Configuration

config.yml

The FSW reads its configuration from config.yml in the project root. This file controls radio parameters and is monitored for live changes via the watchdog library.

Structure

Tempest:
  Comms:
    uplink:
      radio: rfm69           # rfm69 or rfm95
      freq: 915              # Frequency in MHz
      encryption: false
      enc_key: ""

    downlink:
      radio: rfm69           # rfm69 or rfm95
      freq: 915              # Frequency in MHz
      chunk_sizes:
        rfm95: 240            # Max payload bytes for RFM95
        rfm69: 50             # Max payload bytes for RFM69
      encryption: false
      enc_key: ""

    crosslink:
      radio: "none"           # Future feature
      frequency: 921
      encryption: false
      enc_key: ""

Configuration Parameters

Uplink (Ground to Satellite)

Parameter	Type	Description
radio	string	Radio module type: rfm69 or rfm95
freq	number	Operating frequency in MHz
encryption	boolean	Enable AES encryption
enc_key	string	16-byte AES encryption key

Downlink (Satellite to Ground)

Parameter	Type	Description
radio	string	Radio module type: rfm69 or rfm95
freq	number	Operating frequency in MHz
chunk_sizes.rfm95	number	Max payload size for RFM95 (default 240)
chunk_sizes.rfm69	number	Max payload size for RFM69 (default 50)
encryption	boolean	Enable AES encryption
enc_key	string	16-byte AES encryption key

Live Reload

The FSW uses watchdog.observers.Observer with a FileSystemEventHandler to detect changes to config.yml. When modified:

1. The watchdog triggers a reload callback
2. New configuration is parsed from YAML
3. Radio parameters are updated without restarting the service

Backup Restoration

On startup, config.yml.bak is copied over config.yml to ensure a known-good configuration. This prevents a corrupted config from bricking the satellite after a reboot.

Startup: config.yml.bak → config.yml → parse → initialize radios

Radio Hardware Configuration

SPI Bus Assignments

The FSW uses two SPI buses for dual-radio operation:

Primary SPI (Hardware):

Signal	GPIO
SCLK	21
MOSI	20
MISO	19
CS	5
RESET	17

Secondary SPI (Bitbanged):

Signal	GPIO
SCLK	21
MOSI	20
MISO	19
CS	6
RESET	26

Radio Sharing

If both uplink and downlink are configured to use the same radio type (e.g., both rfm69), they share a single radio instance. This means communication is half-duplex — the satellite cannot transmit and receive simultaneously.

ISM Band Frequency Ranges

Band	Min (MHz)	Max (MHz)	Region
433	433.05	434.79	Global
868	863.0	870.0	EU
915	902.0	928.0	US

Chunk Size Limits

These limits account for the radio FIFO size and the Adafruit library header:

Radio	FIFO Size	Library Header	Usable Payload	Config Default
RFM69	66 bytes	4 bytes	62 bytes	50 bytes
RFM95	252 bytes	4 bytes	248 bytes	240 bytes

The \n delimiter appended by radio.py consumes 1 additional byte from the usable payload.

Node Addressing

Radio node addresses identify the sender and destination:

• Default: 0xFF (255) broadcast
• FSW Node ID: Derived from hostname: int(hostname[-2:]) * 10
• Ground Station Node: Configurable via the ground station interface