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102 lines
4.8 KiB
Markdown
102 lines
4.8 KiB
Markdown
# RTK GPS (PX4 集成)
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[Real Time Kinematic](https://en.wikipedia.org/wiki/Real_Time_Kinematic) (RTK) provides centimeter-level GPS accuracy.
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这一章节将介绍 RTK 是如何集成到 PX4 中的。
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:::tip
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Instructions for _using_ RTK GNSS are provided in [Hardware > RTK GPS](../gps_compass/rtk_gps.md).
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:::
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## 综述
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RTK是使用导航信号的载波相位来进行测距的，而不是使用导航信号所搭载的信息。
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它依靠一个单一的参考基站站实时校正，这种校正可以与多个流动站一起工作。
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PX4 配置 RTK 需要两个 RTK GPS 模块和一个数传。
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The fixed-position ground-based GPS unit is called the _Base_ and the in-air unit is called the _Rover_.
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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](https://mavlink.io/en/messages/common.html#GPS_RTCM_DATA) message).
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在自驾仪上，MAVLink消息包被解包得到RTCM的修正信息，并把这些信息发送给移动站，移动站结合修正信息最终解算得到 RTK 解。
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The datalink should typically be able to handle an uplink rate of 300 bytes per second (see the [Uplink Datarate](#uplink-datarate) section below for more information).
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## 支持的 RTK GPS 模块
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The list of devices that we have tested can be found [in the user guide](../gps_compass/rtk_gps.md#supported-devices).
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:::info
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Most devices come with two variants, a base and a rover.
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确保选择正确的变体。
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:::
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## 自动配置
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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).
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As soon as the autopilot receives `GPS_RTCM_DATA` MAVLink messages, it automatically forwards the RTCM data to the attached GPS module over existing data channels (a dedicated channel for correction data is not required).
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:::info
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The u-blox U-Center RTK module configuration tool is not needed/used!
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:::
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:::info
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Both _QGroundControl_ and the autopilot firmware share the same [PX4 GPS driver stack](https://github.com/PX4/PX4-GPSDrivers).
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实际上，这意味着只需要将新协议和/或消息添加到一个地方。
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:::
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### RTCM 报文
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QGroundControl 配置RTK 基地站输出以下 RTCM3.2 帧, 每个帧均为 1 Hz, 除非另有说明：
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- **1005** - Station coordinates XYZ for antenna reference point (Base position), 0.2 Hz.
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- **1077** - Full GPS pseudo-ranges, carrier phases, Doppler and signal strength (high resolution).
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- **1087** - Full GLONASS pseudo-ranges, carrier phases, Doppler and signal strength (high resolution).
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- **1230** - GLONASS code-phase biases.
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- **1097** - Full Galileo pseudo-ranges, carrier phases, Doppler and signal strength (high resolution)
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- **1127** - Full BeiDou pseudo-ranges, carrier phases, Doppler and signal strength (high resolution)
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## 上行数据速率
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The raw RTCM messages from the base are packed into a MAVLink `GPS_RTCM_DATA` message and sent over the datalink.
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MAVLink 消息的最大长度是182字节。 根据RTCM的信息类型，MAVLink信息是不会填满的。
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RTCM 基础位置消息(1005)长度为 22 字节， 而其他卫星的长度则因可见卫星的数量和卫星信号的数量而异（M8P等L1单元只有一个）。
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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.
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If _MAVLink 1_ is used, a 182-byte `GPS_RTCM_DATA` message is sent for every RTCM message, irrespective of its length.
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因此，大约每秒上行需求是700多个字节。
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这可能导致低带宽半双轨遥测模块 (如3DR Telemetry Radios) 连接的饱和。
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If _MAVLink 2_ is used then any empty space in the `GPS_RTCM_DATA message` is removed.
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由此产生的上行链路需求与理论值 (约 300 字节/秒) 大致相同。
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:::tip
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PX4 automatically switches to MAVLink 2 if the GCS and telemetry modules support it.
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:::
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MAVLink 2 必须用于低带宽链接以保证 RTK 性能。 必须注意确保数传链在整个过程中使用 MAVLink 2。
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You can verify the protocol version by using the `mavlink status` command on the system console:
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```sh
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nsh> mavlink status
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instance #0:
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GCS heartbeat: 593486 us ago
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mavlink chan: #0
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type: 3DR RADIO
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rssi: 219
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remote rssi: 219
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txbuf: 94
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noise: 61
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remote noise: 58
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rx errors: 0
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fixed: 0
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flow control: ON
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rates:
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tx: 1.285 kB/s
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txerr: 0.000 kB/s
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rx: 0.021 kB/s
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rate mult: 0.366
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accepting commands: YES
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MAVLink version: 2
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transport protocol: serial (/dev/ttyS1 @57600)
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```
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