Analysis Working Group | WG Integrated Clock Correction Strategies for DORIS | WG NRT DORIS data
Analysis Working Group |
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The Analysis Working Group (AWG) is composed of members of the Analysis Centers and Associate Analysis Centers. It is animated by the Analysis Coordination.
The AWG meets at least once a year. Presentations, minutes, agendas of the meeting are available here.
Working Group "Integrated Clock Correction Strategies for DORIS" (2024 - ) |
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The IDS Governing Board created the Working Group on 11 June 2024, and appointed Patrick Schreiner (GFZ) as chair.
Description
The aim is to establish an IDS working group to deal with the behavior of DORIS clocks, exploiting DORIS clock co-locations both in space and on ground. The goal is to derive methods to better model DORIS USO behavior and reduce a source of systematic error in the DORIS technique. The Ultra Stable Oscillator (USO) of satellites is perturbed by passage through the South Atlantic Anomaly (SAA), which exhibit rapid changes in the phase measurements. The SAA-induced perturbations in the USO affect derived products such as the positioning of stations in the region of the SAA. We note that from Belli (2017) temperature changes can also cause DORIS USO frequency variability, yet at different time scales. Since satellites such as the Sentinels 3A, 3B & 6A are equipped with a GPS/GNSS receiver, in addition to the DORIS receiver, which is connected to the same USO, it is possible to correct the clock of the DORIS observations using estimated GNSS clock corrections. In addition to deriving improved models of the behavior for the orbiting DORIS USO, the scope of the WG will also include the study of the use of ground-based clock co-locations, as those with the GNSS REGINA (REseau GNSS pour l'IGS et la Navigation – global GNSS station network operated by CNES and IGN) stations. Accurate knowledge of the station frequency eliminates a source of systematic error, resulting in improved accuracy in precise orbit determination and the development of geophysical products. The working group will focus on this topic and will develop methods to create suitable products for users of DORIS data.
Terms of Reference
Abstract
The objective of this working group (WG) is to address the issue of DORIS clocks, exploiting clock co-locations both in space and on ground. It is well known that satellite on-board clocks are susceptible to distortions caused by various interferences, such as the SAA (South Atlantic Anomaly). Satellites equipped with additional systems, such as a GNSS (Global Navigation Satellite System) receiver, which is also connected to the same USO (Ultra-Stable Oscillator), provide the opportunity to correct DORIS clock inaccuracies with clock corrections derived from GNSS data. The situation is somewhat different for ground stations, where beacon frequency deviations currently have to be estimated as part of the orbit determination process. By making use of ground-based clock co-location, as in the case of stations such as GNSS REGINA (REseau GNSS pour l'IGS et la Navigation - a global network of GNSS stations operated by CNES and IGN), a highly accurate knowledge of the station frequency can be achieved. As a result, it is no longer necessary to estimate the frequency offset of the stations, resulting in improved accuracy of the determined orbits and derived products.
Description
To generate precise observation epochs for DORIS observations, the satellites use an on-board USO. This instrument measures specific time intervals derived from a beat frequency, which subsequently provides DORIS observations with accurate time stamps. However, USOs are highly sensitive devices that are susceptible to various disturbances. Notably, the SAA is a region characterized by an anomaly in the Earth's magnetic field, resulting in an increased level of energetic particles. Satellites passing through this area are exposed to increased ionizing radiation, which can interfere with the USO's functionality, resulting in distorted measurements.
These time stamp artifacts arise in the processing of DORIS observations, introducing undesired effects that impact derived products, such as station positioning within the SAA region. Certain satellites, such as Sentinel-3A/-3B/-6A (MF), are equipped with both a DORIS receiver and a GPS/GNSS receiver, both linked to the same USO. In GNSS processing, receiver clock offsets are routinely estimated. The essential point here is that both the DORIS and GNSS receivers are linked to the same onboard USO. Therefore, we can use the estimated GNSS clock offsets as corrections for the DORIS observations, improving their accuracy.
Ground stations are in a slightly different situation. The DORIS beacons use a USO which provides the base frequency for the generation of the transmit signal. Since only some of the beacons are linked to high-precision atomic clocks, there is a frequency offset that has to be estimated in the Precise Orbit Determination (POD) process. The use of ground-based clock co-location, as exemplified by stations such as GNSS REGINA, has shown considerable potential to eliminate this need to estimate station frequency biases, resulting in improved accuracy for POD.
