Archive for the 'Hardware' Category

Galileo 5,6 launch and orbit update

Thursday, August 28th, 2014

The satellites are operational as described in today’s ESA press release it indicates that the satellite finally opened their solar panels (there had also been a problem with that!)

I have created 3D images on an excellent iPad application called ‘Orbit Architect’ which I can recommend to everyone as a great tool and wonderful to explain things to children!

OrbitArchitect-Galileo 5 OrbitArchitect-E11

The differences in the orbits is very dramatic as the new satellites (on the left) travel away and towards from the Earth now more than expected. This will affected the received signal power on the ground, initial thoughts are that a 4dB signal strength difference which can mean that the signal may be too weak when at apogee and too strong when at perigee … but it is not too bad, and the signal strength can be adjusted on-board the satellite but surely not continuously so a reasonable level will have to be found.

Therefore to summarise what we have so far;

    • The new orbit ground track is not terrible but optimising the ‘navigation service’ will not be easy as the expected slot in the orbit plane will be empty and providing consistent and correct worldwide coverage will not be possible.
  • Operations will be very complicated so the satellites may never go into operations since the control segment cannot easily adjust to the completely different orbits from other Galileo satellites.
  • Navigation messages and Almanac messages which are essential to use a Navigation service will be more difficult to maintain correct and may not even be able to accommodate the current orbits. These satellite position messages are essential for the receivers to know when to expect the satellite’s signals overhead.
  • The satellites are now generating power from its solar panels and so the navigation payload checkout can be done, since these are the first two satellites from the new OHB contractor they have to be fully tested to validate the new production lines.
  • Overall I am very hopeful that satellites 5 and 6 can be fully tested and that they will be turned ON and be available at least for scientific use, even if normal navigation users will have some issue using these satellites.

    Finally the EC and ESA have established an Inquiry Commission to fully investigate the actual problem with the rocket’s third stage. Lets hope that the problem is not very significant, and I do not expect it to be, since Fregat has been used successfully in the past and the flexibility that it offers is very good for orbit transfers. Being able to turn on and off the thrusting of the Fregat is a great feature which has been validated many times already, but unfortunately some of the recent upgrades have clearly not worked as intended in the launch of Aug 22. I am sure that European space industry will learn from this problem and continue to move forward to deploy Galileo as planned.

    Happy positioning!


    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!)

    SAC helping the ESA/ESOC GPS Reprocessing

    Saturday, November 29th, 2008

    The IGS Reprocessing is a fantastic activity which I have highlighted before in this blog.

    The IGS reprocessing involves recalculating all the GPS products (Station coordinate files, Satellite orbits, Station and Satellite clocks and Earth Rotation Parameters). Most of the IGS Analysis Centers are participating in this effort to take all the IGS GPS data from 1994 to 2008 and recalculating everything with the current state-of-the-art programs and techniques. This will produce more precise and continuous GPS products for the lifetime of the IGS. All the details can be found at the IGS Analysis Center Coordinator Website on Reprocessing.

    At ESA/ESOC/OPS-GN (Navigation Support Office) we are one of the original IGS Analysis Centers so we wanted to participate in the Reprocessing. Unfortunately we had no spare processing capacity, and it was not clear what new machines should be bought to take on this challenge.

    To help resolve this situation our company, SAC s.l., provided access to the ESA/ESOC Navigation support engineers an Intel Quad Core PC with Ubuntu 64bit, named SAC01. On this new machine we could start doing compilation tests, short runs, speed tests, data handling tests, etc. The machine was bought and configured by SAC s.l. in February 2008 and located in Las Palmas, Spain, at the professional housing services of a local Internet Service Provider (ISP). The machine was accessible via ssh and via FreeNX which allows for graphical display export easily to any other internet-able computer. The machine, SAC01, has installed the ESA/ESOC software Napeos POD software:

    SAC01 Ubuntu QuadCore PC for POD ESA/ESOC Napeos POD SW on SAC01

    SAC01: Ubuntu QuadCore PC for POD

    By the end of 2008 we will have reprocessed and delivered to the IGS all the days from year 2000 to 2007, using 150 stations per day. This has involved downloading and checking about 200Gb of GNSS data from the CDDIS. The processing was executed in parallel, leaving one core free for Operating System processes, so that 3 days are run at once. This raises the temperature of the PC considerably but there is enough cooling power at the ISP to handle it!!

