All About AHRS

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All About AHRS

Overview

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. The upgrade includes the attitude sensors, an integrated WAAS GPS receiver, and a barometric altitude sensor. It also adds visual status indicators and a temperature-driven fan speed controller.

Customers who already own a FlightBox can add AHRS by ordering the FlightBox upgrade kit for $160. New customers can add AHRS to their FlightBox order for $150 by checking the “Add AHRS / GPS / Baro board” option on the order page.


AHRS Features

Attitude – Our attitude algorithm combines data from a 3-axis MEMS gyroscope, a 3-axis MEMS accelerometer, and the onboard WAAS GPS to provide pitch and roll values to the virtual attitude indicator or synthetic vision system of many popular EFB applications.

Slip / Skid – Data from the sensors also provides an accurate slip and skid data which drives a virtual turn coordinator “ball”.

G-Force – The 3-axis accelerometer outputs gravitational and pseudo-gravitational force data which can be used to indicate the g-force load on your aircraft.

WAAS GPS – The built-in WAAS GPS provides precise position, track, and ground speed information at 10 Hz (updates per second).

Barometric Pressure Altimeter – The pressure sensor outputs pressure altitude (cabin altitude) which may optionally be adjusted to MSL altitude by your EFB application.

Please note that not all EFB applications support all features of the Open Flight Solutions AHRS system.


FlightBox AHRS Upgrade / Add-On

The FlightBox AHRS Upgrade includes the AHRS sensor module, a replacement case top with status indicator elements, a new variable-speed cooling fan, and mounting hardware. Retrofit installation of the AHRS module typically takes 15 – 20 minutes and requires only standard household tools.

For users who wish to blind-mount their system, we offer a Remote Powered GPS Antenna and a remote mount kit for the ADS-B antennas.


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.


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

Please see the AHRS Upgrade Tutorial page for a step-by-step guide to installing the AHRS software and hardware.


Mounting / Orientation

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.)

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: