The main goal of the GAL project was to revisit the method of strapdown airborne gravimetry (SAG) in light of the Galileo(a) system. In traditional SAG, strapdown inertial measurements and GPS ranging signals (code and phase measurements) are used, where strapdown inertial measurements are obtained from inertial measurement units (IMUs).
Gravimetry is the measurement of the strength of the gravity field of a celestial body. In modern geodesy, many times, gravimetry is synonymous to the measurement of the differences between an actual gravity field and a [global] model of that gravity field. The Earth gravity field is approximated by global and local models that are computed by geodesists from gravity measurements and some other types of geodetic observations like level differences or deflections of the vertical. While global gravity models might be highly accurate they are of limited spatial resolution –i.e., of 100 km wavelengths and lower– and local gravimetric densifications are required.
The knowledge of the Earth gravity is a fundamental asset of modern society. From it, global and local geoid models are derived which, in turn, are the basis of vertical physical coordinate reference frames (CRFs). In cartography, civil engineering, hydrology, geophysics, environmental and Earth monitoring applications, the height coordinate –i.e., the altitude– is always referred to a physical –not geometric– vertical CRF. Thus, for instance, in practice, 3D positioning with GPS but without a geoid model is useless.
Airborne gravimetry using IMUs and GPS is also known as INS/GPS gravimetry or strapdown airborne gravimetry. It was invented by Dr. Klaus-Peter Schwarz (1938-2012) while being Professor of Geodesy at the Department of Geomatics Engineering (University of Calgary, Calgary, AB, Canada). Prof. Schwarz and his team developed the fundamental concepts of INS/GPS gravimetry in the 1990s. Among other results, they came to the conclusion that the noise of the GPS phase observables was the limiting factor of the method with the technology available at the moment. Prof. Schwarz and his team set the goal for INS/GPS gravimetry: a precision of 1 mGal at 1 km spatial resolution or better (1 mGal = 10–5 ms–2). The "1 mGal @ 1 km" target would translate into making INS/GPS gravimetry useful for cm-level geoid determination in geodesy and for geophysical exploration.
After the recent evolution of GPS positioning techniques and the advent of the European Union global satellite navigation system (GNSS) Galileo and other GNSSs like the Chinese BeiDou, the natural questions arise: (1) can the quality of today GPS and of the new GNSSs make the "1 mGal @ 1 km" a realistic achievable target? (2) and/or can the use of just more measurements –i.e., measurements to more satellites– make the "1 mGal @ 1 km" a realistic achievable target? Put in other words, the main goal of GAL was answering the two preceding questions.
The second goal of GAL was to combine the global, highly homogeneous gravity field obtained with the GOCE(b) satellite mission with local INS/GNSS gravimetry.
With the two goals in mind, the project encompassed a number of tasks:
- Estimation of GNSS-derived position, velocity and acceleration parameters with the lowest possible noise. For this purpose, the Precise Point Positioning (PPP) method was selected.
(In conventional INS/GNSS gravimetry, double differenced (DD) GNSS phase measurements are used. These observables are twice as noisy as the raw phase measurements. In GAL the PPP technique was used for the first time in INS/GNSS gravimetry. PPP is, logistically, much simpler than conventional INS/GNSS gravimetry as there is no need of local GNSS reference stations.)
- Direct estimation of gravity anomalies (GA) and/or gravity vector disturbances (GVD) with respect to the global GOCE model with the Kalman Filtering and Smoothing (KFS) and the Dynamic Network (DN) methods.
(Simultaneous estimation of IMU systematic errors and of GA was introduced by Dr. Luisa Bastos and her team in the AGMASCO(c) project at the end of the 1990s. In GAL, GOCE was used for the first time as the global model for INS/GNSS gravimetry.)
- Comparative analysis of the KFS and DN methods.
(Results of DN for INS/GNSS gravimetry with simulated data were presented by Dr. Assumpció Térmens simultaneously to the end of GAL. In GAL, simultaneous estimation of IMU systematic errors and of GA and GVD with actual data was performed by the first time.)
- Analysis of the high frequency noise in the GA and GVD and, if necessary, their low-pass filtering.
