SC 1.1: Coordination of Space Techniques

Chair: Krzysztof Sośnica (Poland)

Terms of Reference

Space techniques play a fundamental role in the realization and dissemination of highly accurate and long-term stable terrestrial and celestial reference frames as well as for accurate monitoring of the Earth orientation parameters linking the two fundamental frames. The current space geodetic techniques contributing to ITRF and ICRF,i.e., Very Long Baseline Interferometry (VLBI), Satellite and Lunar Laser Ranging (SLR/LLR), Global Navigation Satellite Systems (GNSS) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) have particular strengths and technique-specific weaknesses.

Strengths of the techniques are exploited by combining them making use of fundamental sites co-locating more than one technique. Sub-commission 1.1 focuses on the coordination of research related to the geodetic space techniques with emphasis on colocation aspects at fundamental geodetic observatories as well as on co-location targets in space, considering common parameters such as coordinates of stations and satellites, troposphere parameters, and clock parameters. Future terrestrial and lunar missions, such as GENESIS or Moonlight, require developing techniques and methodologies for co-location in space and a proper definition of lunar reference frames and time scales with the proper connection to the terrestrial frames and time. Sub-commission 1.1 will explore prerequisites for future terrestrial and lunar missions and introduce recommendations to ensure highest consistency between reference frames in the Earth-Moon system.

Objectives

• Coordinate research on co-location using common parameters in space;
• Coordinate research on co-location using common parameters at fundamental geodetic observatories;
• Explore the use of new techniques and technologies;
• Interface with IERS WG on Site Survey and Co-location;
• Interface with the GGOS Committee on Performance Simulations and Architectural Trade-Offs (PLATO);
• Interface with JWG 1.1.2 on Atmospheric Ties;
• Foster processing capabilities for the GENESIS mission;
• Interface between terrestrial and lunar reference and time systems with realizations in the form of the reference and time frames.

Working Groups of Sub-Commission 1.1

JWG 1.1.1: GENESIS (joint with IERS)

Terms of Reference

GENESIS is a mission of the European Space Agency (ESA) approved for launch in 2028. It realizes all major space ties on a satellite in a polar orbit at about 6000 km altitude, connecting GNSS receivers, an SLR retroreflector, a DORIS receiver and a dedicated VLBI transmitter. GENESIS holds the potential to significantly enhance the terrestrial reference frame with more accurate ties between the techniques, and it promises to enable scientific investigations with unprecedented levels of accuracy. The objective of this WG is to explore the array of scientific opportunities presented by GENESIS, to formulate optimal observing scenarios, and to develop the methodology for a consistent integration of GENESIS data into future ITRF realizations with simulations and considering already existing space ties. This WG aims at maximizing the utilization of the mission’s capabilities.

Objectives

• This WG is an open forum of the international scientific community to exchange ideas and information, and to work for the best possible implementation of GENESIS and exploitation of its opportunities.
• There is close co-operation and exchange with the GENESIS Science Team of ESA including the GENESIS Science Management Board and the GENESIS Science Exploitation Team. 
• Identify possible scenarios for the utilization of GENESIS for the improvement of the terrestrial reference frame.
• Set up and implement a work plan with timeline to make the most promising scenarios possible once data is available. A detailed plan supported by simulation results will be available after two years. Observation data from GENESIS will become available with the launch of the missions, planned for 2028.
• Get an overview of possible contributions by the various groups to the analysis of GENESIS observations. Help to set up cooperation between the groups to facilitate the best realization of the reference frame with GENESIS.
• Review and investigate existing co-locations in space between GNSS, DORIS, and SLR, as well as VLBI observations to satellites.
• Formulate and raise questions to address open issues with the GENESIS mission, both on a technique-specific level and on the combination level.
• Identify and investigate new scientific opportunities, which will become possible with GENESIS.

