Beagle test page TU Delft

This page is for internal use only.

Bert Vermeersen (TU Delft) boarded the Clipper Stad Amsterdam in Perth (Australia) for the trip to Mauritius. One of the experiments that Bert is supervising on board is the measurement of the topography of Earth's mean sea-level, or, Geoid. For this experiment the Clipper Stad Amsterdam carries a high-precision geodetic GPS receiver which has been supplied by Fugro. This receiver will be able to measure the sea-level height with an accuracy of about 1 decimeter. The receiver will continuously track the US Global Positioning Satellites (GPS) and their Russion cointerpart GLONASS. The receiver will also receive corrections to the GPS and GLONASS satellite from a geostationary satelliet over the Indian Ocean region in order to facilitate the high accuracy of the positions. These corrections are computed by Fugro from a global network of tracking stations.

First results

The GPS receivers and antenna were installed on Monday March 15th. The location for the antenna was far from ideal; it was mounted on a railing on the "sloependek", and tracking of the GPS and GLONASS satellites has been very problematic. The two main problem are that (i) only a very low number of satellites are being tracked, and (ii) for the satellites that are being tracked there are many interruptions due to obstructions on deck and the rolling and pitching of the ship. As a result the position solution often fails due to lack of satellites or satellites moving in and out the solution. Also, after acquiring a sufficient number of satellites, it takes at least ten minutes for the position solution to converge.

Despite these problems the receiver has been collecting data. We were able to downloaded three days of data, until Thursday May 18th, when - as announced - the Internet connection to the Clipper Stad Amsterdam was lost. From these three days of data we have been able to compute ten geoid heights.


This figure on the left shows the color codes EGM96 geoid heights near the coast of Western Australia. The figure on the right shows the bathymetry and topography near the coast of Western Australia. The track of the Clipper Stad Amsterdam is shown in red. The positions were the Clipper Stad Amsterdam has taken GPS height measurements are indicated by crosses. The great-circle route between Perth and Mauritius is shown as well. Clearly, the clipper is taken a more northerly route in order to pick up the favourable trade winds.


This figure shows geoid height as obtained from the GPS measurements on board the Clipper Stad Amsterdam (Beagle II) with error bars, and compares these to the EGM96 model geoid height.

How do we interpret these measurements?

The differences between the GPS measured Geoid heights on board the Clipper Stad Amsterdam and the model Geoid heights can be explained as follows

If the GPS would have performed better would have had a continuous line with measured geoid and dynamic ocean heights instead of the few individual points we have now.

GPS+GLONASS tracking results

Those we are interested in the raw GPS position data can have a look at the following plots for the XP (GPS only) and G2 (GPS+GLONASS) services 15mar_xp.png, 16mar_xp.png, 17mar_xp.png, 18mar_xp.png, 15mar_g2.png, 16mar_g2.png, 17mar_g2.png and 18mar_g2.png. The position solutions that were used in the computation of geoid height are shown in green. The standard deviation which is given for the position solutions is also affected by the motion of the ship. It is clear from these plots that the number of satellites being tracked is very low, we should have almost the double number of what we are tracking now, and also GLONASS is not helping (actually the G2 solutions are worse than the XP).

Can we further improve the GPS results?

Of course, a better location for the GPS+GLONASS antenna could solve some of these problems. However, it is a sailing ship, and there are always mast and other obstructions, but for sure, a better position could have been found. Unfortunaly, we were severely restricted in our choice for the antenna location due to the availability of wiring. It is unlikely that we can change the antenna position.

Post-processing of the data could help. The raw measurements are stored and better results could be obtained from post-processing as compared to the real-time processing on board. However, don't expect wonders, as this will not solve the tracking problems. Finally, in the current processing only measurements for which we have a carrier phase solution (FQ=5, FLOAT) were included. If the less accurate DGPS position, using the code only (FQ=2), is included we would get more data points.

Geoid and bathymetry along the route for Perth to Mauritius


EGM96 Geoid Heights (left) and bathymetry (right) along the great-circle route from Perth to Mauritius.


EGM96 Geoid Heights for the Indian Ocean region. The great-circle route between Perth and Mauritius is shown in black. The Geoid Heights along this track have been plotted in the previous figure. The crosses are the positions Vening Meinesz took gravity measurements with the K18.


Bathymetry and topography of the Indian Ocean region (ETOPO5). The great-circle route between Perth and Mauritius is shown in black. The bathymetry along this track have been plotted in the previous figure.

Test of Fugro GPS data

The GPS receiver on board the Clipper Stad Amsterdam has been supplied by Fugro. This is dual frequency L1/L2 GPS and GLONASS receiver based on the NovAtel OEM5 board. The receiver will also receive propriatory correction messages from a geostationary satellite. These corrections are computed by Fugro from a global network of tracking stations. The receiver will provide two services: XP with GPS only, and G2 with GPS and Glonass.

24 hour test dataset


Results for Fugro G2 service (GPS and GLONASS). The top plots shows the position differences with respect to the mean. The receiver is at rest. The middle plot shows the height above the international reference ellipsoid. The bottom plot shows the number of satellites, horizontal DOP (Dillution of Precision), latency of the G2 corrections and solution quality (fq).


Results for Fugro XP service (GPS only). The top plots shows the position differences with respect to the mean. The receiver is at rest. The middle plot shows the height above the international reference ellipsoid. The bottom plot shows the number of satellites, horizontal DOP (Dillution of Precision), latency of the G2 corrections and solution quality (fq).

2 hour test dataset


Results for Fugro G2 service (GPS and GLONASS). The top plots shows the position differences with respect to the mean. The receiver is at rest. The middle plot shows the height above the international reference ellipsoid. The bottom plot shows the number of satellites, horizontal DOP (Dillution of Precision), latency of the G2 corrections and solution quality (fq).


Results for Fugro XP service (GPS only). The top plots shows the position differences with respect to the mean. The receiver is at rest. The middle plot shows the height above the international reference ellipsoid. The bottom plot shows the number of satellites, horizontal DOP (Dillution of Precision), latency of the G2 corrections and solution quality (fq).

Files

The matlab scripts used to produce the plots on this page can be found in the directory ./matlab. The test data is located in the directory ./testdata. The data collected during the trip can be found in the directory ./data in the

Further links

http://beagle.vpro.nl
http://www.stadamsterdam.com
TU Delft Beagle website

Plots created by Hans van der Marel, TU Delft.
Last updated 19 March 2010.