Freefly's RTK GPS modules provide a drop-in replacement for existing PX4 GPS modules, and integrate the cutting-edge U-Blox multi-band ZED-F9P GPS module to provide you with centimeter-level positional accuracy.

Terminology

• RTK: (Real Time Kinematics) Provides cm-level GPS accuracy

• Base Station: Ground GPS, provides GPS correction data

• Rover: Aircraft GPS, uses base station correction data to calculate heading

• Survey-in: Process in which base station calculates its position

Benefits of Freefly RTK GPS

• RTK provides cm-level positional accuracy

  • Improves sensor data, and flight performance

• The multi-band GPS and antenna translate in higher satellite count, fast lock, even in non-RTK mode

Included in the Box

• 1x GPS modules with multi-band antenna

• USB C to A cable (Type C right-angle to Type A (3 m / 9.84 ft long)

• Magnetic quick-mount, with fasteners for base station module

• Screws to mount onto Alta X

General RTK Procedure

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

Base Station Notes

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 Process

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.

• Ublox ZED-F9P GPS Receiver

  • Ultracap backup power for fast (hot-start) restarts

  • EMI shield over receiver for improved EMI immunity

• IST8310 Magnetometer

• Safety-switch and safety LED

• RGB LEDs for status indication

  • NCP5623CMUTBG I2C Driver

• BMP388 Baro on I2C bus

• External, active antenna (Maxtena M7HCT)

  • SMA connector

• STM32 MCU for future CAN-based communication

  • FW updates through USB connector

• Connectivity:

  • USB-C

    • 2-way USB Switch to MCU and F9P

  • SMA for active antenna (20mA max)

  • 4-pin CAN Bus (dronecode compliant)

  • 8-pin UART/I2C

• Power:

  • Input from either (diode OR'd):

    • USB (5V)

    • CAN (4.7 to 25.2V)

    • UART (4.7 to 25.2V)

  • Power Consumption <1W

RTK Performance over range

The performance of RTK over long distances is typically limited by:

• Telemetry link: You must have a telemetry link between the aircraft and ground station,

• RTK degradation over distance: As the aircraft moves away from the base station, the RTK accuracy will degrade. The F9P RTK GPS receiver spec is 0.01m CEP (Circular Error Probability) + 1ppm for both vertical and horizontal.

  • For example, at 10km distance from base station, the typical accuracy would be 0.01m + (1km * 1/1,000,000) = 0.02m CEP

GPS Origin Point

GPS coordinates reported from this base station are centered around a point 35mm from the base of the antenna.

Mechanical Dimensions

Prerequisites

For use on Alta, the following is necessary:

• Alta X

• Two RTK Modules (Rover and Base Station)

• Alta SW 1.3 or above

• Datalink (FRX Pro recommended)

• Computer (Mac or Windows) running Alta QGC 1.3 or above

• Sturdy mount for base stations GPS. Tripod with 1/4-20 mount is recommended

• USB C cable

I2C Address Table

Connector Pinout

CAN Ports - GH-4 Pin

Peripheral CANbus for future expansion.

UART/I2C Ports - GH-8 Pin

Peripheral CANbus for future expansion.

GPS Antenna Compatibility

The following antennas have been tested with the Freefly RTK GPS module:

• Maxtena M7HCT

• UBlox ANN-MB multi-band antenna

The Freefly RTK modules are shipped with Maxtena M7HCT multiband antennas. However, the GPS is compatible with other SMA, L1/L2 multiband antennas.

Sensor Rotation

The IST8310 defines its coordinate system using left-hand rule, which is corrected in PX4 to be right-hand. Orientation is shown below in both native and PX4-corrected coordinates.

Alta X Orientation

When installed on Alta X, the PX4 orientation is with respect to the front-left vertical mounting surface. In other words, when installed on the front-left (where the standard GPS is installed), the "external mag" orientation should be set to Yaw 0° (ie, No rotation). When installed on the diagonal side (rear-right), the orientation should be set to Yaw 180°.

Alta X Installation - Mechanical

The Rover module replaces the existing GPS module. Follow the instructions below to remove the previous GPS module and install the RTK module.

Alta X Installation - Configuration Changes

After completing the mechanical installation, you will have to configure the autopilot.

RTK

Module LED Indicators

The RTK GPS modules have two different LED indicators: multi-color status LED and red, safety LED.

Multicolor (RGB) Status 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.

Red Safety LED

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.

Alta QGroundControl GUI

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

I get Compass Inconsistency Errors when arming!