Members

• Johannes Böhm (Austria); Chair
• Claudio Abbondanza (USA)
• Mathis Bloßfeld (Germany)
• Alexandre Couhert (France)
• Rolf Dach (Switzerland)
• Claudia Flohrer (Germany)
• Susanne Glaser (Germany)
• Rüdiger Haas (Sweden)
• Bruce Haines (USA)
• Urs Hugentobler (Germany)
• Ozgur Karatekin (Belgium)
• Frank Lemoine (USA)
• Benjamin Männel (Germany)
• Lucia McCallum (Australia)
• Oliver Montenbruck (Germany)
• Arnaud Pollet (France)
• Markus Rothacher (Switzerland)
• Krzysztof Sośnica (Poland)

ESA representatives

Ex-officio Members of IERS:

• Robert Heinkelmann (Germany)
• Daniela Thaller (Germany)

Ex-officio Members of GGOS

• Detlef Angermann (Germany)
• José Rodríguez Pérez (Spain)

Corresponding Members

• Zuheir Altamimi (France)
• Yoaz Bar-Sever (USA)
• Grzegorz Bury (Poland)
• Bingbing Duan (Germany)
• Richard Gross (USA)
• Adrian Jäggi (Switzerland)
• Tomasz Kur (Poland)
• Axel Nothnagel (Germany)
• Toshimichi Otsubo (Japan)
• Felix Perosanz (France)
• Manuela Seitz (Germany)
• Vishwa Sing (Germany)
• Benedikt Soja (Switzerland)
• Peter Steigenberger (Germany)
• Helene Wolf (Austria)

JWG 1.1.2: Atmospheric ties (joint with GGOS)

Terms of Reference

To achieve the 1 mm position and 1 mm/decade velocity requirement of the terrestrial reference frame defined by the Global Geodetic Observing System (GGOS), a rigorous combination of space geodetic techniques should be performed, utilizing as many ties as possible. In addition to the commonly used global (Earth Orientation Parameters) and local (station coordinates) ties, tropospheric parameters between co-location stations can also be combined to enhance the solution, referred to as troposphere ties.

Studies have indicated that the inter-technique agreement of tropospheric parameters is optimal (e.g., around 4 mm for zenith total delay (ZTD) between GNSS and VLBI), and the benefits of employing troposphere ties in multi-technique combinations have been demonstrated, such as improved VLBI scale estimates. However, previous studies are typically based on short-term observations (e.g., during VLBI CONT campaigns), which may not adequately represent long-term reference frame determination.

Additionally, studies have revealed that troposphere parameters estimated from space geodetic techniques can exhibit systematic biases, potentially stemming from different instrument types and data processing strategies. Neglecting these systematic biases when employing troposphere ties could distort the network, introducing artificial biases in station coordinates. Weighting is another critical consideration in utilizing troposphere ties, where appropriate weighting should account for the stochastic noise level of tropospheric parameters and exploit the benefits of the troposphere ties the most.

It is thus essential to assess the stochastic noise of inter-technique troposphere delay agreements and influencing factors such as observation distribution. It is anticipated that appropriate handling of troposphere ties in multi-technique integrated processing will enhance the solution accuracy, such as TRF scale stability. 

Objectives

The primary objective of theWG is threefold: firstly, to assess the long-term systematic and stochastic differences of tropospheric parameters between different space geodetic techniques at different co-location sites; secondly, to investigate optimal methodologies to combine tropospheric parameters in inter- and intra- technique integrated processing; and thirdly, to examine the benefits of utilizing troposphere ties in the combination of space geodetic techniques for long-term terrestrial reference frame determination.

