Telemetry Reference

Packet Structure

All downlink telemetry packets share a common format:

[4-byte ASCII identifier][payload bytes][\n]

The \n newline is appended by radio.py during transmission and serves as a packet delimiter. It is not part of the logical payload.

Binary Packing

Telemetry payloads are packed using Python's struct.pack with little-endian byte order:

Format Code	Type	Size
s	char (ASCII string)	1 byte each
i	signed int32	4 bytes
I	unsigned int32	4 bytes
f	float32 (IEEE 754)	4 bytes

Telemetry Packet Definitions

GYRO — Gyroscope

struct.pack('4s3f', b'GYRO', x, y, z)

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	float	X	°/s
8	4	float	Y	°/s
12	4	float	Z	°/s

Total: 16 bytes

ACCL — Accelerometer

Same format as GYRO. Units: m/s².

MAGN — Magnetometer

Same format as GYRO. Units: µT (microtesla).

GRAV — Gravity Vector

Same format as GYRO. Units: m/s².

EULR — Euler Angles

Same format as GYRO. Units: degrees (°).

BME2 — BME280 Environmental

struct.pack('4s3f', b'BME2', temperature, pressure, altitude)

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	float	Temperature	°C
8	4	float	Pressure	hPa
12	4	float	Altitude	m

Total: 16 bytes

TEMP — Temperature (IMU)

struct.pack('4si', b'TEMP', temperature)

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	int32	Temperature	°C

Total: 8 bytes

QUAT — Quaternion

struct.pack('4s4f', b'QUAT', w, x, y, z)

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	float	W	—
8	4	float	X	—
12	4	float	Y	—
16	4	float	Z	—

Total: 20 bytes

ADCS — Attitude Determination

struct.pack('4s7f', b'ADCS', heading, roll, pitch, quat_w, quat_x, quat_y, quat_z)

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	float	Heading	°
8	4	float	Roll	°
12	4	float	Pitch	°
16	4	float	Quat W	—
20	4	float	Quat X	—
24	4	float	Quat Y	—
28	4	float	Quat Z	—

Total: 32 bytes

SOLR — Solar Panels

struct.pack('4s8f', b'SOLR', v1, i1, v2, i2, v3, i3, v4, i4)

Panel order matches INA219 addresses:

Index	Address	Panel
0	0x40	X-
1	0x41	X+
2	0x44	Y+
3	0x45	Y-

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	float	Panel 1 Voltage	V
8	4	float	Panel 1 Current	mA
12	4	float	Panel 2 Voltage	V
16	4	float	Panel 2 Current	mA
20	4	float	Panel 3 Voltage	V
24	4	float	Panel 3 Current	mA
28	4	float	Panel 4 Voltage	V
32	4	float	Panel 4 Current	mA

Total: 36 bytes

EPSS — EPS Status

struct.pack('4s5if', b'EPSS', error, ch1, ch2, ch3, ch4, battery_voltage)

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	int32	Error Code	—
8	4	int32	Channel 1	0=OFF, 1=ON
12	4	int32	Channel 2	0=OFF, 1=ON
16	4	int32	Channel 3	0=OFF, 1=ON
20	4	int32	Channel 4	0=OFF, 1=ON
24	4	float	Battery Voltage	V

Total: 28 bytes

OBCC / OBCR / OBCD — OBC Single Values

struct.pack('4sf', b'OBCC', value)  # CPU, RAM, or Disk

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	float	Usage	%

Total: 8 bytes

HOST — Hostname

struct.pack('4s11s', b'HOST', hostname_bytes)

Offset	Size	Type	Field
0	4	char[4]	Identifier
4	11	char[11]	Hostname (null-padded)

Total: 15 bytes

BECN — Health Beacon

struct.pack('4sIffff', b'BECN', uptime, cpu, ram, disk, temp)

Offset	Size	Type	Field	Unit
0	4	char[4]	Identifier	—
4	4	uint32	Uptime	seconds
8	4	float	CPU Usage	%
12	4	float	RAM Usage	%
16	4	float	Disk Usage	%
20	4	float	Temperature	°C

Total: 24 bytes

XFRC — Transfer Complete

struct.pack('4sI', b'XFRC', total_packets)

Offset	Size	Type	Field
0	4	char[4]	Identifier
4	4	uint32	Total Packets

Total: 8 bytes

OBCP / OBCL — Chunked Text Data

These are variable-length, multi-packet responses. Each chunk:

[4-byte ID][text data up to (chunk_size - 5) bytes]

The ground station accumulates chunks and flushes after 1 second of no new data.

SEND — Image Data Packet

SEND[base64_data][total_chunks:04d][current_chunk:04d]

Image packets are text-based (not binary struct). See Image Transfer.

PHOT — Photo Response

struct.pack('4sI', b'PHOT', filename_length) + filename_bytes

Variable length. Contains the filename of the captured image.