Satellite laser ranging
Satellite Laser Ranging ( SLR ; German as: satellite laser ranging ) is a highly precise method of Satellitengeodäsie , wherein by means of the transit time of a laser pulse, the distance between a ground station and a satellite is measured. This is a two-way measurement method.
Satellite Laser Ranging is used, on the one hand, to precisely determine the orbit of geodetic satellites and, on the other hand, to determine points in earth measurements and geodynamics . Changes in the earth's body and the earth's rotation can be derived from this - together with other methods of higher geodesy .
A short laser pulse is generated in the transmitter of the ground station and sent to the satellite via an optical system. At the same time an electronic time interval counter is started. The pulse reflected by the satellite is registered, amplified, analyzed and fed to the counter as a stop pulse via receiving optics in the receiving device of the ground station.
The time interval recorded gives the time of flight Δt of the laser pulse and the distance d via the speed of propagation with:
The essential components of the distance measurement system on the ground are accordingly:
- Generator and transmitter of the laser impulses including optical system and mount
- Return pulse detector and analyzer including the receiving system
- Time measuring device for determining the running time
As a space segment, satellites with suitable reflectors are required.
The development of pulsed lasers for orbit tracking of satellites began in the USA as early as 1961/62 as part of the American Explorer program. In 1964 the first satellite was equipped with laser reflectors (BEACON - Explorer - B (BE – B) = Explorer 22). This was brought into an orbit at an altitude of 1000 km and an incline of 80 ° on October 9, 1964 . The first laser distance measurements were made in 1965 with an accuracy of a few meters. Explorer 27 (= BE-C) and the two GEOS satellites Explorer 29 and Explorer 36 were also equipped with laser reflectors.
It was only the GEOS satellites that could be used for satellite geodesy : on the one hand, the satellite orbits could only be insufficiently calculated in advance, on the other hand the interval counters for the time measurement were not yet precise enough and the number of reflected light quanta was too small for high satellites. Lower orbits mean that the satellite is moving too quickly across the sky (only a few minutes pass) and that its orbit is not stable enough for a reliable ephemeris . The breakthrough came with improved control and laser technology , combined with a precisely defined and programmed gate time of the receiver telescope.
Very rapid progress was made in the years that followed. The accuracy reached about one meter in the mid-1970s, today (2015) it is in the millimeter range, so that the shape of the satellite already plays a major role. If the laser echo is strong enough, the apparatus only measures the first of the returning photons. During daytime observations - which have been possible since around 1995 - a larger number of reflexes are also analyzed.
Laser distance measurement systems for satellites have been developed and installed in many parts of the world. Often these were in-house developments in working groups at observatories. In 1986 around 50 high-performance systems were in use worldwide.
Classification of the laser systems
The achievable distance measurement accuracy is closely related to the duration and resolution of the laser pulses.
- The following applies: 1 nanosecond (ns) = 15 cm
It is common to divide the laser systems used into groups (generations) depending on the concept and performance, whereby the transitions are fluid.
- Generation: A pulse duration of 10 to 40 ns corresponds to a distance measurement accuracy of 1.5 to 6 m; mostly ruby lasers
- Generation: Shortening the pulse duration to 2–5 ns, corresponding to 30–120 cm
- Generation: A pulse duration in the subnanosecond range of 0.1 to 0.2 ns, corresponding to 1.5–3 cm; often Nd: YAG laser
With the increase in accuracy of the measuring systems, further areas of application arise. Satellite orbits can be determined more precisely and contributions to geodynamic issues (e.g. crustal movements ) can be made, especially with measuring accuracies of 1–3 cm .
The flashes of light emitted from the ground have a brief output in the range of gigawatts . Therefore, the observation activity must be precisely coordinated with air traffic control . In addition, there is an automatic switch-off, should an aircraft come near the beam.
Laser measurement systems and components
The heart of a laser distance measuring system is the laser oscillator itself. The artificial word LASER (Light Amplification by Stimulated Emission of Radiation) describes arrangements for the coherent amplification of electromagnetic oscillations in the (optical) spectral region through stimulated emission .
In satellite geodesy, in addition to coherence , i. H. the fixed phase relationship between the individual partial beams, two further properties of the laser radiation, namely the high focus sharpness and the high energy density . In this way it is possible to transport extremely short pulses of high energy density over long distances.
In satellite geodesy, two types of lasers have found widespread use, the ruby laser and the neodymium-YAG (= yttrium-aluminum-garnet) laser . The systems of the 1st and 2nd generation are almost exclusively equipped with ruby lasers, those of the 3rd generation largely with Nd: YAG lasers.
