Archive for the 'Comments and Trends' 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!

    Nacho

    Galileo bad orbit insertion

    Sunday, August 24th, 2014

    Unfortunately the launch of the two latest Galileo satellites has left a very bad taste in our mouth.

    The entire GNSS Community was looking forward to the next step in the Galileo constellation deployment. The launch went well from French Guyana aboard the Soyuz rocket, but apparently something went wrong at the orbit insertion point and the Fregat transfer stage did not insert into the correct orbit. Fortunately the separation of the satellites was correct and the new satellites are both under control of the ESA/ESOC center.

    Looking at the TLE’s (orbit two line elements) from celestrak.com, Thank guys! I have plotted using the Mac application SatTrackerBasic (from ClearandDark.com) and for comparison included one of the currently active Galileo satellites (E11), the three newly launched elements are 2 Galileo satellites and one Fregat module and they are all still flying close together as expected, their TLEs are only referred to as 14050A, 14050B, 14050C it is not clear to me which are the satellites and which Fregat, so I have selected 14050A as an example. In the ground track plot below showing two full orbits you can clearly see the effect of the large eccentricity (0.23) in that the 14050A ground track is ’tilted’ over the earth’s map. Whereas the E11 has a very low eccentricity and thus its distance to the surface of the earth is constant in each revolution around the planet at around 23200 Km, the new satellites have a point of closest approach to the earth’s surface of 13800 Km and a farthest point of 25900 Km (both are preliminary values!).

    Screen Shot 2014-08-24 at 16.01.11

    Finally the orbit inclination is also wrong, you can see this as the point furthest south and north of the orbit track over the Earth’s map. A normal Galileo orbit (such as E11 shown in the plot) should reach to +/- 55 deg in latitude, but in the new orbits you can clearly see that they only have a 49 deg inclination.

    The initial report from arianespace confirms all these findings: Arianespace initial report Galileo FOC M1 launch

    I am sure that my excellent Flight Dynamics colleagues at ESOC will do their best to make the satellites useful for the final Galileo constellation. Considering that the satellites are apparently in good working order they could still provide their navigation signal as expected and be useful over their lifetime for position, navigation and scientific investigations.

    The big initial concern as remarked by my colleague Dr. Tim Springer in the last entry of his blog (Groundtrack of the Galileo Satellites) is that the two new satellites will be crossing the orbital planes of other Navigation satellite constellations such as GPS and GLONASS which is indeed scary in terms of possible collision warnings, etc, since it could affect the operations of all the GNSS constellations and cause possible reduction of those other satellite’s lifetimes as on-board station-keeping fuel would be consumed faster than planned if collision avoidance manoeuvres should ever be needed. In-space collisions remain very unlikely in any case but with these orbits there is a remote chance they could happen.

    Let us hope for the best and that the satellites 5 and 6 of Galileo can still be positive for our growing GNSS family!

    Happy positioning!

    Nacho

    Additional news stories;

      Eumetsat Meteo Satellite Conference

      Wednesday, September 22nd, 2010

      I just attended the Eumetsat Meteo Satellite Conference in Cordoba, Spain at a beautiful location right by the Mezquita.

      The event brings together many satellite Meteo researchers. As you know most Meteorological satellites carry on-board many different scientific instruments, and this conference brings together a very heterogeneous group of scientific people which as very informative and useful to put our GNSS engineering work into context.

      I was sent to the conference to present the GSN poster. The GSN is a support service that i have been a part of since 2003 through initial study, conceptual design, implementation, launch, improvements and maintenance. It provides Eumetsat with support GPS products on a very strict time basis to process the Metop satellite GRAS radio occultation instrument. The poster can be seen below, Enjoy!

















      It was a fun short trip and I am glad to be able to represent the GSN project in this international conference.

      All the best!
      Nacho

      Monitoring the Sky for Astronomy with GPS

      Saturday, February 6th, 2010

      SAC has continued to support the IAC (Instituto Astrofisico de Canarias) by calculating the Precipitable Water Vapor (PWV) at the Observatorio Roque de los Muchachos (ORM) in La Palma. The island of La Palma is about 250 Km from Gran Canaria where SAC is based, both in the archipelago of the Canary Islands, Spain, of course, and both beautiful places!!

      There is a public GPS station at the ORM established since 2001 by the IGN-E (Instituto Geográfico Nacional de España) , with the designation LPAL, as part of the coordinate system definition for the Canary Islands and as part of the regional EUREF project. The station’s historical data is available as part of the EUREF permanent network.

      Using precise orbits from the IGS Final products the data from a network of stations including LPAL has been solved to extract the Tropospheric Zenith Delay (TZD) at the ORM every two hours for a period of 10 years from 2001 to the end of 2009. The TZD is used to calculate the PWV as described in the enclosed published paper and presentation which the IAC have produced with SAC’s help. I encourage you to download and read the paper below:

      SPIE2009_PWV1

      The IAC have concluded that the PWV time series produced by SAC for the ORM is correct by correlating with independent radiometer observations that had been undertaken at the ORM during several weeks in 2001 and 2002. The level of correlation of the GPS PWV data to the local radiometer measurements is around 93%, thus confirming the very high-quality of the calculations undertaken by SAC with the LPAL GPS data. As a ‘control’ location SAC included in the calculations also the permanent station MKEA in Mauna Kea, Hawaii, the site of many astronomical Observatories as the ORM is, and the correlation was also very high with independent measurements from Hawaii.

