Thursday, May 3, 2012

Improving Galileo's Accuracy with only 18 Satellites

I'm just back from the European Navigation Conference in Gdansk Poland, where I presented a paper on improving the accuracy of the Initial Galileo navigation constellation.  Europe now has all 18 satellites funded to reach their Initial Operating Capability (IOC), currently slated for 2015 sometime.  With only 18 satellites, there is 100% coverage (at least 4 satellites in view at any time, globally), but the orientation of those satellites will not always be ideal.  In fact, there will be times during the day when Galileo will suffer from Dilution of Precision (DOP) spikes for times longer than 30 minutes.  This will be seen as Galileo outages by receivers.  These times are dispersed throughout the day; other times during the day navigation accuracy is not bad.
In my paper, I outline how the Galileo Control Center (GCC) can improve average user accuracy by improving the upload times for each satellite.  Essentially, the more frequent the uploads of predicted satellite orbital and clock states, the more accurate the Galileo navigation satellites will be.
I use the notion of a Signal-In-Space User Range Error (SISURE) that defines how accurate each Galileo satellite is.  The lower the SISURE, the more accurate a user's position.  When the GCC uploads its predictions more frequently, the SISURE will be smaller.  This is because the SISURE is a difference between the actual satellite position and time and the predicted satellite position and time, uploaded to the satellite and broadcast to the users for use in calculating their positions.  The older the predictions are, the higher the SISURE is, the larger the user's error is.

Below is a picture of 24 hour average navigation accuracy with each Galileo satellite having a SISURE of 2 meters.  Note that because of the geometry of the 18 satellites, there are large mid-latitude bands that suffer more from DOP spikes.  These will go away when more satellites are launched.  The average accuracy across the globe for this configuration is 5.7 meters.

Now, if the GCC can maintain an upload schedule that limits the SISURE to 0.5 meters, a more accurate result can be seen:

In this scenario the average global accuracy drops to 1.9 meters - a big improvement.  Notice also how the DOP spikes are minimized in the mid-latitude bands.

This is only one of many other methods that can be done to improve accuracy with an initial Galileo constellation.  Working to keep the user's navigation error small will gain a lot of trust for Galileo in the user base, making it a better system from the start.

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