All About AHRS


All About AHRS

An AHRS or “Attitude/Heading Reference System” is a set of digital sensors that provide attitude (pitch / roll / yaw) and related information to an electronic display – typically a virtual attitude indicator, PFD, or synthetic vision system.

Open Flight Solutions currently offers AHRS as an add-on / upgrade to our FlightBox kits. Once FCC testing is complete it will be available as an optional part of our assembled systems.

FlightBox customers can order the FlightBox upgrade kit for $160.

Compatible EFB Applications

Please note that this list refers only to AHRS compatibility. The weather, traffic and GPS features of FlightBox work with most modern EFB applications. Applications that have been tested include:

  • Aerovie
  • DroidEFB
  • iFly GPS
  • FlyQ EFB
  • Naviator
  • WingX Pro

We will be adding more apps to this list as we get a chance to test them.

FlightBox AHRS Hardware

The FlightBox AHRS sensor board is built to fit the Raspberry Pi 2/3’s 40-pin feature header. (Sometimes called a GPIO header.) The board includes an advanced inertial sensor, a WAAS GPS receiver, a barometric altitude sensor, a fan controller, a set of status indicator LEDs, and an alternate power input.

The board measures 2.2″ x 2.56″. It includes a socket for a CR1/3N lithium battery which is used to provide “warm start” GPS capabilities. The battery life is estimated at 1.8 years after which it will require replacement. A battery is included with FlightBox upgrade kits.

The onboard GPS includes a 25mm x 25mm board-mount GPS antenna. It also includes an MCX female jack which connects to the GPS through an automatic switch. If an external GPS antenna is connected to the MCX jack, the GPS will automatically disconnect the onboard antenna and begin using the external antenna.

IF YOU ARE NOT PLANNING TO USE AN EXTERNAL ANTENNA: Do not install the MCX-to-SMA pigtail as it can interfere with reception from the onboard chip antenna.

FlightBox AHRS Data (For Developers)

The AHRS algorithm combines data from a set of inertial sensors (gyroscope / accelerometer) with data from the onboard WAAS GPS sensor. The resulting attitude solution is sent to EFB applications as part of the GDL-90 data stream. The data is provided as roll, pitch, heading, slip/skid, and yaw rate. Please see the Stratux source code for details on the format. Note that the heading value provided is generally not suitable for use, as the magnetic / electrical environment surrounding the sensor is quite noisy.

The system also provides pressure altitude (i.e. barometric altitude with the QNH value fixed at 29.92 in Hg). This value can readily be adjusted to MSL altitude using the altimeter value from a nearby airport or other METAR station.

Note that the data is also available as JSON over Websocket in the “situation” structure.

Legal Notice

The AHRS feature of FlightBox is intended to help drive the synthetic vision and attitude feature of various Electronic Flight Bag (EFB) applications. It is NOT intended as a primary attitude source. Under no circumstances should you rely on the AHRS feature as your primary or sole source of attitude information. The system includes the following limitations:

  • The AHRS feature has not been tested or approved by the FAA.
  • The AHRS algorithm relies on GPS data. In the event that GPS data is unavailable, the system will not provide attitude information.
  • The system has not been tested under aerobatic conditions (inverted flight, high-G maneuvers, etc.) and as such should not be used even for situational awareness purposes when conducting aerobatic maneuvers.
  • The software / firmware that provides the AHRS feature is currently in beta and may include issues, limitations, and flaws.

Upgrade Kit Installation

This guide presumes you are upgrading a FlightBox system assembled from a kit or purchased as an assembled, running system (FB1X). If you are using a generic Stratux system the process may differ.

NOTE: If your FlightBox currently has a USB GPS of any sort – remote or internal module – you will need to disconnect it when you install the AHRS upgrade. The AHRS board includes a WAAS-capable GPS receiver.

Video Tutorial

Please watch the installation / upgrade video before starting on the installation process.

Remove The Top / Screws

  1. Start by removing the three 1/4″ screws that hold the top to the case. Keep these screws – they will be re-used in a later step.
  2. Disconnect the fan power cable from the 40-pin header on the Raspberry Pi.
  3. Remove the four 5/8″ screws and nuts that hold the fan to the old top. Keep the screws and the nuts. Discard the fan and the top.

