Slide 1:Michael PearlmanDirectorCentral BureauInternational Laser Ranging ServiceHarvard-Smithsonian Center for Astrophysics Cambridge MA USAmpearlman@cfa.harvard.edu

http://ilrs.gsfc.nasa.gov/index.html International Update on Satellite Laser Ranging (SLR) There are a number of measurement services that operate under the International Association of Geodesy The International Laser Ranging Services oversees the Satellite and Lunar Laser activitiesThere are a number of measurement services that operate under the International Association of Geodesy The International Laser Ranging Services oversees the Satellite and Lunar Laser activities

Slide 2:Icesat, Topex/Poseidon, Grace, Cosmic IceSat, Topex/Poseidon, Grace, Cosmic; GPS required for all aspects NASA’s Space Geodesy Program began with the Williamstown Report published in 1970 and conducted by a who’s who of Earth Science at the time. The report envisioned the utility of precision Very Long Baseline Interferometry and Satellite Laser Ranging in support of new suite of space based observations. These included ocean altimeters, and gravity missions to track ocean circulation and precision geodetic measurements to better measure and understand present day plate tectonic forces. This slide shows some of the applications that the Williamstown vision has spawned. Not shown here is the global geodetic networks that measure and sustain a precision Terrestrial Reference Frame and an equally precise Celestrial Reference Frame within which to place the continuum of global and temporal measurements for such things as crustal deformation, ice dynamics, sea level change, as well as the precision tracking of Earth Observing Satellites and Planetary Probes. GPS and the French Doris system has been added to the suite of space geodetic network instruments since the Williamstown Report.Icesat, Topex/Poseidon, Grace, Cosmic IceSat, Topex/Poseidon, Grace, Cosmic; GPS required for all aspects NASA’s Space Geodesy Program began with the Williamstown Report published in 1970 and conducted by a who’s who of Earth Science at the time. The report envisioned the utility of precision Very Long Baseline Interferometry and Satellite Laser Ranging in support of new suite of space based observations. These included ocean altimeters, and gravity missions to track ocean circulation and precision geodetic measurements to better measure and understand present day plate tectonic forces. This slide shows some of the applications that the Williamstown vision has spawned. Not shown here is the global geodetic networks that measure and sustain a precision Terrestrial Reference Frame and an equally precise Celestrial Reference Frame within which to place the continuum of global and temporal measurements for such things as crustal deformation, ice dynamics, sea level change, as well as the precision tracking of Earth Observing Satellites and Planetary Probes. GPS and the French Doris system has been added to the suite of space geodetic network instruments since the Williamstown Report.

Slide 3:Global Geodetic Observing System has sent a letter to IFOR recommending retroreflectors on the GPS 3 satellites NASA, NGA, NOAA, NRL, USGS, and USNO submitted a Geodetic Requirements Document to the IFOR recommending retroreflectors for PNT Requirement. GPS is very dynamic but faces the challenges of an expanding GNSS infratructure. We must include GLONASS and GALILEO to provide improved remote sensing and PNT capability.Global Geodetic Observing System has sent a letter to IFOR recommending retroreflectors on the GPS 3 satellites NASA, NGA, NOAA, NRL, USGS, and USNO submitted a Geodetic Requirements Document to the IFOR recommending retroreflectors for PNT Requirement. GPS is very dynamic but faces the challenges of an expanding GNSS infratructure. We must include GLONASS and GALILEO to provide improved remote sensing and PNT capability.

Slide 4: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 synchronous satellites (Moon) Unambiguous cm orbital accuracy Long stable time series - change 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 synchronous satellites (Moon) Unambiguous cm orbital accuracy Long stable time series - change

Slide 5:International Laser Ranging Service

Established in 1998 as a service under the International Association of Geodesy (IAG); Collects, merges, analyzes, archives and distributes satellite and lunar laser ranging data for scientific, engineering, and operational needs; Encourages the application of new technologies to enhance the quality, quantity, and cost effectiveness of its data products; Produces standard products for the scientific and applications communities; Includes 75 agencies in 26 countries. ILRS Organization

Slide 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) Orbits and Calibration of Altimetry Missions (Oceans, Ice) Total Earth Mass Distribution Space Science - Tether Dynamics, etc. Relativity Measurements and Lunar Science More than 60 Space Missions Supported since 1970 Four Missions Rescued in the Last Decade Most of the products that we generate (TRF. POD, EO, gravity field, etc) are done in conjunction with the other space techniques (GNSS, VLBI, DORIS) Measurements – POD and time history of station positions and motion Out if these come products: Measurements – POD and time history of station positions and motion Out if these come products:

Slide 7:ILRS Network

33 global stations provide tracking data regularly Tracking about 25 satellites Most of the SLR stations co-located with GNSS

Slide 8: Selected SLR Stations Around the World

Hartebeesthoek, South Africa TIGO, Concepcion, Chile MLRS, TX USA Matera, Italy Tahiti, French Polynesia Yarragadee, Australia Riyadh, Saudi Arabia Wettzell, Germany TROS, China Shanghai, China Kashima, Japan Zimmerwald, Switzerland NGSLR, Greenbelt, MD USA Pictures of some of the Laser Ranging Systems Basic design - different evolutions of technology Some use Domes, Sliding roofs, Some like Mt Stromlo in Australia have some of the newest technologies Other like Yarragadee in Australia - well worn with time, but one of the most productive Pictures of some of the Laser Ranging Systems Basic design - different evolutions of technology Some use Domes, Sliding roofs, Some like Mt Stromlo in Australia have some of the newest technologies Other like Yarragadee in Australia - well worn with time, but one of the most productive

Slide 9:SLR Developments

Higher repetition rate to increase data yield and improve pass-interleaving Eye-safe operations and auto tracking Automation (unattended operation) Event timers with near-ps resolution Web-based restricted tracking to protect optically vulnerable satellites (ICESat, ALOS, etc.) Two wavelength experiments to test refraction models Experiments continue to demonstrate optical transponders for interplanetary ranging; LRO-LR one-way ranging to the Lunar Orbiter presently underway 2-KHz returns from Graz Station Pass Interleaving at Zimmerwald Station One-way ranging to LRO Higher repetition rates – 5 -10 hertz to Kilohertz operations Eyesafe operations Automation Higher repetition rates – 5 -10 hertz to Kilohertz operations Eyesafe operations Automation

Slide 10:NASA New Generation SLR System

NASA’s Next Generation SLR (NGSLR), GGAO, Greenbelt, MD New system under development by NASA – more compact, something that could be mass produced Using newer technologies Looking toward a more standardization Some of the differences in the systems are subtle and become more of an issue as we push to mm accuraciesNew system under development by NASA – more compact, something that could be mass produced Using newer technologies Looking toward a more standardization Some of the differences in the systems are subtle and become more of an issue as we push to mm accuracies

Slide 11:Sample of SLR Satellite Constellation(HEO)

GLONASS GPS GIOVE ETS-8 COMPASS 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 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

Slide 12:ILRS Retroreflector Standards for GNSS Satellitesto increase tracking efficiency

Retroreflector payloads for GNSS satellites in the neighborhood 20,000 km altitude should have a minimum  “effective cross-section” of 100 million sq. meters (5 times that of GPS-35 and -36) Retroreflector payloads for GNSS satellites in higher or lower orbits should have a minimum “effective cross-section” scaled to compensate for the R**4 increase or decrease in signal strength The parameters necessary for the precise definition of the vectors between the effective reflection plane, the radiometric antenna phase center and the center of mass of the spacecraft should be specified and maintained with an accuracy better than 0.1 ppb (few mm).

Slide 13:Current SLR Ranging to GNSS Satellites

Operations include 7 GNSS satellites (GPS 36; GLONASS 102, 109 and 115; GIOVE –A and – B; and COMPASS M1) Satellite priorities set according to satellite altitude; Track 5 minute segments at various points along the pass; Data transmitted after each pass; The data is available on the website within an hour or two; Plenty of spare SLR tracking capacity Getting daylight ranging on new retroreflector arrays on GLONASS 115 and COMPASS M1

Slide 14:ILRS Restricted Tracking

ILRS authorization to track ILRS-approved satellites is constituted and governed by an approved Mission Support Request Form; All SLR stations within the International Laser Ranging Service agree to adhere to any applicable ILRS Restricted Tracking Procedures including: station by station authorization; time and viewing angle constraints; energy/power constraints; go/no-go switch.

Slide 16:Some people think the Earth looks like this:

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Slide 17:But really – it looks like this!

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Slide 18:Terrestrial Reference Frame (TRF)

Provides the stable coordinate system that allows us to link measurements over space, time and evolving technologies; An accurate, stable set of station positions and velocities; Essential for tracking and interpreting flight missions; Foundation for space-based and ground-based metric observations; Established and maintained by the global space geodetic networks; Network measurements must be: precise, continuous, robust, reliable, geographically well distributed proper density over the continents and oceans interconnected by co-location of different observing techniques Strong Ground Network and a Strong Satellite Component with co-location at both ends; “Co-location” at the analysis level.

Slide 19:Value of SLR Tracking of the GNSS Constellations

Geoscience Improve the Terrestrial Reference Frame (space and ground co-location) Distribute the reference frame globally; Improve LEO POD for active satellites (altimeters, etc) GNSS World Provide independent Quality Assurance: - The GNSS orbit accuracy cannot be directly validated from the GNSS data itself; Assure interoperability amongst GPS, GLONASS, Galileo, COMPASS -; Insure realization of WGS84 reference frame is consistent with ITRF; Independent range for time transfer; SLR is NOT required for use in routine/operational RF derived orbit and clock products

Slide 20:Terrestrial Reference Frame 

Two of the most demanding requirements for the TRF: monitoring the water cycle at global to regional scales; monitoring and modeling sea surface and ocean mass changes in order to detect global change signals in ocean currents, volume, mass and sea level; Quantitatively: TRF should be accurate to 1 mm and stable to a 0.1 mm/yr, and Static geoid should be accurate to 1 mm and stable to a 0.1 mm/yr. (GGOS 2020, WCRP) A number of satellite missions are currently observing sea and ice topography with altimetry and mass transport in the water cycle through gravity missions; Future altimetry and gravity field missions with improved capability are in the pipeline; SAR and INSAR missions provide measurements of land surface displacements;

Slide 21:Benefit and RequirementSLR Tracking of GPS Satellites

What are the Benefits: Improve in the accuracy and stability of the reference frame (Earth center of mass, scale, etc) by tracking of satellite constellation at higher altitudes; Determine systematic errors among satellites in each constellation and among the constellations through co-location on the ground and in space; Improve of global PNT, separate orbital errors from clock errors; Use the GPS satellites to distribute the reference frame to everywhere on the Earth, provided we have accurate orbits for each GPS satellite; What do need: Retroreflector arrays on all of the GPS satellites; Accurate center of mass correction on the satellites: Accurate tracking of the satellites;

Slide 22:Concepts for an Operational GNSS Plan

Support GPS, Galileo, GLONASS, and COMPASS; Greater emphasis on more robust tracking and daylight ranging; Increased tracking capacity with high repetition rate systems; Newer retroreflector design; Data available on the website shortly after each pass; Possible tracking strategy Tracking of a subset of each constellation with a rotation through the entire constellation; simulations underway to help develop optimum strategies; For example: one satellite per orbital plane per system at a time; 60-day tracking cycles set to cover all satellites within a 12 month period; Flexible tracking strategies; organized in cooperation with the agencies involved and the requirements for the ITRF; The network can be segmented to track different satellites; ILRS analysts will do the data analysis and make the results available;