GNSS Satellite (GIOVE-A)


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Thursday, 24 December 2009

GNSS Year in Review 2009

So what happened in the GNSS world in 2009!?

Well....not as much as we hoped for but some progress was made. Most progress was made "behind the scenes".


The most exiting and most "visible" was the launch of the GPS satellite SVN-49. This satellite brought new signals to the GPS system as it carries an experimental payload that allows the transmission of the new (future) GPS signals on the L5 band. The L5 experiment was bitterly needed because of the significant delays in getting the GPS IIF (F for Future) satellites ready. Thus the GPS system was at risk of loosing the L5 frequency allocation if they would not get a satellite up and "beeping" on the L5 frequency. The European Galileo system faced, and still faces, a similar challenge for which the launched the Giove-A and Giove-B experimental satellites. Unfortunately, the experimental character of the SVN-49 satellite actually caused some unexpected ill effects on the satellite on which we reported in our BLOG. This is the reason the satellite is still unhealthy although it is planned to turn the satellite healthy soon. However, the satellite will never perform as good as the other GPS satellites due to its anomaly. Besides SVN-49 also SVN-50 was launched marking the last GPS Block IIR-M satellite launch. The next GPS satellite to be launched will be the of the Block IIF type, currently scheduled for May 2010. An other "sad" event in 2009 was that SVN-35 was taken out of service. This satellite was special as it was one of only two GPS satellites that carries a Satellite Laser Ranging reflector array. The loss of this satellite is a grave loss for the scientific world especially because currently no SLR reflector arrays are foreseen on the GPS Block IIF nor on the first batch of the GPS Block III satellites. Hopefully the second batch of GPS Block III satellites will correct this "oversight" of the GPS system.


The most solid progress was made by the GLONASS system. Firstly, one of the three satellites launched in December 2008, GLO-729, is carrying a brand new SLR reflector array design which is 1.5 times better then the previous arrays. This is very exiting because it allows daylight tracking of this satellite which is an absolute "first" in the GNSS world. So far GNSS satellites could only be tracked by the SLR stations during the night. Furhtermore, an other successful triplet launch took place on December 14, 2009. However, also the GLONASS system did have its problems this year. One of the new satellites launched in 2008, GLO-726, developed a problem with its signal generator. As the satellites planned for launch in September 2009 used signal generators from the same batch as this faulty satellite the September launched was cancelled in order to check the satellites and replace the signal generators. The satellites are now scheduled for launch in February 2010. Nevertheless, the progress of GLONASS remains remarkable and they have managed to stick to the schedule that was laid out in 2005! In the space business that is an really astonishing accomplishment!


On the Galileo front things have been very quiet. Giove-A and Giove-B remain to operate which especially for Giove-A is a great accomplishment as it is well past its design life time. However, the schedule of the In Orbit Validation (IOV) seems to remain a "running target". In June the first launch was planned for September 2010. Meanwhile, rumours say the launch has been postponed until May 2011. The reasons for these delays are completely unclear and a more open communication policy would do the project a lot of good. The same holds for the data policy. Since 2005 Giove data has been gathered but the data is only available to ESA "trusted users". Unfortunately, it is practically impossible to obtain a trusted user status with ESA. So the Giove data is only accessible to a very limited number of institutes and thus limits the scientific analysis of the data. Under the surface a lot of things are happening in the Galileo project. The cooperation between ESA and the EU has been improved although it is certainly still not optimal. And a lot of progress has been made for awarding the contracts. The contracts should have been awarded early in 2009 but the process has, not unexpectedly, taken longer then planned. So also for 2010 visibly nothing much will be happening with Galileo. We will have to wait until 2011, at least.


Some progress was made for the COMPASS/Beidou system but since no data is publicly available I can not say too much about it. To my understanding there is still only 1 MEO satellite (MEO is the typical GNSS orbit) and a couple of GEO satellites. One additional GEO satellite was launched but also one was lost and was drifting through the GEO orbit causing quite some concerns for other GEO satellite operators (GEO is the orbits used for most telecommunication satellites). A "wild" satellite in this orbit is very dangerous and can cause a lot of damage.

The Japanese regional QZSS system is making good progress. The signal generator is currently undergoing in space testing as it is being flown on a GEO satellite. The first satellite will be launched in 2010. In principle three satellites are planned but currently funding for only 1 satellite exists.

The only thing remaing to be said is....
Merry Christmas

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Thursday, 25 June 2009

SVN-49 Anomaly Revealed

After writing my two earlier blog's on this issue I decided to write a full article about this SVN 49 issue and submit it to Inside GNSS. The article will be printed in the July/August 2009 issue but because of its high news value it is available online now.

