Satellite Laser Ranging

Satellite Laser Ranging ( SLR abbreviation, German about: satellite measurements) is a highly accurate method of satellite geodesy, in using the term 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 used on one hand for precise orbit determination of the orbit of geodetic satellites, on the other hand to point determination in geodesy and geodynamics. From changes of the earth and the earth's rotation can be derived - together with other methods of the Higher Geodesy.

Basic principle

In the transmitting device of the base station, a short laser pulse is generated and transmitted via an optical system to the satellite. At the same time, an electronic time-interval counter is started. The reflected pulse from the satellite is recorded by a receiving optical system in the receiving device of the ground station, amplified, analyzed and fed to the counter as a stop pulse.

From the recorded time interval, the running time? T of the laser pulse and the propagation velocity, the distance d is given by:

To control and monitor the system and fixing the observation epochs are further sub- systems required (computer, atomic clocks ).

As a Space Segment satellites are needed with suitable reflectors.

History

The development of pulsed lasers for tracking of satellites began in the U.S. as early as 1961/62 within the American Explorer Program. In 1964, the first satellite was equipped with laser reflectors ( BEACON - Explorer - B ( BE- B) = Explorer 22). This was placed in an orbit of 1000 km altitude and 80 ° orbital inclination on 9 October 1964. The first laser distance measurements arrive in 1965 with an accuracy of a few meters. Also, Explorer 27 ( = BE- C ) and the two GEOS satellite Explorer 29 and Explorer 36 were equipped with laser reflectors.

Only the GEOS satellite could be used for satellite geodesy: on the one hand one could have predicted beforehand calculate the satellite orbits only insufficient, on the other hand, the number of reflected photons for high satellites, the interval counter for measuring time were not precise enough and too low. In turn mean lower orbits that the satellite across the sky runs ( runs of a few minutes) and his course for a reliable ephemeris is not stable enough to quickly. The breakthrough came only improved control and laser technology, combined with a well- circumscribed and programmed gate time of the receiver telescope.

In subsequent years, very rapid progress has been made. The accuracy reached mid-1970s, about one meter, now it has arrived in the millimeter range, so that the shape of the satellite is already playing a major role. When the laser echo is strong enough, the apparatus measures only the first of the returning photons. In Tagbeobachtungen - which are possible for 5-10 years - a larger number of reflections is analyzed.

In many places of the world laser distance measurement systems were developed and installed on the satellite. Often, these were to in-house developments in working groups of observatories. 1986 about 50 high-performance systems in use worldwide.

Classification of laser systems

The achievable accuracy of distance measurement is closely related to the temporal duration and resolution of the laser pulses.

It is customary to divide the laser systems used depending on the concept and performance in groups ( generations), the transitions are fluid.

With the increase in accuracy of the measurement systems, there are other areas of application. Especially with measuring accuracies to 1 ... 3 cm can more accurately determined satellite orbits and contributions to geodynamic problems (eg crustal movements ) were made.

The emitted light from the ground flashes have a short-term power in the range of gigawatts. Therefore, the monitoring activities must be agreed exactly with the air traffic control. In addition, there is a shutdown, should an aircraft still fall into near beam.

Lasermessysteme and components

Laser oscillators

Heart of a laser distance measuring system is the laser oscillator itself The artificial word LASER (Light Amplification by Stimulated Emission of Radiation ) refers to arrangements coherent amplification of electromagnetic oscillations in the ( optical ) spectral region by stimulated emission.

In satellite geodesy one uses in addition to coherence, ie the fixed phase relationship between the individual sub-beams, two other characteristics of the laser radiation, namely the high degree of focusing sharpness and high energy density. In this way, to carry extremely short pulses of high energy density over large distances is possible.

In satellite geodesy two types of lasers have been widely used, the ruby ​​laser and the neodymium - YAG - laser. The 1st and 2nd generation systems are almost exclusively equipped with ruby ​​lasers, the 3rd generation largely with Nd: YAG lasers.

Other system components

( a) Mount

In order to measure the distance to variable targets, the laser end part must be placed movable. This can be done on an adjustable in azimuth and elevation mount. It is advisable to install the receiver part on the same mount.

