GADEM
Galileo Atmospheric Data Enhancement Mission

GADEM is driven by the need for a cheap, worldwide measure of atmospheric data which can be used for improving Galileo navigation data, but would also be highly appreciated for climate research and weather prediction. In particular the water vapour content in the troposphere is highly variable and difficult to predict. For that reason a novel measurement concept using radio occultation measurements with two K-band frequencies is proposed for GADEM. During each occultation raw transmission profiles for phase and amplitude are measured at the receiving satellite. Transmission measurements are very sensitive to the contents of water vapour if a frequency near the water vapour absorption line of about 22 GHz is used.

By using a second frequency in parallel in the range of 17 GHz to 20 GHz, the influence of temperature and pressure can be separated. As a result, vertical profiles for temperature, pressure, humidity and water vapour through the troposphere can be calculated with unprecedented accuracy in a post-processing procedure.

The objective of GADEM was the study of concepts for atmospheric sounding by using K-band radio links between Galileo and LEO satellites or between Galileo satellites and ground stations with the goal to provide a worldwide measure of atmospheric data. The study included detailed scientific assessments for different measurement scenarios, the establishment of mission and system requirements, design work and the planning of a demonstrator mission.

Description

During the GADEM project the following study work was performed:

Scientific applications and performance analyses:

• establishment of observational requirements for Galileo radio occultation links;
• modelling of the links and their error characteristics;
• performance analyses results for atmospheric data.

Galileo and commercial applications:

• potential applications for improvement of Galileo system performance;
• possibilities for novel commercial services.

Constellation analysis:

• determination of link geometry for different mission scenarios.
• orbit modelling using satellite tool kit software.

Mission requirements analysis:

• establishment of a complete set of mission and system requirements;
• analysis of scientific requirements;
• analysis of accommodation options for the GADEM payload.

Operational concept:

• analysis of operational cycles for the Galileo-LEO mission concept;
• definition of interactions between space and ground segment.
• System architecture and trades:
• definition of main components of the system;
• selection of a technical baseline.

Detailed system design:

• design of transmitter payload;
• design of receiver payload;
• antenna design;
• final budgets for mass, volume and power.

Demonstrator mission options, requirements and design: - analysis of a feasibility of a demonstrator mission by using the ‘Glonass-M’ and LEO satellites; - selection of the GADEM demo mission baseline concept.

Objectives

The objective of this study is an analysis of the improvement in atmospheric sounding by using some new Galileo capabilities:

• the K-band capability expected for the Galileo inter-satellite link;
• the possibility to receive this K-band in a receiver mounted in low-earth-orbit spacecraft;
• the possibility of receiving this K-band signal in ground stations.

The basic idea behind this is that with the processing of the K-band after crossing through the atmosphere, the atmospheric properties can be derived, especially the water vapour content. This type of information can be very useful for several scientific applications including weather and climate prediction/analysis.

The project includes:

• an analysis of the improvement in scientific and commercial application that this concept would provide;
• the performance assessment for the different scenarios (Galileo-to-Galileo link, Galileo-to-LEO link, Galileo to ground station link, etc.);
• establishment of preliminary mission requirements for the different scenarios, to assess their feasibility;
• defining the system requirements for the K-band system, i.e. transmission and reception;
• the identification/selection of a space-based demo mission to make a prove-of-concept, system complexity and performance assessment of the atmospheric data.