      This has lead the IAC to be able to conclude that in terms of PWV content in the atmosphere (a very significant determinant of sky clarity for ground-based infrared astronomical observations), the ORM is of similar quality than Mauna Kea, a location some 1300 m higher in elevation, you can read the conclusions in the pdf above.

      To finalise the PWV study of the ORM SAC and the IAC came together recently and made a short presentation to explain to the rest of the IAC what we have done in this study pf the PWV at the ORM. You can find the presentation below.

      sample seminar

      ORMPWVseminar_IR.pdf

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

      IGS Infrastructure Committee / Comité de Infraestructura del IGS

      Saturday, January 9th, 2010

      I was honored by my IGS colleagues last year by being selected as the first IGS Infrastructure Committee Chairman. This is a great honor and I hope to be able to make the IC relevant and responsive within the IGS to all infrastructure issues. At the IC 13 IGS members are tasked with providing advice, analysis, in-depth comments, monitoring ideas, etc to the IGS Governing Board and Central Bureau (the executive arms of the IGS)

      At the IC we are taking up very significant issues that have gone unaddressed for too long in the IGS in particular in terms of the IGS station network as it has grown beyond many’s expectations. The network is a collection of resources from many different organizations, which voluntarily have agreed to operate their permanent GNSS stations following the IGS standards and which provide the backbone of GNSS observations that go into the different IGS products: orbits, clocks, station positions, TZD, etc. The IGS network:

      The IGS worldwide network of permanent GNSS stations have recently reached 420. The IC will be initially trying to understand the following: how many IGS are regularly missing from the Data Centers, how many stations have RINEX header inconsistencies with their published station logs, what RINEX data format version are IGS stations using, etc. Many of these basic issues are not being properly documented and tracked to see what the real performance of the network as a whole is.

      The IC is particularly concerned with trying to understand the station and format issues so that they can be properly addressed. The main concern is that the IGS stations are able to provide access to the latest International Terrestrial Reference Frame by having stable position and velocity estimations for the stations, and by having stations with good data and with no jumps.

      The IC will be making recommendations to the IGS GB and CB on the following issues: the raw navigation bit message, improved monitoring for stations, in-depth station performance analysis, RINEX format, RTCM streaming data format, etc.

      I look forward to trying to make a positive impact in the IGS IC!
      Ignacio Romero (Nacho of the IGS!)

      At the EGU General Assembly 2009

      Saturday, April 25th, 2009

      I was just at the European Geophysical Union General Meeting (EGU) in Vienna, Austria. At this meeting there was a strong presence from ESA/ESOC as both Tim Springer and myself had presentations in the Geodesy section and I took a poster for the CGC (Canary GNSS Centre).

      My presentation covered the IGS Reprocessing (the repro1 effort). The main idea was to present the current steps we have taken at the IGS to reprocess all the GPS data from 1994 to 2008 to produce consistent GPS precise products (orbits, clocks, station positions, earth rotation parameters), and the changes at ESA/ESOC to accomplish this ambitious task, you can read it in the pdf below.

      Nacho-igsrepro-tue-21042009-1430

      The EGU was a very well attended meeting and the quality of the speakers in the Geodesy meeting was very high. I was very impressed to see the presentation by Paul Tregoning on the effect of the troposferic mapping funtion and a priori pressure values at the GPS stations, also the presentation by E.J. Petrie on the effect on GPS processing of the second/third order ionospheric effects and Matt King’s presentation on the effects on station coordinate time series of multipath at the stations. From ESA/ESOC we will be following up these findings as they are bound to become very important for the repro2 IGS reprocessing to be undertaken in the 2010/2011 time period.

      The CGC poster that I presented was part of the Geodesy session G14: Geodetic Studies in Africa. The poster covers the efforts of the CGC to process the Macaronesia and Western Africa data and highlights the need for more permanent GNSS installations in our part of the world. As a scientific non-profit association the CGC brings together scientists and engineers from our region interested in GNSS to promote its multidisciplinary role and benefits.

      CGC poster

      CGC poster (click to read)

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

      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
      Director

      Vapor de Agua sobre Observatorio Roque de los Muchachos (LPAL)

      Saturday, September 27th, 2008

      En SAC hemos estudiado los datos de la estación LPAL, La Palma, con el objetivo de caracterizar la cantidad de vapor de agua en la atmósfera cerca de los observatorios astronómicos. Este estudio ha sido realizado para el Centro GNSS de Canarias a partir de una pregunta del Instituto de Astrofísica de Canarias.

      La estación GPS de LPAL pertenece a la red nacional de España que pertenece al Instituto Geográfico Nacional. La estación contribuye sus datos libremente a EUREF, lleva instalada desde Mayo del 2001 y funcionando correctamente.