Prepare The New Top

  1. If you have a FlightBox built from a kit or an early FB1X assembled system that used a USB extension cord rather than a modular extender, you will need to remove the small rectangular tab that extends from the lip of the top. This can easily be done using an Exacto knife similar. If you have an FB1X that has a flush USB port, please skip to the next instruction.
  2. Attach the new fan that came with the kit to the new top, making sure the sticker is facing down (into the case). Secure the fan with the four 5/8″ screws and nuts removed in step #3 above. Use a small amount of LokTite or superglue to prevent the nuts from backing off the screws. USE VERY LITTLE AND MAKE SURE IT IS 100% DRY BEFORE YOU CONTINUE – LIQUID SUPERGLUE IS CAUSTIC AND CAN DAMAGE YOUR SYSTEM!


  1. Remove the dust cap, nut, and washers from the SMA to MCX pigtail cable. Discard the dust cap and the lock washer. Keep the star washer and the nut.
  2. Insert the SMA female bulkhead connector of the pigtail from the inside of the top, such that the female SMA jack is on the outside of the case and the coax cable and MCX jack are on the inside. Do your best to make sure that the MCX connector at the far end of the cable is oriented such that the MCX male plug will extend down into the case when the top is on it.
  3. Secure the SMA female bulkhead connector using the star washer and the nut. DO NOT USE THE LOCK WASHER AS IT TAKES UP TOO MUCH THREAD.

Note that if you do not plan on using an external GPS antenna you should not install the pigtail as it can interfere with the onboard chip antenna’s reception.

Mount The AHRS Board

  1. Remote three of the four black 1/4″ screws holding the Raspberry Pi to the case. Leave the screw nearest the audio jack (the small circular connector that sticks through the case). Keep the three screws – they will be re-used.
  2. IF YOUR SYSTEM HAD A USB GPS INSTALLED – unplug the USB GPS. The AHRS board has a built-in GPS.
  3. Screw the three white nylon standoffs through the holes in the Raspberry Pi and into the case. You may find it easiest to do this by temporarily screwing the black 1/4″ screws removed in the previous step into the top of the standoff, then screwing the assembly (the standoff + screw) into the hole. DO NOT OVER TIGHTEN – THE NYLON THREADS STRIP EASILY. (If you use the screw trick, grip the standoff with small pliers to loosen and remove the screw.)
  4. Mount the AHRS board on the 40-pin header making sure that it lines up correctly. The matching 40 pin socket on the AHRS board should cover all pins. Gently push the board onto the pins until it seats against the three standoffs.
  5. Use the three 1/4″ black screws removed in step #4 to secure the AHRS board to the standoffs.
  6. Insert the CR1/3N backup battery into the battery holder on top of the AHRS board, making sure to match the + symbol on the battery with the + symbol on the holder.

Assemble The System

  1. Place the new top beside the bottom portion of the case.
  2. Carefully connect the MCX male plug on the pigtail to the MCX female jack on top of the AHRS board.
  3. Carefully connect the small white two pin JST connector on the end of the fan cable to the matching two pin JST connector on the board.
  4. Place the top on the case and align the three screw holes in the case bottom with the nuts embedded in the three mount points in the new top.
  5. Use the three 1/4″ screws removed in the first step above to secure the top to the case.
  6. Remove the white circular knockout from the sticker (if not already removed).
  7. Remove the backing from the sticker and align it such that the hole where the knockout was removed surrounds the fan vent in the top, and that the three labels match up with the three indicator LEDs.


Limitation Of Liability

FlightBox is not an FAA approved attitude source. The AHRS feature included in this beta release (and in the eventual production release) is intended exclusively as a means of enhancing situational awareness. Neither Open Flight Solutions nor the developers of the Stratux open source software on which it is based accept any liability for damage, injury or loss of life that may occur while using this software. In the event that laws in your jurisdiction prohibit such a blanket waiver of liability, the total liability for Open Flight Solutions is limited to the amount of money paid for this system.


Beta Software Notice

The firmware that supports the new AHRS board is currently in beta – this means that it has not been tested under every scenario and that it has known issues, limitations, and flaws. Installation may result in certain instabilities in your FlightBox system. Please report any issues to us at If you need, you can follow the standard manual update procedure to reinstall the current production release. You may also send your data card in for re-imaging per the terms of the warranty. (Please include $3 for return shipping.)

