Hi Nate, I’ve been running with the SPOT for a while now and have learned a few more things that might be useful to everyone:
I’ve snooped SPOT’s serial comms some more and determined that the command syntax for sending a “predefined message” is as follows: (0x) AA 07 26 01 01 01 00 (only 7 bytes this time). This still falls into the category of a “Check-in/OK Message”, but it only sends GPS location, time, and whatever predefined message you have setup on the SPOT website for that particular device. The reason this is worthwhile is that unlike the custom “type & send text” messages, these messages are FREE. They are included with the $100 SPOT annual subscription. So, if all you care about is sending GPS position (for locating something for example), you can send as many of these messages as you want, without having to buy a message bundle or pay $0.50 per message.
I recommend going with a higher-output regulator than the MIC5219 for the SPOT 3.3V Main Power (MSP/General) bus. During transmission, the SPOT can pull well over the 500mA rating of the 5219, causing the power to drop out. In some cases, the SPOT stays up, but the transmission is never fully completed and hence, no message is received. I have had lots of reliability issues with this. I replaced it with a LP38692MP-3.3, which is a 1A LDO with a shutdown line. It’s a SOT223, so a bit bigger, but it has been flawless … sending over 1000 messages so far. The other regs (3.0v PIC, 3.0v GPS, 1.8v GPS) can stay the same.
Hope it helps!