ANDE FREQUENCY SELECTION CONSIDERATIONS by WB4APR ------------------------------------------------------------------------ 13 Novt 02 The COMMS mission of ANDE and the other APRS Satellites is unique in the Amateur Satellite Service as they are designed to support the text messaging and position tracking needs of Schools, mobiles and handhelds only and links into the terrestrial internet system. As such, the link margins to user OMNI antennas require downlinks in the 2 meter band. For international operation, 145.825 will be shared with other existing AX.25 packet satellites which all agree to a low dutycycle downlink making channel sharing possible. The following design summary should provides the background. UPLINKS AND RECEIVE GAINS: User up&downlinks fall into several different classes based on ERP and receiver antenna gain as shown below. The column labeled Standby-receive gain is for the user who is not aware of, nor optimized for satellite reception. For example, an HT in a pocket, or a mobile parked under trees. ERP ERP Rcv Rcv Rcv UHF VHF UHF VHF STBY USERS (W) (W) dBi dBi dBi Applications --------- --- ---- --- --- --- ------------------------------ HANDHELDS 3 5 3 3 -6 Sailboats, Hikers, Wilderness MOBILES 70 100 5 5 -6 Boats, Remote Travelers HOME STNS 700 1000 13 13 Not intended for PCsat NETWORK IGATE RCV 7 5 Omni Internet receive site MSG NODE 70 100 Internet to user UPLINK site COMMAND 700 1000 13 13 US Naval Acadmy Ground Station MISSIONS: Since several other amateur satellties have provided imaging and file store-and-forward for the last several years, the APRS satellite mission concentrates on the current trend towards personal real-time wireless messaging using UI digipeating. In this context, the following list prioritizes these needs. 1) Handheld/Mobile message uplink to Internet (emergency and routine) 2) Handheld/Mobile message downlink delivery from Internet 3) Handheld/Mobile to Handheld/Mobile real-time communications 4) Nationwide Bulletin delivery to all users 5) Handheld/Mobile GPS position tracking to internet 6) GPS tracking of buoys, telemetry devices, wildlife, expeditions 7) Other UI digipeating applications (TBD) 8) Message Store-n-forward (limited special applications only) FUNDAMENTAL ASUMPTIONS: 1) Optimum ALOHA CSMA channel effeciency is about 18% due to collisions 2) VHF links have a 9 dB advantage over UHF links (omni to omni) 3) 1200 baud AFSK has a 7 dB advantage (measured) over 9600 baud FSK 4) T/R delays render 9600 only twice as fast as 1200 for short bursts 5) UHF uplinks require wide bandwith to avoid doppler (- 4 dB) 6) UHF downlinks require user tuning throughout pass (not desired) 7) There are available off-the-shelf handheld and mobile radios now that can serve as stand-alone digital satellite transceivers REQUIREMENTS/CONSTRAINTS Design Drivers: The following are some of the obvious first-order alignments of requirements to hardware and frequencies. From these, then, we determined the optimum trade-offs to arrive at our final design and frequency requirement on the 2 meter band. 1) MSG delivery to HT in Standby requires best possible downlink (1200 baud VHF). Igate uplink is relatively unconstrained. 2) MSG receipt from HT requires best possible uplink (1200 baud VHF). Downlink to internet is relatively unconstrained. 3) HT/Mobile real-time messaging requires same up/downlink baudrate 4) GPS HT/Mobile tracking is relatively unconstrained. 5) Low power GPS tracking devices requires best uplink (1200 baud VHF) and the uplink must not be used by any other satellite uplink to avoid unintentional interference to other systems. 6) Reundancy and Backups are desired. 7) UHF downlinks are of little value due to poor link budget and doppler 8) KISS applies (Keep it Simple, Stupid) HARDWARE ALIGNMENT: Using the above as the basis, we chose the KPC-3 TNC to satisfy as many of the above requirements as possible. Operating half duplex in the 2 meter satellite band. Our AMSAT/ITU Frequency request is to share 145.825 with other low duty- cycle AX.25 packet Satellites. Bob Bruninga, WB4APR@amsat.org