rus
Issue #6/2017
V.P.Vasiliev, V.D.Shargorodsky
The Current State of High-Precision Satellite Laser Ranging in Russia
The Current State of High-Precision Satellite Laser Ranging in Russia
The high-precision laser distance measurement is used for the solution of a huge set of tasks today, but among them the invariable relevance and width of application distinguishes determination of coordinates of motionless points on the Earth’s surface (geodesy) and also determination of instant coordinates and parameters of the movement of dynamic objects – navigation.
In the XX century to replace mechanical devices for such measurements – to measured tapes, numerator turns of wheels – devices for measurement of time distribution of electromagnetic waves of different ranges – radio-frequency and optical have come (applicable to underground and underwater measurements also definition of time distribution of acoustic waves is used).
Optical wave band at the solution of such tasks has the advantages and shortcomings in comparison with radio-frequency range: the last provides all-weather capability and in some cases a possibility of over-the-horizon measurements (due to diffraction of radio waves), however accuracy in the radio-frequency range is influenced by distortions of the electromagnetic field by local objects and environment factors (the atmosphere, ionospheres) influencing the speed of distribution of radio-frequency fluctuations make essential impact.
In the optical range these factors reducing the accuracy of measurement appear less, but losses increase in the air environment (especially due to dispersion in aerosols [1][3] and influence of a background of the sunlight creating quite often strong hindrances to carrying out measurements.
Therefore the combination of means of both ranges, and optical means is the most reasonable – today it is generally a laser distance measurement – serve as a standard for calibration and verification of radio engineering systems.
In the middle of the XX century the high-precision distance measurement based on measurement of time of distribution of electromagnetic waves was used mainly for land geodetic works and as during such works of the requirement to accuracy usually prevailed over requirements to efficiency (except for military applications), optical (since the beginning of the 60th years – laser) range finders have gained primary distribution. The measured ranges during such works usually don’t exceed several tens kilometers, and among the applied methods preference was given to a phase distance measurement (measurement of shift phases of the modulating signal imposed on the extending light bunch).
Nowadays, thanks to development of space navigation and geodetic systems, land coordinate networks began to be under construction with their help, and the laser distance measurement applied for this purpose has received the name of a satellite laser distance measurement (SLR – satellite laser ranging). As everywhere the extending satellite navigation relies on geodesy, requirements to the accuracy of navigation and geodetic systems quickly increase, and especially strict requirements are imposed to the laser distance measurement serving as reference and calibration means for these systems.
If in 60th and 70th as rather small was considered an error of range measurements of decimeter order units, then from 2000th years the question on reaching the accuracy of ±1 mm is brought up.
For the solution of these problems the International service of a laser distance measurement (ILRS – International Laser Ranging Service) [3]; since the beginning of the 90th years the Russian Federation became its member. Under the auspices of ILRS over 50 measuring stations on all to the world work now, and some more dozens of stations are in a stage of a construction or reconstruction.
It is necessary to notice that for optimization of this measuring network it has to be whenever possible evenly distributed on the surface of the globe, and in the 90th years the international SLR community had claims: the territory of the biggest country in the world – Russia – has been very poorly equipped with SLR stations (at us it is accepted to call them quantum and optical stations – QOS); today we in this regard came to the advanced positions – in the Russian territory of 20 operating stations, and by the year 2020 will be 29, and besides we make a contribution to condensation of SLR network of the southern hemisphere equipped still more weakly than northern (two of our stations – in Brazil and the Republic of South Africa – already successfully function).
Also SLR stations change – they became more compact, but at the same time more precisely than former. If in the early seventies our first laser stations had optics on the basis of one-and-a-half-meter projector mirrors (the laser locator of SKOL‑2 had 4 such mirrors on the general basic and rotary device), then the laser station "Sazhen-TM" produced in recent years has dual optical system with a diameter of mirrors of 25 cm, providing at the same time the same range and much the best accuracy of measurement.
Types of made at our enterprise and acting now laser stadiametric stations are shown in fig. 1 and 2, and their arrangement in the territory of Russia and the next to it countries – in fig. 3.
