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July 17, 2018, 9:19:23 MDT

GPS/INS method

Inertial navigation systems, not every dot is a GPS dot

CIVL|GPS|PG

https://en.wikipedia.org/wiki/GPS/INS

GPS/INS is commonly used on aircraft for navigation purposes. Using GPS/INS allows for smoother position and velocity estimates that can be provided at a sampling rate faster than the GPS receiver. This also allows for accurate estimation of the aircraft attitude (roll, pitch, and yaw) angles.

A post from Dhayner to https://ozreport.com/forum/viewtopic.php?t=56724

Track Logs: GPS vs. INS and Blending, 16 Jul 2018 8:46 pm

As I recall, the Oudie boasts of containing 3-axis gyros and 3-axis accelerometers as part of an IMU. The correcting altimeter should be pressure based to eliminate and dependence on the GPS. The track log looks like many INS only tracks I’ve seen – not random, just a drift component due to biases and drifts native to all inertial sensors.

What I suspect is that the INS track is used to “filter” the GPS track to remove the GPS jumps and dither. Also, if the GPS data quality falls below some level, the INS track is more heavily weighted than the GPS track. Once a good, high quality GPS data track is re-established, the INS is reset to the GPS track.

The jump, or correction we see in the data near the goal may be Oudie software deciding the GPS data is of sufficient quality to start to weight it more heavily (and the INS track less).

The developers need to disclose this in detail and provide some sort of visual indicator (in all screens) that the GPS data is being ignored or de-emphasized. Unfortunately, depending on the algorithm details, the developers may not even know. There are many pre-packaged INS/GPS blending apps available that are extremely sophisticated, but little details are provided on what is going on deep in the guts of the algorithms.

I’ve also seen many “professional products” developed by teams essentially ignorant of the details of the Kalman or Particle filters employed and how statistical measures of signal quality are used.

As an example of my concern of the Oudie developers understanding of how to use inertial sensors, they claim on their Web site:

Exceptionally accurate Vario (Gyro assisted) for the quickest climb rate

Gyros measure angular rate which has little to do with moving up or down in a gravitational field. Accelerometers, corrected for sensing angle by the gyros, could assist. But it is the accelerometer data that is measuring “up” or “down” accelerations, not the gyros.

The performance of the small, low cost MEMS gyroscopes and accelerometers employed in these products have drift rates and uncorrected biases far beyond practical use for INS only navigation for hang gliding or paragliding competitions. Given these errors and several other reasons, I suspect CIVL will need to specify that GPS only track logs will be the only accepted track.


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