    The reprocessing has involved 150 stations per day (green crosses), it has generally taken 50 minutes per day (red crosses) using 30/31 satellites (blue stars), summaries of four of the years are shown below:

    2000-2003 reprocessing statistics

    2000-2003 reprocessing statistics

    Also noted are the total number of GNSS stations downloaded from the IGS (blue squares), and the stations rejected from the processing (pink squares).

    At the start of 2009 the ESOC reprocessing results from SAC for 2000 to 2007 will have been checked, submitted and validated at the IGS making them the official ESA/ESOC contribution for those 8 years. The excellent results obtained finally convinced ESA/ESOC to buy two similar machines which will be used to produce the 1994-1999 reprocessing internally.

    We are very happy at SAC s.l. to have supported ESA/ESOC in this innovative and successful way,

    Ignacio Romero

    Space is not free anymore ??

    Tuesday, January 29th, 2008

    We all heard about the Chinese destruction of their own weather satellite with a guided missile from the ground. While this appeared impressive the details of the technology were not clear for some time. Hitting a fast moving target is very hard, obviously, but if its yours so you know exactly where it is or if the satellite emits a homing signal the destruction via missile is not so impressive.

    So far we have been bombarded by the news stories of the thousands of pieces of space debris this demonstration has left behind and the Chinese have promised not to do this again since Space is supposed to be a DMZ (De-Militarized Zone), yeah right you may say, but its a nice sentiment and most nations follow it.

    I have also heard countless reports of the difficulty of getting data from the Chinese GNSS network. Aparently Chinese GNSS data is considered secret and it cannot leave the country under threat to any person transporting it! This seems to be a very excesive measure, GNSS data is necessary for internal earth studies and other scientific applications so I cannot see any reason for such secrecy.

    So I have connected these two stories after this news story reported by the Washington Times Jan 11, 2008 (Chinese missile debris) in particular this quote from the article: “Beijing also is asserting national sovereignty over all space above Chinese territory, setting up the potential for a future confrontation with the U.S., which operates intelligence and other satellites that pass over China.” and this second quote: “… China fired a laser at a U.S. satellite in December 2006.”

    This is really worrying for us GNSS users, a claim over all space over a country is so ridiculous it would be laughable if we did not have missiles destroying satellites! Even water around a country is only property of the country up to 120 miles. Claiming Space and all the GNSS data are worrying trends in my mind. I hope some sense returns to Space use and space based applications. Many satellites fly over all the countries of the world, any basic orbital understanding explains that it is not possible to avoid flying over China!! Certain satellites take measurements, some take pictures and some spy, but clearly the benefit of having satellites is very high they help keep us safe, provide warnings of storms, allow us to navigate, etc, etc. You cannot just ban all satellites over your country … and then take advantage and not share the gathered data!

    I hope this is not a taste of what is to come because as I mentioned in a previous blog entry any large scale interference with current L1 navigation messages would damage a growing sector of our economy and many future applications.

    Let us hope for some sense in the near future!

    Best regards to all!!

    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!

    GNSS options

    Friday, August 4th, 2006

    When buying a new receiver for precise applications (where sub decimeter precision is needed) it makes sense to get a multi-constellation receiver.

    A multi-constellation receiver takes the signals from the GPS and GLONASS satellite navigation constellations to calculate the receiver’s position more precisely and more robustly.

    In environments where the open-sky visibility may be compromised the multi-constellation option greatly enhances the usability and precision.

    Good luck and Happy positioning!

    Ignacio Romero