unisat

Communication Protocol

Reference: CCSDS 133.0-B-2 (Space Packet Protocol), CCSDS 132.0-B-2 (TM Space Data Link), AX.25 Link Access Procedure v2.2

Wire-format compatibility contract

Every byte layout described in this document — the beacon, the authenticated command frame, the CCSDS header fields — is frozen within a major version per Semantic Versioning.

Any additive change (new field appended after the last one, new APID value, new flag bit in a reserved range) → minor bump (1.2.x → 1.3.0). Backwards-compatible receivers must ignore unknown trailing bytes / unknown APIDs.
Any breaking change (reorder, resize, or remove an existing field; redefine a reserved bit; switch endianness) → major bump (1.x → 2.0.0).
All multi-byte integer fields in this document are big-endian on the wire, matching CCSDS and ITU convention. This includes the 4-byte replay counter in the command frame (command_dispatcher.h).

Ground stations, flight firmware, and third-party decoders that follow this contract can be mixed and matched across patch releases without coordination.

1. Protocol Architecture

1.1 Protocol Stack

  Ground Station                                 Satellite
  +------------------+                          +------------------+
  | Application      |                          | Application      |
  | (GS Software)    |                          | (Flight SW)      |
  +--------+---------+                          +--------+---------+
           |                                             |
  +--------v---------+                          +--------v---------+
  | CCSDS Space      |  <-- Space Packet -->    | CCSDS Space      |
  | Packet Protocol  |     (TM/TC payload)      | Packet Protocol  |
  +--------+---------+                          +--------+---------+
           |                                             |
  +--------v---------+       UHF Link           +--------v---------+
  | AX.25 Data Link  |  <===================>  | AX.25 Data Link  |
  | (HDLC framing)   |                         | (HDLC framing)   |
  +--------+---------+                          +--------+---------+
           |                                             |
  +--------v---------+                          +--------v---------+
  | GMSK Modem       |  ~~~~~ 437 MHz ~~~~~>   | GMSK Modem       |
  | (9600 bps)       |                         | (9600 bps)       |
  +------------------+                          +------------------+

  +--------v---------+       S-band Link        +--------v---------+
  | CCSDS TM Frame   |  <===================>  | CCSDS TM Frame   |
  | (LDPC coded)     |                         | (LDPC coded)     |
  +--------+---------+                          +--------+---------+
           |                                             |
  +--------v---------+                          +--------v---------+
  | QPSK Modem       |  ~~~~~ 2.4 GHz ~~~~~>   | QPSK Modem       |
  | (256 kbps)       |                         | (256 kbps)       |
  +------------------+                          +------------------+

1.2 Link Parameters

Parameter	UHF (Primary TT&C)	S-band (Payload Downlink)
Frequency	437.xxx MHz (amateur)	2.4xx GHz (ISM)
Data Rate	9600 bps	256 kbps
Modulation	GMSK (BT=0.5)	QPSK
FEC	Conv. r=1/2, K=7	LDPC r=1/2
Data Link	AX.25 v2.2	CCSDS TM Frame
TX Power	1 W (30 dBm)	2 W (33 dBm)
Duplex	Half-duplex	Simplex (downlink only)
Use	Commands, housekeeping TM, beacon	Image data, bulk science
Effective Data Rate	4,080 bps (after FEC + framing)	117,760 bps
Per-Pass Throughput (8 min)	~240 KB	~6.9 MB
Link Margin (5 deg el.)	13.8 dB	19.4 dB

2. CCSDS Space Packet Format (CCSDS 133.0-B-2)

2.1 Primary Header (6 bytes)

Byte:  0         1         2         3         4         5
Bit:   7654 3210 7654 3210 7654 3210 7654 3210 7654 3210 7654 3210
       +---------+---------+---------+---------+---------+---------+
       |VVV|T|S|   APID    |FF| Sequence Count |  Data Length - 1  |
       +---------+---------+---------+---------+---------+---------+
       