Reached now in world practice (including us) the accuracy of measurement is about 0,5 cm and is limited generally by two factors; as it has been stated in 2005 at the special international meeting to Eastbourne (Great Britain) which had a subtitle of Towards 1-mm accuracy [4] (a way to the accuracy of 1 mm), these factors are – difficulty of accurate accounting of index refraction of atmospheric air on the way of distribution of a laser beam and a so-called "error of the purpose" that is necessary to tell slightly in more detail about.
Index of air refraction near the Earth’s surface is ~ 1,0003 [5] and in total brings a delay of time of distribution of a signal from ~ 3 m to ~ 10 m depending on "a place corner" (an inclination of the vising line in relation to the horizon plane); work at small corners of the place when this delay exceeds 10 m, in general is undesirable. The size of index refraction of air depends on its temperature, pressure and humidity, and these factors are measured only in a point of the station arrangement. It is possible to consider all nuances of distribution of these parameters along the route of the beam by means of atmosphere model so far at best with a margin error in several mm (in terms of range) though the research and accounting of local conditions in combination with improvement of the general model of the atmosphere allows to improve this accuracy gradually.
Theoretically there is a solution of this problem – simultaneous measurement of range on two strongly carried lengths of waves of a laser radiation (for example, in visible and infrared sections of a range) with use of the equation of dispersion – dependences of refraction index on wavelength – in principle allows to receive value of signal delay in the atmosphere, without resorting to temperature measurement, pressure and air humidity along the route; however it was shown that for achievement in such a way the error of 1 mm it is required to measure the difference of propagation time for these two lengths of waves with an accuracy in the measured range better ±0,5 picoseconds [5] (i. e. ±0,07 mm in terms of range) that it is not possible to make on a row of technical reasons yet.
The second serious limiter of accuracy – "a purpose error" – is caused by the fact that almost all satellites purposes for high-precision geodetic and geophysical measurements represent metal spheres on which surface the set of retroreflector – angular reflectors from quartz glass (by the way, the majority of such satellites is established – the purposes and reflectors for them is made by our enterprise – see tab. 1 and fig. 4). These spherical designs have diameter from 23 cm to 2,15 m, and the number of angular reflectors on each of them is from 20 to 2142.
Because of simultaneous reflection of a laser impulse from several retroreflectors which are on different removals from the station and because of inevitable dispersion of sizes of the effective reflecting surface of these retroreflectors there is an error reaching for large spherical satellites – the purposes of several cm (tab. 2).
In the 90th years for the solution of specific high-precision problems of geodynamics (in particular, for the Japanese Keystone project on prediction of earthquakes and a tsunami on the basis of observation of small motions of the earth crust) our enterprise has created the special SLR purpose of WESTPAC (see fig. 4) where in each timepoint the laser signal is reflected only from one retroreflector that is reached by restriction of its angular field with special blends. It has allowed to reduce the average size of "a purpose error" for this satellite to ±0,5 mm, but has caused some difficulties at its observation: between series of reflections from different retroreflectors there were breaks, and averaging of results of measurements by means of turning of the satellite round its pivot-center at its start into an orbit gradually worsened because of braking of this rotation by the vortex currents arising in the metal case of the satellite at its movement in magnetic field of Earth (by the way, this lack of a certain measure is inherent in all metal spherical satellite purposes).
For radical overcoming the difficulties connected with "a purpose error" we have offered, made and launched in space the satellite purpose of absolutely new type – BLITS (Ball Lens In The Space) representing a glass spherical body with use of the principle of a lens of Lyuneberg [6], [7] allowing to focus the radiation bunch falling on such lens with the flat wave front on an opposite surface of a lens (fig. 5). Having applied the reflecting covering on a half of the sphere and having given to a spherical lens rotation around the axis lying in the section plane between the reflecting and transparent surfaces it is possible to receive the reflected signal in the form of periodically repeating series of the reflected impulses, and such satellite has no metal parts where there can be vortex currents, and its rotation around the axis of turning remains invariable for all the time of service of the satellite in space.
Such satellite practically represents the "dot" purpose which isn’t causing distortion of a form of the reflected impulse and fluctuations of provision of an effective point of the reflection concerning the center mass of the satellite. The error caused by temperature changes of index of glass refraction in measurement of range doesn’t exceed 0,1 mm.
