Archive for the 'Technique' Category

AGU2013 IGS Infrastructure Committee poster

Wednesday, December 11th, 2013

From 9 to 13 December I participated in the AGU Fall Meeting 2013 presenting a poster of some interesting IGS work. As the IGS Infrastructure Committee Chairman I have coordinated an experiment together with the IGS Network Coordinator, the Reference Working Group, the Antenna Working Group, and the Analysis Center Coordinator. The experiment consisted on identifying the worse performing monument ties for co-located geodetic stations from ITRF08 involving a GNSS antenna covered with an uncalibrated radome.

Uncalibrated radomes introduce an unknown offset into the estimated station positions so via post-processing we have tried to estimate the radome effect in the tie between geodetic techniques. The stations analysed all have two or more geodetic techniques (GNSS, SLR or VLBI) and knowing the local ties precisely (measurements between reference points of each technique) is very important so that the processed solutions from each technique can be correctly fitted together to make the Terrestrial Reference Frame.

AGU13 Poster session

AGU13 Poster session

SAC IGS poster

Nacho at the SAC IGS poster

You can get the poster clicking on the image below. The final conclusions are inconclusive for now. It is not possible to recover the radome effect from the global solutions we have generated and analysed. It is very likely that the radome effect is inside the station position time series noise. We will process the data in short baseline solutions in future developments and present the final results in the IGS Workshop 2014.

poster IGS IC AGU13

IGS IC poster at the AGU 2013

While at the AGU2013 I have also participated in the IGS Governing Board Meeting, the IGS Troposphere Working Group Meeting and the IGS Analysis Center Meeting. All the IGS different parts continue to work correctly and are advancing many issues forward.

Regards to all and Happy positioning!

Nacho

TELECAN project comes to a close

Friday, November 29th, 2013

For the last 12 months our company has been in a collaboration project with the University of Las Palmas in a project called TELECAN.

This activity does not have a GNSS component at this time but it has been a fantastic learning experience for our company in the filed of Earth Observation, and it has brought together disparate members to work together towards a common goal. The partners of the project have been; ULPGC-GPIT, ITC, University of Agadir y SAC.

The final presentation was on 29 Nov, 2013 in the “Jornadas de Teledetección Espacial” and immediately afterwards there was a round table motivating a very interesting discussion based on the presentation.

IMG_20131129_101746IMG_20131129_130234

The project has covered the production, storage, publication and display of Earth Observation products as they are related to renewable energies. This has covered the generation of 13 products, some generic EUMETSAT products, some from a Scientific Application Facility for nowcasting and some dedicated developments by this short project, as shown in the slide below.

TELECAN meteorologia energetica products

TELECAN meteorología energética products

But in truth the biggest component from our part has been capacity building and technology transfer, we see this project as a technology project, even though there has been a scientific component that is not the focus from our point of view. Our company has created a storage and publication system for the many products that the scientists generate in near-real-time. This storage, classification and vision system has been started from scratch internally Layers and GeoServer, two technologies we did not handle before. You can access the service clicking in the image below and find a sample of the products that the TELECAN scientists are generated in near-real-time.

portal web de productos de Meteorologia energetica

portal web de productos de Meteorologia energética

I will link to the presentation once it is loaded in the TELECAN webpage who are the ones authorised to distribute it.

regards to all!!

Nacho

IGS Station Height Tests

Saturday, March 13th, 2010

The IGS permanent GNSS stations are the backbone of the IGS as they provide the overwhelming bulk of the satellite observations used for the IGS products; orbits, clocks, station positions, TZD, etc. The station GNSS receivers record the measurements from the satellite constellations (GPS and GLONASS) as correctly as possible. This means following the IGS guidelines so as not to introduce unknown effects into the recorded observations. If measurements are not properly recorded by the receiver+antenna installation at each station the IGS Analysis Centers and other interested researchers will not be able to properly correct or compensate the effects and the data will be rendered useless in the estimation processes, or even worse the data will negatively affect the IGS products.

No matter how much care is taken into the installation of a permanent GNSS station it is possible that the data are biased due to local effects, either horizon blocks, reflective surfaces, uncontrolled radio-emitting sources, etc can affect the record measurements at the GNSS receiver making them less useful for the IGS.

One of the biggest concerns is the possible dependence of a station’s position calculation to the antenna’s near-field environment. If there are elements close to the antenna that disturb the signal reception then biased measurement can be recorded by the receiver. Since the antenna is a relatively small element compared to its surroundings it is possible to try to see the effect of the surroundings on the measurements by limiting how many of the measurements we actually use in any given calculation. By limiting the observations at low elevations we remove the measurements most likely to be affected by the surroundings.

