GPS Data Injection Target (INJECT_TO)

Description

This parameter controls the routing of "GPS_INJECT_DATA" MAVLink packets. These packets are typically used to send RTK (Real-Time Kinematic) corrections from a Ground Control Station (GCS) to the autopilot, enabling centimeter-level positioning accuracy.

If your vehicle has multiple GPS sensors, you can use this parameter to decide if the corrections should go to a specific GPS instance (e.g., just the primary one) or be "broadcast" to all connected GPS modules simultaneously.

The Mathematics

The value maps to the internal GPS instance index ($i$):

• 0: GPS 1 (First instance).
• 1: GPS 2 (Second instance).
• 127: All GPS instances (Broadcast mode).

The Engineer's View

In AP_GPS::handle_msg() (libraries/AP_GPS/AP_GPS.cpp):

1. The system receives raw serial data from the MAVLink GPS_INJECT_DATA or GPS2_RTK packets.
2. It checks the _inject_to parameter.
3. If set to 127 (GPS_RTK_INJECT_TO_ALL), it iterates through every detected GPS backend and calls inject_data().
4. If set to a specific index ($0$ or $1$), it sends the data only to that specific backend.
5. This ensures that if you have two different GPS models (e.g., a u-blox and a Septentrio) that require different correction formats, you can target the corrections appropriately (though most users simply broadcast to all).

Tuning & Behavior

• Default Value: 127 (All).
• Recommended: Leave at 127 unless you have a specific dual-GPS setup where the sensors are of different types and sharing corrections causes errors.
• RTK Fix: If your GCS shows "RTK Fixed" or "RTK Float" but only on one GPS, check this parameter to ensure it is set to include the second GPS instance if needed.