OPERATIONS OVERVIEW:  There are several modes and operations scenarios on PCsat.  These are SAFE, LOW POWER, NORMAL, GPS, BBS and COMMAND.  The following describes the salient features of these modes:


SAFE (RESET): PCsat was designed with dual redundant systems, labeled A and B.  To assure reliable ground command, PCsat powers up or returns to SAFE mode any time the watchdog timers time-out and reset the TNC.  In SAFE mode PCsat is in its RESET or power-up initial configuration.  Any data and/or file messages are lost.  Note, the primary mission is real-time bent-pipe relay, so normally there are no messages or files on board.  In SAFE mode, PCsat is configured to maximize its ability to respond to ground command even in the presence of multiple failures.  The following conditions are met in SAFE mode:


BATTERIES CROSS CONNECTED:  This assures that power will be available to both A and B systems.


TRANSMITTERS CROSSCONNECTED: This assures that should one transmitter   have failed, that both TNC's are connected to the remaining transmitter so that we can still connect to either TNC.


UHF RECEIVERS ON:  This assures that if there is some problem with the VHF receiver on either system, that we still have at least one receiver per system to establish a command link.


      TELEMETRY ON:  Telemetry is set at a 60 second period.


      DIGIPEATING DISABLED:  The real-time digipeating functions are    disabled.  No users can use the satellite.


      GPS OFF:  The auxiliary GPS payload is OFF to minimize power.


      COOLING RADIATOR is OFF:  This minimizes power.


      XTAL FREQUNECY:  The default crystal for Transmitter B is used.



The only problem with SAFE mode is that it is not the lowest power consumption mode.  It operates at about HALF the nominal design power budget which should be safe enough, but later in life, as the power budget decreases, and power becomes an issue, then the UHF receivers may need to be commanded off as soon as a command station is available to save power.



      ENTERING SAFE MODE:  Safe mode is entered under these conditions:


      * If a TNC fails to transmit a minimum of once in 90 seconds

      * Once every 3 days, if there has been no COMMAND connection.

      * By hardware command via either TNC to RESET the other TNC.

      * By software command via a TNC to RESET itself.






The nominal 6 Watt power budget for PCsat is easily managed via command or by managing the user loading on the packet transponder.  The power requirements for PCsat modules are approximately the following:



---------------   ----- ----------        ------------------------------

TNC’s             .5W  always ON         8%

VHF Receivers     .6W  always ON         10%

UHF Receivers     1  W  default ON        16%

GPS Receiver      2  W  default OFF       33%

Radiator          3  W  default OFF       50%

Transmitter       8  W  variable          13%-50% depending on user load


Under conditions of low power budget, and after all optional payloads are turned off (their defaults), the UHF receivers can be turned off for additional power savings of 8 or 16%.  These receivers are secondary to the primary mission of the VHF receivers.  The greatest variability of power budget depends on user load on the transponder.  This is managed as noted in the OPERATIONAL MODE section. 


The optional GPS receiver will only be enalbled when there is a significant surplus of power on a whole orbit basis.  The Cooling Radiator is similarly for optional experimentation with the thermal balance of the spacecraft or when excessive temperatures require.



NORMAL MODE:  The normal mode for PCsat is with both A and B systems operational, each with both the VHF and UHF receivers ON.  Users select the proper uplink appropriate to their application and the proper PCsat digipeater routing callsign  necessary to be digipeated back down on the proper downlink.  The duty cycle for the transmitter and consequential power budget is based on the assumption that only 10% of the Earths surface has active amateur satellite users in the footprint. 


During passes in these active areas, the upperbound for transmit duty cycle is 50% since the channel is operated half duplex.  But this requires perfect timing from users and continent wide synchronizing of all users to fractions of a second.  The realizable duty cycle is 40% considering the probability of catching a complete packet on the uplink.  The actual duty cycle is expected to be much less.


Under these conditions, the whole-orbit average transmitter duty cycle is less than 4% and well within the 50% power budget allocated.