The working group aims to address this challenge by developing methods to produce usable products for end users, mitigate the effects of satellite USO disturbances, and exploit the potential of ground clock co-location.
The following tentative list of objectives for the WG is proposed (to be determined during WG discussions):
- Create clock series for testing (Test functionality of processing).
- Modify the DORIS data processing tools of the participating groups to use the clock files, if not possible with current version.
- Create a test clock series with various settings (sampling, smoothing) and run the testing campaigns to find optimal parameters of the clock series.
- Define parameters of clock series as an IDS product (or product to be used by IDS), find provider of the product.
- Prepare joint peer-reviewed paper to publish results obtained during
this processing. - Prepare a recommendation for routine production of the clock products and their archival at the IDS data centers as an official IDS product.
- Prepare recommendations for future missions concerning DORIS-GNSS clock co-locations
Chair
Patrick Schreiner (GFZ, Germany)
Members
Hugues Capdeville (CLS, France)
Alexandre Couhert (CNES, France)
Théo Gravalon (CLS, France)
Urs Hugentobler (TUM, Germany)
Frank Lemoine (NASA GSFC, USA)
Jean-Michel Lemoine (CNES, Germany)
Flavien Mercier (CNES, France)
John Moyard (CNES, France)
Heike Peter (PosiTim, Germany)
Anton Reinhold (GFZ, Germany)
Patrick Schreiner (GFZ, Germany)
Petr Štěpánek (GOP, Czech Republic)
Nikita Zelensky (NASA UMD, USA)
COPERNICUS POD Service manager who offers cooperation as a potential provider of Sentinel clock product
Carlos Fernández Martín (GMV, Spain)
Working Group "NRT DORIS data" (2016-2023) |
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The IDS Governing Board created the Working Group on 1 November 2016, and appointed Denise Dettmering (DGFI-TUM) as chair.
On 29 November 2023, the GB decided the end of the current WG whose objectives have been achieved,
Terms of Reference
The general objective of this working group is a thorough assessment on applications, benefits, requirements and prospects of DORIS data with improved data latency. Currently, data is available as daily RINEX files with a latency of about one day. Thus, DORIS real-time and near real-time (NRT) applications of any kind are currently only possible on board of the satellite.
Most of the other geodetic space-techniques provide their data to the users with lower latencies. The IGS disseminates its terrestrial GNSS data as hourly RINEX files and via real-time Ntrip streams. The ILRS asks its stations to provide SLR data within two hours after measurements. For data collected on board of satellites (as it is the case for DORIS) the minimum latency is restricted by the data downlink, usually performed within one to two hours after acquisition. Data sets from GPS radio occultations and satellite altimetry are available with one to three hours latency. In principle, it is also possible to provide the DORIS data with a latency of a few hours. However, this would require significant changes in operations of tools and procedures at the DORIS mission center.
DORIS NRT data sets would be useful for different applications, one of them is the modelling of the Earth's ionosphere. Using DORIS in combination with GNSS (and additional techniques) helps to improve the accuracy and reliability of ionospheric maps, especially in ocean regions with poor GNSS coverage. This has been proved for post-processing applications but will probably also hold for NRT.
The following, non-restrictive list of goals for the WG is proposed (TBD in the WG):
- definition of detailed NRT DORIS data requirements (latency, formats, ...)
- conduction of simulations and/or a short-term test campaign in order to investigate the potential of DORIS NRT in ionospheric applications
- definition of objectives and possible additional applications of NRT DORIS data,
- investigation of possible ionospheric applications for on-board computations and telemetry downlinking (as currently done for pole coordinate estimation)
- identification of potential users
Based on the results of the Working Group CNES may evaluate the possibility to establish a new NRT DORIS data production chain.
Chair
Denise Dettmering (DGFI-TUM, Germany)
Members
Nicolas Bergeot (ROB, Belgium)
Vince Eccles (Utah State University, USA)
Eren Erdogan (DGFI-TUM, Germany)
Zishen Li (CAS, China)
Michael Schmidt (DGFI-TUM, Germany)
Ningbo Wang (CAS, China)
Volker Wilken (DLR, Germany)