Enjoy the reading!

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Thursday, 14 May 2009

A detailed look at GPS Satellite SVN-49/PRN-01

As promised in my post "Problem with new GPS Satellite SVN-49/PRN-01"here some details regarding what we observe when we analyse the data of this satellite.

Below are two plots. The first one showing the pseudo range residuals of a "normal" GPS satellites. We randomly picked satellite SVN-38/PRN08 for this. The residuals are based on all the observations taken by the stations observing this satellite. The data used was from day 100 in 2009, or in normal date April 10, 2009. The residuals are plotted as a function of the elevation of the satellite above the local horizon of the observing station. That means that at 90 degrees elevation the satellite stands right above the station whereas at 10 degrees elevation the satellite is very low on the horizon. In this figure one can clearly see the increase of the noise of the observations at low elevations which is a well known phenomenon mainly caused by so called "multipath" effects.

The second figure shows the same picture but for our the new GPS satellite SVN-49/PRN01. The behavior of the residuals shows a clear signature which is obviously elevation dependent. Interestingly enought the carrier phase observation residuals do not show such a signature. This indicates that the problem is in the pseudo range observations, e.g., in the so called group delay.

However, the residuals show that the problem is only at the few meter level. But somehow the GPS operators see much larger problems. How and where the see those is still unclear to me. However, to my understanding it is possible to measure the differences between the observations on board of the satellite. Possilbe the >100 meter effects are observed in such measurments. However, for normal users as us there is only a few meter problem which we could live with, more or less. The really big problem comes from the fact that the GPS system operators are trying to solve this problem by changing the broadcast ephemerides. This becomes clear if we compare an orbit estimated by ourselves with the orbit information broadcasted by the GPS satellites. Note that our estimated orbits have a precission of a few centimeters whereas the broadcast ephemeris is normally at the few meter level. The figure below shows such a orbit comparison of the GPS satellites with PRN 1, 2, 3, 4, and 5. The differences for the "normal" GPS satellites is at the few meter level. For PRN 1 the differences amount up to 60 kilometers!!! This of course makes the satellite completely useless for any processing.

All in all the problems with this satellite do not seem to be very severe except for the broadcast ephemerides which are truely horrid!

So whatever you do with GPS do not use the data of the new SVN-49/PRN01 unless you know what you are doing!

(ps. I appologize for the poor quality of the plots. Have to find a better tool to do these plots...)

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Monday, 11 May 2009

Problem with new GPS Satellite SVN-49/PRN-01

It looks as if the newest GPS satellite has a significant problem. This is the GPS satellite which also broadcast signals on the new L5 frequency to ensure that frequencies for the GPS system. The L5 signals were planned to be available from the new GPS Block II-F satellites but due to delays in building these satellites it became necessary as kind of an "emergency rescue" of the L5 frequency band to launch launch some other satellite to use the frequency. The "perfect" solution was found to add the L5 capability to one of the Block II-R satellites. So this was done for the SVN-49/PRN-01 satellite that was launched on March 29, 2009.

However, it seems that something has gone wrong with the signals of this satellite. In my work where we try to get the highest precission out of the GPS system, meaning orbits at the ~20mm level and receiver positions at the ~1mm level, we can see a clear pattern in the pseudo range residuals of this satellite. Now since we typically use the carrier phase observations and not the code this is not really a problem for our type of work. However, the few meter (!!!!) pseudo range residuals are disturbing and they show a clear dependency on elevation.

However, what is more disturbing is that on the GPS system side they seem to be getting even larger effects then the few meters we see in our work. So far this is something I have not yet been able to understand. It seems the GPS system operators see problems with this satellite at the 100 to 200 meter level!!! A true catastrophy! As work around for this problem they GPS system is applying a huge antenna offset when broadcasting the ephemerides and clock corrections of this satellite. These problems also explain why this satellite is still not declared healthy. In fact there is some speculation that because of the observed problems it may never be set healthy.

So in summary the facts are:
  1. There is a significant problem difference the code and the phase observations
  2. The phase observations seem to be fine as we can do integer ambiguity resolution without too much problems (but of course one can not use the code observations for aiding the ambiguity resolution)
  3. The observed differences point in the direction of a significant "group delay". This is a delay which effects the code observations but not the phase observations
  4. The GPS system is observing even larger problems (>100m)!? We do not see those problems however!?
In any case it may be that this GPS satellite will never be set healthy making the L5 experiment a rather costly one...

As soon as I find out more details I will let you know!


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