GPS MODE:  PCsat carries an auxiliary GPS experiment to permit PCsat to report its own APRS position integrated with all the other user GPS position data in the downlink.  Thus, the GPS experiment not only supports the PCsat primary mission, but also provides meaningful scientific data on the performance of the GPS on an orbiting spacecraft. The GPS engine draws a substantial 2 Watts of power which is 30% of the PCsat budget.  Careful monitoring of the PCsat power budget during orbital periods of full sun should permit the GPS to remain ON for multiple orbits.  If the GPS comes on via SEU or is left on during poor power periods, the switching circuit is self quenching at at bus voltage of

10% of the time




Command mode is entered as soon as a command station connects to PCsat using the REMOTE SYSOP function in the KPC-9612 TNC.  Both the A and B TNC's are completely independent.  Each TNC has both CONFIGURATION settings and HARDWARE SWITCHES. Thus, to command a TNC, the command station must connect to that specific TNC via either its dedicated VHF or UHF receiver.  For redundancy, all of the crtical hardware COMMAND switches are dual connected to each TNC.  Thus, either TNC can operate any of the critical switches.





BATTERY ISOLATE:  This command permits each TNC and transmitter to operate on its own battery for independence from any possible battery problems on the other side.


TRANSMITTER ISOLATE:  This command permits each TNC to operate with its OWN transmitter only.  This is normal operational mode and permits each SYSTEM to support multiple mission objectives independently.


UHF RECEIVERS OFF:  The UHF receivers can be individually turned off in order to save power as needed.


GPS EXPERIMENT:  The GPS receiver can be turned ON or OFF.  The GPS draws 2 Watts or 30% of power budget.  The switch circuit fails safe (off) if bus voltage drops below 12 volts.


GPS NMEA:  This switch (if GPS is ON) will enable single packet NMEA GPS data on the 1200 baud VHF-A channel.  These position reports will normally occur once every 30 seconds.  This switch can only be controlled from TNC-A.


GPS MITEL:  This switch (if GPS is ON) will enable multiple packet data dumps in the MITEL data format on the 9600 baud downlink of VHF-B.  This switch can only be contro0lled from TNC-B.


COOLING RADIATOR:  This switch enables a 3 watt resistive radiator on the –Z face to radiate 3 watts of electrical energy from the spacecraft in case of overheating.  A possible 8 degree C temperature drop might be possible.  R= 3*32.4 or 97 Ohms for 2.6 Watts.  The switch circuit fails safe (off) if bus voltage drops below 12 volts.



XTAL FREQUENCY:  This switch can change the frequency of VHF transmitter B from its normal mode on the North American APRS frequency to the primary downlink of 145.825 in case the primary transmitter is lost.






Each TNC has hundreds of configuration parameters because of the very capable general purpose applications for AX.25 packet radio protocol.  But for PCsat only the following TNC functions are important:


DIGIPEATER: This is PCsats primary mission.  Real Time Digital Relay


TELEMETRY:  Necessary to see conditions on board the spacecraft


COMMAND:    Necessary to change software or hardware configuration


BBS:        The 32K Bulletin Board System can be used for file store                and forward, but this is not a primary mission on PCsat.


ROUTING:    As a digipeater, the routing of packets between the two

            receivers and two baudrates is under SYSOP control.                     Routing is controlled by the use of digipeater callsigns.



CALLSIGNS:  Communications with PCsat and between users on the ground use station addresses of up to 6 characters called callsigns.  The following default callsigns are assigned to the operation of PCsat:



------- --------  --------- ---------------------------------------------

MYCALL1 W3ADO-1   SYSTEM A, Substituted in user's VHF-VHF relayed packet

                            Also used for GPS NMEA 1200 baud data.

