GRGS > Research > Subjects > Geodetic Measurements

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Geodetic Measurements

Laser Ranging

There are numerous scientific objectives of laser ranging. It serves as a foundation for many other topics, such as the Earth's gravity field at long wavelengths (including its temporal variations), terrestrial and spatial reference systems, for the study of sea levels and glaciers, plate tectonics, the calibration of spatial instruments, theoretical physics and also for lunar or planetary physics.

The team Astrogéo of the laboratory Géoazur, continues to develop this research, first by participating in observations and assimilation of that data into large earth models (gravitational field and terrestrial reference frames), and also in research and development in optical metrology. The T2000 project (2005-2008) resulted in a complete renovation of laser equipment, with new objectives for both science and technology. In addition, a inter-departmental research organisation called Iliad, founded in 2005, focuses on exchanging knowledge on optical links (length metrology) between our research team and ARTEMIS.

Télémétrie laser

The Principle of Measurement

Laser ranging is a technique for measuring distances between a ground observer and a laser reflector placed on an artificial satellite or on the Moon. A ranging station includes a high power laser, which sends a beam to the reflector, a telescope to collect the reflected light, and a dating system that gives the time taken for the light to make a round-trip.
The advantage of laser ranging lies in the simplicity of the measurement concept (measuring the travel time of a light pulse) and in its accuracy.
It uses retroreflectors, which have a modest cost, that are placed onboard satellites or, in the case of the Moon, that were placed there by the Apollo and Luna missions between 1969 and 1973.
However, the technique is dependent upon the weather and the need for specialized personnel to operate the ground stations. The situation is somewhat opposite of that of radio electric technologies such as GPS or DORIS, which work all the time, are very easy to use, and whose considerable cost lies mainly in the technology carried on-board one or several satellites (in the case of a constellation).
Processing laser satellite data often leads to determining a calibration bias for each instrument (at the laser ground station) and, possibly, for each type of satellite. Since we use ranging data on the geodetic satellites LAGEOS (-1 and -2) for calculating the international terrestrial reference frame, we have developed a method that enables us to determine the station coordinates and the bias of its instrument, at the same time as the parameters of polar motion.
The contribution of laser ranging data acquired in recent years on the laser (Geodetic) satellites Starlette, Stella, Ajisai, LAGEOS and LAGEOS-2 has been essential in improving the international terrestrial reference frame (ITRF), as well as the model of the Earth’s gravitation field at long wavelengths. Knowledge of the global gravity field and its temporal variations is fundamental for orbitography and, consequently, for positioning and satellite altimetry.

The Targets of Laser Ranging

Laser Ranging (especially the equipment on the Calern plateau, OCA) has been and is still applied to spatial dynamics (the trajectory of artificial satellites and its scientific applications) and to the study of the Earth-Moon system. See its technological aspects.

There are several types of targets :

  • Earth Observation Satellites (examples and websites):
    • - LAGEOS (6000 km altitude) and GNSS (GPS, GALILEO, GLONASS, equipped with onboard laser reflectors) for positioning information: estimation of station coordinates for the reference frame and of parameters of terrestrial kinematics.
    • - Jason-1 et -2, EnviSAT, etc., satellites for spatial oceanography carrying a system for measuring radar altimetry on board.
Station laser fixe de l'OCA (plateau de Calern)

Station laser fixe de l'OCA (plateau de Calern)

  • the Moon : Lunar laser data obtained by telemetry from reflectors placed on the Moon (in the 70s during the Apollo missions, in particular) from the laser station in Grasse (FRANCE) have enabled and continue to enable significant advances in celestial mechanics (orbital parameters of the Moon and, through the study of gravitational perturbations, those of other planets), in the geodynamics of the Earth-Moon system, and in theoretical physics .
  • distant targets (lunar satellites, uncooperative space debris, planetary probes), such as the LRO satellite currently around the moon.

At the Observatoire de la Cote d'Azur (OCA) in France, there are two laser ranging stations: one mobile (called FTLRS, for the French Transportable Laser Ranging System) which takes measurements in various locations and participates in specific measurement projects, and the other fixed (MeO for Optical Metrology) for the regular observation of satellites and the Moon.

Renovation work was carried between 2005 and 2008 at the Calern plateau (OCA) to upgrade equipment. This project (called T2000), financed by CNES, INSU, the Provence-Alpes-Cote-d’Azur region, and the OCA, has enabled a major reorganisation of laboratories to be able to use the mobile laser station and to renovate the opto-mechanics of the 154cm telescope (MeO, Lunar Laser).

In addition, laser ranging serves as fundamental part of the time transfer space experiment T2L2 (onboard Jason2).

