Satellite Laser Ranging and Its Impact on WEGENER Michael Pearlman 1, Georg Kirchner 2, Werner Gurtner 3, Erricos Pavlis 4, and Carey Noll 5 1 Harvard-Smithsonian Center for Astrophysics, Cambridge MA USA 2 Austrian Academy of Sciences, Graz, Austria 3 Universität Bern Astronomisches Institut, , Bern, Switzerland 4 Joint Center for Earth Systems Technology, Baltimore, MD, USA 5 NASA Goddard Space Flight Center, Greenbelt MD, USA

Satellite Laser Ranging Technique Precise range measurement between an SLR ground station and a retroreflector- equipped satellite using ultrashort laser pulses corrected for refraction, satellite center of mass, and the internal delay of the ranging machine. Simple range measurement Space segment is passive Simple refraction model Night / Day Operation Near real-time global data availability Satellite altitudes from 400 km to 20,000 km (e.g. GPS/GLONASS), and the Moon Cm satellite Orbit Accuracy Able to see small changes by looking at long time series Direct Range measurement to the Satellite Time of flight corrected for refraction and C/M Space segment is passive -retroreflector Optical - Simple refraction model Night / Day Operation Near real-time global data availability Satellite altitudes from 400 km to 20,000 km (Moon) Unambiguous cm orbital accuracy Long stable time series - change Unambiguous centimeter accuracy orbits Long-term stable time series

WEGENER (1/3) Established in 1984 with the WEGENER/MEDLAS activity Mobile SLR measurements of crustal motion around the Mediterranean region WEGENER SLR Occupations TLRS-1 MTLRS-1/2

WEGENER (2/3) Expanded in 1991 with the NRA Proposal (Kootwijk, March 1991) Improving the understanding of the interaction between the Afro-Arabian and Eurasian plates; Study the extent of the rebound phenomena in the Fennoscandia region; Make first assessment of the geodetically identifiable components of change contributing to mean sea level and develop improved techniques for modeling height variation using space space techniques.

WEGENER (3/3) WEGENER/MEDLAS measured ground motions

WEGENER/MEDLAS GPS Measurements Noto, Italy Matera, Italy (photos courtesy of EUREF)

SLR Science and Applications Measurements Precision Orbit Determination (POD) Time History of Station Positions and Motions Products Terrestrial Reference Frame (Center of Mass and Scale) Plate Tectonics and Crustal Deformation Static and Time-varying Gravity Field Earth Orientation and Rotation (Polar Motion, length of day) Precision Orbits and Calibration of Altimetry Missions (Oceans, Ice) Total Earth Mass Distribution Space Science - Tether Dynamics, etc. Lunar Science and Relativity More than 60 Space Missions Supported since 1970 Four Missions Rescued in the Last Decade

ILRS Network 33 global stations providing tracking data regularly Majority of SLR stations co-located with GNSS

Selected SLR Stations Around the World Zimmerwald, Switzerland Shanghai, China NGSLR, Greenbelt, MD USA Matera, Italy MLRS, TX USA Kashima, Japan Riyadh, Saudi Arabia TROS, China Wettzell, Germany Tahiti, French Polynesia Pictorial Basic design - different evolutions of technology Stromlo - newest, advanced technology Yarragadee - well worn with time All are collocated with GPS Some are collocated with other techniques TIGO, Concepcion, Chile Yarragadee, Australia Hartebeesthoek, South Africa

Sample of SLR Satellite Constellation (Geodetic Satellites) Etalon-I & -II LAGEOS-1 LAGEOS-2 Starlette Ajisai Stella GFZ-1 Two categories of satellites Stable reference in space - station position, reference frame, gravity field(temporal, low o/d) Passive, Heavy, spherical - long lifetime Special orbits, Well defined orbits Limitation - retros Inclination 64.8° 109.8° 52.6° 50° 98.6° 51.6° Perigee ht. (km) 19,120 5,860 5,620 1,490 810 800 396 Diameter (cm) 129.4 60 215 24 20 Mass (kg) 1415 407 405.4 685 47.3 20.6

Sample of SLR Satellite Constellation (POD Support) Terra-SAR-X CHAMP GFO-1 ERS-1 ERS-2 Inclination 64° 98.5° 66° 98.6° 87.27° Perigee ht. (km) 19,140 780 1,350 800 474 Mass (kg) 1,400 2,400 2,516 400 Meteor-3M Jason-1 GRACE Envisat ANDE Active satellites - LEO and High Satellites Terrain mapping altimeters Ocean and Ice surface, Land Topography POD and validation - critical Gravity - GRACE Navigation Satellite - GPS, GLONASS Calibration, Phase center of the antennas Galileo complex - retros (first two - 30) Need to understand the performance Inclination 99.64° 66° 89° 98.5° 90° Perigee ht. (km) 1,019 1,336 450 800 650 Mass (kg) 5,500 500 432/sat. 8,211 3,334

