Add support for Rockblock 9704 satellite modem

[stephendade]

This PR adds support for the Rockblock 9704 satellite modem (https://www.groundcontrol.com/product/rockblock-9704/), using the MAVLink High Latency protocol. It sends a HIGH_LATENCY2 packet once per 30 seconds.

The software required at the GCS to process messages is at stephendade/rockblock2mav#5

I also:

• Added Lua bindings for CRC16 functions
• Disabled stream rate setting for MAVLink High Latency protocol

Tested in hardware with a CubeOrange+.

(Thanks to GroundControl for supplying the hardware and airtime)

[IamPete1]

It would be interesting to do some bench-marking vs a native lua implementation. I'm a little reluctant to start a president crc bindings just because there are so many people may want.

[IamPete1]

A crc implementation in lua for example:

ardupilot/libraries/AP_Scripting/examples/TraxxasVelineon.lua

Lines 15 to 40 in da01dac

	--- Apply a byte to the crc and return the result
	---@param crc integer
	---@param byte integer
	---@return integer
	local function applyCRCByte(crc, byte)
	crc = crc ~ byte
	for _ = 1, 8 do
	if (crc & 0x80) ~= 0 then
	crc = ((crc << 1) ~ 0xE7) & 0xFF
	else
	crc = (crc << 1) & 0xFF
	end
	end
	return crc
	end
	--- Apply a table of bytes to the crc and return the result
	---@param crc integer
	---@param data integer[]
	---@return integer
	local function applyCRC(crc, data)
	for i = 1, #data do
	crc = applyCRCByte(crc, data[i])
	end
	return crc
	end

[stephendade]

It would be interesting to do some bench-marking vs a native lua implementation

@IamPete1 Here's one I put together:

-- Show various CRC calculations

--CRC16 Xmodem
local inString = "Hello, world! This is a test string. 50 bytes long"

--CRC16 via pure Lua
local function crc_xmodem_update(crc, data)
    crc = crc ~ (data << 8)
    for i = 0, 7 do
        if (crc & 0x8000) ~= 0 then
            crc = (crc << 1) ~ 0x1021
        else
            crc = crc << 1
        end
    end
    return crc & 0xFFFF
end

local function crc_xmodem_lua(data)
    local crc = 0
    for i = 1, #data do
        crc = crc_xmodem_update(crc, string.byte(data, i))
    end
    return crc
end

local function update()
    local start = micros()
    -- so 3 crc and take the average time
    for i = 1, 3 do
        crc = crc_xmodem(inString)
    end
    local stop = micros()
    local delta = (stop - start)/3

    gcs:send_text(3, "CRC16 (bindings) is: " .. string.format("0x%04X", crc))
    gcs:send_text(3, "CRC16 (bindings) calculated in " .. tostring(delta) .. " us")

    start = micros()
    for i = 1, 3 do
        crc = crc_xmodem_lua(inString)
    end
    stop = micros()
    delta = (stop - start)/3

    gcs:send_text(3, "CRC16 (pure Lua) is: " .. string.format("0x%04X", crc))
    gcs:send_text(3, "CRC16 (pure Lua) calculated in " .. tostring(delta) .. " us")

end

-- wrapper around update()
function protected_wrapper()
    local success, err = pcall(update)
    if not success then
        gcs:send_text(MAV_SEVERITY.ERROR, "CRC: Internal Error: " .. err)
        -- when we fault we run the update function again after 1s, slowing it
        -- down a bit so we don't flood the console with errors
        return protected_wrapper, 5000
    end
    return protected_wrapper, 5000
end

-- start running update loop
return protected_wrapper()

Calculating the CRC16 of a 50 byte string has the following execution time on a CubeOrangePlus:

• 15-20usec using the Lua bindings
• 900-1400usec using the pure Lua implementation.

So the Lua bindings are ~50x faster