MYCALL1 W3ADO-2   SYSTEM B, Substituted in user's VHF-VHF relayed packet

                            Also used for GPS MITEL data at 9600 baud                           

MYCALL2 W3ADO-11  SYSTEM A, Substituted in 9600 UHF to 9600 VHF packets

MYCALL2 W3ADO-12  SYSTEM B, Substituted in 9600 UHF to 9600 VHF packets


MYREM   COMAND-1  SYSTEM A, COMMAND Channel.  Password protected.

MYREM   COMAND-2  SYSTEM B, COMMAND Channel.  Password protected.


MYBBS   MAIL-1*   SYSTEM A, MAILBOX (not a primary mission)

MYBBS   MAIL-2*   SYSTEM B, MAILBOX (not a primary mission)


UIDIGI  APRSAT*   SYSTEM A  VHF/UHF digipeating call. User uses this

    path on uplink and PCsat substitutes its own

    MYCALL in the downlink


UIDIGI  SECRET*   SYSTEM B  VHF/UHF digipeating call for Special users.

                            Callsign substitution of MYCALL2 will occur.


UIDIGI  WIDE*     BOTH      Alias for terrestrial compatibility


MYGATE  XBAUD*    SYSTEM A VHF-1200 to VHF-9600 digipeating callsign

MYGATE  XBAUD*    SYSTEM B VHF-1200 to VHF-9600 digipeating callsign


MYGATE  XBAUD*    SYSTEM A UHF-9600 to VHF-1200 digipeating callsign

MYGATE  XBAUD*    SYSTEM B UHF-9600 to VHF-1200 digipeating callsign


* Functions show with (*) are disabled in SAFE mode.







       1. ISOLATE BATTERIES                           CTRL A ON

       2. ISOLATE TRANSMITTERS                        CTRL B ON

       3. Turn OFF UHF Receiver A                     OUTPUT 0xxxxxxx

       4. Turn OFF UHF Receiver B                     OUTPUT x0xxxxxx

       5. Reset OTHER TNC                             OUTPUT xx0xxxxx

       6. GPS ON                                      OUTPUT xxx0xxxx

       7. A: NMEA ON / B:MITEL ON                     OUTPUT xxxx0xxx

       8. Cooling Radiator ON                         OUTPUT xxxxx0xx

       9. Switch B to 145.825 MHz                     OUTPUT xxxxxx0x

      10. Enable Auxilliary Relay                     OUTPUT xxxxxxx0



       1. Enable digipeating via MYCALL               DIGI ON

       2. Enable digipeating VIA APRSAT and ALIASES   UIDIGI APRSAT, etc

       3. Enable cross speed conversion (XBAUD)       MYGATE XBAUD

       4. Enable Bulletin Board(not primary mission)  PBPERSON off

       5. Enable access to GPS                        CONOK ON, CMSG ON



       1. Set TELEMETRY rate                          TE N (in 10 secs)

       2. Set Beacon Rate                             BE E N (in mins)

       3. Set Bulletin Rates                          BLT N



       1. Set Beacon Text                             BT ……………

       2. Set CONNECT Text                            CT ……………

       3. Set BULLETIN Texts (four)                   LT N ………


2)  CONNECT TO MYCALLS (W3ADO-1,2 and W3ADO-11,12).  This is not normally done unless there is an auxilliary RS-232 payload such as a GPS or auxiliary processor.



3)  TRANSPONDER OPERATIONS:  Users address the various transponder paths through PCsat based on the uplink and downlink channel and the path callsign.  To minimize collisions and contention on the uplink these users are separated amongst the 4 different receivers:


A. 5 Watt HANDHELD USERS:  Receive on VHF-A 1200 baud downlink

1. Handhelds will use the path VIA APRSAT on the VHF-A Uplink

2. Handhelds may use the path VIA XBAUD on the VHF-A  uplink

   to communicate with mobiles directly at 9600 baud


B. 50 Watt MOBILE USERS:   Receive on VHF-A 9600 baud downlink

1. Mobile users will use the path VIA APRSAT on the UHF-A uplink

2. Mobile users may use the path VIA XBAUD on the UHF-A uplink

   to communicate directly with Handheld users at 1200 baud


C. Internet linked special ground stations or IGates:

1. Igates will monitor the VHF-A downlink for all packets.

2. Igates will use the path via XBAUD on the UHF-A uplink to send

   messages to Handhelds monitoring the VHF-A 1200 baud downlink

3. Igates will use the path via APRSAT on the UHF-A uplink to send

   messages to Mobiles.



1.    Special tracking devices will be authorized for low power uplink on the unpublished VHF B command channel via SECRET.  Downlink is on the North American 144.39 APRS channel.

2.    Other Special uses, such as tracking the Olympic Tourch, etc

   can uplink on VHF B via SECRET or via XBAUD



1. Authorized PAGING stations will use the unpublished 9600 baud UHF-B on a limited basis to initiate North American wide Paging on the 144.39 downlink.  Normally the path of XBAUD will be used so that the downlink comes down on the normal 1200 baud used on 144.39. 

2. "Semiprivate" pages would use XBAUD for the packet to come

down on the unusual 9600 baud on that frequency.



1. USNA will control PCSAT via the unpublished 9600 baud UHF-B if the UHF receivers are on.  Otherwise, it will use VHF-B.

2. If System B fails, USNA will use UHF-A or VHF-A as needed.



G. RECOVERY FROM SAFE MODE to NORMAL MODE:  With all systems and

components operating normally, the following steps will usually be

taken to re-configure from SAFE MODE:

1.  If power is scarce even with all user load disabled and all

receivers are fully functional, then the UHF receivers should be

commanded off to conserve additional power.

2.  The ISOLATE TRANSMITTERS command should be sent to separate

the two transmitters so that signals intended for the separate

transmitters do not collide with each other during NORMAL Ops.

3.  The ISOLATE POWER command should be sent to separate the

battery systems.  This allows us to watch the degradation of

battery system A by allowing it to run with the greater load

anticipated on System A.  Once System A has degraded below the

point of reliable operation, then the systems will remain cross-



5) FAILURE MODES:  Commands are provided for operating with the following failures:



      1. Send UHF Receivers OFF command

      2. Send UIDIGI OFF to disable general users of the APRSAT alias

      3. Send DIGI OFF command to disable all users

      4. Increase TELEMETRY period above 1 minute to 80 seconds.


   B. FAILED VHF-A Transmitter

      1. Send Cross Connect XMTR command (and change XTAL Freq)

   C. FAILED VHF-A Receiver

      1. Move Handheld users to VHF-B uplink (downlink stays on 144.39)

      2. Optionally cross connect XMTRS (which also swaps frequencies)

   D. FAILED UHF-A Receiver

      1. Move Mobiles to VHF-A at 1200 baud

      2. Move MObiles to UHF-B and cross connect XMTRS.


      1. Move Handhelds to VHF-B


      1. Send Cross-Connect-Power command


   G. FAILED VHF-B Transmitter

      1. Send CROSS-CONNECT XMTRS command (downlink now is on VHF-A)

   H. FAILED VHF-B Receiver:

      1. Suggest all special tracking devices move to VHF-A

   I. FAILED UHF-B Transmitter

      1. USNA and Paging applications move to VHF-B



      1. Send Cross-Connect-Power command




   A. Both Batteries Fail (May work in Sunlight at lower power)

   B. Both TNC's fail

   C. Both Transmitters Fail

   D. Solar power drops below X% of original

   E. Both 1-min AND 3-day Fail-Safe-Timers fail and both TNC's lockup.

   F. Both VHF receivers fail and UHF Receivers are off and TNC's both

are OK and the 3-day Fail-Safe-Timers fail

   G. Stuck Transmitter and both UHF Receivers OFF and TNC's are both OK

will cause Batteries to drain.  THus resetting the UHF receivers to ON.









