Since the beginning of SLR, the original effort performed by the French geodesy community (‘Groupe de recherche de géodésie spatiale’, GRGS) has to be emphasized. Starting in the 1970s, the French space agency CNES, the French mapping and survey agency IGN, and CERGA (now OCA) have developed a dedicated site for both SLR and LLR activities. It is located in southern France (‘plateau de Calern’, alt. 1280 m), near Grasse. The two stations were equipped with telescope of 1 and 1.5 m, respectively; they have been operating continuously since the beginning of the 1980s (Table 2).

As a result of the increasing role of the French SLR and LLR systems in tracking both low and high earth orbiting satellites and the Moon, the Grasse site has been becoming a fundamental geodetic reference point. A permanent GPS station has been deployed in 1997, and several campaigns have been realized with mobile systems (VLBI, and FG-5 absolute gravimeter), thus reinforcing global and also regional studies for geodesy and geodynamics (e.g., tectonics of Alps, hydrology, etc.).

In view of participating in oceanography space missions (notably TOPEX/Poseidon, T/P, launched by CNES in 1992), an original and still unique mobile system, called the French Transportable Laser Ranging Station (FTLRS, see Fig. 1), has been developed. The idea behind this project was to build a very small SLR station (telescope of 13 cm in diameter, weight: 300 kg) that would be very easily transportable and could be installed, for example, in oceanic zones on islands and oil drilling platforms [28]. The main objectives of the FTLRS are to play an important role in space oceanography missions via satellite tracking, centimetre calibration of radar altimeters, positioning, and geodynamics [18]; during the past few years, the FTLRS has been deployed four times on different areas (see Table 2).

As initially expected, LAGEOS has been (and is still) extremely used to determine the very-long-wavelength part of global Earth gravity field models, including the gravitational constant GM that defines the distance scale [16]. As also expected, Starlette, which is sensitive to the mean wavelengths (due to its lower altitude) has been used to determine numerous geodynamical parameters, particularly for the tidal potential [5,24]. In the 1990s, the growing number of available satellites gradually increased the resolution of the large-scale geoid to wavelengths of less than 1000 km at the Earth's surface [3,22,33]. For example, at its altitude (398 km; lifetime of four years) GFZ-1 was the lowest geodynamic satellite ever so far being ranged to by laser stations: it has demonstrated the possibilities and difficulties of tracking such low targets with state-of-the-art SLR systems (altitude at the end of the mission: 230 km).

While measurement precision for all (gravity) model parameters benefit from the ever-increasing improvement in precision of individual range measurements, some parameters of geophysical interest, such as time variations of the Earth's oblateness [37], particularly benefit from the long time period of range measurements. To directly detect the fine corresponding signals contained in the geodetic satellite orbits (secular and long-period perturbations), we have developed a dedicated method based upon the averaging of the motion equations [25]. It allows us to simultaneously determine both precise averaged satellite orbits and geophysical parameters over periods of 20 years and more [17]. Thus, the early SLR data are still important in the separation of effects with long characteristic time scales, such as post-glacial rebound and 18.6-yr tide [14]. Currently, data spanning 28 years from six to eight satellites are used for measuring the large-scale mass movements on the Earth [11]. This long-term history of the SLR measurements makes it possible for geodesists to determine the changes over time in melting glaciers and polar ice sheets and the associated sea level change. The main idea is that significant variations in the Earth's gravity field might be partially linked to the long-term global climate changes.

As an example of the continuous improvement of the dynamical model driving orbit precision and parameter estimation, orbital laser residuals computed over successive 10-day periods (orbital arcs) from normal point acquired on LAGEOS, range from 50 cm in 1976 to 10 cm in 1980, to 5 cm in 1985–1986, and then to 2 cm in 2000 (average rms). Today, typical SLR residuals obtained from LAGEOS precise orbit determinations show an rms of around 1.8 cm from 12–15 tracking stations, even if it is still possible to reduce this result to about 1 cm by carefully editing data and site coordinates, and by adjusting empirical coefficients in the dynamical model. Inversely, these coefficients have been used to precisely quantify non-gravitational forces linked to phenomena appearing on LAGEOS satellites [20,26].