Taking random days over all seasons during 2009 more than 325 are processed with fixed orbits . The runs go from 5 to 40 deg elevation cut-off angles. The height differences are calculated with respect to the 5 deg solution. It would be expected that the position solutions at a station not be affected too much by excluding low elevation data. Indeed this is correct for most of the stations, but some stations show very large differences when limiting the low elevation data the height estimate changes by decimeters sometimes! Here is the example of one of our (ESA/ESOC) stations MAS1, in Maspalomas, Gran Canaria. It can be seen by the red line that as the elevation cutoff is increased the station height estimate changes by up to 2 cm, and the error band increases of course as less data is used in the estimation.

MAS1_09032

This kind of processing is also helpful when the observation residuals are analyzed in azimuth and elevation. Taking all the residuals over each of the processed days in 2009 a “station fingerprint” (Huisman, et a., 2009) can be interpolated from the un-differenced residuals. In the case of MAS1 the station fingerprint shows the direction in which blocks and reflective surfaces are located, causing either a hole in the observations or significant negative/positive residuals.

mas1_MP_map

These kind of investigations are important when evaluating the performance of a GNSS permanent station, and is the kind of in-depth analysis I have been doing as part of the IGS Infrastructure Committee.

Happy positioning
Ignacio Romero (Nacho of the IGS!)

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)

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!)

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

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

P1-C1 bias … and other Code biases

Saturday, March 15th, 2008

The C/A code and the P code encoded in the L1 “should” be coincident so that a pseudo-range derived with the C/A code; “C1″ should be equivalent to one derived with the P code; “P1″ so that we could use them interchangeably, in the IGS we agreed years ago to use P1,P2 & L1,L2 to create the iono-free observables to estimate GPS orbits, clock biases, etc.

The problem is that some receivers do not generate a P1 so we just use C1 in its place. Therefore we end up mixing in one epoch of data measurements for the satellites from many receivers and we have to mix C1 and P1 measurements from the same satellite and this is the problem, that the satellite clock estimated with this “mixture” will not be correct and our whole estimation of clock biases will be affected since therte is now way to separate the P1-C1 bias at this point.

An additional problem is that old receivers that do not generate P2 in the modern sense of an independent observation of the L2 code. The P2 pseudorange observable from this reduced list of receiver types (mainly older ones) are actually formed by a process which tracks the cross-correlated (P2-P1) pseudorange to produce a P2 which also needs to be corrected by the P1C1 bias!:

P2 = C1 + (P2-P1)

The corrections f(i) are applied as follows to C1 and the P2 if it needs correcting:

C1 –> C1 + f(i)
P2 –> P2 + f(i)

Since the signals encoded on L1 in the satellites are NOT coincident, it is easy to see that C1 and P1 are delayed one with respect to the other, so we cannot simply mix them in the processing without correcting one of them, in this case the agreement is to bring C1 to P1 by adding the P1-C1 bias to the original measurement.

Since we agreed to use P1 for all the receivers if the P1 is not available we take the C1 and add this correction factor which is very stable for each satellite but it is satellite specific and can be quite large:

biases f(i) are always in units of ns and are listed in PRN order from PNR01 thru PRN40:


data bias / -0.233d0, -0.033d0, -0.163d0, 1.234d0, -0.944d0,
+ 0.444d0, -1.132d0, -0.416d0, 0.366d0, -1.651d0,
+ 0.591d0, -9.999d9, 1.529d0, 0.108d0, -1.326d0,
+ -0.525d0, 1.391d0, -0.012d0, -2.026d0, -1.228d0,
+ -0.392d0, 0.532d0, 0.130d0, -0.069d0, 0.575d0,
+ 1.083d0, -0.189d0, -0.232d0, 0.643d0, 2.016d0,
+ -0.073d0, -9.999d9, -9.999d9, -9.999d9, -9.999d9,
+ -9.999d9, -9.999d9, -9.999d9, -9.999d9, -9.999d9 /

The biases are an output of the Ionospheric processing which characterise the Ionosphere state from the delay due to the effect the free electrons have on each frequency (L1,L2) as they enter the atmosphere, therefore since many receivers have both C1 and P1 the IGS Ionospheric Analysis Centers can see the “code bias” and they report it regularly to the IGS and everywhere else.

There is another bias from P1-P2 which the Iono people calculate since the P code should be encoded in L1 and L2 at exactly the same time but this is not so, therefore a bias between the P codes exist as well.

Yet another code bias between P2 and C2 (the new GPS L2C signal) but not much work has been done of this since it is new and few satellites have L2C.

In the beginning we corrected new modern receivers to mix them with the older ones (pre-2000 era):
Handling mixed receiver types

Later we started correcting the older receivers to make them consistent with the modern receiver measurements (from April, 2000 onwards):
New pseudorange bias convention

We now have the biases published officially by CODE for everyone to use:
Monitoring (P1-C1) code biases

The latest updated set of values:
Updated p1c1bias.hist

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.