This satellite which was successfully observed by all stations of the international SLR network since the end of 2009 prior to the beginning of 2013 has been put to circular orbit 835 km high and has served as a prototype for the new BLITS-M satellite, of a little bigger size and weight which is prepared for start into an orbit by height ~ now 1500 km where influence of the atmosphere on stability of an orbit is almost absent that will allow to use effectively this spacecraft for geophysical surveys and geodetic measurements with an accuracy, which achievement was difficult earlier.
Important addition to the range-metering systems considered above is (BKOS)[8] "requestless" system developed recently intended for transmission of laser pulse from the earth-based station on airborne receivers of the GLONASS spacecrafts and a binding of the pulses accepted by this receiver to an onboard time scale of the appropriate spacecraft (fig. 6). It allows, using a binding of high-precision "hours" of the earth-based laser station by this time, to make a check of onboard time scales of GLONASS devices with a terrestrial standard, providing at the expense of small errors of the laser line the substantial increase of accuracy of synchronization of onboard time of GLONASS and the appropriate increase in accuracy of navigation and geodesic determination by means of this global space system.
Equipment in the short term of all new earth-based laser stations by network of such stations of the territory of the globe will provide with such system taking into account broad coverage further substantial increase of precision characteristics of GLONASS system and will create her in this regard advantage before other global satellite navigation and geodetic systems. In a bigger degree it will be promoted by equipment of all spacecrafts of new modifications by inter-satellite laser lines (MLNSS) for fast repeated data exchange about a divergence of time scales on these devices (fig.7). The equipment of these lines in essence represents a combination of a requestless range finder and the low-informative communication line.
What further prospects of development of a high-precision laser satellite distance measurement – after equipment of measuring network with new, containing the BKOS system, stations like "Sazhen-K" and "Sazhen-l" (fig. 8)[9] prepared for mass production and having potential of achievement of an instrument error of 1–2 mm and after essential expansion of network of exact stations? What is conceived, planned and can be realized in the next years at our enterprise?
First of all, input is provided in a system of the second turn of the Altai optiko-laser center[4] with the telescope with a diameter of 3,12 m (fig. 9) and a laser range finder which will be capable to measure effectively with a millimetric accuracy of distance to the retroreflectors established on the Moon.
Here it is necessary to notice that within last three years we took measurements at ranges, comparable with distance to the Moon – it means the work on the retroreflex panel installed on the "Radioastron" spacecraft with the extended elliptic orbit which apogee is at distance of nearly 350 thousand km from Earth. However the accuracy of the ranging equipment used at the same time – about a decimeter that it is quite enough for the solution of problems of this space mission, but isn’t enough for the solution of the fundamental tasks connected with researches of the Moon and system Earth Moon (here appropriate to mention that our terrestrial geodesy and navigation are carried out in system where the center of gravity position is in the related system Earth Moon, and even "terra firma" fluctuates depending on position of the Moon, not to mention levels of ocean waters.
Now only three stations (two in the USA and one in France) having the corresponding equipment – powerful short-pulse laser transmitters and big reception telescopes are engaged in a lunar laser distance measurement (LLR–Lunar Laser Ranging). In the near future our station in Altai also will join them; this work in time is coordinated to new space missions during which will be delivered to the Moon and new retroreflex systems including created at our enterprise.
Now in three points on the Moon there are retroreflex panels delivered there by astronauts of missions of Apollo‑11, Apollo‑14, Apollo‑15 and in two points – the panels, smaller by the sizes, installed on the Soviet devices "Moon Rover‑1" and "Moon rover‑2". But all these systems were delivered to the Moon more than 40 years ago and their efficiency decreased because of the natural reasons (dust pollution and erosion of a surface, and maybe also because of possible radiation degradation).
Unfortunately, in short article there is no opportunity at least briefly to mention other numerous applications of a high-precision laser distance measurement – from the solution of the major problems of fundamental science to various practical devices including household purpose.
At distribution in the water environment (especially in blue-green area of a visible range) losses in the optical range, on the contrary, turn out less, than in radio-frequency [2] thanks to what the optical distance measurement (as well as information transfer) finds application during the underwater works.