Troubleshooting steps:

• Make sure the sensor rotations are set correctly

  • In the Alta QGC parameters screen, search for "CAL_MAG0_ROT." In Alta X, this parameter should be set to "No Rotation" when the GPS is installed in the front, or "YAW_180" when the GPS is installed in the rear

  • In the Alta QGC parameters screen, search for "SENS_BOARD_ROT." In Alta X, this parameter should always be set to "ROTATION_YAW_180"

• Perform compass calibration away from steel structures

  • Proximity to steel structures (such as rebar in buildings or manhole covers on roads) can sometimes interfere with compass calibration. As such, we suggest you perform compass calibration outdoors, away from buildings and any underground steel structures.

Survey-in takes a long time to complete or does not complete

Cause: The Base Station GPS probably does not have good GPS lock

Fix: Move the GPS to a location with more open sky. Ensure that GPS is mounted on a stable platform, and that GPS is not moved after initial installation.

Base Station GPS does not show up in Alta Ground Station

Cause: GPS serial connection was not recognized

Troubleshooting steps

• Make sure no other software is open that might be using the GPS com port

• In Windows, check device manager to see if GPS serial port appears.

  • If it does not appear, check USB cable

• Reboot computer

Base Station safety LED is solid red

Cause: The GPS module has entered bootloader mode. This happens when the safety button is pressed while plugging the GPS module into the computer.

Fix: Unplug and re-plug the GPS module, taking care that the safety button is not pressed while plugging in.

I accidentally changed the GPS settings!

By default, PX4 and Alta Ground Control only write configuration settings to the GPS's volatile RAM. This means that all settings are reset to their default settings when the module is re-powered.

If you have made permanent changes to the GPS by writing to its flash memory and the module no longer responds, you will have to reset its settings via USB.

To reset the GPS to its factory settings, you will have to download U-Center from U-Blox, using the following link: https://www.u-blox.com/en/product/u-center. Once you've installed U-Center, you can load the default Freefly GPS parameters found in the "Configuring" page of this Wiki.

My aircraft GPS module does a cold restart on every boot

The GPS module has a supercap onboard to provide backup power, which allows you to perform a warm-restart, resulting in super-quick lock when re-powering the boards. However, this supercap only provides power for a limited time, typically less than one day. So, if the module has been disconnected for long enough, it will perform a cold boot, taking ~30s to acquire a lock.

Due to the need to perform survey-in, the base station GPS is commanded to perform a cold start every time you connect it to Alta Ground Control.

My base station is in the wrong location in the Ground Control map

Cause: The base station was moved without restarting survey-in

Fix: Make sure you're using Alta QGC 1.3 or above. Stock QGC does not always restart RTK survey-in. To restart survey in in Alta QGC (1.3 and above!), use the RTK "Restart" button shown in the "Using RTK Modules" section.

My aircraft is in the wrong location in the Ground Control map

Cause: Refer to the "Base station in the wrong location" section above.

v1.0.0 Release Package

• Summary: This is the initial release of RTK GPS

• Release Date: March 2020

• Versions in this package:

  • RTK GPS firmware version 1.0.0

  • RTK GPS bootloader version 1.0.2

  • Configurator (Max + Windows) version v1.0.0 (Not released public)

• Notes:

  • Initial release!

Configuration

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.

Improving Height Hold Performance in RTK Mode

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"

Ground Station RTK Configuration

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:

Default PX4 Configuration with RTK GPS

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.

Default Freefly U-Blox F9P Configuration

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.

