Goddard Geophysical and Astronomical Observatory (GGAO)

GGAO logolocation of the GGAO on a map of the world

The Goddard Geophysical and Astronomical Observatory (GGAO) is one of the few sites in the world to have all four geodetic techniques co-located at a single location: SLR, VLBI, GNSS, and DORIS. Thus, sites like GGAO provide scientists a unique opportunity to assess system performance and perform multi-technique analysis. Furthermore, co-located techniques are an integral part to the maintenance of the International Terrestrial Reference Frame (ITRF), a set of points with their 3-dimensional cartesian coordinates which realize an ideal reference system. Co-location of systems in near proximity also enable (according to the ITRF website):

  • calibration of mobile systems, for instance SLR or GNSS antennas, using simultaneous measurements of instruments referring to the same technique
  • repeated measurements on a marker with mobile systems (for instance mobile SLR or VLBI), using non simultaneous measurements of instruments referring to the same technique
  • changes in antenna location for GNSS or DORIS
  • co-locations between instruments referring to different techniques, which implies eccentricities, except in the case of successive occupancies of a given marker by various mobile systems
GGAO map

GGAO site map

The GGAO is located as part of the NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, and is known as the birthplace of Satellite Laser Ranging (SLR). Development activities for current and prototype SLR and VLBI systems occur at the GGAO, which consists of nearly 50 hectares of land located 2 miles from Goddard's main campus.

Other activities at GGAO include supporting one-way laser ranging experiments to the Lunar Reconnaissance Orbiter, neutron spectroscopy experiments, search for optical sources of gamma ray bursts, the X-Ray beam-line, low frequency interferometry, the Astronomy Club's telescope facility, and many others.

Due to site topography issues and the long timeframe of implementing each of the original systems, the layout of this site is not ideal. At issue is poor geometry of each technique (all mostly in a straight North/South alignment), and poor geometry of calibration piers due to site topography. Other site infrastructure including power, safety, security, and access are excellent, being part of the GSFC infrastructure. Local commitment and support are excellent with the GGAO being the center for the development of the SLR and VLBI next generation systems, the proximity to GSFC, and availability of abundant Government, contractor, and other support.

The weather in the region is not ideal for SLR with about 50% cloud cover.

Recent news:

Gimbal and Mass Simulator installed at GGAO on September 19, 2019
September 30, 2019

Gimbal #1 and mass simulator (GMS) were crated and shipped from Cobham in Lansdale PA and delivered to GGAO early in the week of September 16th. The riser, which is needed to mate the pier with the gimbal, was also delivered to GGAO early that week. The riser was lifted by crane through the dome slit into place in the SGSLR shelter early in the morning of the 19th and bolted to the pier.

The riser bolted to the pier

The riser bolted to the pier

Later that morning, once the riser installation was complete, the GMS was lifted by crane through the dome slit into place in the SGSLR dome area and attached to the top of the riser.

The GMS being lifted by crane

The GMS being lifted by crane

Power and servo controller cables were connected to the GMS. Later that same day, Cobham employees were able to move the mount under computer control through the servo controller. This was followed by NASA/KBR connecting their computer and moving the mount.

Installed GMS

Installed GMS

The final levelling of the mount still needs to be done, and offsets for the encoders must be determined before star calibrations can be attempted. The mass simulator has a tertiary mirror (no primary or secondary mirrors) but we expect to be able to see enough stars to do a rough initial star calibration.

This installation will allow NASA/KBR to test the performance of the gimbal and also checkout the GTA test software. Meanwhile in Lansdale, Cobham is working to complete gimbal #2 and test it. Once the first telescope is ready, gimbal #1 will be shipped back to Lansdale and mated with the telescope in preparation for the first Gimbal and Telescope Factory Acceptance Test (FAT).