In addition to this important contribution of SLR to dynamical modelling, absolute range measurements have been obtained also on the very high navigation satellites GLONASS and GPS-35 and -36 during the IGEX-98 campaign [36]. The goal was to calculate orbit errors at an accuracy level of 1 to 2 cm maximum, thus establishing external and absolute local and regional control points of such constellations that are tracked mainly by their own technique [2]. Likewise, the gravity missions launched in the early 2000s – CHAMP (Germany, 474 km high) and GRACE (US and Germany, 485 km) – to measure fine-scale features of Earth's gravitational field and time variable gravity require orbits that are accurately referenced to the Earth centre of mass. For these missions, GPS is used for permanently tracking the spacecraft orbits while SLR provides range vertical measurements that give the orbit scale very accurately.

The precise orbit knowledge required by recent altimeter missions – TOPEX/Poseidon (T/P) and Jason-1 (US and France, 1336 km), and ERS and Envisat (ESA, 800 km) – is a critical prerequisite to their ability to significantly contribute to oceanography. Right now T/P, which uses SLR and DORIS as its primary tracking systems, has been seeing an average yearly increase of about 3 mm in global sea level. Jason-1 continues the global sea-level measurements begun by T/P more than 11 years ago, building up a record of sea level change that may help explain the past and predict the future [6]. During this period, the main contribution of laser ranging has been to achieve ever more accurate range measurements, particularly at high elevations where the uncertainty related to atmospheric propagation corrections is the lowest. These SLR data, seen as absolute ones, have enabled us to compute orbits of accurate altitude and to control their radial error budget on an operational basis [4]; for example, the radial orbit precision of Jason-1 has now come down to 16 mm rms. Also, very good results have been obtained with altimeter satellites only tracked by laser: ERS-1, GEOSAT follow-on, and T/P at present (e.g., [32]).

Radar altimeter calibration experiments, or campaigns, play also an important role in addition to orbit validation activities; this is particularly true in view of mixing satellite altimeter data from several missions over decades. While the changes may be less than 1 cm (e.g., due to instrument drift), we take these changes in the altimeter very seriously as they clearly affect the scientific goals of space oceanography missions. For this reason, we have set up a semi-permanent site in Corsica with the lowest possible installation and monitoring costs, in collaboration with CNES and NASA. The FTLRS system, the role of which is to achieve centimetre accuracy locally for the orbit, has been deployed two times on this site already (in 2002 and very recently in 2005). T/P and Jason-1 have benefited from laser tracking during the calibration campaigns [19]. The size of the changes that have been investigated for T/P and Jason-1 and the large amount of effort already expanded show the difficulty of conducting long-term altimetry at the millimetre-per-year level. In this respect, radar altimeter calibration and orbit validation work undertaken for altimetry projects is extremely important.

The determinations of Earth orientation and UT1 at the 1-millisecond of arc level on the one hand, and of terrestrial reference frame parameters at the sub-decimetre level on the other hand began 35 years ago with SLR and VLBI. Since drastic improvements have been obtained at the international level regarding the accuracy and the operational features, ITRF2000 is the most extensive and accurate terrestrial reference frame ever developed and includes positions and velocities for about 800 stations located at about 500 sites [1]. From the available space-geodetic techniques, SLR is the one which provides unique information on the location of the earth's geocenter and, shared with VLBI, absolute scale. In particular, SLR is the most accurate technique currently available to monitor vertical motion, providing an absolute reference system for post-glacial rebound, sea-level and ice-volume changes. We should note that GPS and DORIS play a very important role in the ITRF solution, considering the homogeneous networks and the increasing quality and quantity of available data.

