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.
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:
We will be adding more apps to this list as we get a chance to test them.
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.
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.
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:
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.
Please watch the installation / upgrade video before starting on the installation process.
IF YOU WILL BE USING AN EXTERNAL GPS ANTENNA:
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.
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.
IF YOU DO NOT ACCEPT THE ABOVE TERMS, DO NOT DOWNLOAD / INSTALL THIS SOFTWARE. INSTALLATION AND OPERATION OF THIS SOFTWARE SHALL BE CONSTRUED AS ACCEPTANCE OF THESE TERMS.
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 firstname.lastname@example.org. 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.)
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.
NOTE: THE ‘FLIGHTBOX UTILITY’ iOS APP IS NOT CURRENTLY COMPATIBLE WITH THE BETA. DO NOT USE THE UTILITY WITH A SYSTEM THAT HAS BEEN UPGRADED WITH THE BETA CODE.
Option 1 – Update File – Beta Version 2 – UPDATED 3/1/17
* 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
Option 3 – Order A Replacement Card
After completing the software update / card replacement, verify that the system is working using the verification procedure.
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.
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.
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.