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Michael Pearlman Director Central Bureau International Laser Ranging Service Harvard-Smithsonian Center for Astrophysics Cambridge MA USA [email protected] International Update on Satellite Laser Ranging (SLR). http://ilrs.gsfc.nasa.gov/index.html.

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Michael PearlmanDirectorCentral BureauInternational Laser Ranging ServiceHarvard-Smithsonian Center for Astrophysics Cambridge MA [email protected]

International Update on

Satellite Laser Ranging (SLR)


GPS impact on Environmental Monitoring






  • GPS is essential to the measurement of environmental change.

  • sea level change,

  • ice budget,

  • ocean circulation,

  • land sequestration of water,

  • atmospheric and

  • space weather (GPS occultation)



Atmospheric Dynamics

Sea Level Change

Products of the Global Geodetic Observing System

Terrestrial Reference Frame

Accuracy of 1 mm and stability of 0.1 mm/yr.


International Terrestrial Reference Frame (ITRF)

International Earth Rotation Service



Precision GPS Orbits and Clocks, Earth Rotation Parameters, Station Positions

Very Long Baseline



Satellite Laser



Global Navigation

Satellite Systems


Doppler Orbit Determination

and Radiopositioning

Integrated on Satellite


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 300 km to synchronous satellites, and the Moon

  • Cm satellite Orbit Accuracy

  • Able to see small changes by looking at long time series

  • Unambiguous centimeter accuracy orbits

  • Long-term stable time series

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.

International Laser Ranging Service

ILRS Organization

Slr science and applications

Measurements Association of Geodesy (IAG);

Precision Orbit Determination (POD)

Time History of Station Positions and Motions


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)

SLR Science and Applications

Ilrs network
ILRS Network Association of Geodesy (IAG);

  • 33 global stations provide tracking data regularly

  • Tracking about 25 satellites

  • Most of the SLR stations co-located with GNSS

Selected slr stations around the world
Selected SLR Stations Around the World Association of Geodesy (IAG);

Zimmerwald, Switzerland

Shanghai, China


Greenbelt, MD USA







Saudi Arabia

TROS, China

Wettzell, Germany


French Polynesia

TIGO, Concepcion, Chile

Yarragadee, Australia

Hartebeesthoek, South Africa

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

SLR Developments

One-way ranging to LRO

Pass Interleaving at Zimmerwald Station

2-KHz returns from Graz Station

Nasa new generation slr system
NASA New Generation SLR System pass-interleaving

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

Sample of slr satellite constellation heo
Sample of SLR Satellite Constellation pass-interleaving(HEO)






Ilrs retroreflector standards for gnss satellites to 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).

ILRS Retroreflector Standards for GNSS Satellitesto increase tracking efficiency

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

Current SLR Ranging to GNSS Satellites

Ilrs restricted tracking
ILRS Restricted Tracking 109 and 115; GIOVE –A and – B; and

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.

Missions for 2009 109 and 115; GIOVE –A and – B; and







COMPASS (Beidou-2)








Some people think the earth looks like this
Some people think the Earth looks like this: 109 and 115; GIOVE –A and – B; and


But really it looks like this
But really – it looks like this! 109 and 115; GIOVE –A and – B; and


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.

Terrestrial Reference Frame (TRF)

Value of slr tracking of the gnss constellations

Geoscience measurements over space, time and evolving technologies;

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

Value of SLR Tracking of the GNSS Constellations

Terrestrial reference frame

Two of the most demanding requirements for the TRF: measurements over space, time and evolving technologies;

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;


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;

Terrestrial Reference Frame

Benefit and requirement slr tracking of gps satellites

What are the Benefits: measurements over space, time and evolving technologies;

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;

Benefit and RequirementSLR Tracking of GPS Satellites

Concepts for an operational gnss plan

Support GPS, Galileo, GLONASS, and COMPASS; measurements over space, time and evolving technologies;

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;

Concepts for an Operational GNSS Plan

We invite you to visit our website @ measurements over space, time and evolving technologies;