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APRS Satellite Details



Now that the launch of the Naval Academy student built APRS Satellite
project is announced for August 2001, here are some design details.  This
was presented at the annual AMSAT conference in Portland Maine in October
2000.  The students call it "PCSAT" for Prototype Communications
Satellite.

NOTE:  Use a FIXED FONT for best formatting of tables...
------------------------------------------------------------------------
USNA PCSAT:

The mission of PCsat is unique in the Amateur Satellite Service as it
is designed to support the text messaging and position tracking needs of
mobiles and handhelds only and to provide links into the worldwide
internet linked APRS system. 

UPLINK/DOWNLINK QUALITIES:  Typical uplink and downlinks fall into several
different classes based on user 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 mobile parked under trees.  It is assumed that stations
intending to transmit to the satellite will be in the clear.
  
             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 Academy Ground Station

Notice that this satellite is not designed nor intended for shack-to-shack
communications.  It is optimized for mobiles and handhelds with OMNI whip
antennas.

MISSIONS:  Since several other digital amateur satellites have provided
imaging and file store-and-forward for the last several years, PCsat
concentrates on the popular trend towards personal real-time wireless
communications 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 (paging)
 3) Handheld/Mobile station-to-station digital communications
 4) North American wide Bulletin delivery to all users (occassional)
 5) Handheld/Mobile GPS position tracking to internet
 6) GPS tracking of buoys, telemetry devices, wildlife, expeditions,etc
 7) Other UI digipeating applications (TBD)
 8) Message Store-n-forward (non HAM populated areas only, if at all)

FUNDAMENTAL ASUMPTIONS DRIVING THE DESIGN:
 
 1) Optimum ALOHA TDMA 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 now on the market off-the-shelf amateur digital handheld
    and mobile transceivers suitable for this SATELLITE mission.
 8) Anyone can assemble a similar station from existing components
    readily available over the last 20 years for 1200 baud.

REQUIREMENTS/CONSTRAINTS Design Drivers:  The following are some of
the obvious first-order alignments of these requirements to hardware and
to frequencies.  From these, then, we determined the optimum trade-offs to
arrive at our final design and frequency requirements.

 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 downlink on existing 
    dedicated terrestrial APRS frequency (144.39 in North America)
 4) HT/Mobile real-time messaging requires same up/downlink baudrate
 5) GPS GAIN-HT and 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 (rare) is unconstrained with equal baud rates
 9) Applications should be spread among multiple receivers to minimize
    collisions.
10) Reundancy, Backups and Fail-Safe RESET timers are desired.
11) UHF downlinks are of little value due to poor link budget and doppler
12) UHF Uplinks without tuning can be compensated with a 30 Khz SAT RCVR
13) KISS applies (Keep it Simple, Stupid)
   
HARDWARE ALIGNMENT:  Using the above as the basis, our design evolved to
a pair of dual-port KPC-9612 TNC's.  Each KPC-9612 can cross relay from
either of its two inputs to its two outputs.  Since we only have two
transmitters on VHF for best downlink, we can output both the 1200 and
9600 baud channels to the same transmitter (with appropriate PTT/DCD cross
strapping to avoid colllisions).  ALthough we do not yet have frequency
coordination for our two 2m downlinks and two 2m uplinks and two 70cm
uplinks, we have gone ahead and penciled in the SUNSAT APRS channel and
North American 144.39 APRS channels that we hope to share.
 
USERS             UPLINK            TNC              DOWNLINK
----------------  -------         ------------       ---------------------

                                  KPC-9612 #1
HT-to-HT          145.825 ------> 1200 Baud -*-----> 145.825 (ITU Subband)
                                     \ /     |
                                     / \     |
Mobile-to-Mobile  UHF-U1  ------> 9600 Baud -*
 

                                  KPC-9612 #2
Low Pwr Trackers  VHF-U2  ------> 1200 Baud -*-----> 144.39  (over USA )
                                     \ /     |               
                                     / \     |
Igate Paging      UHF-U2  ------> 9600 Baud -*
 
Igates will monitor both downlinks to capture every packet heard anywhere
and feed to the worldwide APRServe internet linked network for
distribution everywhere.

DISCUSSION OF THE FREQUENCIES::

1) 145.825 could be a shared UI digipeating frequency for such low
dutycycle operations in synergism with other similar satelltes on the same
frequency with agreement amongst them.  See my AMSAT SYMPOSIUM Paper 1999.

2) VHF UPLINK#2 on any 2m frequency hopefully not shared with any other
satellite uplink so that we do not QRM any other sats.  We have no
objections if this frequeny is another satellite downlink.  Our user
uplinks on this frequency will be about 5W ERP only for special
applications in remote areas and will have minimum terrestrial range.

3) UHF UPLINK#1.  Any 20 KHz frequency in the ITU UHF satellite band

4) UHF UPLINK#2.  Any other 20 KHz frequency in the ITU UHF sat band

5) 144.39 has been blessed in North America for APRS by TAPR, ARRL and
AMSAT-NA and their Canadian counterparts.  Downlink on this frequency
over North America is a unique opportunity hopefully welcomed by all
current users in the region.  This downlink will only be SPARSELY USED by
special authorized stations for special events or special comms needs or
special messaging to users not reachable by the terrestrial system.

CONCLUSION:  We hope that other amateur satellites will consider adding a
transponder of this simplicity to join our constellation in supporting
handheld and mobile amateur radio operators throughout the world.  The
power of APRS is its use of generic digipeater callsigns.  Thus it does
not matter what satellite is in view, they all will function equally to
link the users to the rest of the world.  Secondly, each user's need is
only a few 1 second packets.  THus the throughput and number of users
supported can be far greater than the other FM satellites (voice).
Similarly there is no advantage to channel hogging.  Each user only gets
one POSITION, One STATUS packet and any one-line messages pending.

For more information on operating the current APRS capable Satellites
now in orbit 

* For info on APRS Satellites http://web.usna.navy.mil/~bruninga/astars.html
* To see the LIVE worldwide APRS data,  TELNET to 199.227.86.221 port 23.
* To see a LIVE map, BROWSE www.aprs.net and click on the WORLD map.
* To find any APRS Mobile/Handheld/Station BROWSE Steve Dimse, K4HG's,
http://map.findu.com/CALLSIGN

There were some problems over the weekend with these sites, but they
should be OK now.  See ya on the birds!

de WB4APR, Bob

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