LARUS Gliding Sensor Unit

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.

In principle, LARUS calculates:

• 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)

In the Essential variant, LARUS has installed a precise GNSS receiver with an external active antenna. In the LARUS Dual-GNSS variant, on the other hand, a high-precision dual-band receiver is connected to two active, multi-frequency band antennas.

IMU

Inertial Measurement Unit (IMU) describes an inertial measurement unit that combines several inertial sensors. LARUS relies on the high-quality IMU XSENS MTi-1. It measures three-dimensional acceleration, yaw rate and magnetic field.

The flight attitude is calculated from this data by comparing it with the speed data of the GNSS (AHRS function).

When circling, the heading is determined by comparing the acceleration (determined from the flight path using the GNSS positions) and the AHRS. You don't need a magnetic compass for this. The Larus sensor unit automatically determines the calibration of the magnetic compass during circling flight, so that it can then be used to measure the magnetic heading in straight flight. Together with the adjustable magnetic declination, true heading (relative to true north) can be measured with an accuracy of about 1-2 degrees.

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).

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:

A special algorithm detects and eliminates deviations in circular flight that result from a change in the angle of attack.

Influence of the heading accuracy on the wind calculation in forward flight

As described above, the LARUS Essential (uBlox M9N single-frequency receiver) already achieves a high heading accuracy of 1-2° (under optimal installation conditions, see below).

In contrast, the accuracy of heading determination with the Dual-GNSS variant is outstanding at 0.05°. This is particularly advantageous at high airspeed in level flight. As the wind triangle illustrates, at an airspeed of 200 km/h, 2° corresponds to a 7 km/h inaccuracy in the crosswind component, while 0.05° corresponds to only a 0.2 km/h inaccuracy.

In addition, the accuracy of the dual GNSS version is not affected to the same extent as LARUS Essential if iron parts or magnetic fields in the vicinity of the LARUS sensor unit cannot be avoided due to the installation situation. Due to the two GNSS receivers, only a small part of the measured values from the IMU flows into the algorithm.  

Explanations regarding LARUS Essential

A requirement for the high accuracy of LARUS Essential is that the disruptive influence of magnetic fields in the vicinity of the sensor unit is minimized. This is particularly high priority for LARUS Essential. In particular, the correct operation of the inertial measuring unit IMU, which among other things measures the magnetic induction, is severely impaired by magnets, changing magnetic fields or iron parts. We recommend keeping the LARUS sensor unit as far away as possible (at least 20 cm), especially from speakers and magnets (often contained in GPS / GNSS antennas, simply check with magnets). In addition, the fastening elements in the immediate vicinity of the sensor should be made of stainless steel, brass, plastic, aluminum or fiber composite materials and the usual nuts and bolts made of steel should be avoided. Iron parts are unsuitable because they generate a variable interference field with every movement of the aircraft.

The GNSS antenna, on the other hand, is not sensitive to magnetic fields.

Explanations regarding LARUS Dual-GNSS

Bei LARUS Dual-GNSS ist dagegen wichtig, dass die beiden GNSS Empfänger einen Abstand von mind. 1,0 Metern zueinander aufweisen.

The master GNSS antenna should be mounted on the inside of the fuselage tube (luggage compartment / end of the canopy) approximately above the center of gravity of the aircraft. Because the shorter the distance between the antenna and the center of gravity, which is also the center of rotation of the aircraft around the pitch axis, the less the influence of pitching movements (forward or backward movement of the stick) on the measured altitude.

The second antenna (slave GNSS antenna), on the other hand, is mounted at the level of the instrument panel or in front of it in the nose of the aircraft.

For all GNSS antennas, it is important that the laminate above the antenna is made of GRP and not CFRP, otherwise the electrical conductivity of carbon fibers would cause the attenuation between the GNSS satellite and the receiver in the aircraft to be too great. In the case of aircraft with a CFRP fuselage, however, mounting the antennas in the front and rear area of the canopy can also be considered, provided the antennas are then at a sufficient distance (approx. 1.0 m) from one another.

LARUS Vario Display (Seperate Round Instrument)

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

License fees ensure long-term further development.