The Location Aids system needs to operate in two modes: 

(1) while the main vehicle batteries are still discharging (normal mode)
(2) after the main vehicle batteries have been exhausted, 
   and the system is running off the backup batteries (sleep mode).

Normal mode:

The Basic Stamp needs to run a 15 minute "Dropweight Watchdog Timer."
The Main Vehicle Computer normally sends a "Reset Dropweight Watchdog" message
to the Basic Stamp on the RS232 line (J9).  If the Basic Stamp does not receive 
the "Reset Dropweight" message, when the timer completes the countdown,the
Basic Stamp needs take HIGH the signal DROP_WT_STAMP_CMD (P9),
which will energize relay K2 and burn the wire to release the drop weight.

When the Acoustic Modem (J7) activates, the signal DROP* (P6) will go LOW.  
The Basic Stamp should then take HIGH the signal DROP_WT_STAMP_CMD (P8).

The signal WATCHDOG_TOGGLE (P15) needs to be touched by the software at intervals 
shorter than the timeout period set by C26 (U5 pin 4), approximately 30 minutes.

The DG16 sends position and time information to the Main Vehicle Computer during the
normal mode of operation. 
   
Also during the normal mode of operation, the main vehicle batteries get queried by the
Main Vehicle Computer; they return battery voltage information and individual cell pack
voltages.  

The Basic Stamp needs to receive a string from the Main Vehicle Computer containing   
time, position, and voltages, and then the Basic Stamp needs to send this string to
the Iridium modem.

Sleep mode:

The signal 12VDC_MONITOR (P4) monitors the 12 VDC from a voltage regulator 
in the Main Vehicle Computer.  When the vehicle’s batteries are exhausted and shut down, 
the regulator will turn off, and the signal on P4 will go LOW.  At this time, 
the system needs to go into the sleep mode where it conserves all energy possible.

When the system is placed into the "sleep" mode, the Basic Stamp needs to 
turn off the 5V high current power supply, by taking the signal SHDN_PS* 
to the LOW level (P3).  At the same time, the signal SHDN_MODEM*  (P7) 
should be taken LOW, to turn off the 9601 Iridium modem.  
Also during the sleep mode, the Ashtech DG16 GPS receiver needs to go into 
its sleep mode, which is accomplished via a command on the RS232 line (P13).

The system timer (U5) should issue a RESET* signal LOW approximately every half hour, 
"waking up" the Basic Stamp.

When the Stamp wakes up, it needs to acquire the GPS position and time, 
and send a string containing position and time out the 9601 Iridium modem.  

This will mean waking up the 5V high current power supply, 
by taking the signal SHDN_PS* signal HIGH (P3), as well as the signal SHDN_MODEM*.

The Basic Stamp needs to wake up the DG16, get time and position,
and send these to the Iridium modem.

Then the Basic Stamp should turn off the 5V high current supply, turn off the Iridium
modem, turn off the Ashtech GP16, then put itself back into the sleep mode, 
and wait for RESET* to go LOW.