# Config changes format version 1.0
# created by u-center version 19.08.01 at 15:37:31 on Monday, 07 Oct 2019
[del]
[set]
  RAM CFG-UART1-BAUDRATE   0x1c200              # write value 115200  0x1c200      to item id 40520001 in layer 0
Flash CFG-UART1-BAUDRATE   0x1c200              # write value 115200  0x1c200      to item id 40520001 in layer 2
  RAM CFG-UART1-STOPBITS   1                    # write value 1 - ONE              to item id 20520002 in layer 0
Flash CFG-UART1-STOPBITS   1                    # write value 1 - ONE              to item id 20520002 in layer 2
  RAM CFG-UART1-DATABITS   0                    # write value 0 - EIGHT            to item id 20520003 in layer 0
Flash CFG-UART1-DATABITS   0                    # write value 0 - EIGHT            to item id 20520003 in layer 2
  RAM CFG-UART1-PARITY     0                    # write value 0 - NONE             to item id 20520004 in layer 0
Flash CFG-UART1-PARITY     0                    # write value 0 - NONE             to item id 20520004 in layer 2
  RAM CFG-UART1-ENABLED    1                    # write value 1                    to item id 10520005 in layer 0
Flash CFG-UART1-ENABLED    1                    # write value 1                    to item id 10520005 in layer 2
  RAM CFG-UART1INPROT-UBX  1                    # write value 1                    to item id 10730001 in layer 0
Flash CFG-UART1INPROT-UBX  1                    # write value 1                    to item id 10730001 in layer 2
  RAM CFG-UART1INPROT-NMEA 1                    # write value 1                    to item id 10730002 in layer 0
Flash CFG-UART1INPROT-NMEA 1                    # write value 1                    to item id 10730002 in layer 2
  RAM CFG-UART1INPROT-RTCM3X 1                    # write value 1                    to item id 10730004 in layer 0
Flash CFG-UART1INPROT-RTCM3X 1                    # write value 1                    to item id 10730004 in layer 2
  RAM CFG-UART1OUTPROT-UBX 1                    # write value 1                    to item id 10740001 in layer 0
Flash CFG-UART1OUTPROT-UBX 1                    # write value 1                    to item id 10740001 in layer 2
  RAM CFG-UART1OUTPROT-NMEA 0                    # write value 0                    to item id 10740002 in layer 0
Flash CFG-UART1OUTPROT-NMEA 0                    # write value 0                    to item id 10740002 in layer 2
  RAM CFG-UART1OUTPROT-RTCM3X 1                    # write value 1                    to item id 10740004 in layer 0
Flash CFG-UART1OUTPROT-RTCM3X 1                    # write value 1                    to item id 10740004 in layer 2
  RAM CFG-UART2-BAUDRATE   0x1c200               # write value 115200  0x1c200        to item id 40530001 in layer 0
Flash CFG-UART2-BAUDRATE   0x1c200               # write value 115200  0x1c200        to item id 40530001 in layer 2
  RAM CFG-UART2-STOPBITS   1                    # write value 1 - ONE              to item id 20530002 in layer 0
Flash CFG-UART2-STOPBITS   1                    # write value 1 - ONE              to item id 20530002 in layer 2
  RAM CFG-UART2-DATABITS   0                    # write value 0 - EIGHT            to item id 20530003 in layer 0
Flash CFG-UART2-DATABITS   0                    # write value 0 - EIGHT            to item id 20530003 in layer 2
  RAM CFG-UART2-PARITY     0                    # write value 0 - NONE             to item id 20530004 in layer 0
Flash CFG-UART2-PARITY     0                    # write value 0 - NONE             to item id 20530004 in layer 2
  RAM CFG-UART2-ENABLED    1                    # write value 1                    to item id 10530005 in layer 0
Flash CFG-UART2-ENABLED    1                    # write value 1                    to item id 10530005 in layer 2
  RAM CFG-UART2-REMAP      0                    # write value 0                    to item id 10530006 in layer 0
Flash CFG-UART2-REMAP      0                    # write value 0                    to item id 10530006 in layer 2
  RAM CFG-UART2INPROT-UBX  1                    # write value 1                    to item id 10750001 in layer 0
Flash CFG-UART2INPROT-UBX  1                    # write value 1                    to item id 10750001 in layer 2
  RAM CFG-UART2INPROT-NMEA 0                    # write value 0                    to item id 10750002 in layer 0
Flash CFG-UART2INPROT-NMEA 0                    # write value 0                    to item id 10750002 in layer 2
  RAM CFG-UART2INPROT-RTCM3X 1                    # write value 1                    to item id 10750004 in layer 0
Flash CFG-UART2INPROT-RTCM3X 1                    # write value 1                    to item id 10750004 in layer 2
  RAM CFG-UART2OUTPROT-UBX 1                    # write value 1                    to item id 10760001 in layer 0
Flash CFG-UART2OUTPROT-UBX 1                    # write value 1                    to item id 10760001 in layer 2
  RAM CFG-UART2OUTPROT-NMEA 0                    # write value 0                    to item id 10760002 in layer 0
Flash CFG-UART2OUTPROT-NMEA 0                    # write value 0                    to item id 10760002 in layer 2
  RAM CFG-UART2OUTPROT-RTCM3X 1                    # write value 1                    to item id 10760004 in layer 0
Flash CFG-UART2OUTPROT-RTCM3X 1                    # write value 1                    to item id 10760004 in layer 2
  RAM CFG-HW-ANT_CFG_VOLTCTRL 0                    # write value 0                    to item id 10a3002e in layer 0
Flash CFG-HW-ANT_CFG_VOLTCTRL 0                    # write value 0                    to item id 10a3002e in layer 2
  RAM CFG-HW-ANT_CFG_SHORTDET 0                    # write value 0                    to item id 10a3002f in layer 0
Flash CFG-HW-ANT_CFG_SHORTDET 0                    # write value 0                    to item id 10a3002f in layer 2
  RAM CFG-HW-ANT_CFG_OPENDET 0                    # write value 0                    to item id 10a30031 in layer 0
Flash CFG-HW-ANT_CFG_OPENDET 0                    # write value 0                    to item id 10a30031 in layer 2
  RAM CFG-TMODE-MODE       0                    # write value 0 - DISABLED         to item id 20030001 in layer 0
Flash CFG-TMODE-MODE       0                    # write value 0 - DISABLED         to item id 20030001 in layer 2

Configure Here2 GPS with AltaX

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.