Installation Process

There are three options for updating the software on your FlightBox system. You can install a beta update file, re-image the SD card with a complete beta image, or order a pre-imaged card from the web store. The update file is the quickest option but may have issues depending on your system’s current configuration (remember – this is BETA software). The re-image process is useful if you’ve experienced trouble with the update process, but it does require some computer skill.


Option 1 – Update File – Beta Version 2 – UPDATED 3/1/17

  1. PLEASE NOTE: There are two slightly different versions of the update file depending on what version of the FlightBox firmware you’re starting out with. Check the version and build values on the Status page of the web interface – it looks like “Version 1.0r1 (d1607e0729)”. If your version string reads 1.0r1, you need the first option below. If it reads 0.8x (x = anything after the 8) then you need the second option.
    1. Download the BETA firmware update file for v1.0r1 or newer [ here ].*
    2. Download the BETA firmware update file for older v0.8x versions [ here ].*
  2. Follow the manual update process using the web user interface detailed [ here ]. (The current FlightBox Utility app is not able to install beta firmware at this time.)
  3. After the system has rebooted and your laptop / desktop has re-established a Wifi connection, you will need to do a hard reset (unplug power / reconnect power). A hard reset is required before the GPS on the board will be recognized.

* The only difference between the two versions is the capitalization of “FlightBox” in the filename. The older systems require it be lower case. The newer systems require it be upper case.

Option 2 – Re-Image The Card

  1. Download the BETA image file from our web server [ here ].
  2. Follow the re-image process using the procedure outlined [ here ]. (Use the file downloaded in step 1  rather than the file linked in the re-image tutorial.)

Option 3 – Order A Replacement Card

  1. Order a replacement data card from the Open Flight Solutions web store.
  2. Select the “AHRS Beta” option from the list of card images.
  3. Carefully disassemble your FlightBox and replace the current data card with the replacement.

After completing the software update / card replacement, verify that the system is working using the verification procedure.

Verifying The Upgrade / Enabling Sensors

Please see the following video for assistance enabling the AHRS features:

YOU MAY HAVE TO DO A HARD REFRESH ON YOUR BROWSER TO GET THE NEW PAGES TO LOAD. Use Ctrl + F5 on Windows or Command + Shift + R on Mac. You may need to do this on each of the pages in the web admin.

After the update is complete you may need to go to the settings page and turn on the “Sensors” button. This will enable the drivers that read the AHRS sensors. Once you have the sensor button enabled you can check the Status and GPS / AHRS pages on the web interface to make sure the system recognizes the board.

The GPS will immediately begin searching for satellites to track and you should see a fix / lock within 2 – 3 minutes (presuming the system can receive signals in your location – metal hangar roofs are notoriously impervious to GPS signals).

To test the attitude sensors, go to the AHRS / GPS page and observe the AHRS test indicator (attitude indicator). When you move the FlightBox you should see the indicator display the attitude corresponding to the box’s orientation in space. To adjust the orientation of the system for your preferred mounting, please see below.

Status LEDs and Fan Controller

Two of the lesser features of the AHRS board are status annunciation and control of the cooling fan. The board has three LEDs, one for power, one for GPS status, and one for ADS-B status.

Power – indicates green when the board is receiving power and the core software is running. The LED typically takes 20 – 30 seconds after power is applied to turn on.

GPS – blinks orange while the system is attempting to acquire a fix; solid orange once a fix has been established.

ADS-B – off indicates no reception on either band; blinking blue indicates reception on one band within the past 60 seconds; solid blue indicates receiving on both bands within the past 60 seconds.

Fan – automatically comes on when the CPU temperature reaches 40° C. The fan will turn on periodically after several minutes operating in a room-temperature environment.

Mounting / Orientation

The current version of the beta firmware allows you to select your own preferred orientation (i.e. the “forward” and “up” for the system). Please see the video linked here to set the orientation. Below is the default orientation for the system:

The system should be mounted as securely as is practical / legal. Vibration and motion relative to the airframe decrease the accuracy of the attitude solution provided by the system. With regard to pitch, the unit should be mounted at or very near the aircraft’s level pitch attitude in level flight. (Note that this can be significantly different from the aircraft’s pitch while parked on the ground.)

Please note that the final release of the firmware will include a utility that allows you to calibrate the orientation as you wish.