Site setup

To get maximum performance from flight instruments and advanced attitude and position state estimators it’s useful to have some environmental data of our airfield.Having environmental data isn’t crucial, but it significantly reduces the limitations caused by uncertainty. Knowledge of the exact altitude of the airfield and ground barometric pressure enables us to recalibrate the aircraft altimeter prior to take off and, if an uplink is available, also during the flight; with altimeter calibration prior to every flight the sensor drift issues are mitigated.

It is always useful to send information on ground wind to the aircraft. It can be used by advanced controllers or state estimators [1]. Even in the absence of such advanced systems, this data is useful for enhancing telemetry quality.

In many applications our position information relies solely on the GPS data. Alternatively it can be a good choice use data from an INS and GPS. In the last case the error variance is effectively bounded [2]. To reduce the weight, volume and cost of the airborne equipment the later solution can also be implemented offline, where the flight data will be processed post flight to obtain attitude, position and speed measurements.

Another approach consists of transmitting flight data to the ground station for real-time processing, but this can lead to less overall accuracy; A larger data set processed as a whole gives results with less variance. However, this may be the only approach to a problem where real-time feedback is needed.

A GPS system with the modern, integrated, low cost, receivers is typical to have an uncertainty around 2,5 meters CEP right out the box.

To increase GPS position accuracy we can use RTK techniques at the airfield. I leave to the reference the details of this approach and stress only on the fact that implementation of such a system needs a reliable fixed reference point at an arbitrary base station and a data connection between the base and the airborne vehicle. It’s convenient to acquire the position of a permanent, local reference point, using a good GPS receiver on which we can place our base station position with an uncertainty of 20 cm.

DIY reference point and altitude calibration point

Here are some pointers for setting up your reference point. Keep in mind that if you calibrate a sensor with respect to a reference point at a certain accuracy and then calculate the same reference point with better accuracy, you can correct the previous measurements.

In Italy there is a network of reference geodetic points called IGM95, the Italian part of EUREF89 or ETRF89. IGM point locations can be browsed on line on the site maintained by IGM Military organization.

The positional information available online is rounded to one second for latitude and longitude and 10 meters for altitude, wich is rather coarse. To have centimeter accurate data is necessary to buy coordinates from IGM. Some organizations provide real time differential position correction to their subscribers. You can visualize an IGM point into Google Earth by downloading the following unofficial file, extracting it and opening the .kml file.

To find a the precise position of a point there are common GNSS methodologies, commonly referred to as position modes [3]. To fix the position of your reference point you need to put a receiver over an IGM point and another on the desired reference point and use, for example, a RTK technique. There is an interesting fully documented open project linked here, You can also download calculations libraries here. Without entering in deep detail RTK, can also be useful when employed in reverse mode: when you have fixed your reference point you can check your measurement with 3 or more other IGM points. You fix the base station on your reference point and check the position of the selected IGM points.

Final accuracy, is heavily dependent on your equipment, roughly for sub-meter accuracy you need a GPS receiver which has reading resolution of 1/100 of second on lat/lon, and probably you will needed to do some averaging on a medium-sized measurement set.

At a glance:

Having a known set of coordinates, altitude and current wind information for your airfield is useful. This information can be transmitted to the airborne unit or used for post flight processing and dramatically decrease uncertainty in measurements.

References

[1]
A. Brezoescu, P. Castillo, and R. Lozano, “STRAIGHT-LINE PATH FOLLOWING IN WINDY CONDITIONS,” ISPRS – International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, vol. XXXVIII-1/C22, pp. 283–288, Sep. 2012 [Online]. Available: https://www1.ethz.ch/igp/geometh/uav_g/proceedings/brezoescu
[2]
J. A. Farrell, Aided navigation: GPS with high rate sensors. New York, NY: McGraw-Hill, 2008 [Online]. Available: http://www.mecatronica.eesc.usp.br/wiki/upload/6/64/2008_Aided.Navigation.GPS.with.High.Rate.Sensors_Kalman.pdf
[3]
A. El-Rabbany, Introduction to GPS: the Global Positioning System. Boston, Mass.: Artech House, 2002 [Online]. Available: http://www.artechhouse.com/uploads/public/documents/chapters/El-Rabbany_016-CH05.pdf;
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