VVV    = Version Number (3 bits) = 000 (always)
T      = Packet Type (1 bit): 0 = TM, 1 = TC
S      = Secondary Header Flag (1 bit): 1 = present
APID   = Application Process Identifier (11 bits)
FF     = Sequence Flags (2 bits): 11 = standalone packet
Seq    = Sequence Count (14 bits): 0-16383, wrapping counter
Length = Data Length - 1 (16 bits): bytes in data field minus one

2.2 Secondary Header (10 bytes)

Byte:  6    7    8    9   10   11   12   13   14   15
       +----+----+----+----+----+----+----+----+----+----+
       |       Timestamp (8 bytes, CCSDS epoch ms)       |
       +----+----+----+----+----+----+----+----+----+----+
       |SubsysID| Subtype |
       +---------+---------+

Timestamp: 64-bit unsigned, milliseconds since 2000-01-01T00:00:00 UTC (CCSDS epoch)
SubsysID:  8-bit subsystem identifier
Subtype:   8-bit message subtype within subsystem

2.3 Full Packet Layout

+---Primary Header (6B)---+--Secondary Header (10B)--+---Data Field (N B)---+--CRC (2B)--+
| Ver|T|S|APID|Flg|SeqCnt | Timestamp(8B)|SID|SubTyp |  Payload bytes ...   | CRC16-CCITT|
| Len                      |                          |                      |            |
+--------------------------+--------------------------+----------------------+------------+

Minimum packet size: 6 + 10 + 0 + 2 = 18 bytes (empty payload)
Maximum packet size: 6 + 10 + 240 + 2 = 258 bytes (CCSDS_MAX_PACKET_SIZE in firmware)
CRC polynomial: 0x1021 (CRC-16/CCITT), initial value 0xFFFF

3. APID Assignments

3.1 Telemetry APIDs (0x001 - 0x0FF)

APID	Hex	Subsystem	Subtype	Rate	Size (bytes)	Description
1	0x001	OBC	0x01	1 Hz	48	Housekeeping (temps, uptime, mode, errors)
1	0x001	OBC	0x02	On event	32	Mode change notification
1	0x001	OBC	0x03	On event	64	Memory dump response
2	0x002	EPS	0x01	1 Hz	72	Power telemetry (V, I, SOC, solar, loads)
2	0x002	EPS	0x02	On event	24	Power mode change
2	0x002	EPS	0x03	On event	16	Overprotection triggered
3	0x003	COMM	0x01	0.1 Hz	32	COMM status (RSSI, packets TX/RX, errors)
3	0x003	COMM	0x02	On event	16	Link acquired/lost
4	0x004	ADCS	0x01	1 Hz	84	Attitude (quaternion, rates, mag, sun)
4	0x004	ADCS	0x02	0.1 Hz	36	ADCS mode and control torques
5	0x005	GNSS	0x01	0.1 Hz	48	Position (lat, lon, alt, vel, PDOP, sats)
6	0x006	Camera	0x01	On event	24	Capture status (image ID, size, time)
6	0x006	Camera	0x02	On request	240	Image data chunk (max data field)
7	0x007	Payload	0x01	0.2 Hz	32	Radiation dose rate and cumulative dose
7	0x007	Payload	0x02	On event	64	IoT relay message
16	0x010	Error	0x01	On event	48	Error report (code, severity, context)
255	0x0FF	Beacon	0x01	1/30 Hz	48	Beacon (callsign, status, position)

3.2 Telecommand APIDs (0x100 - 0x1FF)

APID	Hex	Description
256	0x100	General telecommand (routed by opcode)
257	0x101	Time-tagged command (delayed execution)
258	0x102	Macro command (sequence of commands)