As initially expected, LAGEOS has been extremely used to determine station coordinates and velocities, and has largely contributed to the monitoring of polar motion and rotation of the Earth in association to other space techniques. At present, the laser technique through tracking data on both LAGEOS and LAGEOS-2 (around 2300 normal points collected per week) is able to determine a combined solution on a weekly operational basis: laser station coordinates computed every week and Earth orientation parameters computed at a 1-day interval [10]. The average internal consistency of the weekly coordinate solutions is of 12–15 mm, depending on the used process (constrained inverse methods), which corresponds to an accuracy of around 1–2 mm yr⁻¹ in term of velocity. Concerning the great features of the global SLR network, if the total number of participating stations is stable since several years (around 40), the mean number of stations that observe 1500 satellite passes a year has increased from 15 to 19. Nevertheless, the network is still poorly distributed, particularly in the Pacific zone (southern hemisphere), although Haleakala (Hawaii), Tahiti, Arequipa (West of South America), and Mt Stromlo (East of Australia) systems. Currently, several of these stations have had technological, logistical, and/or financial problems, because what is perhaps less appreciated is how important the terrestrial reference frame is to other scientific experiments. One example is satellite altimetry, which uses DORIS, GPS, and SLR as tracking systems. Assessments of sea-level rise derived from studies of several and independent missions (e.g., T/P and Jason-1) directly benefit from accurate ITRF coordinates being used for the DORIS, GPS, and SLR tracking sites.

NASA put three panels of laser reflectors, on the landing sites of Apollo 11, 14 and 15. After more than 30 years, Lunar Laser Ranging (LLR) remains the only active Apollo experiment [15]. Two French panels were also put on the Soviet vehicles Lunakhod even if one was never detected. Very few stations are able to get returns, but it is enough to measure the Earth–Moon distance and thus to determine the accurate rate of getting away of the Moon (around 3.80 cm yr⁻¹ or equivalent to −25.9 arcsec/century square for its secular acceleration). It is one of the few measurements of the energy dissipation within the Earth–Moon system. On the other hand, recent results based on TOPEX/Poseidon data enabled to locate in the mid North Atlantic an area where a significant part of energy is deposited through the dissipation of internal waves excited by the lunar tides effects against the bottom of the ocean topography [23,30].

With the passive nature of the reflectors and steady improvement in observing equipment and data analysis, LLR continues to provide state-of-the-art results. Gains are steady as the data-base expands (see Table 3); the big advantage of LLR is the long time span of lunar observations (1970–2005). Results exist in the field of solid Earth sciences, solar system ephemeris, terrestrial and celestial reference frames, lunar physics, and general relativity gravitational theory; they are shortly developed below.

The lunar ephemeris entirely relies on LLR data. The complications of the orbit are handled by numerical integration (translation and rotation) in the JPL's solution. However, analytical theories fit to the integrations provide information on the behaviour of the orbital elements. A specific characteristic of the Paris Observatory's approach of the lunar problem is the use of analytical solutions for the orbital motion and for the libration of the Moon [7]. Analyses of LLR observations from 1972 till now have allowed us to determine the lunar and solar parameters of the motion (in particular lunar and solar mean motions useful for long term predictions), the tidal deceleration of the lunar mean longitude, the ‘observed corrections’ for the motions of perigee and node [8]. Taking advantage of the natural separation of the forced and free libration in an analytical solution, Chapront et al. have also fit the free libration parameters providing a solution for the ‘free component’ of the lunar rotation vector. The precision (rms) of the over-all fit nowadays is 3 cm. Because the Moon is the most important tide-generating body, the capability of LLR to determine tidal parameters has been carefully investigated by several authors [35].

In addition, the LLR technique is sensitive to the orientation of the Earth with respect to the lunar orbit. Hence, the components of a rotation matrix to refer the dynamical frame (motion of the Moon) to the celestial reference frame have been determined: a by-product of this determination is a correction to the precession constant [9]. The analysis of LLR data also enabled the determination of several geometrical parameters, such as reflector coordinates on the Moon's surface.

