GNSS Satellite (GIOVE-A)


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Wednesday, 15 October 2008

Galileo versus GPS

Several people have asked me "what does Galileo offer that I can not get from GPS?"

Well if you just think of your average car navigation device, be it a Garmin, a Tom Tom, Magellan, or Medion, as user you will probably not notice much difference if it is based on GPS, Galileo, or both. Only in so called "urban canyons", areas where your horizon is obscured by tall buildings (e.g. downtown Manhattan), you would see some improvement. The same holds for any GNSS receiver in a mobile phone.

However, mobile phones you typically use in the center of town where you are likely to suffer more from obstructions of the sky. Thus using a receiver which tracks both GPS and Galileo will certainly give a significant improvement. Especially the time required to get a position estimate will be shorter, which, for mobile applications is very important. So although Galileo will offer a higher accuracy compared to GPS the average usage and users will not really see much impact of that. The main impact will be the combined usage of both systems since it will double the number of available satellites and thus help in cases where the observation geometry is poor, e.g. in the afore mentioned urban canyons.

Of course the fact that Europe has been planning to build Galileo already impacted the GPS system. There are many people who believe, and I am one of them, that selective availability was turned off because Europe is building Galileo. Selective availability (SA) was a significant artifical degradation of the GPS system which gave rise to position estimates at the level of 25 meters rather than the 1 meter level achieved today. With SA turned on GPS would not be "competitive" compared to Galileo.

Furthermore, the Galileo GPS "competition" has given rise to the fact that both systems will offer civilian signals on two frequencies. The access to signals on two frequencies will allow the removal of the signal delay effects caused by the upper layers of the Earth's atmosphere, the so called ionosphere. With two frequencies this effect, which easily reaches the 10 meter level, can be removed at the millimetre level. Thus the advent of civilian signals on two frequencies constitutes a very major improvement compared to the current GNSS systems. Without the advent of Galileo it is questionable if GPS alone would have moved in that direction.

So, in conclusion, the major impact of Galileo is that it has driven the developments of GPS forwards in such a way that GPS has tried to keep up to "spec" with the Galileo plans. In this way the end users are the real "winners". Since it is also the end users, that is us, who pay the systems through their tax money this is quite fair.

One thing I am very glad about is that GPS and Galileo will be fully interoperable. This means that the receivers will not change very much compared to today and that one single receiver will be able to track both systems. Of course the increased number of satellites calls for a larger number of tracking channels but I am sure that technology can easily cope with that. To what extend dual frequency impacts the receiver complexity and thus price is unclear at the moment. This will greatly depend on what the "standard" receiver will be, single or dual frequency. I personally am convinced that dual frequency receivers will be the standard. Full GNSS constellation(s) providing dual frequency civil signals will not be avaiable before 2015. By then I am pretty sure that the receiver manufacturers will be able to make a dual frequency receiver with the same characteristics as todays single frequency receivers. Most importantly: power consumption, size, mass, and price.

So all in all the future looks bright. Especially if one considers that also GLONASS, the Russian GNSS system, is planning to follow the GPS and Galileo developments and want to be interoperable with both. That would be a truly great development.

Stay on track!

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