Further system components
In order to be able to measure the distance to variable targets, the laser transmitter part must be set up so that it can move. This can be done on a mount that is adjustable in azimuth and height. It is advisable to install the receiver on the same mount.
With devices of the 1st generation, it is common to attach the laser oscillator to the mount, lasers of the 3rd generation are very sensitive and must be installed in an air-conditioned, dust-free environment. With stationary lasers, a separate room ( clean room ) is used for this. The laser pulses are directed into the transmitting telescope via optical conductors. The mount must be aligned with the moving target with sufficient accuracy so that the laser pulse hits the satellite. If the accuracy requirements are lower (1st generation), the tracking can be carried out manually by visual control. In the case of lasers of the 3rd generation, which also work in daytime operation, the tracking takes place automatically on the basis of precalculated satellite ephemeris .
(b) light receiver
The energy of the laser pulse per unit area decreases on the way to the satellite and back with the square of the distance. Furthermore, the signal is weakened by the earth's atmosphere . In spite of the very high output energy and strong bundling, very little energy is returned, so that a very powerful receiving device is required for greater satellite distances.
The receiving part consists of an optical system and an electronic light receiver. As optical systems , reflector telescopes or telescopes come into consideration, which focus the photons of the reflected laser pulse onto the light receiver . Because of the larger aperture ratio , reflector telescopes with a large aperture are preferred, especially since the measurement of weak brightnesses and not geometric quality is important. To avoid interference light, a filter with a narrow bandwidth (Δλ ~ 1 nm) is used for the frequency range of the laser light.
As electronic light detector are photodetectors with a very short rise time as photomultiplier tube (PMT), microchannel plates -Photomultiplier (MCP-PMT) or avalanche photodiode (APD) is used. To reduce interfering signals , the photodetector is only activated for a short pre-calculated period of time of Δt from 1 to 10 microseconds ( microseconds ). The rise time should not exceed 100 to 300 ps ( picoseconds ).
(c) pulse analysis
The signal sent back is deformed due to numerous interferences. Causes include a. atmospheric disturbances, superimposition by reflection on several reflectors, relative movement of transmitter and reflector. Careful pulse analysis is required to determine the pulse center. Several methods are possible. Establishing the center of gravity by measuring the area under the signal curve has proven itself .
If working on the basis of single photons (e.g. Lunar Laser Ranging , LLR), the pulse analysis is not required. Processes must then be used that allow individual photons to be recognized and processed.
(d) time base
For transit time measurement electronic counters are used, the resolution may be 10 ps. The counters are controlled by atomic frequency standards , which are characterized by high short-term and long-term stability. Rubidium and cesium standards as well as hydrogen masers come into consideration for such a time base . The atomic frequency standards also define the station time for setting the epoch and must then be regularly compared with higher-level time services.
(e) process computer
A powerful process computer and comprehensive system software are required for the precalculation of the setting values, tracking of the mounting, system monitoring, calibration and checking of the system parameters as well as for data preparation and control.
(f) aircraft detector
In densely populated areas and near airports, precautions are sometimes required to prevent the laser beam from passing an aircraft. For this purpose, an optical system for aircraft location can be installed, which automatically switches off the laser operation.
(g) Gate time and noise analysis
Modern SLR telescopes use the same optics for sending and receiving the laser. Switching takes place by means of the gate time , the short period of time after which the reflected signal can be expected at the earliest. It is also used to facilitate noise analysis.
The latter is essential for daytime observations , where a thousand times more photons arrive from daylight than from the satellite echo. The picture on the right shows an example of the noise analysis, where the software of the Wettzell satellite station only lets through those photons from the reception noise that deviate from the gate time by at most 5 nanoseconds.
Satellites with laser reflectors
Laser distance measurements can only be carried out to satellites that are equipped with suitable laser reflectors . The task of the reflectors is to reflect the light back in the same direction from which it is incident. Such reflectors are also called retroreflectors .
In order to achieve the desired measurement accuracy, reflectors must be designed very carefully for each satellite shape and orbit height. The reflector must be large enough to reflect enough light. For this purpose, several individual reflectors with a diameter of 2–4 cm are usually combined into specific arrangements (arrays). Very high requirements are placed on the correct mutual assignment of the individual reflectors in order to keep pulse deformations due to signal overlap as low as possible. In addition, the light path in the reflector must be known.
Since retroreflectors are passive systems that can be installed relatively easily as additional components on satellites, a large number of spacecraft are now equipped with them. With most satellites equipped in this way, the aim is to obtain precise orbit information for the actual satellite missions with the help of laser distance measurements. However, since these satellites fulfill other tasks, the reflectors cannot be arranged concentrically to the center of mass. Therefore, a clear relationship between the appropriate reflector and the satellite center must be established.