[3] At distribution in the water environment (especially in blue-green area of a visible range) losses in the optical range, on the contrary, turn out less, than in radio-frequency [2] thanks to what the optical distance measurement (as well as information transfer) finds application during the underwater works.
[4] The first stage of this measuring and research center functions more than 15 years.
Optical wave band at the solution of such tasks has the advantages and shortcomings in comparison with radio-frequency range: the last provides all-weather capability and in some cases a possibility of over-the-horizon measurements (due to diffraction of radio waves), however accuracy in the radio-frequency range is influenced by distortions of the electromagnetic field by local objects and environment factors (the atmosphere, ionospheres) influencing the speed of distribution of radio-frequency fluctuations make essential impact.
In the optical range these factors reducing the accuracy of measurement appear less, but losses increase in the air environment (especially due to dispersion in aerosols [1][3] and influence of a background of the sunlight creating quite often strong hindrances to carrying out measurements.
Therefore the combination of means of both ranges, and optical means is the most reasonable – today it is generally a laser distance measurement – serve as a standard for calibration and verification of radio engineering systems.
In the middle of the XX century the high-precision distance measurement based on measurement of time of distribution of electromagnetic waves was used mainly for land geodetic works and as during such works of the requirement to accuracy usually prevailed over requirements to efficiency (except for military applications), optical (since the beginning of the 60th years – laser) range finders have gained primary distribution. The measured ranges during such works usually don’t exceed several tens kilometers, and among the applied methods preference was given to a phase distance measurement (measurement of shift phases of the modulating signal imposed on the extending light bunch).
Nowadays, thanks to development of space navigation and geodetic systems, land coordinate networks began to be under construction with their help, and the laser distance measurement applied for this purpose has received the name of a satellite laser distance measurement (SLR – satellite laser ranging). As everywhere the extending satellite navigation relies on geodesy, requirements to the accuracy of navigation and geodetic systems quickly increase, and especially strict requirements are imposed to the laser distance measurement serving as reference and calibration means for these systems.
If in 60th and 70th as rather small was considered an error of range measurements of decimeter order units, then from 2000th years the question on reaching the accuracy of ±1 mm is brought up.
For the solution of these problems the International service of a laser distance measurement (ILRS – International Laser Ranging Service) [3]; since the beginning of the 90th years the Russian Federation became its member. Under the auspices of ILRS over 50 measuring stations on all to the world work now, and some more dozens of stations are in a stage of a construction or reconstruction.
It is necessary to notice that for optimization of this measuring network it has to be whenever possible evenly distributed on the surface of the globe, and in the 90th years the international SLR community had claims: the territory of the biggest country in the world – Russia – has been very poorly equipped with SLR stations (at us it is accepted to call them quantum and optical stations – QOS); today we in this regard came to the advanced positions – in the Russian territory of 20 operating stations, and by the year 2020 will be 29, and besides we make a contribution to condensation of SLR network of the southern hemisphere equipped still more weakly than northern (two of our stations – in Brazil and the Republic of South Africa – already successfully function).
Also SLR stations change – they became more compact, but at the same time more precisely than former. If in the early seventies our first laser stations had optics on the basis of one-and-a-half-meter projector mirrors (the laser locator of SKOL‑2 had 4 such mirrors on the general basic and rotary device), then the laser station "Sazhen-TM" produced in recent years has dual optical system with a diameter of mirrors of 25 cm, providing at the same time the same range and much the best accuracy of measurement.
Types of made at our enterprise and acting now laser stadiametric stations are shown in fig. 1 and 2, and their arrangement in the territory of Russia and the next to it countries – in fig. 3.
Reached now in world practice (including us) the accuracy of measurement is about 0,5 cm and is limited generally by two factors; as it has been stated in 2005 at the special international meeting to Eastbourne (Great Britain) which had a subtitle of Towards 1-mm accuracy [4] (a way to the accuracy of 1 mm), these factors are – difficulty of accurate accounting of index refraction of atmospheric air on the way of distribution of a laser beam and a so-called "error of the purpose" that is necessary to tell slightly in more detail about.
