The European Space Agency’s Navigation Directorate is planning a new satellite whose results will enable it to generate an updated global model of Earth – the International Terrestrial Framework of Reference, used for everything from land surveying to measuring sea level rise – with an accuracy of up to 1 mm, while tracking ground motion of approx. Only 0.1 mm per year. This improvement, in one fell swoop, will have a major impact on multiple navigation and Earth science applications, including enhancing the accuracy of the Galileo navigation system. This mission, called GENESIS, has been proposed to next month’s ministerial-level ESA Council meeting.
GENESIS will work by bringing together all four major technologies currently used to measure and co-locate the geodetic Earth on the same platform for the first time, aboard a satellite in orbit at an altitude of 6000 km. In the process, this satellite will also have one of the most precise specific orbits of any object in space, down to the millimeter scale.
“Measurement is about exact reference points,” explains Javier Ventura-Traveset, head of the Galileo Office of Navigation Science at the European Space Agency.
“Thanks to GENESIS we will enhance the accuracy of the Earth’s space reference system by about an order of magnitude. This is essential for positioning and navigation in civil society and for proper georeferencing of all geospatial information. GENESIS will also allow to enhance the ‘accurate orbiting’ of Galileo and other satellites, thus Immediately improve GPS performance, because we will have a more correct reference to the exact distances at which its signals travel from space to Earth.”
In addition to improving our knowledge of the Galileo orbits – and the Global Navigation Satellite System, and GNSS satellites – the data collected by GENESIS will also allow for “phase center calibration” of the GNSS antennas, determining the displacement between their mechanical and electrical centers, which plays a key role in many aspects of positioning and navigation and timing.
In addition, the high-resolution orbital tracking that GENESIS needs to perform its mission will enable one of the most accurate modeling yet of the non-gravitational forces acting on satellites in space — such as solar radiation pressure, the slight but constant thrust that objects in orbit receive from the sun itself.
GENESIS itself will be a relatively small satellite, but the challenge will be to synchronize and calibrate the quadruple payload array in a very stable environment, positioning them relative to the satellite’s center of mass down to a millimeter or less over the entire mission duration.
A new view of our planet
Equivalent benefits will also accrue to many other space missions, such as the radar altimeter satellites used to track sea level rise, as well as geosciences and location-based services.
Javier adds: “Thanks to GENESIS, we will improve the current accuracy of the International Terrestrial Reference Framework (ITRF), which is the basis for all space and terrestrial observations in navigation and Earth sciences, and therefore, its improvement will provide major benefits in all related applications. The GENESIS results will also enhance many of location-based services in everyday life, such as land surveying, intelligent transportation systems, precision agriculture, and infrastructure maintenance.”
The ITRF, in turn, is built from a network of specific reference points: Global Geodetic System stations scattered around the world whose locations are precisely and regularly determined using a quadruple combination of space-based geodetic techniques.
Javier adds: “The problem is that when you combine all of these methods of ITRF generation, they are affected by the accuracy with which we define the differential coordinates between the reference points for each technology, the so-called local links, some systematic errors, and the overall error level construct. “
“But now by operating them all together from the same satellite, with instruments duly calibrated and synchronized, we can identify and correct these biases over time, achieving greater accuracy and overall stability. GENESIS will in turn become a dynamic space geodetic observatory, which efficiently complements the structure existing terrestrial substructures, providing a breakthrough in improving the accuracy and consistency of the terrestrial frame of reference.”
Combined scientific measurements
GENESIS will embark on the following geodesic payloads, used today to locate monitoring stations:
|Global Navigation Satellite System . Receiver
|Satellite navigation, otherwise known as satnav, is one of the most widely used geodetic techniques. The position of a particular site is continuously fixed, using multiple star constellations from the satellite for greater accuracy, and with time it becomes more and more accurate down to the millimeter scale, in addition to detecting gradual drifts due to the movement of the Earth.
|satellite laser range
|This measurement method involves bouncing the laser pulses off a back reflector aboard a satellite, then measuring their two-way travel time to determine the distance the laser light shone, down to a few millimeters.
|The Doppler Integrated Satellite Orbitography and Radiopositioning System, or DORIS, has been in operation for the past three decades and has become a standard data source for the ITRF. DORIS signals from satellites are compared to those from a network of 60 receivers spaced across the planet in terms of a “Doppler shift” – the frequency rises on approach and sinks on distance – to fix its relative position down to the centimeter scale.
|Very long fundamental interferometry, or VLBI, began as a technique in radio astronomy in which observations of celestial bodies such as quasars from multiple radio telescopes are precisely combined together to produce a resolution equivalent to a single giant telescope. For geodesy, this technique can be reversed so that the exact timings of joint observations of celestial targets can derive the exact distance between locations. Thus, an artificial radio source aboard GENESIS would serve as a common target for multiple VLBI stations on the ground, broadcasting on at least two frequency bands.
One or more of these instruments, once duly qualified and marked for the GENESIS mission, could also be transported on future Galileo satellites.
Strong scientific and international support
Following the publication of the recent ESA Science White Paper and GNSS Science Symposium in Sofia, Bulgaria, the proposed mission has received very strong support from the scientific community as well as interest from NASA.
Moreover, the capabilities of this mission were recently recognized during the meeting of the United Nations International Committee on the Global Navigation Satellite System 16The tenth Meeting in Abu Dhabi October 9-14, where the wide range of benefits of this mission for micro-navigation, geodesy, earth science and climate change monitoring were highlighted.
ESA’s “Request for Information” was recently published for the GENESIS mission to gather more input from industrial partners who could undertake the mission, once approved.
Forward to FutureNAV
GENESIS is supported by the European Space Agency’s Navigation Directorate FutureNAV program, which also includes in-orbit demonstration satellites for the low-Earth orbit constellation Satnav. The FutureNAV programme, including GENESIS, is on view for decision at the next ESA Council at the ministerial level in November.
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