Lunar Laser Station

Lunar Laser Station (fixed station MeO of the OCA)

The technical objective for the Moon Laser Station that belongs to the Observatoire de la Cote d’Azur (France) is to obtain distances from a fixed point on the Earth (crossing the two rotation axes of the telescope MeO) to five points on the moon, where reflectors were placed by U.S. missions Apollo XI, XIV and XV, and also by the Soviet missions Lunakhod 17 and 21.

lune

Scientific Objectives of the Lunar Laser Station

They include :

  • testing theories of gravitation: models of lunar motion within a relativistic framework enable us to include the PPN parameters γ, β in our adjustments. Moreover, the violation of the Strong Equivalence Principle can be studied on the magnitude of a periodic term equal to a lunar month. Note that for β and δ, the lunar laser is the best technology available.
  • lunar physics: shape of the moon, libration, tidal effects. This was one of the first applications of lunar laser measurements.
  • celestial mechanics: theory of lunar motion. Ideal laboratory for testing the most advanced theories, precision is still a challenge for analytical or semi-analytical theories. There are projects on librations and the interpretation of free librations. The accuracy of observations involves advanced relativistic modeling of lunar motion.
  • reference systems: terrestrial and celestial: through the observation of a solar system body and a theory of movement, the lunar laser allows us to link the two fundamental reference systems, a dynamic system and a celestial system. Besides this practical aspect, at a more fundamental level, we are actually testing Mach's principle.
  • Earth’s rotation: Measurements allow us to determine Universal Time in real time. The role of the lunar laser in this area has decreased with the lack of stations, the access to VLBI, and of course the development and the success of GPS (U.S.) and DORIS (radio-positioning system of CNES and the IGN). The parameters of the Earth’s rotation make it very difficult to model the Earth's interior.
  • precession and nutation: this involves the analysis of long series of observations, combined with those made by long baseline interferometry (VLBI), which provide access to Earth's motion in space and therefore to fundamental constants of precession and nutation.

Among the major ground facilities in the field of Experimental Gravitation, the lunar laser station of the Calern Plateau occupies an enviable position, because of the quality and regularity of the measures that are made there. The comparison of these measurements with theoretical models enables us to estimate very subtle physical effects, in particular those concerning theories of gravitation.

  • observations of distant satellites such as LAGEOS (6000 km) and ETALON (at 19000 km) for geodetic satellites, and GPS and GLONASS for those related to navigation. These observations including LAGEOS, also allow us to carry out a co-localization between the lunar laser stations and laser satellites (currently being dismantled); all of this serves as a reference point for the international terrestrial reference frame.
  • diameter measurements of stars that are hidden by the moon (i.e., the experiment TELOC)
  • observations of Miras, suspected to be double by Hipparcos, with the help of a speckle camera (MIRAS DOUBLES)
  • testing adaptive optics.

Targetting the Moon

The lunar laser station uses a laser and a telescope. The laser pulse is dated as it leaves. Part of this pulse captured by the telescope is dated when it returns. The difference between the two dates gives the time taken by light to make the round trip and, knowing the speed of light (a physical constant), we can calculate the distance.
Of course, we also have to take into account many other complex phenomena such as, the change in the speed of light depending on the altitude and the various layers of air, lengthening of the photon’s path due to the gravitation of the Sun and other massive planets, terrestrial and lunar tides that vary the distances, both between the telescope and the center of the Earth, and secondly between the reflector array on the Moon and center of the Moon, etc...all in all, about thirty different parameters.
Since mid-1995, the station has operated in millimeter mode with an internal accuracy of about 3 mm on an observation point corresponding to about a hundred echoes collected over a period of ten minutes. There is an internal evaluation of the residual noise during the construction of a normal point, not the accuracy of the distance in a vacuum, a measurement which depends, among other things, on various calibrations and the evaluation of refraction.

On the Moon (currently the most distant target, at approximately 384,000 km), 5 retro-reflector panels were placed during different lunar missions (including Apollo and Soviet robotic missions).

Plusieurs stations laser ont acquis des mesures sur la Lune depuis les années 70, notamment MacDonald aux EU et Grasse.

Plusieurs stations laser ont acquis des mesures sur la Lune depuis les années 70, notamment MacDonald aux EU et Grasse.

Exemple de Points Normaux acquis sur la Lune entre 2003 et 2006 par la station MeO

Exemple de Points Normaux acquis sur la Lune entre 2003 et 2006 par la station MeO

The station is also very active in observing high targets such as the GNSS satellites :

Répartition des tirs laser sur cibles élevées par Station

Répartition des tirs laser sur cibles élevées par Station

Exemple d'un réflecteur laser Lune, embarqué sur le Rover soviétique Lunakhod-1

Exemple d'un réflecteur laser Lune, embarqué sur le Rover soviétique Lunakhod-1

Doris

DORIS (Doppler Orbitography and Radiopositionning Integrated by Satellite) is a French civil system of precise orbit determination and precise ground location. The system operates on the principle of the Doppler effect between a network of ground stations that transmit (dual frequency beacons at 401.25 and 2036.25 MHz) and instruments (antenna, radio receiver and ultra-stable oscillator at 2 10-13 on 10s) onboard different satellites (SPOT n, TOPEX / Poseidon, Jason-n ...). A detailed description of aspects of the DORIS system, its principles, technology, the satellites involved, and the network of stations can be found on the website Aviso: http://www.aviso.oceanobs.com/fr/doris/index.html.

