Almabraxas/2/Transmission frame format

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