      La caracterización del vapor de agua en la atmósfera es posible usando el cálculo preciso con los datos de LPAL. Los datos (las distancias a los satélites GPS visibles en esos momentos) se procesan en pasos de 300 segundos. Se han tomado dos años de datos (2005-2007) y se han procesado sacando un valor de troposfera cada 2 horas durante los dos años procesados. De este valor de troposfera se ha restado el valor del retraso hidroestático asumiendo la presión atmosférica debido a la altura sobre el nivel del mar de LPAL y la época del año. El resultado de todos estos cálculos es la cantidad de vapor de agua en la atmósfera. El conocimiento de la cantidad de vapor de agua es fundamental para poder hacer observaciones astronómicas en la banda de infrarojos con garantias.

      El estudio y sus resultados se lo puede bajar aquí; el-cielo-de-la-palma-gen-iac-stu-0001-sac

      Espero que le sea de interés y no dude de contactarnos con cualquier duda o pregunta,

      Saludos,

      Ignacio Romero
      Director

      What is EGNOS?

      Saturday, August 23rd, 2008

      I was asked this question at a job interview many years ago and I did not know the answer … I didn´t get the job, but I have no regrets! let me explain a bit …

      The European Geostationary Navigation Overlay System (EGNOS) is an enhancement and extension of GNSS (GPS/GLONASS). It is the European SBAS (Satellite Based Augmentation System). The other SBAS systems are WAAS (Wide Area Augmentation System in the USA, MSAS in Japan, SNAS in China, etc. This fact sheet from ESA explains it all very well.

      factsheet14

      The fact that all the different systems from around the world work together is a testament to international cooperation/collaboration in this case through ICAO (International Civil Aviation Authority) so that the messages transmitted using the different SBAS systems can be interpreted by all the SBAS qualified GNSSS receivers.

      The Overlay part of the system refers to the fact that EGNOS transmits from Geostationary orbit (36000 Km) on L1 (1575.42 MHz) a message with additional information to help GNSS users. This additional information helps users

      The information transmitted by EGNOS is received similarly to the GPS and GLONASS navigation messages. The EGNOS message is used on-board the user receiver to provide integrity to the GNSS service (GPS/GLONASS). This integrity provides protection levels to the GNSS users against constellation problems and malfunctions and it can improve the positioning accuracy.

      EGNOS is therefore considered the pre-cursor of Galileo “The European GNSS system”

      Additional resources:

      http://www.esa.int/esaNA/egnos.html

      Salabert, F., “Interoperability, from Signals to Integrity”, GPS World, pp 10-12, Nov 2008.

      IGS reprocessing, an exciting project!

      Sunday, July 13th, 2008

      After the IGS workshop which we recently concluded to great success, one of the most important and interesting activities for the near future is the reprocessing of all the historical IGS data using current state-of-the-art techniques.

      Reprocessing of GPS data back to 1994 is a high-priority of the IGS since modern techniques can better estimate the IGS products of the past. The original products produced week to week continue to be the “gold standard” of GPS precision as they are store in Data Centers around the world. These products can be used by scientist, engineers and researchers as the best GPS orbit, clocks and ITRF values for their own research and post-processing. The reasons for the urgent need for reprocessing all of the IGS products are as follows:

      1. The IGS is now using absolute Antenna Phase Center calibrations for ground and space antennas (ANTEX), this has brought the IGS ITRF station position estimations closer to other techniques and reduced uncertainty. This jump introduced in GPS week 1400 (Nov, 2006) creates a discontinuity with the previous IGS products, which needs to be removed by reprocessing the older data with the new standards.
      2. The IGS product consistency and accuracy has improved over time as better processing methods and standards have been implemented at each of the Analysis Centers. This high-level of consistency and increased accuracy which now exists needs to be extended backwards in time for the benefit of all. With more precise past products better science can be produced by the users of the IGS products.
      3. The IGS has adopted many reference frame updates as the ITRF reference frame has been updated over time. This creates a set of products that is not continuous over time as each new ITRF realization introduces a new set of precise coordinates and velocities for the stations. The reprocessing will produce a consistent set of products using the latest ITRF05 reference frame.
      4. Data processing limitations of the past are removed. As computer speed and power has increased , it is now possible to include many more stations as available in the reprocessing of the IGS products. The inclusion of more data for the reprocessing allows for better products and for many more reference station coordinates to be estimated precisely.

      The improvement of the IGS products over time can be best highlighted by the IGS Analysis Center Coordinator orbit quality plot:

      IGS Final Orbits from each AC since the IGS start

      IGS Final Orbits from each AC since the IGS start

      The justification for the IGS reprocessing are therefore clear, and most of the IGS Analysis Centers have committed time and resources to contribute to this effort to reprocess the data for the time period between 01/1994 to 12/2007. The reprocessing efforts are being coordinated by the IGS ACC from this website which I encourage anyone interested to visit: ACC Reprocessing website.

      The main output, apart from the much improved GPS products, will be the station positions to be used in the next realization of the International Terrestrial Reference Frame (ITRF).

      Take care
      Nacho Romero