Its main applications are on one hand the precise orbit determination of satellites, particularly altimetry satellites (Jason, ENVISAT, CRYOSAT) as well as image satellites such as SPOT, and the other hand scientific applications in the areas of precise localization of ground stations (terrestrial reference system) and determining the parameters of the Earth’s rotation (pole coordinates, Universal Time).

doris

The scientific aspects of the DORIS system are coordinated by the IDS (International DORIS Service, http://ids.cls.fr/ ), in particular DORIS’ participation in defining the terrestrial reference system (ITRS International Terrestrial Reference System) through its various projects (see the website of the ITRF, International Terrestrial Reference Frame: http://itrf.ensg.ign.fr/ ).

The GRGS includes two analysis centers that treat DORIS data for this purpose:

The time series of both centers are available on the IDS website: http://ids.cls.fr/html/doris/ids-station-series.php3, and all the products generated by the two centers for the IDS are available on the Data Centers of the IGN ( ftp://doris.ensg.ign.fr/pub/doris/) and of the CDDIS ( ftp://cddis.gsfc.nasa.gov/pub/doris/).

doris

GNSS

The Global Navigation Satellite Systems (GNSS) provide an increasingly critical and increasingly broad contribution to the various disciplines of earth sciences. In this context, the national and international contribution of the GRGS revolves around three points:

1. International services: The objective is to participate in the International GNSS Service (IGS) as an Analysis Center (AC) by providing products of orbits and clocks for satellites in the GPS constellation as well as SINEX files containing station coordinates and orientation parameters of the Earth. The Analysis Center works in partnership with CLS (see: http://igsac-cnes.cls.fr). The IGS combines the solutions from different AC and makes the final product available to the international community of users. The normal equations produced by the GRGS are also sent to the Observatoire de Paris to be combined under the Working Group COL (Combination at The Observation Level) of the IERS.

2. Cooperation with research teams: The GRGS supports scientific projects that implement GNSS survey data through collaborations with other laboratories and by providing theGINSsoftware for those users. Thus, this software is used for processing of GPS data by several teams for applications as diverse as the study of terrestrial kinematics, the monitoring of glacier flows, or the calibration / validation of spatial altimeters or the study of crustal loading effects.

3. GNSS Hybridization: the European programme Galileo is of significant interest for the geosciences, in particular the configuration of hybridization with GPS and / or GLONASS. To prepare for the upcoming deployment of Galileo, the GRGS develops and validates the multi-GNSS capacities of GINS in processing hybrid GPS-GLONASS data. The goal is to soon be able to provide GLONASS products to the IGS.

These GNSS activities of the GRGS fall within the framework of the CNES programme : "Tools and Processing of GNSS data for the Geosciences."

Contribution of GNSS measurements for atmospheric dynamics and rising sea levels:

GNSS techniques enable us to precisely estimate the content of water vapor in the vicinity of measuring stations. Moreover, they allow us to study temporal variations of the vapor content at high frequency; and they thus contribute to meteorological studies. For example, a study of the monsoon in West Africa is in progress within the framework of AMMA project. Conversely, these estimates are also necessary for correcting measurements from satellites that are affected by crossing the troposphere. Research is underway to improve estimates of tropospheric delays and thus to better estimate other geodetic products, notably vertical positioning. One of the major challenges of the TIGA project, which the LAREG team participates in, is to determine vertical velocities in the vicinity of tide gauges using GPS, and thus provide a more solid estimate of the rising sea levels.

Comparaison de la cohérence des produits d’orbite et d’horloges fournis à l’IGS par les différents CA dont le GRGS (grg). Les écarts RMS sur les 3 composantes North-East-Up de la solution PPP de 36 stations sont représentés (GPS Week 1516) / a Guide to Using International GNSS Service (IGS) Products, Jan Kouba, Geodetic Survey Division, Natural Resources Canada, May 2009

Comparaison de la cohérence des produits d’orbite et d’horloges fournis à l’IGS par les différents CA dont le GRGS (grg). Les écarts RMS sur les 3 composantes North-East-Up de la solution PPP de 36 stations sont représentés (GPS Week 1516) / a Guide to Using International GNSS Service (IGS) Products, Jan Kouba, Geodetic Survey Division, Natural Resources Canada, May 2009

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