Program of Activities

The WG will concentrate on the following activities:

• Evaluating the inter- and intra-technique agreement of troposphere parameters obtained from space geodetic techniques (GNSS, VLBI, DORIS), in-situ instruments (such as water vapor radiometer, radiosonde), and Numerical Weather Models with different spatial and temporal resolutions. Exploring the systematic biases of different techniques and identifying potential causes. For example, analyzing factors that could systematically impact GNSS troposphere delay estimates, such as receiver and antenna type, as well as data processing strategy (e.g., cut-off elevation angle, down-weighting strategy, mapping function).
• Investigating the stochastic noise level of troposphere parameter differences between different techniques and their influencing factors, including the inter-station distance, water vapor content, station location, and the spatial and temporal distribution of observations. The effect of atmosphere turbulence will also be included.
• Developing an optimal strategy for the rigorous combination of atmosphere parameters for space geodetic techniques (GNSS, VLBI, DORIS, and SLR) to leverage the contribution of troposphere ties, while minimizing the impact of systematic biases and stochastic noise.
• Demonstrating the advantages of utilizing troposphere ties in multi-technique integrated processing, encompassing the determination of both terrestrial and celestial reference frames, and Earth Orientation Parameters.
• Determining high-resolution and high-precision multi-technique combined troposphere delay and water vapor products for climate-related applications.

Members

• Jungang Wang (Germany); Chair
• Kyriakos Balidakis (Germany)
• Mateusz Drożdżewski (Poland)
• Claudia Flohrer (Germany)
• Susanne Glaser (Germany)
• Changyong He (China)
• Robert Heinkelmann (Germany)
• Iván Dario Herrera Pinzón (Switzerland)
• Chaiyaporn Kitpracha (Tailand)
• Tobias Nilsson (Sweden)
• Krzysztof Sośnica (Poland)
• Dariusz Strugarek (Poland)
• Xiaoya Wang (China)
• Zhilu Wu (China)
• Florian Zus (Germany)

JWG 1.1.3: Lunar reference frames (joint with IAU)

Terms of Reference

Recently, several organizations have established plans to visit the Moon for exploration and science. These led to the recognition that updated localization standards for both surface and orbital activities at the Moon are needed and should therefore be a priority for operations leading to exploration. The objective of this WG is to address the issues of the connection between Celestial, Earth and Lunar Reference Frames for the future missions in coordination with the IAG, IAU, and IERS and to formulate recommendations regarding the definition, the realization, and the dissemination of Lunar Reference Systems, across agencies and user communities. Experience acquired with the establishment of the Earth Reference Frame (ITRF, GCRF) will serve as the foundation for this task. The work of this group will be connected with on-going Lunanet International interoperability standardization work being performed by NASA, ESA and JAXA.

Objectives

The group will work towards identifying areas or fields, including models, methodologies, and instruments necessitating enhancement to align with the requirements of forthcoming lunar surface and orbital activities. It will also assess the consistency between time reference definition as provided by other institutions and space reference frame definition. It will recommend directions for improvements and assemble specific recommendations for users and future IERS conventions.

Program of Activities

The group will:

• Coordinate with IAU and IERS for Lunar Reference System and Frames definition;
• Organize meetings during international conferences (EGU, AGU, IAU GA and symposia, . . . );
• Organize online meetings in order to assess present limitations in terms of models and instrumentation with the goal of identifying directions for improvement; 
• Suggest recommendations for IERS conventions, in the form of a white book.

Members

• Agnès Fienga (France); Chair
• Kayla J. Brinkley (USA)
• Pasquale Defraigne (Belgium)
• Trevor Garner (USA)
• Cheryl Gramling (USA)
• Robert Heinkelmann (Germany)
• Luciano Iess (Italy)
• Maria Karbon (Spain)
• Sergei Klioner (Germany)
• Erwan Mazarico (USA)
• Stephan Merkowitz (USA)
• Jürgen Müller (Germany)
• Flora Paganelli (USA)
• Dmitry Pavlov (Russia)
• Nicolas Rambaux (France)
• Krzysztof Sośnica (Poland)
• Susan Stewart (USA)
• Richard Swinden (ESA)
• Patrizia Tavella (BIPM)
• Javier Ventura-Traveset (ESA)

Corresponding members

• Masaya Murata (Japan)
• Jinsong Ping (China)