Archive for 2009

PRN01 … not quite working as expected

Sunday, September 13th, 2009

PRN01 is one of the latest GPS satellites launched on 24 March 2009. Over the summer we have been investigating what the deal is with this satellite which is still not set to healthy.

When a GPS satellite is launched it goes through a period of a few weeks of commissioning. This period involves in-orbit checkout, etc, including the activation of the navigation payloads and others. As soon as the navigation payloads are activated many of the stations of the IGS network will start recording observations for the satellite, so in the IGS we will produce precise clocks and orbits for the satellite. A few days after that normally the satellite is declared healthy for all navigation users and the satellite is available.

For PRN01 the situation has been a bit peculiar as you can read here GPS SVN49 and L5 Signal. The satellite was essential for GPS to lock the L5 frequency for GPSIII use in the future. A reserved frequency to be used in space applications has to be used from space within a certain number of years or it is released by the ITU for anyone else that needs it. Therefore as SV-49 was going up anyway to completent the GPS constellation they decided to add an L5 demonstration payload to the satellite so that the L5 frequency filing would not be in jeopardy. But after months and months the satellite never became “healthy” so it is not available for general navigation use and it is a doutbful “spare” satellite in the GPS constellation.

My wonderful colleagues at ESOC Tim Springer and Florian Dilssner have now investigated the situation an published the results in InsideGNSS, one of the best industry publications. Read their investigations online: SVN49 and Other GPS Anomalies, Saving GPS SVN49, GPS Signal Anomalies. There are other further investigations out there of this issue and possible solutions, but these three from my colleagues at ESOC are the first three I saw published.

I will not try to summarize all the issues which are very well explained above, basically they connected the L5 demonstration payload in a spare port to feed the signal to the transmitting antenna. In doing so they have introduced significant elevation dependent errors mainly in the L1 signal. While some elevation dependent issues are sometimes observed with different GPS satellites, none have ever had it as bad as this one!

The issue continues to be under investigation and the satellite will remain unhealthy until the Air Force is satisfied the problem is completely understood and a workaround implemented.

Good positioning!!
Ignacio Romero (Nacho of the IGS!)

GPS-GLONASS biases … a receiver issue

Tuesday, June 30th, 2009

GNSS receivers decode all the signals ‘more or less’ at the same time, the time is really set by the GPS but the other systems (Glonass) have a bias so that apart from the clock bias per satellite there is an inter-system bias between the constellations (at least that is what we thought!! here is a paper I wrote sometime ago on the subject: GLONASS_POD

The plot below shows the state of the art when I was still in charge of glonass processing at ESOC some years ago!! you can see that we estimated one bias per receiver and per arc (2 days of data) and the biases are very stable each day and they are ‘grouped’ per receiver type. all the Javads tend to behave ‘similar’ and all the Ashtech Z-18 the same … but the estimated clocks were still not very good so then we estimated ‘one bias per satellite per data arc per receiver’ this is more reasonable, as the previous was an average over all the glonass satellites. The average (in the plot) is enough to get reasonable orbits.

InterSystem Bias

Now at the ESA/ESOC IGS Analysis Center we estimate the intersystem biases per receiver and satellite pair, and we have excellent orbits and clocks for glonass, the size of the intersystem biases is similar to what you see in the plot for each receiver type except each satellite/receiver pair varies a bit around those initial averages per site.

The explanation is that internally the receiver takes longer to decode the glonass satellite signals and since glonass works in different frequencies for each satellite (FDMA and not CDMA) then the bias must be frequency dependent.

Certain ‘batches’ of receivers using the same electronics produce very similar biases per satellites but it still has to be estimated all the time.

For Galileo I would assume a general intersystem bias with GPS will work but this is still unclear to me at this time, maybe many biases are need per satellite and per measurement type?? … not sure yet!!

Happy positioning!
Ignacio Romero (Nacho of the IGS!)

At the EGU General Assembly 2009

Saturday, April 25th, 2009

GPS launch opens new capability

Wednesday, April 1st, 2009

New GPS satellite!! The launch and start of operations of the new SVN49/PRN01 is excellent news. You can follow the report on the news of the launch here, for example: Delta II launches penultimate GPS-IIR satellite.

This satellite is interesting because it has an experimental L5 signal transmitter. This would be the third signal for the GPS constellation which currently transmits only on L1 and L2. The new signal on L5 is needed by the GPS space segment at this time to ensure that the ITU filing for frequency use from space does not expire which it would do so by 26 August, 2009 as reported here. So even though L5 use will be sporadic and not useful for users until Block IIF and GPS III they needed to use the frequency as planned or other users could file for use.

The satellite just launched; SVN49/PRN01 has already started transmitting the navigation signal and several IGS stations and Analysis Centers have started including the satellite in the IGS products and data (IGSMAIL-5924)

Happy positioning!

Ignacio Romero