Radio_Software

The radio software is designed to be the same regardless of hardware variant (i.e. the base code is the same, but uses different variant files) The code has been modified to support three hardware configurations: The Silversat Radio Board, an Adafruit Metro w/ custom AX5043 hat (there are 2 in existence, but still in use), and the Silversat Ground Radio Module. The variant files for each board also set certain config variables that are specific to the hardware (e.g. single ended versus differential output).
By selecting the correct board from the board configuation menu, all of the correct settings are applied.

To make this work, the files in the variants subfolder of this repository need to be copied to the variants subfolder for the Adafruit SAMD21 boards. Make sure to copy over 'boards.txt' as well. It goes into the parent directory of the variant files.

This code includes a port of the AX library available here: https://github.com/richardeoin/ax/blob/master/sw/ On the Adafruit Metro version, the AX5043 is connected via SPI on the 6 pin header. All others are wired directly.

The main satellite/ground station code base is in the 'silversat_radio' folder (was e2ecommands_metro). The radio code now supports IL2P tailored to the specifics of the Silversat mission. All packets now contain an IL2P header (scrambled, with RS(255,253) added), an IL2P payload (scramled, with RS(255,239) added), and a hamming encoded CRC. Detailed implementation notes are here.

The Packetbuffer_rateconverter code uses two back to back RPi/Metro combination boards. The RPi to Metro connection is serial running at 115kbaud. The two systems are cross-connected via serial running at 9600 baud. The second serial port runs in half-duplex mode (only one side transmitting at a time). This was used in early testing to determine what rate we could expect of a half duplex link.

The gnuradio folder contains the flow graph for an FSK demod designed for the current modulation (GFSK) in the AX test file. Basic header detection has been implemented. Captured packets are written to a file. This requires an Analog Devices PLUTO SDR to operate, but should also run on an RTL-SDR. This was designed using gnuradio 3.10.
A post processor ('main.py') is included that takes the captured data, identifies packets starts based on the IL2P

Variants have been added that support the Adafruit Metro prototype, Silversat Radio board and Silversat Ground Radio board. Signals are defined by their schematic net name (or something close) to make code easier to read.

Implementation of radio code

The basic flow of the radio code is similar to the following:

Code description

1. The loop runs continuously. It is non-blocking (to the maximum extent possible) in the FULL_RX state. On GROUND, Serial1 is connected to the Proxy Computer (PXC), Serial0 to the Base Station Controller (BSC). On SATELLITE, Serial1 is connected to PAYLOAD, Serial0 to AVIONICS. Note that you can only read from the FIFO when it is in the FULL_RX state, and you can only write to the FIFO when it is the FULL_TX state. The data serial port runs at 19200, command at 57600. The debug port (Serial) also runs at 57600.
2. Radio packets are HDLC formatted with a 16 bit CRC. That means that there is no length byte and that there are 2 CRC bytes appended to the packet. The 2 CRC bytes are checked by the AX5043, packets that fail the CRC check are silently dropped. The RADIO does not do anything with the CRC bytes and drops them from the received packet.
3. The code consists of four main parts: A Interface Handler, a Packet Processor, a Receive Handler and a Transmit Handler.
4. The interface handler accepts incoming bytes from serial ports and immediately writes them to a circular buffer. Incoming packets are natively KISS encoded and this is used to delimit packets in the buffers. The KISS address nibble on data arriving on Serial0 is decoded to determine local (configuration) commands versus remote commands.
5. Once per loop each buffer is processed to determine if a valid packet (delimited by 0xC0's) has arrived. This function returns the length of the received packet.
6. If the length is non-zero, the data is passed to the Packet Processor. In order to be ready to transmit once the state is changed (FULLTX or Transmit = True), one packet is decoded and queued for transmit (in the transmit buffer). While one packet is being transmitted, another is prepared. Note that ax_transmit_packet() is blocking until the last chunk is written. If writing less than max chunk (240 bytes) it doesn't block.
7. AVIONICS sends commands in KISS format, but limited to ASCII characters. Local commands are processed (and executed) immediately when a full command packet is available. Cmd/data is determined from the first byte of the received packet. All received packets are then KISS encoded and forwarded over the appropriate port upon receipt. Note that calling ax_rx_packet empties the FIFO, allowing another packet to be received while handling the current one. See “Silversat Local Commands” for more information on commands. Most command code is currently only a placeholder.
8. Remote commands (packets on Serial0 with address byte 0xAA) that are destined for the other end of the link) are transferred to the data buffer (Serial1 buffer). Typically, this happens when the data port (Serial1) is idle, however in the case of a HALT command, it might be in the middle of a data transfer. No attempt is currently made to synchronize the transfer to the data packet boundaries, but since the data packet can already be considered corrupt (otherwise, why send the halt?), it isn't an issue.
9. The FULL_TX state can only be entered when the receiver is idle (ax_rx_packet returns 0), the clear channel criteria are met, and if there is something to send (txbuffer is not empty).
10. A CCA (Clear Channel Assessment) must be performed. The clear channel assessment consists of two factors. First enough time since the last packet was received (TXDELAY). The timer is reset when a valid packet is received. The second factor is that the average (right now a simple average over 2 samples) RSSI is below a threshold (CLEARTHRESH).
11. Prior to being written to the AX5043 FIFO, KISS encoding is removed (it has to be removed in order to get fixed max length packets, otherwise packet length is data dependent).
12. Transmit completion is verified by polling the RADIOSTATE register. The loop continues to queue transmit packets while waiting for the current transmission to complete. Packets queued for transmit are sent until the buffer is empty.