Almabraxas/2/Transmission frame format

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Encoding

The frame is fixed size with a length of 50 characters. This seems to be the biggest possible message for 1 billing unit with my Iridium provider.

Binary coding is not used, so I don't need to use CTS / DTS lines on the modem without doing anything special.

Since the frame is fixed size, there's no need for an initial synchronization symbol (like start bit in UART or $ in GPRS sentences): if the frame is less than 50 characters, something was missed.

Numbers are encoded in groups of 6 bits, least significant group first, encoded with a radix-64 using characters 0-9 then a-z then A-Z then - and _. So for example, for a 2-digit (radix 64) number, decimal 0 is encoded as 00, 1 is 10 (least significant first), 9 is encoded as 90, 10 as a0, 63 as _0 and 64 as 01. Note that the coding allows for 6 bits for each radix 64 digit (thus 4096 values for a 2-digits number, 262144 for 3-digits, and so on).

I'm trying to have "natural" values to save some computing of scale, so for example heading goes from 0 to 3600 (in 1/10th of degrees), instead going to the full 4095.

Since not everything I want to transmit fits into the frame with the selected coding scheme, the less important data (number of error messages from GPS and modem for example), are transmitted in a rotating manner. The "]" field is a bitfield and allows for discriminating the differents cases, according to the tables below. Also note that MSB of this field is a one bit flag indicating that logging on the SD card in running.

The frame

Index in frame Field size Field name Expected range Description
1 1 N 0-63 Incrémental message number (cycling on 64 positions) reset on computer reset
2 5 CCCCC 0-2^30-1 (~sometimes in 2034) Absolute clock: Number of secs since January 1 2000 à 00h00:00. NB: the GPS is the clock source
7 3 UUU 0-262143 Uptime in sec (teensy clock)
10 3 MMM 0-262143 Free memory (bytes)
13 4 LLLL -90-+90 with mapped in 2^24 Latitude (range allow for 1 mm precision)
17 4 llll -180-+180 mapped in 2^24 Latitude (range allow for 2 mm precision)
21 2 VV 0-4095 (0V-40.95V)
23 3 PPP 0-262143 Barometric pressure (Pa)
26 3 AAA 0-262143 GPS altitude (meter)
29 2 TT -220+XXX Temperature (OAT) in 10th of celsius degree, starting at -220 (that is 0 = -22.0)
31 2 tt -220+XXX board temperature in 10th of celsius degree, starting at -220 (that is 0 = -100.0)
33 2 ss 0-4095 GPS ground speed in knots
35 2 hh 0-3600 (degree / 10) (last) GPS heading
37 2 SS Servo position
39 1 [ 0-31+MSB MSB = SD card logging ok

If ]&0xf == 0, the rest of the fields are as follows

Index in frame Field size Field name Expected range Description
40 2 ww 0-4095 No of current waypoint
42 4 xxxx -90-+90 with mapped in 2^24 Next waypoint latitude
46 4 yyyy -180-+180 mapped in 2^24 Next waypoint longitude

If ]&0xf == 1, the rest of the fields are as follows

Index in frame Field size Field name Expected range Description
40 3 ggg 0-262143 Nbr of messages from GPS
43 3 iii 0-262143 GPS GRMC messages with status void

If ]&0xf == 2, the rest of the fields are as follows

Index in frame Field size Field name Expected range Description
40 3 eee 0-262143 Nbr of ko messages from GPS
43 3 fff 0-262143 Number of ERROR from satellite modem
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