PCsat uses an off-the shelf AX.25 packet radio terminal node controller for all of its communications, telemetry, command and control.  The Kantronics KPC-9612Plus is a dual port version of this popular communications standard, and it includes enough basic input/ouput capability to handle all of PCsat’s needs.  The following table summarizes its capabilities…


Radio Ports:

Radio Port 1:     Uplink/downlinks using AX.25 packet data at 1200 baud

Radio Port 2:     Uplink/downlinks using AX.25 packet data at 9600 baud


Spacecraft I/O ports:


Serial Port:      A bi-directional RS-232 4800 baud serial data port when the TNC is in the CONVERSE mode.


Control Bits:     Four FET switches.  Use CTRL A,B/A,B command


Output Bits:      Eight input/output bits.  As outputs, these are open collector switches.  Use the OUTPUT bbbbbbbb command.


Input Bits:       The same eight I/O bits may be used as inputs if a 10k pullup resistor is used.  The state of these bits is shown in the Telemetry packets.


Input Bit:        One additional single input bit in the telemetry.


Telemetry:        There are 5 analog (0-5volt) inputs.  These appear in every telemetry packet.  In PCsat, an external multiplexer switches 4 banks of 5 values into sequential packets so that 20 values may be read.



PCsat Functional Descriptions:



Solar Panels:     There are three types of solar cells on PCsat.

Sides +/-X and +/-Y:  These four sides contain 4 terrestrial

1W Teflon coated panels connected as an A system pair and a

system pair. 


+z Panels:  Four space rated high efficiency glass covered cells

producing about 1.7 times as much as the terrestrial cells.  These

are also connected as an A pair and a B pair.


-Z Panel:  One custom panel consisting of a single 16 volt string

supplies 1.4 times the power of the terrestrial cells.


Battery Charging Circuits:  The PCsat charging system was designed with

a 16 volt unregulated bus to be self-regulating.  By using 12

NICAD cells, with a fully charged open circuit terminal voltage of

16.8 volts, the solar panel output current tapers to under X ma

under full charge conditions so there is no chance for overcharge. 

As a precaution, there are 4 zener diode shunt regulators that

will dump any excess current from voltages above 16.9v to ground.

Under load, the nominal bus voltage will be 14.4 volts.


Power Regulators:  The transmitter is operated directly from the

unregulated 14.4 volt bus.  All other circuits are powered by dual

redundant 8 volt switching regulators.  Internally, the TNC’s

regulate down to +5 volts for their use.


Special Circuits:

Fail-Safe Reset Timers:


72 Hour fail-safe Reset Circuit:


Transmitter Synchronizing and Holdoff:




Telemetry Multiplexer:




Terminal Node Controller (TNC):





Battery Isolate Test:  B receivers and TNC were not connected

      Setup:  Supply 16.60 volts on Bat-A.  Idle 120 ma.  XMT 550 ma

              Supply 16.62 volts on Bat-B.  Idle  40 ma.  XMT 290 ma

      Command BAT-ISOLATE:   XMT-A = 820 ma.  XMT-B = 40 ma.


Transmitter Isolate:  Continuity was measured through the TXA and PTT lines between the A and B side while commanding the XMIT-ISOLATE relay.

Default was 0 ohms.  XMIT-ISOLATE commanded ON was OPEN.


UHF RECEIVER A OFF:   OUTPUT 0xxxxxxx changed UHF-A B+ from 7.6 to 0 v

UHF RECEIVER B OFF:   OUTPUT x0xxxxxx changed UHF-B B+ from 7.6 to 0 V

GPS ON:               OUTPUT xx0xxxxx changed GPS-ON   from 0.3 to 16 v

TNC-RESET:            OUTPUT xxx0xxxx Changed pull-up from 5 to 0

NMEA ON/MITEL ON:     OUTPUT xxxx0xxx Internal to TNC not yet built

XTAL-B:               OUTPUT xxxxx0xx Missing wires in harness

RADIATOR-ON:          OUTPUT xxxxxx0x C&C interfacec not built!

AUXILLIAR-A ON:       OUTPUT xxxxxxx0 Bad bit from TNC