Index of air refraction near the Earth’s surface is ~ 1,0003 [5] and in total brings a delay of time of distribution of a signal from ~ 3 m to ~ 10 m depending on "a place corner" (an inclination of the vising line in relation to the horizon plane); work at small corners of the place when this delay exceeds 10 m, in general is undesirable. The size of index refraction of air depends on its temperature, pressure and humidity, and these factors are measured only in a point of the station arrangement. It is possible to consider all nuances of distribution of these parameters along the route of the beam by means of atmosphere model so far at best with a margin error in several mm (in terms of range) though the research and accounting of local conditions in combination with improvement of the general model of the atmosphere allows to improve this accuracy gradually.
Theoretically there is a solution of this problem – simultaneous measurement of range on two strongly carried lengths of waves of a laser radiation (for example, in visible and infrared sections of a range) with use of the equation of dispersion – dependences of refraction index on wavelength – in principle allows to receive value of signal delay in the atmosphere, without resorting to temperature measurement, pressure and air humidity along the route; however it was shown that for achievement in such a way the error of 1 mm it is required to measure the difference of propagation time for these two lengths of waves with an accuracy in the measured range better ±0,5 picoseconds [5] (i. e. ±0,07 mm in terms of range) that it is not possible to make on a row of technical reasons yet.
The second serious limiter of accuracy – "a purpose error" – is caused by the fact that almost all satellites purposes for high-precision geodetic and geophysical measurements represent metal spheres on which surface the set of retroreflector – angular reflectors from quartz glass (by the way, the majority of such satellites is established – the purposes and reflectors for them is made by our enterprise – see tab. 1 and fig. 4). These spherical designs have diameter from 23 cm to 2,15 m, and the number of angular reflectors on each of them is from 20 to 2142.
Because of simultaneous reflection of a laser impulse from several retroreflectors which are on different removals from the station and because of inevitable dispersion of sizes of the effective reflecting surface of these retroreflectors there is an error reaching for large spherical satellites – the purposes of several cm (tab. 2).
In the 90th years for the solution of specific high-precision problems of geodynamics (in particular, for the Japanese Keystone project on prediction of earthquakes and a tsunami on the basis of observation of small motions of the earth crust) our enterprise has created the special SLR purpose of WESTPAC (see fig. 4) where in each timepoint the laser signal is reflected only from one retroreflector that is reached by restriction of its angular field with special blends. It has allowed to reduce the average size of "a purpose error" for this satellite to ±0,5 mm, but has caused some difficulties at its observation: between series of reflections from different retroreflectors there were breaks, and averaging of results of measurements by means of turning of the satellite round its pivot-center at its start into an orbit gradually worsened because of braking of this rotation by the vortex currents arising in the metal case of the satellite at its movement in magnetic field of Earth (by the way, this lack of a certain measure is inherent in all metal spherical satellite purposes).
For radical overcoming the difficulties connected with "a purpose error" we have offered, made and launched in space the satellite purpose of absolutely new type – BLITS (Ball Lens In The Space) representing a glass spherical body with use of the principle of a lens of Lyuneberg [6], [7] allowing to focus the radiation bunch falling on such lens with the flat wave front on an opposite surface of a lens (fig. 5). Having applied the reflecting covering on a half of the sphere and having given to a spherical lens rotation around the axis lying in the section plane between the reflecting and transparent surfaces it is possible to receive the reflected signal in the form of periodically repeating series of the reflected impulses, and such satellite has no metal parts where there can be vortex currents, and its rotation around the axis of turning remains invariable for all the time of service of the satellite in space.
Such satellite practically represents the "dot" purpose which isn’t causing distortion of a form of the reflected impulse and fluctuations of provision of an effective point of the reflection concerning the center mass of the satellite. The error caused by temperature changes of index of glass refraction in measurement of range doesn’t exceed 0,1 mm.
This satellite which was successfully observed by all stations of the international SLR network since the end of 2009 prior to the beginning of 2013 has been put to circular orbit 835 km high and has served as a prototype for the new BLITS-M satellite, of a little bigger size and weight which is prepared for start into an orbit by height ~ now 1500 km where influence of the atmosphere on stability of an orbit is almost absent that will allow to use effectively this spacecraft for geophysical surveys and geodetic measurements with an accuracy, which achievement was difficult earlier.