Sample of SLR Satellite Constellation (HEO) GLONASS COMPASS GIOVE ETS-8 GPS Inclination 64° 55° 55.5° 56° 0° Perigee ht. (km) 19,140 20,100 21,500 23,920 36,000 Mass (kg) 1,400 930 1,000 600 2800 Active satellites - LEO and High Satellites Terrain mapping altimeters Ocean and Ice surface, Land Topography POD and validation - critical Gravity - GRACE Navigation Satellite - GPS, GLONASS Calibration, Phase center of the antennas Galileo complex - retros (first two - 30) Need to understand the performance

GPS Tracking Campaign (25-Mar-2008 through 26-May-2008) Site Name Station # No. Passes No. Normal Points Beijing 7249 1 3 Changchun 7237 2 8 Graz 7839 28 251 Greenbelt 7105 4 Herstmonceux 7840 23 77 Katzively 1893 6 Koganei 7308 9 Matera 7941 McDonald 7080 10 42 Monument Peak 7110 Mount Stromlo 7825 11 40 Riyadh 7832 20 99 San Juan 7406 60 375 Simeiz 1873 50 Tanegashima 7358 29 149 Wettzell 8834 18 79 Yarragadee 7090 70 267 Zimmerwald 7810 15 61 Totals: 18 stations 299 1535 33 Passes/week

International Laser Ranging Service (ILRS) Established in 1998 as a service under the International Association of Geodesy (IAG) ILRS collects, merges, analyzes, archives and distributes satellite and lunar laser ranging data to satisfy a variety of scientific, engineering, and operational needs and encourages the application of new technologies to enhance the quality, quantity, and cost effectiveness of its data products Components Tracking Stations and Subnetworks Operations Centers Global and Regional Data Centers Analysis and Associate Analysis Centers Central Bureau ILRS produces standard products for the scientific and applications communities SLR World 40 stations, 20 Analysis Center, Engineering Groups Data Centers Groups on many countries How do we keep track ILRS established under the IAG Coordinated obervations Priorities, Formats, reporting Data Archiving, Engineering notes Missions support decisions Product definition - standard product

Satellite Tracking through the Years

Technology Developments 2 kHz operation to increase data yield; Automation and autonomous tracking to increase tracking coverage; Pass interleaving techniques to improves acquisition efficiency; Eye-safe operations for safety during autonomous operations; Event timers with near-ps resolution for higher accuracy; Web based restricted tracking to protect optically vulnerable satellites (ICESat, ALOS, etc.) Two wavelength experiments to test refraction models; Hollow retroreflector cubes for lighter, higher efficiency arrays; Experiments to demonstrate optical transponders for interplanetary ranging

LAGEOS Pass from Graz Station High repetition rate, short pulse lasers allow us to see retroreflector array details

Pass Interleaving Pass interleaving allows stations to track satellites that are simultaneously visible

NASA SLR Systems NASA’s Next Generation SLR (NGSLR), GGAO, Greenbelt, MD

Retroreflector Technology Better designed arrays with uncoated cubes now being placed on high satellites Several stations now ranging to ETS-8 in synchronous orbit; Much improved results on COMPASS; Hollow cube technology for space now being developed (GSFC and Laboratori Nazionali di Frascati, LNF, Italy); Dialog underway with relevant agencies on the importance of including reflectors on GPS-III satellites Single hollow cube Hollow cube array configuration

Analysis Activities ILRS “official products” (station coordinates and EOP) issued weekly; Seven ILRS Analysis Centers contribute to the official products: ASI, Agenzia Spaziale Italiana BKG, Bundesamt für Kartographie und Geodäsie DGFI, Deutsches Geodätisches Forschungsinstitut GA, Geosciences Australia GFZ, GeoForschungsZentrum Potsdam JCET, Joint Center for Earth Systems Technology NSGF, NERC Space Geodesy Facility GRGS, Groupe de Recherches de Geodesie Speciale Combination and Combination Back-up Centers at ASI and DGFI compute the combination products and furnish them to the IERS Time series of weekly solutions is provided to the IERS for the development of the ITRF; Analysis of early LAGEOS (1976-1993) data underway for ILRS product submission to the next reference frame; New “official POD product” for geodetic satellites under development

Geocenter Motion As an example… There is mass transport (like tides and seasonal effects) which redistribute mass in a fashion which is not uniform w.r.t. a crust-fixed reference frame. Since a satellite orbits the center of mass of the entire earth system, this redistribution of mass moves crust-fixed observers w.r.t. the geocenter. Monitoring this motion at the mm-level is now required for many global monitoring applications. SLR uniquely defines the geocenter motion at mm-levels. mm-level Geodesy requires understanding of the reference frame and its distortions to acute levels of precision. Shown here is the change in the origin of the crust-fixed frame w.r.t. the center of mass due to non tidal mass transport in the atmospheric and hydrospheric systems.

Gravity changes from SLR showing long-wavelength water redistribution Large interannual variations related to strong El-Niño-Southern Oscilation (ENSO) events seasonal variation only From Cheng and Tapley (2004) Seasonal variation from mass redistribution in atmosphere, ocean and continental water

Multi-year gravity changes from SLR (seasonal variation removed) Cheng and Tapley (2004) Secular trend significantly affected if time series is not long enough Cox and Chao (2002)