0. 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

1. 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

2. 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

3. 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.

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

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

6. 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

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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. Satellite Tracking through the Years

15. 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

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

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

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

19. 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

20. 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 ( ) data underway for ILRS product submission to the next reference frame;
New “official POD product” for geodetic satellites under development

21. 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.

22. 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

23. 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)