PCsat DESIGN DETAILS: STATIONS: This table shows the uplink power and receive antenna gains for all expected stations in the PCsat system. 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 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 USNA MISSION GOAL: Handheld and Mobile digital communications with Whips. 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 digital comms 4) Nationwide Bulletin delivery to all users 5) Handheld/Mobile GPS position tracking to internet 6) GPS tracking of buoys, telemetry devices, wildlife, spies 7) Other UI digipeating applications (TBD) 8) Message Store-n-forward (limited special applications only) BOUNDARY CONDITIONS: 1) Optimum ALOHA CSMA channel effeciency is about 20% 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) Commercial amateur radio Handheld and Mobile all-in-one digital transceviers are now readily availaable (Kenwood and Alinco) REQUIREMENTS/CONSTRAINTS Design Drivers: The following constraints were used to arrive at the optimum hardware/operational configuration: 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) Continent wide Bulletin Delivery requires existing 144.39 (1200 baud) 4) HT/Mobile real-time messaging requires same up/downlink baudrate 5) GPS HT/Mobile tracking is relatively unconstrained. 6) 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. 7) Other UI digipeating applications should be crossband full duplex and should use same up/downlink baud rates 8) Store-n-forward is relatively unconstrained with equal baud rates 9) Spread of applications among multiple receivers to minimize collisions is desired. 10) Synchronizing of downlink transmissions is desired to maximize the available half-duplex satellite receive time. 11) Reundancy and Backups are desired. 12) UHF downlinks are of little value due to poor link budget and doppler 13) UHF Uplinks without Doppler tuning is possible with a 30 Khz Sat Rcvr 14) KISS Principle should reign. (Keep it Simple, Stupid) HARDWARE: PCsat consists only of two KPC-9612 dual port TNCs that can gate packets between either port. Since we only have two transmitters on VHF for best downlink, we can output both the 1200 and 9600 baud channels to the same transmittter, one for each TNC as shown below: KPC-9612 #1 145.825 ------> 1200 Baud -*-----> 145.825 (ITU Satellite subband) \ / | / \ | UHF-U1 ------> 9600 Baud -* KPC-9612 #2 VHF-U2 ------> 1200 Baud -*-----> 144.39 (over USA and AUS) \ / | (or more later...) / \ | UHF-U2 ------> 9600 Baud -* TRANSMITTER SCHEDULING: To maximize receive (uplink) time, a cycle timer is used to drive the channel-busy inputs of each of the four TNC channels. By holding off both transmitters for N seconds and then allowing them to both transmit simultaneously, we minimize the VHF transmit time and thus maximize VHF receive time. We also bundle multiple packets into one burst thus, amortizing the TX Delays into only one TXD for further savings.. UHF receive time is unaffected. Since both the 1200 baud and 9600 baud TNC outputs will use the same transmitter, we must also use these external channel-busy inputs to prevent the 9600 baud channel from trying to transmit at the same time as the 1200 baud channel. This can be done with a two step timing circuit: HOLDOFF to 1200 BAUD TNC Channels ...----------------* *------------------------* *------- . . . | | | | *----* <- xmit time *----* HOLDOFF to 9600 BAUD TNC CHANNELS ...---------------------* *------------------------* *-- . . . | | | | *----* <- xmit time *----* When each of these signals goes low to allow the transmitter to key, if there are packets pending, they will be transmitted. If there are none pending, then no transmission occurs. Bob Bruninga, WB4APR