Every field of the GeoNetworking Source Position Vector this firmware sent
was a compile-time constant: the bench coordinates, speed 0, heading 0,
TST 0, station type passengerCar and one fixed MAC. The CAM inside
described a moving cyclist while the GN header around it described a car
parked at the bench.
SERIAL_MSG_CAM_TX_PV (0x05) puts a 24-byte prefix ahead of the CAM UPER:
MAC, station type, PAI, TST, latitude, longitude, speed and heading, all
values the phone already has when it builds the CAM and none of which
this chip can know. geonet_wrap_shb now takes them as a gn_lpv_t, and
tx_radio_task hands the same MAC to dot11p_build_frame, so the 802.11
source address and the GN_ADDR MID stay one address across a pseudonym
change. Speed is clamped rather than masked, since an overflowing 15-bit
value flips its sign bit and reads as travelling backwards.
This reverses the Phase 03 decision that the firmware owns the
pseudonym. A pseudonym only protects anyone if the MAC, the GN_ADDR and
the CAM's stationID change together, and the phone owns the stationID.
The heartbeat gains a capability byte (payload[7], bit0 = CAM_TX_PV),
appended so an app reading the first 7 bytes is unaffected. The app sends
0x05 only once it sees that bit, so app and firmware can be updated in
either order. CAM_TX (0x01) is still handled and falls back to the bench
values, with the station type corrected to cyclist to match the CAM.
Verified on air from the COM10 test board, decoded independently by the
CiT One's gnHeader: 24 of 24 CAM_TX_PV frames matched the sent position
vector field by field, and so did the CAM station ID. The legacy path
delivered 23 of 24 frames with no field mismatches. Flashed on the COM3
OBU and its boot log is clean.
Also corrects the SERIAL_LINK_MAX_PAYLOAD comment, which still named the
400-byte receive capture buffer as the ceiling on the RX path. That
buffer is 800 bytes now, so the serial link is the ceiling, and larger
payloads are dropped and counted there.
Ashin Walpola
3eeccfb268