4. Command Table

4.1 OBC Commands (Opcode 0x01xx)

Opcode	Name	Parameters	Response	Auth Level
0x0100	CMD_NOP	None	ACK	Basic
0x0101	CMD_REBOOT	confirm: u8 (0xAA)	ACK, then reboot	Critical
0x0102	CMD_SET_MODE	mode: u8	ACK + new mode	Elevated
0x0103	CMD_GET_STATUS	None	Status TM packet	Basic
0x0104	CMD_SET_TIME	epoch_ms: u64	ACK + time sync TM	Elevated
0x0105	CMD_MEM_DUMP	addr: u32, len: u16	Memory dump TM	Critical
0x0106	CMD_MEM_WRITE	addr: u32, data: [u8]	ACK	Critical
0x0107	CMD_SW_UPDATE	segment: u16, data: [u8]	ACK per segment	Critical
0x0108	CMD_CLEAR_ERRORS	None	ACK + error count = 0	Elevated
0x0109	CMD_SET_TM_RATE	apid: u16, rate_hz: f32	ACK	Elevated

4.2 EPS Commands (Opcode 0x02xx)

Opcode	Name	Parameters	Response	Auth Level
0x0200	CMD_EPS_STATUS	None	EPS TM packet	Basic
0x0201	CMD_LOAD_ON	channel: u8 (0-7)	ACK + channel state	Elevated
0x0202	CMD_LOAD_OFF	channel: u8 (0-7)	ACK + channel state	Elevated
0x0203	CMD_SET_HEATER	mode: u8, setpoint_C: i8	ACK	Elevated
0x0204	CMD_BATTERY_TEST	None	Battery TM sequence	Elevated
0x0205	CMD_SET_POWER_MODE	mode: u8	ACK + mode change	Critical
0x0206	CMD_EPS_EMERGENCY	None	ACK, shed loads	Critical

4.3 COMM Commands (Opcode 0x03xx)

Opcode	Name	Parameters	Response	Auth Level
0x0300	CMD_COMM_STATUS	None	COMM TM packet	Basic
0x0301	CMD_SET_TX_POWER	power_dbm: u8	ACK	Elevated
0x0302	CMD_SET_DATA_RATE	rate_bps: u16	ACK	Elevated
0x0303	CMD_BEACON_ON	interval_ms: u32 (min 10000)	ACK	Elevated
0x0304	CMD_BEACON_OFF	None	ACK	Elevated
0x0305	CMD_S_BAND_ON	None	ACK	Elevated
0x0306	CMD_S_BAND_OFF	None	ACK	Elevated

4.4 ADCS Commands (Opcode 0x04xx)

Opcode	Name	Parameters	Response	Auth Level
0x0400	CMD_ADCS_STATUS	None	ADCS TM packet	Basic
0x0401	CMD_SET_ADCS_MODE	mode: u8 (0-4)	ACK + mode	Elevated
0x0402	CMD_SET_TARGET	lat: f64, lon: f64, alt: f64	ACK	Elevated
0x0403	CMD_DETUMBLE	None	ACK + start detumble	Elevated
0x0404	CMD_MTQ_TEST	axis: u8, duty: f32	ACK + MTQ state	Critical
0x0405	CMD_RW_TEST	axis: u8, speed_rpm: f32	ACK + RW state	Critical
0x0406	CMD_CALIBRATE_MAG	None	ACK + cal data TM	Elevated
0x0407	CMD_DESATURATE	None	ACK + start desaturation	Elevated

4.5 Payload & Camera Commands (Opcode 0x06xx - 0x07xx)

Opcode	Name	Parameters	Response	Auth Level
0x0600	CMD_CAPTURE_IMAGE	resolution: u8, compression: u8	ACK + image ID	Elevated
0x0601	CMD_DOWNLOAD_IMAGE	image_id: u16, offset: u32	Image data TM sequence	Basic
0x0602	CMD_LIST_IMAGES	None	Image catalog TM	Basic
0x0603	CMD_DELETE_IMAGE	image_id: u16	ACK	Elevated
0x0604	CMD_LIST_FILES	dir: u8	File catalog TM	Basic
0x0605	CMD_DELETE_FILE	file_id: u32	ACK	Elevated
0x0700	CMD_RAD_START	None	ACK	Basic
0x0701	CMD_RAD_STOP	None	ACK	Basic
0x0702	CMD_RAD_DOWNLOAD	start_time: u64, end_time: u64	Radiation data TM	Basic
0x0703	CMD_PAYLOAD_CMD	cmd_data: u8[64]	ACK + payload response	Elevated

