IGS Workshop 2017

The 2017 edition of the IGS Workshop was held in Paris from 3-7 July. I had the pleasure to attend this meeting for the first time and was quite pleased with the quality of the talks and the networking opportunities. Below is a summary of what I consider were important topics: these notes are based on my own understanding of the presentations and I encourage you to look for the original presentations for all the details once they are made available on the workshop website.

The antenna working group, in parallel with MGEX operations, is preparing for a significant upgrade of the ANTEX format to version 2.0. At the root of these changes is the release of ground measurements for the phase center offsets (PCOs) of the Galileo IOV satellites and, in the upcoming months, for the FOC satellites as well (Gonzalez 2017). ESA has provided PCO values with respect to an antenna reference point (ARP) which is itself defined with respect to an arbitrary origin on the spacecraft. Since PCOs are generally provided with respect to the center of mass of the satellites, the working group is using this opportunity to revisit ANTEX conventions. The problem with the actual PCO information is that the center of mass of the satellite varies during its lifetime (due to changes in fuel quantity onboard for instance). Since the values provided in ANTEX are typically not updated to account for such changes, they slowly drift from the values originally defined. For this reason, it has been proposed to define new satellite blocks in the SINEX format to describe both the position of the satellite center of mass and antenna reference point. ANTEX would then be modified to contain the ARP to phase center offsets.

 

The redefinition of ANTEX would also be an opportunity to remove redundant information from the files such as the duplication of PCO information when a satellite is changing its PRN number. This change would be made possible by adding a SVN to PRN block in the SINEX format. It will probably take a little while before these changes are fully implemented, but it is good to know that they are on the way. Meanwhile, the Galileo PCO values provided by ESA will be translated to the current ANTEX format definition. The antenna working group is also recognizing the need to provide phase center offsets and variations (PCOs/PCVs) for L5 on GPS and Galileo. An initial calibration of ground antennas in an anechoic chamber has revealed PCO differences of about 9 mm with respect to the L1/L2 signals for some Javad antennas which is clearly not negligible (Villiger et al. 2017). It is obvious that these discrepancies must be solved before obtaining the most accurate GNSS products.

 

Again related to antennas, the type-mean PCV values provided in the ANTEX files do not seem to be of sufficient accuracy for all applications (Moore and Hu 2017). There are significant (mm-level that is) differences between antennas of the same type which are typically amplified in the positioning domain. Therefore, it has been recommended to study the impact of these PCV discrepancies in GNSS products and station positions.

 

Another important shift is taking place within the bias working group. The new Bias-SINEX 1.0 format has been proposed for adoption by the IGS. The Center for Orbit Determination in Europe (CODE) has done a major reprocessing of both clock and differential code biases (DCB) starting in 1996 (?) which allows the definition of “uncombined” or “observation-specific” biases (Schaer et al. 2017). These values from the repro should be made available to the public in about a month. Meanwhile, the latest values for the running-average monthly means of these biases are provided at ftp://ftp.aiub.unibe.ch/CODE/CODE.BIA. The working group is also suggesting that we move towards a flexible definition of the clock datum, currently provided by the C1W/C2W signals. In the future, analysis centers could choose any signals to define their clock products (for example C1C/C2C), and users will be able to compensate for different datum definition by using the RINEX clock files along with the biases provided in the Bias-SINEX files.

 

Disseminating satellite attitude to better represent maneuvers during eclipses has also been proposed (Loyer et al. 2017). Based on the ORBEX format, the attitude of the satellites would be provided in the form of quaternions. The benefit for users will be to model the satellite-induced wind-up effect during eclipses which can cause cm-level positioning errors in kinematic mode if not properly accounted for. The GRG analysis center should start providing attitude files around September which will allow testing by users.

 

The ionospheric working group is working on developing a weighting scheme for a combination of IONEX products. If I am not mistaken, I believe that the current combined IONEX maps provided by the IGS only include 2 analysis centers… The idea would then be to combine the GIMs from 7 analysis centers and to do a repro campaign going back to 1998. Also note that the new version of the IONEX format will not contain DCBs anymore, since these values will be provided in the Bias-SINEX format.

 

Even though I do not have the knowledge to fully understand the details of the orbit working group discussions, sub-daily Earth orientation parameters (Sibois and Desai 2017), antenna thrust power (Steigenberger et al 2017) and solar radiation pressure (Ziebart et al 2017, Peter and Springer 2017, Sakamura et al 2017) were the main topics of this year’s discussions.

 

In conclusion, there is a lot of change ahead with the IGS products, mainly driven by the new signals and constellations. Even though we often take the IGS products for granted in our work, we are reminded to always reference the following contribution in our publications whenever we use such products:

 

Dow, J.M., Neilan, R. E., and Rizos, C., The International GNSS Service in a changing landscape of Global Navigation Satellite Systems, Journal of Geodesy (2009) 83:191–198, DOI: 10.1007/s00190-008-0300-3

 

 

References

 

Gonzalez F (2017) Phase centre calibration of the Galileo satellite navigation antenna. IGS Workshop, Session on Antennas and Biases.

 

Loyer S, Banville S, Mercier F, Perosanz F (2017) Disseminating GNSS attitude for improved clock correction consistency. IGS Workshop, Poster PS01-04.

 

Moore M, Hu G (2017) Type mean antenna PCV models are no longer sufficient to support geodetic analysis. IGS Workshop, Session on Antennas and Biases.

 

Peter H, Springer T (2017) Impact of improved GPS-IIF box-wing models of LEO orbit determination. IGS Workshop, Session on Orbit Modelling.

 

Sakumara C, Sibois A, Sibthorpe A, Murphy D (2017) Improved modeling of GPS block IIF satellites for the GSPM13 solar radiation pressure model. IGS Workshop, Session on Orbit Modelling.

 

Schaer S, Villiger A, Dach R, Prange L, Jäggi A (2017) New GNSS bias products from CODE. IGS Workshop, Session on Antennas and Biases.

 

Sibois A, Desai S (2017) Impact of modern Earth orientation models on GPS precise orbit determination. IGS Workshop, Session on Analysis Centers.

 

Steigenberger P, Thoelert S, Montenbruck O (2017) Measuring GNSS satellite transmit power. IGS Workshop, Session on Orbit Modelling.

 

Villiger A, Prange L, Dach R, Schaer S, Jäggi A (2017) Consistency antenna products in the MGEX environment. IGS Workshop, Session on Antennas and Biases.

 

Ziebart M, Bhattarai S, Grey S, Allgeir S, Springer T, Tobias Gonzalez G (2017) Next generation radiation pressure modelling for GNSS space vehicles – grid file model tests on GPS IIR. IGS Workshop, Session on Orbit Modelling.



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