Archive for 2007

Selective Availability OFF forever

Saturday, December 15th, 2007

I was a recent arrival to the GNSS field in the year 2000 when president Clinton turned off by surprise, to us at ESA at least, the SA degradation to the GPS satellite clocks. At that time at the IGS we were only estimating orbits and clocks and for our work this SA-off had no immediate effect. It did have a longer term effect in the IGS as we started to plan for a clock prediction product latter materialized in the ultra-rapid product updated 4 times a day with 24 hours of predicted orbits and clocks. This product can now substitute the broadcast navigation message for real-time users to provide improved real-time positioning results.

Now president Bush has cleared the air and declared SA dead, by not adding the capabilty in the design of the new GPS satellites, as had been recommended by the DoD. I believe that this reflects the fact that a re-introduction of SA would harm the economic fabric that has been built of single frequency users which rely on stable satellite clocks for increased positioning performance.

SA was artificially “moving the satellite clock” so that the timing measurement which underlie the navigation system could not be as precised as it should be. The GPS satellites have 4 very precise atomic clocks on-board which are used to generate the timing messages, if you purposefully degrade the atomic clock output superposing a signal the timing messages are affected and the position cannot be calculated as precisely. The degradation was based on a function which only authorised military users could effectively remove from the timing message to recover the inherent accuracy of the system. Since SA has been off for almost 8 years and with the improvements in the quality of the GPS navigation message the GPS currently delivers accuracies much higher than originally specified for the civilian single frequency users.

The clock bias values calculated for that day for PRN10 easily show the jump from SA to no-SA on May 4, 2000 at 04:00 UTC indicating the recovery of the stable satellite clock bias :

Clock Bias PRN10, May 2, 2000, SA turned OFF!!

Galileo has the money!!

Wednesday, December 12th, 2007

It was obvious that at this stage we are not going to stop Galileo but how to pay for it was never a trivial matter. Even though the system will cost significantly less than a line of high-speed rail its seems like satellite based systems have a harder time of finding money than land-based transport systems. Even though I have to say that overall the benefits of both are great the satellite based systems are much more cost effective, but that is my personal feeling without knowing all the numbers.

Anyway the good thing is that the European Union fund money left over from the CAP (Common Agricultural Policy) for Galileo. I love the idea that the farmers had a good year so they did not need so much funding from the EU and the left-over money goes to the high-flying, high-technology Galileo system.

That Europe needs Galileo is obvious but we are also finding that the world needs it too and the Galileo development slow as it has been has already spurred tremendous progress on GLONASS (Rusia), Compass (China), and GPS III making all the players more open to interoperability and reaching more global understandings. All of this from the promise of Galileo!! … what will happen when it is actually operational … :)

GGTO – an interesting correction

Thursday, November 8th, 2007

It turns out, as reported by Inside GNSS (Sept/Oct 2007), that of course the GPS and Galileo time-scales will not be the same. GGTO is the GPS-Galileo Time Offset The navigation solution calculated by receivers using signals from both navigation systems will incur in an error if the GGTO is not accounted for. The Galileo and GPS navigation messages will include the GGTO which will allow the measurements from each system to be properly combined to a common time. the pseduoranges determined with Galileo will be referenced to the Galileo System Time, and the GPS pseudoranges to the GPS System Time. But since the system times are not the same the offset between the times will be given as the GGTO. As reported by Inside GNSS (Sept/Oct 2007) position calculations are not too sensitive to errors in the GGTO reported in the Navigation message, we will see!

We do have some experience with this as we currently have many dual-system receivers tracking GPS and GLONASS. Using the raw data from these receivers requires that at least we estimate a common time offset between the GLONASS pseudoranges and the GPS pseudoranges. The time offset in this case are hardware dependent since they depend greatly on the type of receiver for the values, so it has been speculated that decorrelating and processing the GLONASS signal takes longer in the dual-system receivers even if the receivers timestamps all the pseudoranges as occuring at the same time, cleary they are not at the same time!!

Recently it has been shown that actually each receiver-GLONASS satellite link requires an individual time offset calculation for the estimation of all the GLONASS quantities to be correct. IF an individual correction is not calculated in each receiver-GLONASS satellite link the error goes into the estimated satellite clock bias and it can be as large as 5 or 6 ns.

GPS Control Center updated!

Tuesday, October 16th, 2007

Finally the GPS Control Center has been updated so that it can deal with the increasing numbers of active satellites.

No doubt this is an excellent development as it comes together with improved navigation messages (they have been improving for some time now) and better management of satellite outages.