5. Response Format and Error Codes

5.1 Acknowledgment Packet

Every command receives an ACK/NAK response:

ACK Packet (APID 0x100, Subtype 0x00):
  +--------+--------+--------+--------+--------+--------+
  | Opcode (2B)     | Status | ErrCode| SeqNum (2B)     |
  +--------+--------+--------+--------+--------+--------+
  
Status byte:
  0x00 = ACK_OK       (command accepted and executed)
  0x01 = ACK_QUEUED   (command accepted, execution deferred)
  0x02 = ACK_PROGRESS (command in progress, partial result)
  0xFF = NAK          (command rejected, see error code)

5.2 Error Codes

Code	Name	Description
0x00	ERR_NONE	No error
0x01	ERR_UNKNOWN_CMD	Unrecognized opcode
0x02	ERR_INVALID_PARAM	Parameter out of range or invalid type
0x03	ERR_AUTH_FAILED	HMAC-SHA256 verification failed
0x04	ERR_SEQ_INVALID	Sequence number out of acceptance window
0x05	ERR_TIME_STALE	Timestamp outside +/- 60 s freshness window
0x06	ERR_BUSY	Subsystem busy executing prior command
0x07	ERR_NOT_READY	Subsystem not initialized or in wrong mode
0x08	ERR_DISABLED	Target subsystem or channel disabled by EPS
0x09	ERR_HARDWARE	Hardware fault detected during execution
0x0A	ERR_CRC_FAIL	Packet CRC-16/CCITT mismatch
0x0B	ERR_OVERFLOW	Buffer, queue, or storage capacity exceeded
0x0C	ERR_TIMEOUT	Operation timed out (I/O or processing)
0x0D	ERR_PERMISSION	Auth level insufficient for requested command
0x0E	ERR_SAFE_MODE	Command rejected because satellite is in safe mode
0x0F	ERR_REPLAY	Replay attack detected (sequence reuse)
0xFF	ERR_UNKNOWN	Unclassified internal error

5.3 Severity Levels (for Error Report TM, APID 0x010)

Level	Value	Action
DEBUG	0	Log to FRAM ring buffer only
INFO	1	Log + include in next housekeeping packet
WARNING	2	Log + immediate TM if link active
ERROR	3	Log + trigger anomaly counter, attempt recovery
CRITICAL	4	Log + enter safe mode, beacon-only operation

6. Command Authentication (HMAC-SHA256)

6.1 Authentication Fields

Command packet with authentication:

+--CCSDS Header--+--Cmd Payload--+---Auth Block (44 bytes)--------+--CRC--+
|  (16 bytes)     |  (variable)   | SeqNum(4B) | Timestamp(8B)    |       |
|                 |               | HMAC-SHA256 (32B)              |       |
+-----------------+---------------+--------------------------------+-------+

HMAC input = CCSDS_Header || Cmd_Payload || SeqNum || Timestamp
HMAC key   = 256-bit pre-shared key (loaded during integration, stored in FRAM)

6.2 Authentication Levels

Level	Commands	Requirements
Basic	Status queries, data download	No HMAC required (read-only, no state change)
Elevated	Mode changes, config updates	HMAC + valid sequence number + timestamp
Critical	Reboot, memory write, SW update	HMAC + sequence + timestamp + confirm byte (0xAA)

6.3 Replay Protection

Sequence window: Accept if seq_num > last_accepted - 16 AND seq_num != previously used
Timestamp freshness: TC_timestamp - onboard_UTC < 60 seconds
Time sync relaxation: CMD_SET_TIME accepts +/- 300 s window (to allow clock correction)
Key management: Single pre-shared 256-bit key, no in-orbit key rotation
Key storage: Duplicated in two FRAM chips (FM25V20A) for redundancy

