RTK GPS (PX4 통합)

Real Time Kinematic (RTK) provides centimeter-level GPS accuracy. RTK와 PX4 통합 방법을 설명합니다.

TIP

Instructions for using RTK GNSS are provided in Hardware > RTK GPS.

개요

RTK는 신호 보다는 신호의 반송파 위상을 사용합니다. 여러 모바일 스테이션에서 실시간 수정을 제공하기 위하여 단일 참조 스테이션에 의존합니다.

PX4로 RTK를 설정에는 2개의 RTK GPS 모듈과 데이터 링크가 필요합니다. The fixed-position ground-based GPS unit is called the Base and the in-air unit is called the Rover. The Base unit connects to QGroundControl (via USB) and uses the datalink to stream RTCM corrections to the vehicle (using the MAVLink GPS_RTCM_DATA message). On the autopilot, GPS_RTCM_DATA packets are reassembled according to the MAVLink fragment and sequence fields before the RTCM byte stream is forwarded to the Rover unit, where it is processed to get the RTK solution.

The datalink should typically be able to handle an uplink rate of 300 bytes per second (see the Uplink Datarate section below for more information).

지원 RTK GPS 모듈

The list of devices that we have tested can be found in the user guide.

INFO

Most devices come with two variants, a base and a rover. 올바른 변형을 선택하여야 합니다.

자동 설정

The PX4 GPS stack automatically sets up the GPS modules to send and receive the correct messages over the UART or USB, depending on where the module is connected (to QGroundControl or the autopilot).

As soon as the autopilot receives GPS_RTCM_DATA MAVLink messages, it reassembles fragmented packets when needed and then forwards the RTCM data to the attached GPS module over existing data channels (a dedicated channel for correction data is not required).

INFO

The u-blox U-Center RTK module configuration tool is not needed/used!

INFO

Both QGroundControl and the autopilot firmware share the same PX4 GPS driver stack. 실제로, 새 프로토콜 또는 메시지 지원시 한쪽에만 추가하면 됩니다.

GPS_RTCM_DATA handling

If you are sending RTCM corrections to PX4 yourself, follow the MAVLink GPS_RTCM_DATA definition:

• Each MAVLink packet carries up to 180 bytes of RTCM data.
• If the RTCM payload exceeds 180 bytes, split it across up to 4 packets using the Fragment ID and Sequence ID (encoded in GPS_RTCM_DATA.flags). Every packet except the last one must be filled to its maximum 180-byte capacity; only the final packet may be partially filled.
• PX4 reassembles fragmented packets according to the MAVLink rules and supports out-of-order delivery for one in-progress fragmented message at a time.
• A fragmented message is considered complete when either 4 fragments with the same Sequence ID have been received, or when you receive a partial fragment and you have already recieved all the fully-packed fragments that precede it (by Fragment ID) in the current sequence.
• If the RTCM payload length is an exact multiple of 180 bytes and uses fewer than 4 fragments, the sender must still send a final zero-length fragment to mark completion. A 720-byte payload (all 4 fragments full) is complete after the last fragment is received.
• As a compatibility fallback for older QGroundControl builds that omit that final zero-length fragment, PX4 also flushes a buffered RTCM message to the GNSS when a GPS_RTCM_DATA message with a different Sequence ID arrives, but only if the buffered fragments are a gap-free run of full 180-byte fragments starting at fragment 0.

Current limitations:

• PX4 keeps only one in-progress fragmented GPS_RTCM_DATA message at a time. A packet with a different sequence_id starts a new buffer.
• Stale partial state is dropped after 1 second if the rest of the fragments do not arrive.
• The legacy exact-multiple compatibility fallback only works if another sequence_id arrives before that 1 second timeout. Otherwise the buffered partial message is dropped.

RTCM 메시지

명시되어 있지 않은 경우에는 QGroundControl은 RTCM3.2 프레임을 1Hz로 출력하도록 RTK 베이스 스테이션을 구성합니다.

• 1005 - Station coordinates XYZ for antenna reference point (Base position), 0.2 Hz.
• 1077 - Full GPS pseudo-ranges, carrier phases, Doppler and signal strength (high resolution).
• 1087 - Full GLONASS pseudo-ranges, carrier phases, Doppler and signal strength (high resolution).
• 1230 - GLONASS code-phase biases.
• 1097 - Full Galileo pseudo-ranges, carrier phases, Doppler and signal strength (high resolution)
• 1127 - Full BeiDou pseudo-ranges, carrier phases, Doppler and signal strength (high resolution)

업링크 데이터 속도

The raw RTCM messages from the base are packed into a MAVLink GPS_RTCM_DATA message and sent over the datalink. MAVLink 메시지의 최대 길이는 182바이트입니다. RTCM 메시지에 따라 MAVLink 메시지는 채워지지 않을 수도 있습니다.

RTCM Base Position 메시지(1005)의 길이는 22바이트이고, 나머지는 모두 가시 위성의 수와 위성의 신호 수에 따라 가변 길이입니다(M8P와 같은 L1 장치의 경우 1만). Since at a given time, the maximum number of satellites visible from any single constellation is 12, under real-world conditions, theoretically an uplink rate of 300 B/s is sufficient.

If MAVLink 1 is used, a 182-byte GPS_RTCM_DATA message is sent for every RTCM message, irrespective of its length. 결과적으로, 대략적인 업링크 요구 사항은 초당 약 700+바이트입니다. 이는 저대역폭 반이중 원격 측정 모듈(예: 3DR 원격 측정 라디오)에서 링크 포화로 이어질 수 있습니다.

If MAVLink 2 is used then any empty space in the GPS_RTCM_DATA message is removed. 결과적인 업링크 요구 사항은 이론적인 값(초당 ~300바이트)과 거의 같습니다.

TIP

PX4 automatically switches to MAVLink 2 if the GCS and telemetry modules support it.

MAVLink 2는 고성능 RTK을 위하여, 낮은 대역폭 링크를 사용합니다. 텔레메트리 체인에서 MAVLink 2를 사용하는지를 확인해야합니다. You can verify the protocol version by using the mavlink status command on the system console:

nsh> mavlink status
instance #0:
        GCS heartbeat:  593486 us ago
        mavlink chan: #0
        type:           3DR RADIO
        rssi:           219
        remote rssi:    219
        txbuf:          94
        noise:          61
        remote noise:   58
        rx errors:      0
        fixed:          0
        flow control:   ON
        rates:
        tx: 1.285 kB/s
        txerr: 0.000 kB/s
        rx: 0.021 kB/s
        rate mult: 0.366
        accepting commands: YES
        MAVLink version: 2
        transport protocol: serial (/dev/ttyS1 @57600)