The GRACE Satellites

The two satellites of the first GRACE mission were largely based on the design of GFZ's earlier CHAMP (Challenging Mini-Satellite) mission, which was launched in July 2000 and operated for over 10 years to measure both the Earth’s gravity and magnetic fields. The CHAMP platform was designed and built on behalf of the GFZ by Daimler Chrysler Aerospace Jena Optronik GmbH (DJO) in cooperation with Dornier Satellitensysteme GmbH (formerly DSS, now Airbus) and Raumfahrt und Umwelttechnik GmbH (RST). CHAMP was the prototype for a number of subsequent Airbus missions, such as TerraSAR-X (DLR, launched in 2007), TanDEM-X (DLR, launched in 2010) or Swarm (ESA, launched in 2013). The technology known as "Flexbus" is based on a number of outstanding platform features that were developed and realised in particular for gravity field missions and for this reason were commissioned by JPL/NASA for GRACE (2002-2016) and GRACE-FO (since 2018) to Airbus in Immenstaad.

The continuous measurement of the distance between the two GRACE and GRACE-FO satellites is the primary observation for measuring temporal and spatial changes in the gravity field. The satellites  trailing each other at a distance of around 200 kilometres are attracted by the Earth slightly differently - sometimes more strongly and sometimes more weakly - depending on how much mass is underneath them. This leads to a continuous small change in the distance between the satellites. The primary instrument for measuring this distance on both missions is the very precise distance measuring system HAIRS (High Accuracy Inter-satellite Ranging System) developed by NASA's Jet Propulsion Laboratory (JPL) especially for GRACE, whose accuracy is a few thousandths of a millimetre. This corresponds to about one tenth of the diameter of a human hair.

A Laser Ranging Interferometer (LRI) is also operated on GRACE-FO as a technology demonstrator for future gravity field missions. The LRI is a collaboration between JPL and German partners under the leadership of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover. The LRI concept, its prototypes and the technical specifications originate from the AEI, whose researchers were intensively involved in the development and testing of the flight hardware. 

The German contributions to the LRI comprise the entire optical system, consisting of

• a triple mirror assembly (TMA) manufactured by Hensoldt (formerly Zeiss) in Oberkochen, 
• the optical bench assembly (OBA) from SpaceTech GmbH in Immenstaad, which was responsible for the entire German LRI contribution on the industry side including also the optoelectronics from the German Aerospace Center (DLR) in Berlin-Adlershof, and electronic assemblies from Apcon AeroSpace & Defence in Neubiberg near Munich,
• calibration instruments developed and supplied by the DLR Institute of Space Systems in Bremen,
• the LRI electronics (Optical Bench Electronics, OBE) for which JPL is responsible, the LRI processor (Laser Ranging Processor, LRP), the optical frequency reference (Cavity), and the procurement of the laser from Tesat Spacecom GmbH & Co. KG in Backnang near Stuttgart.

The LRI exceeded all expectations at GRACE-FO. Since its first commissioning on 14 June 2018, it has delivered measurement data virtually continuously, unless operational reasons on the satellites (e.g. manoeuvres or special in-flight tests) prevented the laser operations. With just 2 nanometres, the high-frequency measurement noise of the LRI is significantly lower than its specification of 60 nm and thus right of the order of magnitude of the thickness of a DNA strand This is realized in flight at a distance of about 200 kilometres and a satellite speed of roughly 27,000 km/h. It has been confirmed also that high-frequency (small-scale) signatures in the Earth's gravitational fieldThe Earth's gravitational field is made up of the Earth's gravitational pull and the latitude-dependent centrifugal acceleration caused by the Earth's rotation. The matter in and on the Earth is not evenly distributed. Water, loose sediments (such as... can be determined much more accurately with the LRI. The laser system has thus proven to be clearly superior to the microwave method and will therefore be routinely used instead of microwave interferometers on the future gravity missions.

Text: Prof. Dr. Frank Flechtner, GFZ