Where is GNSS heading

Sunday, September 9th, 2007

We currently only have GPS flying complete and after many years working with GLONASS (at ESA/ESOC we are one of only two worldwide Center’s estimating GLONASS orbits continuously for over 7 yrs), I am not too optimistic that GLONASS will deploy a complete constellation anytime soon. We have been going from GLONASS constellation of 14 to 11 to 14 to 12 satellites for all these years the types of satellites change, as the old ones are decomissioned and new ones launched, but the numbers are very stable, and its not positive so far!

So we just have GPS and the politics of the GPS upgrades and the longevity of the satellites has kept new features and upgrades from being introduced. Maybe the problem was the obsolete GPS Control System which is being upgraded in Sept/Oct 2007 (I will write more about this soon). It is possible that the new control system will allow the constellation to better manage increasing number of satellites so that the new satellites waiting on the ground can be launched soon.

In general, though, I am not positive that this will happen due to overall decisions based on this so-called ‘launch on need’ policy. This idea is not to launch unless it is needed due to an incomplete constellation. Of course we are far away from having an incomplete constellation, on last count we have 31 satellites flying, the old PRN23 has just been introduced as PRN32 and only PRN15 appears unused. So at this rate we will not see significant new capabilities until many more years go by!

To me this is really unfortunate but also an opportunity for our Galileo. IF we can somehow get Galileo up and running in the next 5 to 6 yrs with its extra frequencies and measurements it could be possible that LBS (Location Based Services) companies will adopt them and make Galileo the GNSS leader. GPS will be the backbone of any LBS system, to be sure, but any added edge that using Galileo can add would make the European constellation a very important player in the GNSS world much faster than expected. This would also help to change the GPS attitudes and so that all new satellites get launched and introduced as quickly as possible.

What do you think?

RealTrack football (I mean soccer!)

Wednesday, August 15th, 2007

I am proud to highlight this product from my home country, I am of course spanish and this company is from Almeria in southern spain. The product RealTrack football is a real-time traning system using GPS positions of each to aid in training regimes, total distances covered, tactical mapping using the recorderd positions of the players, etc.

The company that has invented this system is C&M Communications . >There are two version as described on their web the ‘Lite’ and ‘Pro’ versions. This product seems to be a really innovative LBS (Location Based Service) which I could have never imagined.

I certainly think that such a system can be very useful to keep records of the players that are training correctly and putting in a good effort on the training field and those that are slacking off! we all remember ‘pretending’ to run around the field but actually cutting all the corners. RealTrack Football can easily provide feedback to the training staff so that they can customised, adapt and monitor their training plans in real-time and over the long-term.

My congratulations to the company and I wish them all the best!

Good luck to all of you

what GNSS signals do we have …

Wednesday, June 13th, 2007

It is interesting sometimes to take a step back and remember what signals we actually have as they limit completely what we can accomplish. It has been amazing what engineers worldwide have been doing with these basic and limited signals. Noone at the start of GPS could have imagined the number of applications and how well the brilliant world-wide engineers have made them work.

So to take the step back we go back to the basic GPS and GLONASS signals. There are two frequencies being used by each GNSS satellite on the L-band. GPS uses the same frequencies for each satellite 1575 MHz (L1) and 1227 MHz (L2) and encodes ranging messages on both. GLONASS uses two distinct frequencies for each satellite (although antipode satellites may use the same frequencies) the GLONASS frequency ranges are 1610.6-1613.8 MHz (L1) and 1240-1260 MHz (L2).

The ranging messages encoded on the L1 signal give the following ranging information:

  • C1 ; Ranging to the satellite based on the Civil message.
  • P1 ; Ranging to the satellite based on the Encrypted (high-security) message.
  • L1 ; Number of cycles since satellite acquisition.

on L2 we have:

  • P2 ; Ranging to the satellite based on the Encrypted (high-security) message.
  • L2 ; Number of cycles since satellite acquisition.

a limited number of satellites are transmitting C2 now but not many receivers are providing this signal regularly. until a significant number of satellites have C2 capabilities it will remain an oddity in the constellation. Preliminary studies suggest the signal is good but for receivers that provide P2 this one is preferred. The issue of the P2-C2 calibration has also to be addressed as it has been for the P1-C1 bias in case of mixing the measurements in any calculations.

Good luck!

On the real Galileo benefits

Monday, June 4th, 2007

Satellite Navigation cannot improve with just one system. No matter how well GPS has worked so far it is becoming antiquated and could be surpassed in terms of signals in space by GLONASS in the very near future. It is VERY difficult for any user community to have ANY impact on any decision in the GPS program, noted by the continued delays in the GPS III decisions and funding. How many comittes are there advising or supervising GPS?! and who has the final say …

There are good reasons for some of these GPS III delays and management complications, but lets face it for the past 20 yrs we have had the same signal, very similar spacecraft and a slowly improving mission control which has finally improved the navigation message when at the IGS we were doing it years before.