7. AX.25 Frame Format (UHF Link Layer)

7.1 AX.25 UI Frame Structure

+-------+----------+--------+--------+-------+----------+-------+
| Flag  | Address  | Ctrl   | PID    | Info  | FCS      | Flag  |
| 0x7E  | (14B)    | (1B)   | (1B)   | (var) | (2B)     | 0x7E  |
+-------+----------+--------+--------+-------+----------+-------+
  1 B     14 B       1 B      1 B    <=256 B    2 B       1 B

Total frame overhead: 20 bytes (excluding info field)

Field	Value	Size	Description
Flag	0x7E	1 B	HDLC frame delimiter, bit-stuffed
Dest Address	e.g. GS callsign UN7xxx-0	7 B	6 char callsign + SSID byte
Source Address	e.g. UN8SAT-1	7 B	Satellite callsign + SSID
Control	0x03	1 B	UI frame (unnumbered information)
PID	0xF0	1 B	No layer 3 protocol
Information	CCSDS Space Packet	<=256 B	Full CCSDS packet as payload
FCS	CRC-16/AX.25	2 B	Polynomial: x^16 + x^12 + x^5 + 1
Flag	0x7E	1 B	End-of-frame delimiter

Note: AX.25 is used only on the UHF link. The S-band link uses CCSDS Attached Sync Marker (ASM) framing directly over the synchronous serial stream.

7.2 Beacon Frame Content (Periodic, 30 s interval)

AX.25 UI Frame:
  Destination: CQ-0 (broadcast)
  Source: UN8SAT-1

  Info field — Beacon Data Layout (48 bytes):
  +--------+------+------+------+------+------+------+------+
  | Offset | 0-3  | 4    | 5-6  | 7-8  | 9    | 10-11| 12-13|
  | Field  | Upt  | Mode | Vbat | Ibat | SOC  | Psol | Tcpu |
  | Type   | u32  | u8   | u16  | i16  | u8   | u16  | i16  |
  | Unit   | sec  | enum | mV   | mA   | %    | mW   | 0.1C |
  +--------+------+------+------+------+------+------+------+
  | Offset | 14-15| 16-19| 20-23| 24-27| 28-31| 32-33| 34-37|
  | Field  | Tboard| Qw  | Qx   | Qy   | Qz   | Omega| Lat  |
  | Type   | i16  | f32  | f32  | f32  | f32  | u16  | i32  |
  | Unit   | 0.1C | --   | --   | --   | --   |.01d/s| 1e-7 |
  +--------+------+------+------+------+------+------+------+
  | Offset | 38-41| 42-43| 44   | 45   | 46-47|
  | Field  | Lon  | Alt  | Fix  | Errs | SeqCnt|
  | Type   | i32  | u16  | u8   | u8   | u16   |
  | Unit   | 1e-7 | m    | enum | count| count |
  +--------+------+------+------+------+-------+

Total beacon: 48 B data + 16 B CCSDS headers + 2 B CRC = 66 B packet
With AX.25: 66 + 20 = 86 B frame
TX duration: 86 * 8 / 9600 = 71.7 ms
Duty cycle: 71.7 ms / 30 s = 0.24%

8. Protocol State Machine

8.1 Receiver State Machine (Satellite)

                    ┌──────────┐
         ┌──────────│   IDLE   │◄──────────────────────┐
         │          └────┬─────┘                        │
         │               │ Byte received                │
         │               ▼                              │
         │          ┌──────────┐                        │
         │    ┌─────│   SYNC   │ Wait for AX.25 flag   │
         │    │     └────┬─────┘ (0x7E)                 │
         │    │ timeout   │ Flag matched                 │
         │    │ (500ms)   ▼                              │
         │    │     ┌──────────┐                        │
         │    │     │  HEADER  │ Collect address + ctrl  │
         │    │     └────┬─────┘ (16 bytes)             │
         │    │          │ Header complete               │
         │    │          ▼                              │
         │    │     ┌──────────┐                        │
         │    │     │   DATA   │ Collect info field      │
         │    │     └────┬─────┘ (until next 0x7E)      │
         │    │          │ End flag received             │
         │    │          ▼                              │
         │    │     ┌──────────┐                        │
         │    └────►│ VALIDATE │ Verify AX.25 FCS       │
         │          └──┬───┬───┘ then CCSDS CRC-16      │
         │     CRC OK  │   │ CRC FAIL                   │
         │             ▼   └────────────────────────────┘
         │   ┌────────────────┐    (increment error counter)
         │   │ AUTHENTICATE   │
         │   │ Verify HMAC if │
         │   │ auth required  │
         │   └──┬──────┬──────┘
         │ OK   │      │ FAIL → send NAK (ERR_AUTH_FAILED)
         │      ▼      └───────────────────────────────►│
         │   ┌────────────────┐                         │
         │   │ PROCESS PACKET │                         │
         │   │ Dispatch to    │                         │
         │   │ command queue  │                         │
         │   │ Send ACK       │                         │
         │   └───────┬────────┘                         │
         │           │                                  │
         └───────────┘◄─────────────────────────────────┘

8.2 UHF Communication State Machine

                     +-------------+
                     |   IDLE      |
                     | (beacon TX) |
                     +------+------+
                            |
                     GS detected (preamble + valid AX.25)
                            |
                     +------v------+
              +----->|   LINK      |<-----+
              |      | ESTABLISHED |      |
              |      +------+------+      |
              |             |             |
         No response   Valid CMD      Invalid CMD
         (timeout 30s)     |          (NAK sent)
              |      +------v------+      |
              |      |  COMMAND    |------+
              |      |  PROCESSING |
              |      +------+------+
              |             |
              |        Response ready
              |             |
              |      +------v------+
              +------+ TX          |
                     | RESPONSE   |
                     +------+------+
                            |
                     Pass ends (LOS) OR idle timeout (120s)
                            |
                     +------v------+
                     |   IDLE      |
                     | (beacon TX) |
                     +-------------+

8.3 S-band Downlink State Machine

                     +-------------+
                     |  S-BAND OFF |
                     +------+------+
                            |
                     CMD_S_BAND_ON received (via UHF)
                            |
                     +------v------+
                     |  WARM-UP    | (PA stabilization, 5 s)
                     +------+------+
                            |
                     +------v------+
                     |  SYNC TX    | (ASM pattern, 1 s)
                     +------+------+
                            |
                     +------v------+
                     |  DATA TX    | (CCSDS TM frames, continuous)
                     +------+------+
                            |
                     Buffer empty OR CMD_S_BAND_OFF OR timeout (15 min)
                            |
                     +------v------+
                     |  S-BAND OFF |
                     +-------------+

9. Timing Diagrams

9.1 Typical Command-Response Sequence

Ground Station                          Satellite
     |                                      |
     |--- AX.25[CMD_GET_STATUS] ----------->|  t=0
     |                                      |  t+5ms: Parse AX.25
     |                                      |  t+7ms: Extract CCSDS
     |                                      |  t+10ms: Verify CRC-16
     |                                      |  t+12ms: No auth (Basic)
     |                                      |  t+15ms: Execute command
     |                                      |  t+20ms: Build response
     |<-- AX.25[ACK_OK] -------------------|  t=50ms
     |                                      |
     |<-- AX.25[OBC_HK TM] ----------------|  t=100ms
     |                                      |

9.2 Authenticated Command Sequence

Ground Station                          Satellite
     |                                      |
     |--- AX.25[CMD_SET_MODE(SAFE)] ------->|  t=0
     |   (includes HMAC + seq + timestamp)   |
     |                                      |  t+5ms: Parse AX.25
     |                                      |  t+10ms: Verify CRC
     |                                      |  t+12ms: Extract HMAC
     |                                      |  t+15ms: Verify HMAC-SHA256
     |                                      |  t+18ms: Check seq > last
     |                                      |  t+20ms: Check timestamp window
     |                                      |  t+25ms: Execute mode change
     |<-- AX.25[ACK_OK] -------------------|  t=50ms
     |<-- AX.25[MODE_CHANGE TM] ------------|  t=80ms
     |                                      |

