We are proud to announce that with the activation of the Laser ranging interferometer (LRI) on GRACE Follow-On on June 13th 2018, the first optical instrument flight hardware of SpaceTech is fully operational in orbit! This is a major milestone for our activities in the field of laser-optical intrumentation and proof of STIs capabilities to provide top notch optical instrument equipment.
The LRI measures the changes of the inter-satellite distance of the two GRACE FO satellites flying in approx. 220 km distance to each with unprecedented accuracy down to several ten nanometers (about 1/1000th of the thickness of a human hair).
Under contract to the Geoforschungszentrum Potsdam (GFZ) (and under the scientific lead of Albert Einstein Institute Hannover - AEI) SpaceTech signed responsible for the development of the optical bench, the retroflector, and the instrument baffles of the LRI, starting from prototypes to EM, QM and FMs for both satellites. On the German side, the photoreceivers of the optical bench were provided by DLR Institute for optical systems in Berlin, while STI subcontracted Airbus DS and Hensoldt Optronics for the steering mirror on the optical bench and the manufacturing and assembly of the Zerodur parts of the retroreflector . On the US side we had a close cooperation with JPL who where responsible for the mission and provided the US part of the LRI (the laser, the cavity & the phasemeter).
Left: Optical bench (flight spare); Right: Retroreflector (EM)
The optical bench (shown above), consisting of an titanium optical bench with integrated and attached high performance laser optics, receives the laser signal via an optical fiber interface, launches the beam out of the fiber into free space, shapes it and directs it to µrad accuracy to the second spacecraft by means of a fine steering mirror. In addition it receives the laser signal from the second spacecraft and superimposes it with the local signal onto quadrant photoreceivers to aqcuire the DWS and heterodynes signal for the ranging measurement. Main challenges in the development were the high wavefront planarity requirement of lambda/12 (pv), the beam alignment error of less than 10 µrad and the ranging noise contribution of less than 5 nm/sqrt(Hz). To achieve this a low thermal noise, low mechanical stress, highly stable optical bench design was developed, including a newly designed ultra-stable monolithic beam collimator, which is now available for further applications.
The retroreflector (shown above) , consisting of an ultra-stable carbon-fiber structure with attached zerodur optics, routes the laser beam around the center of mass of the respective spacecraft, essential to achieve Nanometer accuracy to the ranging measurement. Main challenges of this development were the limited available space in the satellite in conjuction with the demanding mirror alignment error of less than 40 µrad, less than 400 nm/K vertex stability and less than lambda/15 wavefront planarity(pv).
A first evaluation of the measurement of the laser ranging interferometer is shown in the picture below
Top: Ranging data taken by the GRACE Follow-On Laser Ranging Interferometer as it flew over the Himalayas. Middle and bottom: The topography beneath the satellite tandem.[less]
graphic: B. Knispel/G.Heinzel/Max Planck Institute for Gravitational Physics; GRACE FO data: NASA, GFZ, JPL; map data: SRTM
In addition the the contribution to the LRI equipment, STI has been responsible for:
- the LRI instrument integration @ STI facitlities in cooperation with JPL, DLR Bremen, Airbus DS and AEI
- the spacecraft primary structures (structural analysis and procurement)
- the ASTSS tertiary structure (manufactured by CST)
- the deployable S-Band boom
- the Coarse Earth-Sun-Sensors (CESS)
- the satellite MGSE & transport containers
We did this in contract to the Geoforschungszentrum Potsdam (GFZ) for the LRI (and under the scientific lead of Albert Einstein Institute Hannover - AEI) and in subcontract to Airbus DS for the other contributions and in close cooperation with our collegues at JPL, the DLR Institutes in Bremen and Berlin Adlershof as well as Hensoldt Optronics.
STI is proud to be part of this mission and thankful for the great cooperation of all project partners!
Much more information on GRACE Follow-On can be found here: