LARUS Gliding Sensor Unit

SteFly produces and distributes the high-end variometer under license developed by the LARUS project team led by Prof. Dr. Klaus Schäfer, Horst Rupp and Maximilian Betz.

In short: Data from one (or two) GNSS receiver(s), an IMU (inertial measurement unit) and pressure sensors for static pressure and dynamic pressure are combined and evaluated in the LARUS sensor unit using special algorithms. The results are transferred to a display device via a serial interface or Bluetooth. For example, all OpenVarios or other devices running XCSoar are compatible with the LARUS sensor unit.

• Energy-compensated climb or sink (variometer)
• Horizontal wind speed as an instantaneous value (live / real-time wind) and a value averaged over an adjustable period (e.g. 30 s)
• Attitude for display in an artificial horizon
• Pressure altitude / flight level FL
• True airspeed (TAS)
• Course over the ground (track)
• Drift angle (difference between track and heading)

GNSS

In recent years, location determination using GNSS (Global Navigation Satellite System) has become much more accurate. Because GNSS also includes satellites from GLONASS, Galileo and BeiDou in addition to GPS satellites, the modern GNSS receivers installed in the LARUS sensor box can achieve a horizontal positioning accuracy of 1.5 - 2 m. In addition, the ground speed is measured three-dimensionally and used to support the flight attitude measurement and as a measured variable for the variometer.

The accuracy of the speed measurement is:

• LARUS Essential with uBlox M9N single-frequency receiver and GNSS antenna: horizontal approx. 0.1 km/h; vertical approx. 5 cm/s
• LARUS Dual GNSS with uBlox F9P dual GNSS receiver: horizontal approx. 0.05 km/h; vertical approx. 2.5 cm/s

With LARUS Dual-GNSS, the accuracy of the heading can be improved to approx. 0.05 degrees by using a D-GNSS "compass" (2 coupled differential GNSS receivers, here uBlox F9P).

The flight attitude measurement can be used for an artificial horizon as well as for the precise display of the current drift angle (track – heading).

Pressure Sensors

The Larus sensor box contains pressure sensors for measuring static pressure and dynamic pressure. The static pressure is required to display the pressure altitude and the flight level FL. It is also used to determine air density.

The indicated airspeed IAS is formed from the dynamic pressure.

The true airspeed TAS is calculated from the indicated airspeed, taking into account the current air density. LARUS calculates the air density in real time by comparing the GNSS altitude with the pressure altitude, without having to measure the temperature or humidity. LARUS then uses the TAS to calculate the wind speed.

Algorithms

Calculation of (real-time) wind speed

LARUS determines the wind speed from the three-dimensional difference between ground speed and flight speed. Both a short-term mean value and a long-term mean value are output. The periods differ as follows:

• In straight flight, the time period for short-term wind is 5 s and for long-term wind 30 s (default setting; both times are configurable)
• When circling, the short-term wind is the mean of the last circle, while the long-term wind is the accumulated mean over the entire time of the last circling phase

With the introduction of the LARUS Vario display, all information can now be displayed clearly and attractively on a separate round instrument. The round display with a diameter of 57 mm offers plenty of space.

LARUS Sensor Box and LARUS Vario Display can also be installed as a stand-alone unit. A navigation computer or smartphone with XCSoar / OpenSoar is not required in this case.

OpenSoar with LARUS driver

The variometer display familiar from XCSoar accesses the LARUS data via the RS232 interface. OpenSoar / XCSoar continues to generate the vario sound.

Two separate wind arrows can be displayed directly in the moving map. This visualises the direction and strength of the current wind and the average wind over a longer period of time. In addition, new wind-related info boxes are available in OpenSoar.

Another novelty is the artificial horizon, which is displayed on a separate page in OpenSoar. It is helpful, for example, to check the correct function and calibration of the IMU after installing the sensor unit.

Installation Instructions

• Mount the sensor unit absolutely fix so that the position of the sensor unit in relation to the aircraft structure does not change under the influence of acceleration forces
• The LARUS sensor must be positioned as far away as possible from magnetic fields and bigger iron parts
• The installation orientation of the LARUS box is possible in all directions and only needs to be adjusted in the configuration file "sensor_configuration.txt". Nevertheless, we recommend to select the orientation with regard to the longitudinal axis of the aircraft during assembly according to the coordinate system printed on the LARUS housing. Ideally, the LARUS sensor unit is also mounted in such a way that it is roughly horizontal in normal flight and that the SD card remains accessible.
• The GNSS antennas must be mounted in such a way that there is ‘visual contact’ with as many satellites as possible. This means that they should be positioned approximately horizontally and above electrically conductive materials (including CFRP). Suitable installation locations are on or directly below the instrument cover, on the inside of an acrylic glass canopy, in the upper half of a GRP fuselage tube, under GRP engine cowlings, or using a special external antenna on the top of the fuselage.

LARUS Essential or LARUS Dual-GNSS?

• Precision thanks to high-quality sensors for measuring acceleration, air pressure and magnetic fields
• GNSS technology (incorporating GPS, Galileo, GLONASS and BeiDou satellites)
• Sophisticated algorithm that is constantly being optimised
• Future-proof system thanks to free firmware updates
• Reduction of error and search cycles
• A thermal lift is never round. LARUS indicates the segment with the best lift, allowing you to centre it perfectly.

In addition to the features of LARUS Essential, it is characterised by:

• Unprecedented precision in determining the position through differential GNSS 
• Due to D-GNSS Heading even during long straight flights without circling (cloud streets) always correct determination of the wind direction and strength
• Lower sensitivity to interfering magnetic fields, which facilitates installation and setup of the system
• Thanks to the best possible wind determination, the quality of the E-Vario has been further improved. 

Larus hardware is licensed accordingly to Creative Commons NonCommercial Share-alike 4.0 International, Larus software accordingly to GNU General Public License v3.0.

Licence fees paid by SteFly to the development team ensure long-term further development.

1. Copy the update file (file extension .bin) to the LARUS SD card via a computer
2. Disconnect all cables from the LARUS except for the one that supplies power.
3. Insert the SD card into the switched-off LARUS
4. Switch on the LARUS and wait for one minute
5. Check whether the new version has been installed: either activate sensor.readings or open the latest EEPROM file in the logger folder on the LARUS SD card with a text editor.

Please note: There are two .bin files for each firmware version. The relevant file depends on the currently installed firmware version:

• If version 0.4.0 or older is currently installed, use larus_sensor_V2_image.bin, delete sensor_config.txt and replace it with the file larus_sensor_config.ini
• If version 0.5.0 or lower is currently installed, use larus_sensor_v1_v0-x-x-x.bin; a file named larus_sensor_config.ini.used should already be stored on the SD card, otherwise copy larus_sensor_config.ini to the card.
• There must only be one .bin file in the main folder of the SD card, so delete all .bin files from the SD card before copying a new .bin file to the card.