9.3 Image Downlink via S-band

Ground Station                          Satellite
     |                                      |
     |--- UHF: CMD_S_BAND_ON ------------->|  t=0
     |<-- UHF: ACK_OK ---------------------|  t=50ms
     |                                      |  t=5s: PA warm-up complete
     |                                      |  t=6s: ASM sync pattern
     |                                      |
     |--- UHF: CMD_DOWNLOAD_IMAGE --------->|  t=7s
     |   (image_id=42, offset=0)            |
     |<-- UHF: ACK_OK ---------------------|  t=7.05s
     |                                      |
     |<~~ S-band: Image chunk 0 (240B) ~~~~|  t=7.1s
     |<~~ S-band: Image chunk 1 (240B) ~~~~|  t=7.12s
     |<~~ S-band: Image chunk 2 (240B) ~~~~|  t=7.14s
     |    ...                               |
     |<~~ S-band: Image chunk N (last) ~~~~~|
     |                                      |
     |--- UHF: CMD_S_BAND_OFF ------------>|
     |<-- UHF: ACK_OK ---------------------|
     |                                      |

S-band chunk rate: 258 B * 8 / 256000 bps = 8.1 ms per chunk
3 MB image: ~12,500 chunks = ~102 s (~1.7 min)

10. Data Prioritization and Queue Management

10.1 TX Queue Priority

Priority	Packet Type	Max Latency	Queue Depth
0 (highest)	Emergency TM (ERR_CRITICAL)	Immediate	4
1	TC Acknowledgement (ACK/NAK)	< 100 ms	8
2	Beacon	30 s period	1 (overwrite)
3	Housekeeping TM (OBC, EPS)	< 5 s	16
4	GNSS/ADCS TM	< 10 s	8
5 (lowest)	Payload / Image data	Best effort	32

10.2 Store-and-Forward

When no ground station contact is available, telemetry is stored to the SD card in a circular buffer. During the next pass, stored data is downlinked in chronological order after real-time housekeeping.

SD Card Layout:
  /tlm/YYYY-MM-DD/HH-MM-SS.ccsds    (raw CCSDS packets, ~4 MB/day)
  /img/NNNN.jpg                       (camera images, 50 KB - 10 MB each)
  /rad/YYYY-MM-DD.bin                 (radiation data, ~100 KB/day)
  /err/error_log.bin                  (error log mirror from FRAM)

Storage policy: Oldest telemetry deleted first when SD > 90% full.
Images and radiation data protected until explicitly deleted by TC.

11. Data Integrity and Reliability

11.1 Error Detection Layers

Layer	Mechanism	Coverage	Standard
Physical	FEC (Conv r=1/2 or LDPC r=1/2)	Bit-level correction	CCSDS 131.0-B-3
Data Link	AX.25 FCS (CRC-16)	Frame-level	AX.25 v2.2
Network	CCSDS CRC-16/CCITT	Packet-level	CCSDS 133.0-B-2
Application	HMAC-SHA256 (commands)	Command integrity + auth	CCSDS 352.0-B-2
File Transfer	CRC-32 per image + sequence numbers	File-level completeness	Custom

11.2 Retransmission Policy

Beacon: No retransmission (periodic, best-effort)
Command ACK: GS retransmits command if no ACK within 5 seconds (max 3 retries)
TM packets: No automatic retransmit; GS can request specific data via TC
Image chunks: GS tracks received chunks, requests missing by offset using CMD_DOWNLOAD_IMAGE
Maximum retries: 3 per command from GS side, then abort and log

12. References

CCSDS 133.0-B-2: Space Packet Protocol, 2020
CCSDS 132.0-B-2: TM Space Data Link Protocol, 2015
CCSDS 131.0-B-3: TM Synchronization and Channel Coding, 2017
CCSDS 231.0-B-3: TC Synchronization and Channel Coding, 2017
CCSDS 352.0-B-2: CCSDS Cryptographic Algorithms, 2019
AX.25 Link Access Protocol for Amateur Packet Radio, v2.2, 1998
IARU Satellite Frequency Coordination Guidelines
ITU Radio Regulations, Article 25 (Amateur Service)

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