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The RTK system requires two GPS modules:
The Base Station GPS should be mounted in a stable location on the ground.
During the survey-in period, it observes its location and averages it to acquire a very accurate estimate of its absolute location on earth. The longer the survey-in, the more accurate the location will be.
After the survey-in period, the base station GPS will broadcast GPS corrections to the Rover, which can use the corrections to calculate an accurate absolute location.
Note: For optimal performance, the base station GPS should not be moved after survey-in has begun!
The Rover GPS is mounted on the aircraft, and receives corrections from the base station to calculate very accurately its location.
Additional components are required for the system to work:
Ground station computer & software (Alta Ground Station)
Communications link to aircraft (such as FRX Pro)
PX4-based autopilot
The standard signal flow is the following:
The base station GPS sends correction data to the ground station software
The ground station software sends the correction data to the aircraft's PX4-powered autopilot
The autopilot forwards correction data to the Rover GPS
The Rover GPS calculates its accurate location, sends it back to the PX4 autopilot
For best performance, please note the following:
Install the base station on a stable mount
Do not move the base station once survey-in has begun
Place the antenna in a location with as much sky view as possible. The more sky the GPS can see, the more satellites, the faster the lock, the faster the survey-in
Survey-in will begin as soon as it is plugged into the computer and the ground station software is started
Survey-in is the process where the ground station GPS sits stationary and performs many measurements to improve its location estimate. In essence, over time, any errors will average out and the estimate error will decrease. This survey-in period is critical, since it will determine the overall accuracy of the aircraft.
During the survey-in, Alta QGC will display the mean 3D error, and you should notice it steadily decreasing. Survey-in is complete when both of the following criteria are met:
Mean 3D error is below a pre-defined threshold (Alta QGC default is 2m)
Minimum observation time has been met (Alta QGC default is 180s)
After survey-in, the base station GPS will start broadcasting packet in the "RTCM3" standard, which the aircraft ("Rover") GPS utilizes to calculate very accurate positional data. The process of the Rover establishing high-accuracy position does take a few seconds after survey-in is complete, and will be indicated by a GPS status of "RTK (fixed)."
It's critical that the base station is not moved, either during survey-in, or after. Every time you move the base station, you should restart survey-in, which can be done by pressing the button "Restart" in the "RTK" window on the main toolbar.
The RTK GPS modules have two different LED indicators: multi-color status LED and red, safety LED.
This LED is located in the corner of the modules, and can vary in color.
When installed on AltaX (or another PX4-based autopilot), the LED follows PX4 convention: https://docs.px4.io/v1.9.0/en/getting_started/led_meanings.html
On the base station, the LED lights solid white when the module is powered via USB.
This LED is located next to the module's button.
When installed on Alta X, the LED indicates arm status
Double-blinking red: aircraft is powered up, but not armed
Solid red: Aircraft is armed
On the base station, the safety LED is used to indicate bootloader mode (used for firmware updates). The LED will briefly blink when the module is first powered up via USB, and will stay solid red when the module is in bootloader mode.
When the base station is plugged into your computer, Alta QGC will display two GPS-related indicators:
Aircraft GPS: This is the same dialog as with normal GPS, and it indicates:
GPS Count: The number of satellites seen by the Alta
GPS Lock: The type of lock.
3D GPS = normal (non-RTK)
3D RTK GPS Lock (fixed) = RTK lock, best precision
Other modes:
3D RTK GPS Lock (floating) is pretty much equivalent to 3D GPS
2D GPS is equivalent to not having GPS (ie, it must be at least 3D to be of any use to the aircraft)
HDOP: Horizontal Dilution of Precision: A measure of how good the horizontal position is (lower is better)
VDOP: Vertical Dilution of Precision: A measure of how good the vertical position is (lower is better)
Course over ground: Direction of flight path. Only valid when the aircraft is moving
RTK Base Station: This icon and window indicates the status of the base station
The RTK icon color indicates status:
Red or Grey: Not connected, no base station GPS detected
Blue: Survey-in initializing
Yellow: Survey-in active
White: Survey-in complete, base station is sending RTK corrections
GPS Status (inside the window when the icon is clicked) indicates the Survey-in Status
Not Connected: No base station GPS detected
Survey-in in process
Survey-in complete: Base station is providing RTK corrections, aircraft can compute RTK lock
Accuracy: Current accuracy of position estimate. Accuracy number should decrease during survey-in, and will remain the same after survey in is complete
Satellites: Number of satellites observed by the ground station. This number might not match the aircraft's number
Duration: Duration of survey-in. Number will increase until survey-in completes
Lat/Lon: Computed Global location of base station
Alt: Altitude computed by base station
Instructions for RTK with the Pilot Pro can be found on our Pilot Pro wiki through the link below
Instructions for RTK with the Astro's Herelink controller can be found on our Pilot Pro wiki through the link below
Before operating the RTK GPS, you should have the following:
Alta X with RTK GPS
Base Station RTK GPS module on a stable mount (tripod is recommended!) and a USB-C Cable
Any time you move the base station GPS, it is recommended that you manually restart survey-in by clicking on the "Restart" button. This ensures that the base station updates its location and provides optimal absolute positioning.
If you are retrofitting RTK GPS onto an existing Alta X, follow the.
