- Since 2006 the Navigation Support Office at ESOC has been working on its new analysis software, called Napeos, for all its activities. Although the main focus for Napeos was GNSS processing the developments kept the SLR and Doris processing capabilities intact. The development was completed by September 2007 at which point the software was extensively tested for its IGS operational capabilities. For that purpose the whole year 2007 was reprocessed generating a GPS only and a GPS/GLONASS combined solution. The main focus of this reprocessing was robustness of the software to ensure that it could process the full year without any malfunctioning. After successful completion of the tests Napeos was put into operations for the ESOC IGS activities. It replaced the old software for the ESOC IGS final and IGLOS products starting with GPS week 1463 (20-January-2008). Next followed the ESOC rapid products on day 2 of GPS week 1469 (4-March-2008). The Ultra-rapid products were replaced starting with the solution of 18 hours, on day 2 of GPS week 1471 (18-March-2008). This poster highlights the significant improvements of the quality of the ESOC IGS contributions. The key features of the Napeos IGS analysis are:
- Full IERS 2003 compliance
- Undifferenced GNSS processing. GPS, GLONASS, and Galileo
- Non-overlapping processing using 24-hour solutions
- No day boundary continuity constraints
- Fast!!! (Final orbit solution takes 30 minutes using 100 stations on a Linux PC with Intel Fortran compiler)
- Currently for improving our IGS products we are focusing on the following themes:
- Earth Albedo and infra-red radiation modeling
- Orbit predictions to improve the Ultra-rapid orbits
- Normal equation stacking for longer (GLONASS) arcs.
- Effects of network geometry and number of stations
- ESOC is heavily involved in LEO precise orbit determination (POD), especially ENVISAT and METOP-A. ENVISAT POD is based on SLR and Doris tracking whereas METOP-A is based on GPS tracking. With the IGS results and the comparisons of the LEO orbits we have demonstrated that Napeos is fully state of the art in these areas. This implies that we could use Napeos also to become an analysis centre for the IDS and ILRS. We are currently studying the efforts required regarding joining these technique services. In particular we are interested in contributing a TRF solution from each of these three techniques for the upcoming ITRF realization.
- ESOC Innovation Corner
- At ESOC we have the ambition to be the “best in class” within the IGS. For this purpose we have weekly meetings of our IGS group and within these meetings we have what we call an “innovation corner”. In this innovation corner group member can propose special study topics. If the topic is accepted one of the group members can volunteer to do the study. Results from two innovation corner topics, which were studied earlier this year, are shown below.
- Station Selection
- In this study the effect of the number of stations and the station selection feature of Napeos were investigated. For this purpose we used our normal IGS final routine settings. The only change was that we did not select any preferred stations. The routine was run starting with 30 stations, increasing the number by 10 for each next step, and relying on the Napeos station selection feature to select a network with a good spatial distribution. The quality of the solutions was assessed to determine which number of stations is “optimal”. The figure below shows the result for the orbits. There are two interesting features:
- 60 to 70 well distributed stations seem to give sufficient quality, additional stations do not bring any significant improvements.
- The ambiguity free results do not seem to “converge” to the same level as the ambiguity fixed results. This is somewhat surprising!?
Besides our IGS GNSS activities our group at ESOC is also heavily involved in LEO POD using DORIS, SLR, PRARE, and Altimetry observations, in particular for the ESA missions ERS-1 and 2 and Envisat. We do, however, have concrete plans to become a reprocessing Analysis Center for both the IDS and the ILRS. For the IDS a first 1-year solution is currently being generated. Below some interesting results from our DORIS activities.
DORIS Phase Center Variations
In GNSS we have had significant issues with the location of the receiver and transmitter phase center position. DORIS, also being a microwave technique, suffers from the same problems. The lessons learned in GNSS in this area are very useful for the DORIS technique. The top two figures below show the residuals of the DORIS observations as function of azimuth and elevation for two DORIS stations. One station, SCRB, located around the equator, the other one, FAIB, near the North-Pole. The bottom two figures show the residuals for the Envisat transmitting antenna, once in colour and once in black and white. In all cases some clear signals may be observed which are most likely caused by the phase center variations of the DORIS antennas.
- True GNSS Processing
- The ESOC IGS final analysis solution is generated in two variants. The first variant is a “classical” GPS only solution. The second variant is a true GNSS solution in which the GPS and GLONASS observations are processed simultaneously and contribute equally to all estimated parameters. We have ensured that both solutions use the same station selection by sharing the same preprocessing step. So the only difference between the two solutions is the inclusion of the GLONASS data. This allows us to study the impact the GLONASS observations have on our solutions.