For units of the 1st generation, it is customary to attach the laser oscillator on the Mount, Laser 3rd generation are very sensitive and must be constructed in conditioned, dust-free environment. Under steady-state lasers, a separate room ( clean room ) is to be used. The laser pulses are routed via optical circuit in the transmit telescope. The mount must be aligned with sufficient accuracy on the moving target, so that the laser pulse hits the satellite. At lower accuracy requirements ( 1st generation ), the tracking can be done manually by visual inspection. In lasers of the 3rd Generation, who also work in the daytime, the tracking is done automatically because of pre-calculated satellite ephemeris.

( b ) light receiver

The energy of the pulse laser per unit area decreases on the way to the satellite and back respectively to the square of the distance. Further, the signal is attenuated by the atmosphere. Despite the very high output power and strong hugging consequently comes back very little energy, so that for larger distances a very powerful satellite receiver is needed.

The receiving part is composed of an optical system and an electronic light receivers. As optical systems are mirror telescope or telescopes into consideration, which focus the photons of the reflected laser pulses to the light receiver. Due to the increased aperture ratio reflector telescopes of large aperture are preferred, especially since it depends on the measurement of weak brightness and not on geometric quality. In order to avoid stray light, a low-bandwidth filter ( Δλ ~ 1 nm ) is used for the frequency range of the laser light.

When electronic photoreceiver photodetectors with a very short rise time, such as a photomultiplier (PMT), microchannel plate photomultiplier tube ( MCP- PMT) or avalanche photodiode (APD ) may be used. To reduce noise of the photodetector is enabled only for a short predicted period of t = 1 .10 microseconds ( microseconds). The rise time should not exceed 100 to 300 ps ( picoseconds ).

( c ) Pulse Analysis

The returned signal is distorted due to numerous disturbances. Causes are, inter alia, atmospheric interference, superposition by reflection on several reflectors, relative motion of the transmitter and reflector. To determine the pulse center a careful pulse analysis is required. Several methods are possible. Has proven to be the definition of center of gravity by measuring the area under the waveform.

If working on the basis of single photons ( eg, Lunar Laser Ranging LLR ) eliminates the pulse analysis. It must then be used methods which allow a detection and processing of individual photons.

( d ) Time Base

At runtime measurement electronic meters are used, the resolution can be 10 ps. The counters are controlled by atomic frequency standards, which are characterized by high short - and long-term stability. Considering come rubidium and cesium standards and hydrogen masers. The atomic frequency standards also define the station time to the epoch defining and must then be compared regularly with parent -time services.

( e) process computer

To advance calculation of the adjustment, the mount tracking, system monitoring, calibration and verification of the system parameters as well as data processing and control a powerful and comprehensive process computer system software is required.

( f) plane detector

In densely populated areas and near airports arrangements are sometimes required to avoid flying through an airplane through the laser beam. For this purpose, an optical system may be installed on aircraft location, which automatically switches the laser operation.

( g) gating and noise analysis

Modern SLR telescopes use the same optics for transmission and reception of the laser. Switching is done by means of gate time, that brief period of time, can be expected after the reflected signal with the earliest. It also serves to facilitate the noise analysis.

The latter is essential in Tagbeobachtungen where the daylight coming in from the thousand times more photons than the satellite echo. An example of the noise analysis shows picture on the left, where the software for the satellite station Wettzell noise from the receive only those photons pass through, which differ from the gate time by more than 5 nanoseconds.

Satellites with laser reflectors

Laser distance measurements can be performed only to satellites which are equipped with suitable laser reflectors. The reflectors have the task reflect back the light in the same direction from which it occurs. Such reflectors are also called retro-reflectors.

In order to achieve the desired accuracy, reflectors must be designed very carefully for each satellite and the Railway height. The reflector must have a sufficient size to reflect enough light. To this end, several individual reflectors of 2-4 cm diameter at certain arrangements (arrays) are summarized in most cases. At the correct interrelation of the individual reflectors very high demands are placed to keep momentum deformations as low as possible by signal superposition. Also the light path in the reflector must be known.

Since it is retroreflectors are passive systems that can be installed relatively as additional components of satellites simply, today a large number of spacecraft are equipped with them. For most satellites, as it comes equipped with the help of laser distance measurements to obtain exact path information for the actual satellite missions. However, since these satellites perform other tasks, the reflectors can not be arranged concentrically with the center of mass. Is a clear relationship between the respectively appropriate reflector and the satellite center must be set up.