Important addition to the range-metering systems considered above is (BKOS)[8] "requestless" system developed recently intended for transmission of laser pulse from the earth-based station on airborne receivers of the GLONASS spacecrafts and a binding of the pulses accepted by this receiver to an onboard time scale of the appropriate spacecraft (fig. 6). It allows, using a binding of high-precision "hours" of the earth-based laser station by this time, to make a check of onboard time scales of GLONASS devices with a terrestrial standard, providing at the expense of small errors of the laser line the substantial increase of accuracy of synchronization of onboard time of GLONASS and the appropriate increase in accuracy of navigation and geodesic determination by means of this global space system.
Equipment in the short term of all new earth-based laser stations by network of such stations of the territory of the globe will provide with such system taking into account broad coverage further substantial increase of precision characteristics of GLONASS system and will create her in this regard advantage before other global satellite navigation and geodetic systems. In a bigger degree it will be promoted by equipment of all spacecrafts of new modifications by inter-satellite laser lines (MLNSS) for fast repeated data exchange about a divergence of time scales on these devices (fig.7). The equipment of these lines in essence represents a combination of a requestless range finder and the low-informative communication line.
What further prospects of development of a high-precision laser satellite distance measurement – after equipment of measuring network with new, containing the BKOS system, stations like "Sazhen-K" and "Sazhen-l" (fig. 8)[9] prepared for mass production and having potential of achievement of an instrument error of 1–2 mm and after essential expansion of network of exact stations? What is conceived, planned and can be realized in the next years at our enterprise?
First of all, input is provided in a system of the second turn of the Altai optiko-laser center[4] with the telescope with a diameter of 3,12 m (fig. 9) and a laser range finder which will be capable to measure effectively with a millimetric accuracy of distance to the retroreflectors established on the Moon.
Here it is necessary to notice that within last three years we took measurements at ranges, comparable with distance to the Moon – it means the work on the retroreflex panel installed on the "Radioastron" spacecraft with the extended elliptic orbit which apogee is at distance of nearly 350 thousand km from Earth. However the accuracy of the ranging equipment used at the same time – about a decimeter that it is quite enough for the solution of problems of this space mission, but isn’t enough for the solution of the fundamental tasks connected with researches of the Moon and system Earth Moon (here appropriate to mention that our terrestrial geodesy and navigation are carried out in system where the center of gravity position is in the related system Earth Moon, and even "terra firma" fluctuates depending on position of the Moon, not to mention levels of ocean waters.
Now only three stations (two in the USA and one in France) having the corresponding equipment – powerful short-pulse laser transmitters and big reception telescopes are engaged in a lunar laser distance measurement (LLR–Lunar Laser Ranging). In the near future our station in Altai also will join them; this work in time is coordinated to new space missions during which will be delivered to the Moon and new retroreflex systems including created at our enterprise.
Now in three points on the Moon there are retroreflex panels delivered there by astronauts of missions of Apollo‑11, Apollo‑14, Apollo‑15 and in two points – the panels, smaller by the sizes, installed on the Soviet devices "Moon Rover‑1" and "Moon rover‑2". But all these systems were delivered to the Moon more than 40 years ago and their efficiency decreased because of the natural reasons (dust pollution and erosion of a surface, and maybe also because of possible radiation degradation).
Unfortunately, in short article there is no opportunity at least briefly to mention other numerous applications of a high-precision laser distance measurement – from the solution of the major problems of fundamental science to various practical devices including household purpose.
At distribution in the water environment (especially in blue-green area of a visible range) losses in the optical range, on the contrary, turn out less, than in radio-frequency [2] thanks to what the optical distance measurement (as well as information transfer) finds application during the underwater works.
[3] At distribution in the water environment (especially in blue-green area of a visible range) losses in the optical range, on the contrary, turn out less, than in radio-frequency [2] thanks to what the optical distance measurement (as well as information transfer) finds application during the underwater works.
[4] The first stage of this measuring and research center functions more than 15 years.
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