With so-called laser satellites , the task of laser ranging is in the foreground. For this, the satellite orbit has to be very stable. This is why laser satellites are built with a core made of solid metal (sometimes even particularly dense material such as uranium ) so that a football-sized satellite such as Starlette weighs almost 50 kg. As a result, it suffers only minor orbital disturbances from non-gravitational forces (high atmosphere, light pressure, solar wind, etc.), and the orbit can be precisely determined - for example for satellite triangulation or for calculating the earth's gravity field .
Of the 20 or so laser satellites launched since 1970, the most important are:
- LAGEOS ( Laser Geodynamics Satellite , USA 1975), approx. 5,000 km high polar orbit , therefore a lifespan of several million years, diameter 60 cm, mass 411 kg (see picture above)
- Starlette (France, 1975), track height currently approx. 900–1100 km, size ≈20 cm, 50 kg
- LAGEOS 2 (Italy, 1992), identical to the original LAGEOS, launched as part of the space shuttle mission STS-52
- Stella (identical to Starlette), launched in 1993 with the European Ariane launcher
- a Bulgarian satellite (around 1985) and two Japanese laser satellites.
Global SLR network
The International Laser Ranging Service (abbreviated ILRS) was founded in the 1990s to coordinate laser measurements with satellites internationally . The ILRS organizes and coordinates laser range measurements to support global geodetic projects and satellite missions. He also develops suitable standards and strategies for measurement and analysis in order to ensure a high, consistent quality of the data.
The measurements of the SLR stations, of which there are a few dozen worldwide, are computationally combined to form precise surveying networks, from which coordinates and earth rotation in the millimeter range can be derived. The fundamental products of the ILRS include precise ephemeris (orbits) of the LASER satellites, the coordinates and plate tectonic changes of the observatories, variations of the geocenter and the earth's gravity field , as well as fundamental constants of physics, the earth's moon and the lunar orbit .
The so-called Lunar Laser Ranging ( LLR ) is used to determine the latter, i.e. the distance measurement from terrestrial stations to the lunar surface. For this purpose, some laser reflectors are used that were placed on the moon during Apollo missions and those of the USSR . For each strong laser pulse emitted, only individual light quanta are received during these measurements over twice the moon distance (approx. 750,000 km) , so that the method is very complex overall. The measurements showed that the radius of the lunar orbit increases by around 40 mm every year.
International Earth Rotation Service
Since all laser observatories rotate around the earth's axis in 23.9345 hours with the earth's rotation , the spatial position of the earth can be precisely determined from the measurements. A special department of the IERS (International Earth Rotation Service) serves this purpose .
The above-mentioned ILRS service (ILRS: International Laser Ranging Service) provides the IERS with the measured SLR data, which has been reduced to a uniform model. From this, it calculates the three most important earth rotation parameters (ERP) at short intervals , namely the polar coordinates x, y (point of intersection of the earth's (rotational) axis in the Arctic) and the world time correction dUT1 (irregularity of the earth's rotation ).
The value pair (x, y) varies locally in a spiral in the rhythm of the Chandler period (about 430 days, overlaid by a 365-day period), but remains within a 20 meter circle. The value of dUT1 changes mostly monotonously (always in one direction) and is the cause of the so-called leap seconds by which the UTC world time is adjusted every 1–3 years on December 31 or June 30 of the mean earth rotation.
Combination with related processes
In order to bridge the weather dependency of the SLR and to increase the accuracy, the laser measurements are combined with other methods. These methods are particular
- the VLBI -Radiointerferometrie to distant radio sources (several hundred almost point-like quasars )
- the Global Positioning System (GPS) and the related systems GLONASS and in future Galileo ,
- the DORIS Doppler radio system and
- the PRARE microwave system , which is small enough to equip other satellites with it.
These different systems form an uninterrupted monitoring of the earth and are combined into a new terrestrial reference system every few years . These earth models (see ITRS and ITRF 2000 ) currently have global accuracies of a few centimeters. In a few years, the next global model will be even more precise than ITRF 2005 .
- Image errors of the measuring beam, time measurement , time signal transmitter
- The high goal , Stellartriangulation , Pseudoranging , geoid and orbit determination
- Fundamental stations in Wettzell (Bavaria) and in Chile (menu = TIGO)
- swisstopo laser station Zimmerwald and image of a laser satellite
- International Laser Ranging Service
- Lunar Laser Ranging-The high-precision measurement of the moon movement, TU Munich