Image Transfer

Overview

Image transfer over radio is the most complex operation in the TEMPEST system. Images are captured by the Pi Zero camera, compressed, encoded, chunked, and transmitted packet-by-packet. The ground station reassembles, decompresses, and displays the image.

Transfer Pipeline

┌─────────────┐    ┌──────────┐    ┌──────────┐    ┌──────────┐    ┌──────────┐
│ rpicam-still│───►│   gzip   │───►│  base64  │───►│  chunk   │───►│  radio   │
│  (JPEG)     │    │ compress │    │  encode  │    │ + metadata│   │   TX     │
└─────────────┘    └──────────┘    └──────────┘    └──────────┘    └──────────┘

                            SATELLITE (Pi Zero)
═══════════════════════════════════════════════════════════════════════════════
                            GROUND STATION (Browser)

┌──────────┐    ┌──────────┐    ┌──────────┐    ┌──────────┐    ┌──────────┐
│  radio   │───►│  parse   │───►│ reassemble│──►│  base64  │───►│  gunzip  │
│   RX     │    │ metadata │    │  chunks  │    │  decode  │    │ decompress│
└──────────┘    └──────────┘    └──────────┘    └──────────┘    └──────────┘
                                                                     │
                                                              ┌──────▼──────┐
                                                              │  Display    │
                                                              │  JPEG       │
                                                              └─────────────┘

Step 1: Capture (Satellite)

TAKE_PHOTO

The camera subsystem executes:

rpicam-still -q 40 -o capture/{count}.jpg

Then gzip compresses the JPEG. The PHOT response returns the filename.

Step 2: Initiate Transfer (Ground)

SEND_IMAGE 5.jpg.gz

The FSW loads the gzip file and processes it into chunks.

Step 3: Chunking (Satellite)

The file is split into chunks sized for the active radio:

Radio	Raw Chunk Size	Base64 Size	+ Metadata	Total
RFM69	33 bytes	44 chars	+ 8 digits	~52 bytes
RFM95	168 bytes	224 chars	+ 8 digits	~232 bytes

Each packet is formatted as:

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

Example (RFM69, 100 total chunks, chunk #7):

SENDaGVsbG8gd29ybGQhIFRoaXMgaXMgYSB0ZXN0IGNodW5r01000007
    ├─── 44 chars base64 ───┤├─total─┤├─current─┤

A .chunked file is also written to disk for retransmission support.

Step 4: Paced Transmission (Satellite)

The FSW transmits packets with a configurable delay to avoid overwhelming the receiver:

• Delay: 0.25 seconds between packets (DEFAULT_TX_DELAY)
• Progress: Logged every 50 packets
• End marker: XFRC packet with total packet count

SEND packet 0 → 0.25s → SEND packet 1 → 0.25s → ... → SEND packet N → XFRC

Step 5: Reception (Ground Station)

The browser processes each incoming SEND packet:

1. Extract the last 8 characters as metadata
2. Parse total_chunks (first 4 digits) and current_chunk (last 4 digits)
3. Store the base64 data in a packetBuffer Map, keyed by chunk number
4. Track received chunks in a Set

// Metadata extraction
const metaStr = textData.substring(textData.length - 8);
const totalChunks = parseInt(metaStr.substring(0, 4));
const currentChunk = parseInt(metaStr.substring(4, 8));
const b64Data = textData.substring(0, textData.length - 8);

Step 6: Completion Check

When the XFRC packet arrives:

1. Compare total expected packets against received count
2. If all received: trigger image reconstruction
3. If missing: list missing packet numbers and suggest retransmit command

Image 5.jpg.gz: 95/100 received (95.0%), 5 missing
Missing packets: 12, 34, 56, 78, 99
Use: RETRANSMIT 5.jpg.gz.chunked 12 34 56 78 99

Step 7: Reconstruction (Ground Station)

When all chunks are received (or manually triggered):

1. Sort chunks by number (0 to N)
2. Concatenate base64 data (remove padding == from all but the last chunk)
3. Base64 decode to binary (atob())
4. Verify gzip header (0x1F 0x8B)
5. Decompress using DecompressionStream('gzip')
6. Create download links for both the raw .jpg.gz and decompressed .jpg

Retransmission

Automatic Detection

A 30-second timeout checker runs in the background. If no new packets arrive for 30 seconds during an active transfer, it lists missing packets and suggests a retransmit command.

Manual Retransmit

Use the retransmit input field in the Image Management section:

5.jpg.gz.chunked 12,34,56,78,99

Or send directly as a command:

RETRANSMIT 5.jpg.gz.chunked 12 34 56 78 99

Retransmit Batching

Large retransmit requests are split into batches to fit within the command size limit (60 bytes). Batches are sent with a 2-second delay between them. Maximum 3 retransmit attempts per image.

Active Image Management

• Active Images: Shows all in-progress image receptions with packet count and completion percentage
• Clear Buffers: Discards all image reception state

Timing Considerations

Parameter	Value	Notes
TX delay	0.25s	Between consecutive packets
Timeout	30s	Before suggesting retransmit
Retransmit batch delay	2s	Between retransmit batches
Max retransmit attempts	3	Per image

For a 100-packet image on RFM69: ~25 seconds transfer time (100 × 0.25s).