In so-called laser satellites, the task of the Laser Ranging in the foreground. For the satellite orbit must be very stable. Therefore, to build satellite lasers with a core of solid metal ( sometimes even more dense material such as uranium, ) so that even a football great such as satellite Starlette weighs almost 50 kg. He suffered only minor perturbations characterized by non- gravitational forces ( high atmosphere, light pressure, solar wind, etc.), and the web can be accurately determined - for example, for satellite triangulation or to calculate the Earth's gravity field.

Of the approximately 20 launched since 1970 laser satellites are the most important:

  • LAGEOS (Laser Geodynamics Satellite, USA 1975), about 5000 km high polar path, therefore a lifetime of several million years, diameter 60 cm, weight 411 kg ( see figure above )
  • Starlette (France, 1975), train height currently about 900-1100 km, size ~ 20 cm, 50 kg
  • LAGEOS 2 (Italy, 1992), identical to the original LAGEOS, launched in the wake of the Space Shuttle mission STS -52
  • Stella ( identical with Starlette ), starting in 1993 with the European launcher Ariane
  • A Bulgarian satellite ( to 1985) and two Japanese laser satellites.

Global SLR Network

To the international coordination of the laser measurements to satellites was founded in the 1990s by the International Laser Ranging Service ( ILRS abgek. ). The ILRS organizes and coordinates the laser distance measurements to support global geodetic projects and satellite missions. He is also developing appropriate standards and strategies for measuring and analysis to ensure a high and consistent quality of the data.

The measurements of the SLR stations, of which there are several dozen world will come together arithmetically precise measurement networks, resulting coordinates and Earth rotation can be derived in the millimeter range. One of the fundamental products of the ILRS include precise ephemeris ( orbits ) LASER satellites, the coordinates and plate tectonic changes of the observatories, variations of Geozentrum and the Earth's gravity, and fundamental constants of physics, the Moon and the Moon's orbit.

To determine the latter is the so-called Lunar Laser Ranging (LLR ), ie the distance measurement of terrestrial stations to the lunar surface. For some laser reflectors are used, which were placed with the Apollo missions and those of the USSR on the moon. ( 750,000 miles) will only receive single photons per ausgesendetem strong laser pulse in these measurements over the 2-fold Moon distance, so the method is generally very expensive. The measurements showed that increasing the radius of the moon's orbit every year by about 40 mm.

International Earth Rotation Service

Since turning all laser observatories with the Earth's rotation in 23.9345 hours to the earth's axis, the spatial position of the Earth can be determined from the measurements exactly. The purpose of a special department of the IERS (International Earth Rotation, International Earth Rotation Service ).

The O.A. ILRS Service ( ILRS: International Laser Ranging Service) is the measured and reduced to a single model SLR data to the IERS available. This calculates at short intervals of the three main Earth rotation parameters (ERP ), namely the pole coordinates x, y ( the intersection points of the Earth's axis ) and the World Time Correction DUT1 ( irregularity of the earth's rotation ).

The values ​​of x, y vary spiral in the rhythm of the Chandler period ( about 430 days superimposed by a 365-day period), but stay within a 20 -meter circle. The value of DUT1 usually varies monotonically (always in one direction) and is the cause of the so-called leap seconds, by which the UTC time is adjusted every 1-3 years ended December 31 or June 30, the average Earth's rotation.

Combination with related methods

To bridge the weather dependence of the SLR and to increase the accuracy, the laser measurements are combined with other methods. These methods are especially

  • The VLBI radio interferometry to distant radio sources ( several hundred almost point-like quasars )
  • The Global Positioning System (GPS) and related systems GLONASS and the future Galileo,
  • The Doppler radio system DORIS and
  • The microwave system PRARE that is small enough so that other equip satellites.

These various systems form a continuous monitoring of the Earth and are united at intervals of several years to a new terrestrial reference system. This 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 more accurate than ITRF 2005.

All of these fundamental systems are in addition to the geodesy for other disciplines fundamentally, particularly for astronomy, physics and space travel.

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