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Telemetry decoding

Describes how to decode data from recieved telemetry. This includes evaluation of temperature, pressure and humidity from measured values or GPS coordinates.

FRAME_PAYLOAD_DATA structure

Each frame contains payload consisting of subframes with varying length.

SUBFRAME structure

offset (B)	lenght (B)	name	content
0	1	SUBFRAME_TYPE	Type of subframe ?
1	1	SUBFRAME_LEN	Length of subframe payload
2	2*SUBFRAME_LEN	SUBFRAME_PAYLOAD	subframe payload
2*SUBFRAME_LEN+2	2	SUBFRAME_CRC	CRC 16 of subframe payload, polynom = 0x11021

Known subframe types

SUBFRAME_TYPE	description
0x65	Calibration and Configuration fragment
0x69	Measurement data
0x67	GPS data
0x68	typ of content not indentified
0xff	padding ?

Calibration subframe payload (0x65)

expected payload length: 32

offset (B)	lenght (B)	name	content	description
0	2	CALIB_FR_NUM	uint16	frame number, (starts from 1)
2	10	CALIB_ID	string	0x20 0x20 + sonde ID (same as on cover)
12	1	CALIB_STATE_0	uint8	0 - OK; 0x08/0x0A - battery low / battery low, powe off countdown running?
13	1	CALIB_STATE_1	uint8	see table bellow
14	1	CALIB_UNKNOWN_0	0x00	???
15	1	CALIB_FRAG_NUM	unit8	callibration and configuration fragment number
16	16	CALIB_FRAG_DATA	bytes	calibration and configuration data fragment

STATE_1

TODO: verification needed, possible documentation bug

bit offset	description
(LSB) 0	start detected (set at the same time as bit 5)
1	???
2	humidity sensor heating: 0 = off, 1 = on
3	humidity sensor for measurement: 0 = U1 (CH2), 1 = U2 (CH3)
4	set to 1 when current drown from battery is increased by 50mA
5	start detected (set at the same time as bit 1)
6	???
(MSB) 7	???

Measurement subframe payload (0x69)

expected payload length: 24

offset (B)	lenght (B)	content	name	description
0	3	int24	MEAS_T	Temperature sensor (wire at end of stick)
3	3	int24	MEAS_U1	Humidity (remoter to sonde)
6	3	int24	MEAS_U2	Humidity (closer to sonde)
9	3	int24	MEAS_REF1	reference upper bound, increase with temperature (or vice versa, TODO)
12	3	int24	MEAS_REF2	reference upper bound, decrease with temperature (or vice versa, TODO)
15	3	int24	MEAS_P	Pressure
18	3	int24	MEAS_REF3	reference lower bound
21	3	int24	MEAS_REF4	reference lower bound

Humidity value equations:

ref_lo = (MEAS_REF3 + MEAS_REF4) / 2
ref_hi1 = MEAS_REF1 - ref_lo
ref_hi2 = MEAS_REF2 - ref_lo
U1_rel = ref_hi2 / (MEAS_CH1 - ref_lo)
U2_rel = ref_hi2 / (MEAS_CH2 - ref_lo)
U1 = cal[40] + cal[41]*U1_rel + cal[42]*U1_rel^2 + cal[43]*U1_rel^3 + cal[44]*U1_rel^4 + cal[45]*U1_rel^5
U2 = cal[50] + cal[51]*U2_rel + cal[52]*U2_rel^2 + cal[53]*U2_rel^3 + cal[54]*U2_rel^4 + cal[55]*U2_rel^5
U = max(U1, U2)

Similar set of equations, but with some aditional tricks should work for P, and T.

GPS subframe payload (0x67)

expected payload length: 122

offset (B)	lenght (B)	content	description
0	4	GPS_TIME (int32)	GPS time - ms from week start
4	2	GPS_BLOB_0	???, 4*4 bits
6	8	GPS_PRN	PRN for each tracked satelite
14	12	GPS_STAT_1	signal strength, fix status …
26	96	GPS_DATA_1	doppler, pseudorange, …

GPS_PRN

List of pseudorange sequence number for each recieving satelite, 0 == no satelite.

12*5 bit - PRN number (x - unused ?)

11100000 x2222211 44433333 x5555544 77766666 x8888877 AAA99999 xBBBBBAA

GPS_STAT_1

GPS satelite channel state.

offset (b)	length (b)	description
0	4 (int4)	signal strength or fix quallity, 0xf == NaN / no signal
4	1	time fix - set to 1 when time is succesfully obtained from subframe
5	1	pseudorange fix ??? - pseudorange fix is correlated to time ?
6	1	pseudorange fix ??? (not corelated to time ?)
7	1	signal tracking - some signal found at certain frequency

GPS_DATA_1

offset (B)	length (B)	type	description
0	4	int32/fract32 ?	value in range < -2e6, 0x1400000 ), pseudorange in metters?, 0x7FFFxxxx == NAN
4	3	int24/fract24 ?	some number, might be signed, not sure, doppler shift?
7	1	???	number, min/max: < -38, 39 > usually -5..10

Unknown subframe payload (0x68)

expected payload length: 10

No clue what this means.

offset (B)	length (B)	description
0	10	???

ussual content:

0x03 0x03 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

Padding subframe (0xff)

expected payload length: 4

This type of frame probably identifies only padding

no CRC?
constant pattern of repeating (0x02 0x00) ?

Configuration and calibration

Calibration data are composed from fragments send in first subframe. Calibration data block is 512B long. Content is identical with data from EEPROM.

- there should be somewhere date/time (GPS send only seconds offset from start of week)

offset (B)	lenght (B)	content	description
0	2	???	??? (checksum?)
2	2	unit16	downlik frequency: f = (400E6 + 10E3*value) Hz upper limit is around 423MHz (value = 0x08FC)
4	2	uint16	always 0xffff - kill timmer ?
6	2	int16	always 0x0100 - set transmit power to full ?
???
22	10	string	probe ID (same as printed on conver, starts with 0x20 0x20)
???
64	5*x	CALIB_COEFF	calibration coefficients table

CALIB_COEFF

Consist of sequence of repeated fields with following structure:

offset (B)	lenght (B)	context	description
0	1	uint8	calibration constant index
4	4	float32	calibration constant value

Example of calibration data

coefficient index row/col (x)	0	1	2	3	4	5	7
1x	-366.026	-43.9063	393.234	-131.314	25.6268	-2.02341	0.96
2x	-803.34	363.815	317.784	127.429			0.96
3x	-187.802	89.6416	-26.9122	9.15022	-1.45707	0.0868068	0.849488
4x (humidity 1)	-1223.17	3548.18	-2646.42	588.217			0.96
5x (humidity 2)	-1198.04	3497.31	-2611.15	580.153			0.96
6x	0.0318379	0.0374458	-0.0433925 0.00630527
7x	-0.000132316	0.000414854	-0.000402439 6.31402e-05
8x	-2.04505e-06	1.5662e-067.05777e-07
11x	0.5227	-0.0284	0.000286	8.97e-07	-1.26e-09
12x	1.07573	-0.204	0.009009	-5.83e-05	-1.04e-06
13x	0.5227	-0.0284	0.000286	8.97e-07	-1.26e-09
14x	1.07573	-0.204	0.009009	-5.83e-05	-1.04e-06
15x		4.75029e-06-5.47494e-08 4.851e-10 -1.736e-12					0.7
16x							0.7

Coefficients at rows 1Nx are never chenge.

brmlab

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