(In GAL, it was proven that GA and GVD after their determination with KFS or DN are much less affected by GNSS noise as compared to the original INS/GNSS gravimetric methods thus rendering the traditional last step low-pass filtering unnecessary.)
- Integration of the GA and/or GVD with the GOCE global models to derive cm-level accuracy local geoid models.
(In GAL, local geoid models were derived by the first time by just using satellite and airborne gravity measurements.)
- Execution of kinematic gravimetry campaigns, airborne and also terrestrial, for empirical validation of the said methods.
- Generation of realistic simulated Galileo ranging measurements along the trajectories of the actual gravimetric campaigns.
(In GAL, the contribution of GPS/Galileo –as compared to GPS alone– to the determination of a vehicle's acceleration in the KFS or DN INS/GNSS integration step was investigated for the first time.)
After a two year measurement, research, development and processing effort, the research team of GAL concluded that the combined use of GPS and Galileo along the method outlined above can deliver GA and GVD at the "0.5 mGal @ 1 km" accuracy and precision level with practically no need to final low-pass filtering of GA or GVD. KFS and DN performed similarly although the DN method was recommended since it eliminates the need for a final cross-over adjustment.
In the project, GeoNumerics concentrated on trajectory and GA, GVD determination with the DN method for navigation-grade IMUs. For this purpose, the capacity of the GENA platform was significantly increased (virtually unlimited number of observations) and the model toolbox dynamicSURVEY was developed.
GAL was supervised by geodesist and GNSS expert Marta Krywanis-Brzostowska (GSA project officer) and by telecommunications engineer and GNSS expert Guillermo Salgado (external project reviewer). Mr. Salgado was, at the time, the Galileo Mission Segment Programme Director of Thales Alenia Space.
More details of the project can be found in the GAL project web page (see below).
(a) Galileo is the European Union (EU) satellite-based positioning, navigation and timing system and, as such, the EU contribution to a global, multi-system GNSS together with the US GPS, the Russian GLONASS and the Chinese BeiDou. It will consist of 30 satellites positioned in three circular Medium Earth Orbit (MEO) planes at 23222 km altitude above the Earth. Its Final Operational Capability (FOC) configuration of 30 satellites is scheduled, as of today, for 2020.
(b) GOCE (Gravity field and steady-state Ocean Circulation Explorer) was an ESA (European Space Agency) mission and satellite (launched: 2009-03-17, ended: 2013-11-11) with the goal (1) to determine gravity-field anomalies with an accuracy of 1 mGal, (2) to determine the geoid with an accuracy of 1-2 cm and (3) to achieve the said accuracies at a spatial resolution better than 100 km. GOCE mapped variations in Earth’s gravity with extreme detail. As a result, for instance and among other applications, scientists exploited GOCE data to create the first global high-resolution map of the boundary between Earth’s crust and mantle –the Moho– and to detect sound waves from the massive earthquake that hit Japan (2011-03-11).
(c) AGMASCO (Airborne Geoid MApping System for Coastal Oceanography) was an EU FP4 (4th Framework Programme for Research and Development) MAST III (Marine Science and Technology Programme, 1994-1998) project conducted between 1996 and 1999. In AGMASCO, an airborne geoid mapping system was successfully implemented in different aircraft and flight tests conducted in in close and an open ocean (Skagerrak, Fram Strait and Açores) areas. After comparison of the airborne gravity results with ground truth and satellite data an accuracy better than 2mGal at spatial resolutions of 5 to 6 km was demonstrated. This was a remarkable result considering that a tactical-grade IMU (Northrop Grumman LN-200) was used instead of a navigation-grade one.
Title: Galileo for Gravity
Period: 2012-01-14 to 2014-02-14
Funding: European GNSS Agency (GSA), European Commission (EC) grant 287193, 7th Framework Programme for Research and Development (FP7), FP7-GALILEO-2011-GSA-1-a
Coordinator: Galileian Plus (Rome, IT)
Other participants: DEIMOS Engenharia (Lisabon, PT), École Polytechnique Fédérale de Lausanne (Lausanne, CH), GeoNumerics (Barcelona, ES), Institut Geològic de Catalunya (Barcelona, ES), Institute of Geomatics (Castelldefels, ES), Politecnico di Milano (Milano, IT)
Web page: http://www.gal-project.eu