I am not complaining at all I love GPS and all the people that have made it happen but it is stuck and only thanks to Galileo being approved has real movement started to happen.

I am also confused by articles such as this from the Editors of GPSworld (Out in Front ) I like GPSworld very much and I like the article but come on, so Galileo is not exactly better than the GPS of the future in all its claims!! get a life … I am convinced we would probably NOT have a GPS of the future if Galileo had not been started.

The benefits of Galileo come from other places:

  1. It reduces the pressure on GPS and promotes healthy competition.
  2. It helps to push GPS stakeholders “to stay ahead” of Galileo developments, otherwise we would live in a C1, L1, L2, P2, P1 world for how much longer?
  3. It allows others to develop capabilities complementary to the ones in the US. In the long run having both continents with competent professionals and capable companies helps the overall climate of cooperation and understanding. Pretending GPS can solve all future problems leads us to a terrible state monopoly situation.
  4. Galileo will provide integrity for ALL constellations, after the EGNOS project Europe is a key player in both GPS and GLONASS integrity algorithms so we can get integrity for all from one signal. I doubt GPS or GLONASS would do the same now or in the future.
  5. We have already done it in Europe! following the models of INMARSAT, EUTELSAT and EUMETSAT we have already created large multi-national satellite operators at first fully dependant on ESA for technology and national governments for funding and control and later independent and even completely private. Galileo will follow this same model and all the better for it.

The Galileo discussion always has to be put in the context of what GPS is actually doing and as long as GPS III and Galileo developments are close to each other both projects will be OK. They will push each other through some tough financial and technical challenges ahead. In the end no-one will develop huge new mass-market applications until both systems offer similar levels of service so that redundancy is at a maximum.

Happy Positioning!

GLONASS and GPS differences

Saturday, April 14th, 2007

You have probably already heard that there are actually TWO satellite constellations providing positioning signals available to everyone. the first is the well-known American GPS and the second is the Russian system GLONASS.

Both system do exactly the same thing. They provide a signal-in-space beamed down to earth from so-called Medium Earth Orbit (MEO) using dedicated satellites. Both systems provide a Civilian service and a “restricted” (Military) service.

The main differences as of now are:

  1. GLONASS is not complete. Whereas GPS has 32 satellites transmitting signals GLONASS only has 14 which does not guarantee world-wide coverage.
  2. GLONASS uses FDMA for its transmissions whereas GPS uses CDMA. in FDMA each satellite uses a different frequecy to transmit the same ranging code. in CDMA each satellite uses the same frequency with different ranging codes per satellite.
  3. GLONASS is providing a second Civilian signal on L2 which helps civilian users to properly eliminate the ionospheric error.
  4. GLONASS satellites are upgraded more often. This is due to a russian satellite design constraint or due to cost-saving measures but the GLONASS satellites have to be replaced much more often than GPS ones (every 5 to 7 yrs). This appears as a problem but it allows GLONASS to introduce new capabilities much faster than GPS.

The IGS provides precise orbit products and clocks for both constelations GLONASS and GPS as a public service. These orbits can be used together with precise GLONASS observations to perform the same type of calculations as with GPS observations. Many researchers are also supplementing GPS observations with GLONASS observations to increase precision and robustness of derived products.

Best Regards to all and Happy positioning.

Semana Geomatica

Wednesday, February 28th, 2007

Hemos estado en la Semana Geomatica que tuvo lugar en la Fira de Barcelona del 20 al 23 Febrero, y tambien hemos participado en el evento de CTAE (Technology Centre for the Aeronautical and Space industry) que organizó un evento para empresas de GNSS el 21 Feb en las mismas instalaciones. Las presentaciones tecnicas sobre Geodesia fueron muy concurridas y de un alto nivel, sobre todo las que hablaban de las mediciones del nivel del mar combinando datos de altimetria de satélites en Orbita baja y mediciones GNSS.

We have recently attended the Semana Geomatica in Barcelona for two days and at the same time participated in a CTAE (Technology Centre for the Aeronautical and Space industry) day which held a one day business exchange event where 20+ businesses participated in exchanging ideas and sharing interest to foster future collaboration projects. The scientific presentations were very interesting in particular the ones that dealt with the sea-level studies in the Mediterranean which combine satellite base altimetry measurements with in-situ GNSS measurements.