Co-authored-by: Andrew Tridgell <andrew@tridgell.net>
uses the elements out of the simulated GPS data "d", rather than going to the parameters in teh simulated GPS "backends".
- checksum calculation was including header parts in CRC twice
- need to send DOP message to make EKF happy
- need to supply own number of satellites
- must pad packets to a multiple of 4 bytes