are still well above the FM capture effect. APRSat PAYLOAD DESIGN: THe link budgets above require a horizontal receive polarization on the satellite. A single dipole is sufficient. Thus a stabalized platform is required. BUT by giving up the 8 dB polarization isolation, a non-stabalized satellite with an omni antenna is also entirely possible. To maintain enough isolation between the mobile users and the existing terrestrial users, though, may require some reduced power and directional antennas for northern and canadian stations running power levels of 50 watts. But it is possible. Also, the downlink power of only 10 watts to the few OSCAR class ground stations makes the satellite power budget small enough to be able to use only the solar panels that would fit on a one foot or so cube satellite. THus we could either be a stand-alone no attitude controlled satellite or we could be a 20 watt payload on someone elses bus.