- For testing our new software, before putting it into operations for the IGS, we did a full reprocessing of the year 2007 generating both a GPS only and a GPS-GLONASS combined solution. The figure below shows the RMS and Median of comparing the GPS parts of the orbits against the IGS final orbits. For comparison also the CODE orbits and the original ESOC solutions are included in this picture.
- We may make the following observations:
- The new ESOC software solutions (labeled GPS and GPS/GLO) outperform the old ESOC solutions.
- The GPS and GPS/GLO solutions perform equally good.
- No negative influence of GLONASS!
- The inclusion of GLONASS data has a noticeable impact on the solutions
- It is our ambition to stop submitting our GPS-only solutions and start submitting our true GNSS solutions to the IGS in the near future. The required software modifications have been made and are now in testing. Two further enhancements are envisioned:
- Improved pre-processing of the GLONASS observations
- GLONASS ambiguity fixing
Orbit Prediction Tests
Improvement of ESOC IGS Contributions
The two figures below show the improvement of the ESOC IGS contributions. The top figure shows it for the ESOC Rapid orbits. The ESOC rapid products are based on our new software Napeos starting with the products of day 2 of GPS week 1469. The bottom figure shows the improvement for our ESOC Final clocks. The ESOC final products are based on our new software Napeos starting with GPS week 1463. In both cases a significant improvement of the results may be observed.
- Orbit Prediction Tests
- The second innovation study is investigating the quality of our orbit predictions. Despite the fact that the estimated part of our Ultra-rapid orbits is amongst the best within the IGS, the predicted part does not perform as good. Main goal of this investigation is to improve the ESOC orbit predictions to become “best in class”. For this study we use:
- ESOC final orbits as input, IGS final orbits to compare the predictions
- Different combination of all 18 orbit parameters, and different arc lengths
- 9 CODE parameters (D0 Y0 B0 Dp Yp Bp)
- 9 CPR parameters (R0 A0 C0 Rp Ap Cp)
- The figure below gives a summary of the obtained results from which we may derive:
- For 1-day fit a few parameters may be saved but the predictions do not get much better
- The 3-day and 4-day predictions can be improved using more parameters but still do not reach the same level as the 2-day predictions.
- We will continue these orbit predictions studies:
- Focusing on the Albedo and IR effects
- Look at ERP issues using the real Ultra Rapid results
- Using 1.5 and 2.5-day arc lengths for the fitting interval.
- GIOVE-A Processing
- At ESOC we have analyzed the GIOVE-A microwave data (150 days in 2006/2007):
- IGS final orbits and clocks fixed.
- Estimated parameters:
- GIOVE-A orbit and clock.
- GESS station coordinates and clocks.
- Receiver dependent bias between GPS and GIOVE-A.
- Daily solutions saving the normal equations.
- Generate n-day solutions based on normal equation stacking (n = 1, 2, 3, 4, and 5).
- The figure below shows the orbit quality based on a 2-day fit through the middle day of the solutions. For comparison a GPS satellite (SVN-35) was treated in the same way as the GIOVE-A satellite. The performance of the GIOVE-A orbit estimates is not as good as that of the GPS satellite.
- Improvement of ESOC IGS Contributions
- ESOC has joined the “best of class” for the IGS rapid, final, and reprocessed products!
- True GNSS Processing
- ESOC is running a GPS-only and a true GNSS solution. Soon, hopefully before the end of 2008, only the true GNSS solution will be submitted for the IGS final products.
- GLONASS data can be included in the IGS processing without any negative impact on the GPS results. But…no improvement observed either.
- ESOC is actively involved in processing the GIOVE-A and GIOVE-B data. The new observables offered by these satellites are really interesting.
- ESOC Innovation Corner
- ESOC has an active “innovation corner” which should ensure that we remain amongst the “best of class” for all IGS products, by pro-actively investigating possible improvements and enhancements.
- A significant list of ideas exist, several being pursued at present, e.g.:
- Intersystem bias stacking, improved data cleaning and cycle slip detection, N-day solutions (e.g. weekly orbits to have orbits and SINEX fully consistent), M-hourly solutions (for real time applications), Ambiguity fixing improvements, and advanced clock modelling.
- Non-GNSS Activities
- ESOC is active in routine processing of DORIS and SLR observations from ESA satellite missions. Also involved in reprocessing of all historic observations of these missions.
- ESOC is planning to become an reprocessing Analysis Centre for all space geodetic techniques. Starting with the IDS and ILRS in 2008. IVS possibly in 2009.
ESOC has the ambition to remain amongst the
“best in class” in the IGS and in addition
contribute significantly to the IDS, ILRS, and IVS!