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Decoding the FSK transmissions

A project log for Rocket Receiver Teardown

Teardown of the "Beckett Rocket" fuel level sensor receiver.

david-woodhouseDavid Woodhouse 12/09/2015 at 00:050 Comments

Using RTL-SDR, we've managed to listen to the transmissions of the device and decode them in software. This is the FM-demodulated waveform of one of its transmissions:

This is a Manchester encoding where high->low indicates a 0 and low->high indicates a 1, with a frame preamble of three high half-bit periods followed by three low half-bit periods, and a frame postamble which just maintains the level of the last half-bit (low for a 0, high for a 1) for a further two half-bit periods to make three. The "half-bit" period is about 1ms. The data in this particular packet are as follows:

Or 0x28 0xc6 0xd6 0x1d 0x81 0x40 0x51 0x83. The first byte is always 0x28 and might be part of the framing or packet identification. The final byte is a 1Wire CRC. We'll get to the rest later.

At this point it should be possible to configure the RFM01 or similar modules to receive the signal. Support for decoding this framing has also been added to rtl_433 which is probably easier to experiment with.

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