The US community is also pushing the LLR experiment as well, especially on a long-term basis, considering the challenge of having at our disposal a long time series to explore the Equivalence Principle through our unique Earth–Moon system. The fitting of 28 years of LLR data for a possible range signal indicating an Equivalence Principle-violating difference in the gravitational acceleration rate of Earth and the Moon toward the Sun has been performed and then examined, both analytically and by computer simulations [27]. A synodic post-model residual signal of characteristic size 1 cm was found in the data but, as far as we know, not yet interpreted. It strongly indicated that LLR observations should, for some time into the future, preferentially be made on the new Moon side of the quarter Moon phase.

At the moment, LLR remains the best natural tool in gravitational physics to test the equivalence principle (at several 10⁻¹³) [12]. Its interest will be dwindling with space projects like Mini-Step (US) or MicroSCOPE (France, to be launched in 2010) [34] for testing the weak equivalence principle up to 10⁻¹⁸ and 10⁻¹⁵, respectively. But it is not true for the strong EP only accessible through observation of the Earth–Moon system's dynamical behaviour. Due to the increasing amount of LLR data, the EP should be tested, hence, up to few 10⁻¹⁵ in the 2010s.

The laser community is studying several ideas concerning the network, the efficiency of the laser pulse (particularly when tracking the Moon's targets), and the precision and accuracy of range measurements. In our opinion two factors are essential in the future of SLR technique: the improvement of the worldwide coverage and of the measurement stability (the ability of the technique to permanently ensure the same data quality and thus accuracy) over long time periods of typically 1 year and more. The coverage will be improved first by improving the current less precise and productive stations, and second by deploying mobile and other ‘light’ systems (FTLRS, other LLR stations, and the expected US SLR2000) in areas where the impact of the tracking could be very great (southern hemisphere, Pacific and Indian oceans). The improvement of the precision and accuracy of range measurements to the absolute millimetre level is currently studied by several groups through a very short pulse duration (from 30–35 to 10 ps), the increase of the fire's rate (from 10 to 1000 Hz), and the adoption of new event timer (<5ps). In addition to these great technological features, SLR and LLR analyses have demonstrated the necessity of decreasing the target signatures to 1 mm or less without loss of returning signal.

In the time and frequency domain, the comparisons of up-to-date standards are still very complicated to set up, but this kind of experiments is more and more crucial to realize. In addition, due to the development of space projects (notably the Galileo navigation system), because time and frequency standards could be better in space than at ground level, several space techniques of time transfer have been developed. The Time Transfer by Laser Link experiment should be placed on-board Jason-2 (2008). The objective is to realize this time transfer with an accuracy of about 50 ps and a temporal stability better than 1 ps over 1000 s of integration, which represents an improvement of more than one order of magnitude with respect to present techniques. This experiment should demonstrate for the first time the capability of the laser tracking technique to realize the calibration of other navigation systems like GPS and the future Galileo project. Other techniques are under development; they are based on radio-electric frequencies, which permit a use on an operational basis and are easy to be deployed, in opposition of ones based on laser.

The laser technique that characterises the ‘Télémétrie interplanétaire optique’ (TIPO) project is based on a one-way range measurement mode. It should permit to realize measurements over several billions of kilometres through the solar system without a very complicated technology to be developed as in space (on a solar system probe) as at ground. The performances of the one-way technique, as it is proposed with TIPO, are extremely dependent of the clock performances to be used on-board the probe. In a short future, the development of space clocks having a frequency accuracy of 10⁻¹⁶ should permit to measure an absolute range at the 1 meter level and to reach the sub-centimetre level in measuring relative distances over several days of integration. The idea is to avoid drift of the embarked clock relative to Earth's clocks. Then, future applications in the solar system can be investigated, as for navigation or celestial mechanics as for fundamental physics. Such a project exists now and could be developed.