Alta X running firmware v1.3 or above ()
Telemetry Radio, such as the
Alta QGroundControl v1.3 or above running on Windows or macOS ()
Title | Title |
1. | Get Alta powered on and ready to fly |
2. | Install base station module onto mount |
3. | Connect Alta QGroundControl to aircraft |
4. | Click the "RTK" symbol to open the status window |
5. | Connect base station module to laptop via USB |
6. | Ensure QGroundControl recognized the module
|
7. | Open RTK status window and press "Restart" button to initiate survey-in. |
8. | Wait until GPS Status says "GPS Initializing." Note: If the status does not change after 10 seconds, refer to the troubleshooting section. |
9. | Wait for GPS Status to change to "Survey-in Complete". Note: The survey-in process will complete after the minimum survey-in time has completed (typically 180s) AND the accuracy is below the configured threshold (2m). See the configuration page for adjusting these parameters. |
|
10. | Wait for Aircraft GPS status of "RTK GPS Lock (fixed)" |
11. | Ensure that Aircraft location is correctly indicated on the map |
12. | Fly and enjoy! |
Follow this guide if you are retrofitting RTK GPS onto an existing Alta X.
The Rover module replaces the existing GPS module. Follow the instructions below to remove the previous GPS module and install the RTK module.
After completing the mechanical installation, you will have to configure the autopilot.
Note: You must be running Alta SW 1.3 or above for RTK GPS operation
1.
CUT cable tie
2.
REMOVE three bracket mounting screws attaching the bracket to the Alta X
3.
REMOVE the three mounting screws attaching the bracket to the GPS module
4.
REMOVE cover screw
5.
Carefully LIFT cable cover, slide out
6.
With a driver or long device, DEPRESS connector latch and slide connector and cable out
7.
REMOVE GPS device and mounting bracket
8.
INSTALL RTK Module, button should be facing away from the aircraft
9.
PLUG existing 8-pin GPS cable into RTK Module
10.
Apply threadlocker and SCREW the 3, M3x16 mm bolts
Hint: The M3x16 use a 2.5mm driver (ie "Freefly Blue")
Update RTK module parameters in accordance with Service Bulletin 007 ->
1
Open Alta Ground Control
2
Power Alta X up by connection at least one battery
3
Connect to Alta X via Alta Ground Control
4
Click on the Vehicle Setup (gear icon) and on the "Sensors" button.
5
Press "Compass" to start compass calibration
6
When prompted for autopilot orientation, set ROTATION_YAW_180
7
Follow instructions to calibrate compass
8
When prompted for external compass orientation, set NO_ROTATION (set to YAW_180 if placed in the alternate [rear] location)
9
In Vehicle Setup, click on "Parameters" button
10
In the "Standard" group, select "Serial"
11
Set "SER_GPS1_BAUD" to 115200 8N1
12
Reboot Alta
The default configurations have been tested to provide good performance for all users. However, there are a few additional settings that users can change to tweak RTK usage.
By default, Alta utilizes the barometer as the principal source of height, since the baro accuracy (10s of cm) is much better than that of the normal GPS (many meters).
However, RTK provides very good height precision (centimeters), so it can be used to improve the Alta height performance by switching Alta to use GPS as the primary altitude source.
To change the height mode, navigate to the Parameters tab of QGC, and search for " EKF2_HGT_MODE." Set mode to "GPS" and reboot.
When not using RTK, it is recommended you return the EKF2_HGT_MODE to "Barometric Pressure"
Setting EKF Height Mode to GPS is only recommended when flying in RTK mode. In non-RTK mode (or with a normal GPS), the baro will provide much better height accuracy.
While the default RTK settings are likely to be sufficient for most people, Alta Ground Control Station allows you to tune the following RTK base station survey-in parameters:
Survey-in accuracy: The lower the number, the more accurate the base station location will be, though it will also take longer to complete survey-in. Default value: 2.0m
Minimum observation time: Longer time typically results in a better survey, though this number can be reduced if the base station GPS is in a location where the survey-in accuracy quickly reaches the desired number. Default value: 180 seconds
To change the parameters, perform the following steps:
The RTK module serial port is configured for 115kbaud. PX4 may not be able to automatically set to this baudrate, so it is recommended to set the parameter "SER_GPS1_BAUD" to 115200 8N1 to ensure reliable GPS communication.
The default GPS configuration is shown below. By default, PX4 only modifies RAM values, which reset to the flash values on every power-up.
However, if you inadvertently modify the Flash values, you can use the data below to reset them to the Freefly defaults.
To configure Here2 GPS with Alta X, you will need to upload the param file "ALTAX-HERE2-enable.params" to QGC.
Steps:
Connect the computer to Alta X using a USB cable while no batteries are plugged in.
Open ALTA X QGC on your computer.
Download the file "ALTAX-HERE2-enable.params" (attached below).
In QGC, navigate to the Parameter section, on the top right you will see a "Tools" button. Click on that, and you will find an option to upload parameters.
Upload the Here2 param file you just downloaded, power-cycle Alta X and you should be good to go after this.
1.
Alta SW
OPEN Alta Ground Control Station
2.
Settings Tab
Click on QGC logo, then "General" Button
3.
RTK GPS
Scroll Down to RTK GPS Section, change settings
4.
Restart Alta SW
Restart Alta Ground Control Station to apply settings
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