General Idea:
The base station GPS will take into its coordinates from surveying and radio signals from at least 4 satellites and generate an RTCM message that it will send to the GPS module on the drone, the GPS on the drone will generate NMEA sentences with information regarding position, time and velocity depending on the type of NMEA message and communicate it with Ardupilot.
NMEA sentence parser driver Ardupilot - https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_GPS/AP_GPS_NMEA.cpp
Preliminary Test Harness (Without Wireless Radios):
Configuration of Base using GNSS viewer:
Configure RTK Mode, select RTK base mode and update to SRAM+Flash
Select Configure RTK Mode And Operational Function, select Survey Mode and set survey Length to 60 seconds and Update to SRAM+Flash
Configure Binary Measurement Data Output and RTCM Measurement Data Output.
Refer for more details about how to do this in the User Guide:
Configuration of Rover
Step 1: Using GNSS viewer:
From GNSS Viewer RTK pull-down menu, select Configure RTK Mode, select RTK rover mode, and update to SRAM+Flash
Step 2: Using Misson Planner:
Select GPS_TYPE to 5 (i.e NMEA)
Select RXD_2 Baud Rate to 115200
Output To Observe:
We should see the rover state going from Position Fix 3D or DGPS mode, to Float RTK or to Fix RTK in the GNSS viewer depending on the number of Satellites tracked.
A minimum of 6 GPS satellites could result in fix RTK solution during zero-baseline GPS simulator testing, 8 or more satellites spread across 4 quadrants of the sky is preferred in getting a faster RTK fix with reliable results.
References:
https://ardupilot.org/copter/docs/common-gps-for-yaw.html#antenna-offsets
https://www.navsparkforum.com.tw/viewtopic.php?f=23&t=127&p=348&hilit=pixhawk#p348
https://ardupilot.org/copter/docs/common-pixhawk-overview.html
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