The new laser ranging facility on
Mt Stromlo is making fundamental measurements to satellites in order
to monitor the motions of
the dynamic Earth.
Mt Stromlo is just half an hour's drive from downtown Canberra. There have been observatories here for almost as many years as there have been politicians chattering in Parliament House. Both came for the location; the politicians because it is more or less half way from the vested interests of both Sydney and Melbourne; the astronomers because it had the best mix of weather, altitude, dark skies and accessibility on the eastern seaboard.
In spite of the fact that the city lights of Canberra now make a mess of deep sky photography, serious astronomy is still done here. Last year, telescopes on the mountain were generating science sufficiently important to be reported in Nature, possibly the most significant science journal in the world. That research was part of the search for Dark Matter, the 90% of the universe that is not visible. This year, they are involved in a search for trans-Neptunian objects - a huge swarm of small, dark planets believed to orbit the sun at the very edge of the solar system.
But the latest instrument to be installed is not focused on the very distant or the very large. It's focused on the very near - and the very small. It's a new satellite laser ranging station operated by the Australian Surveying and Land Information Group (AUSLIG). It looks at satellites only a few thousand kilometres away, and it measures their distance to within millimetres.
Measurements made with the station are so accurate that they can be used to detect the endless pulsing of the earth as the crust responds to the gravity of the Moon. They can detect changes in the way the Earth spins on its axis. In fact, people here talk seriously about dynamic geodetic datums: datums which are reset every year to account for plate tectonics.
The station is part of the International Laser Ranging Service. ILRS exists to provide global satellite and lunar laser ranging data to support geodetic research as well as products important to the maintenance of an accurate International Terrestrial Reference Frame.
The service develops the necessary global standards and specifications. It collects, merges, archives and distributes satellite ranging and lunar laser ranging observations. Lunar ranging observations use the corner reflectors left on the moon by some of the Apollo missions, and thus allow very accurate determination of Moon-Earth dynamics, as well as turning up interesting information on the characteristics of the moon itself.
One of its most important functions is to provide a linkage between this ITRF and the Stellar Reference Frame, i.e: between distant quasars and the surface of the earth. Very Long Baseline Interferometry provides accurate positions to the stars, and thus defines the SRF. If the relative positions of the antennae in the interferometer and the SLR stations are known, it is then possible to define the ITRF in terms of the stars.
The ILRS is not old. Its terms of reference were approved 13 June 1997 at a meeting in Maratea, Italy. Working group charters were also posted. In the last quarter of 1997 a call for participation in the new service was issued. The proposals were evaluated in April 1998 and an ILRS governing board was established.
As a result, some 46 tracking stations, four operations centres, three analysis centres, four lunar analysis centres, 18 associate analysis centres, two global data centres and a regional data centre were established. A central bureau was established at NASA's Goddard Space Flight Center with John Bosworth as its first director and Michael Pearlman of the Harvard-Smithsonian Center for Astrophysics as secretary.
NASA is the largest contributor to the network. It paid the capital cost of many of the stations and continues to pay the running costs for a number of them. NASA originally funded both the existing West Australian station at Yaragadee and the old eastern station at Orroral Valley. AUSLIG took over the running of Yaragadee in 1998. Under a joint agreement, NASA will replace its old equipment in 2002 for fully automatic operation. NASA declined to do the same at Orroral Valley, ruling that its equipment could not be economically upgraded.
This would have left an even more serious hole in the Southern Hemisphere observations of the ILRS, so AUSLIG decided to replace it with a new station on Mt Stromlo.
One of the first actions of the ILRS was to establish four standard working groups, for missions co-ordination, analysis co-ordination, networks and engineering, and data formats and procedures. John Luck, the director of SLR network management at AUSLIG, currently heads this latter group, as well as being on the governing board of the ILRS.
The Mt Stromlo station was built under government contract by Queanbeyan- based Electro-Optical Systems. Dr Ben Greene heads EOS, one of the largest private suppliers of geodetic telescopes in the world. The station was first operated in May 1998, and accepted by AUSLIG for service on 28 October that year. EOS now operates it under a five-year contract with AUSLIG.
So, how does SLR work? As is often the case, the theory is quite simple; the practicalities are where the going gets a little rough.
To give the simple version first: a laser is used to measure the distance from the primary mirror of the telescope to the satellite and back again. Given that the orbit of the satellite is known, the position of the telescope can be deduced from the journey time of the light. Changes in the distance can be attributed to movement in the telescope.
There is a certain circularity in this argument: the inverse process determines the position of the satellite, i.e: the orbit is calculated from the distance to the satellite, given the position of the ground station. However, the system can be made to work because there is more than one station. In fact, there are some 40 stations around the world dedicated to measuring the position of some half a dozen satellites. At the level of precision required here, all these stations can be considered to be sliding around and bumping up and down on a plastic Earth. But by using results from all of them, a so-called golden orbit can be calculated which ignores or minimises the movement of any one station.
An ideal network would have all the stations spread evenly around the world. In fact, the SLR network has only three operational stations in the Southern Hemisphere, and there is a dense clump of them in Europe, but this is sufficient to generate a very accurate determination of the position of a satellite at any point in time. The position and movement of any one station relative to this orbit can then be calculated.
There are certain complexities in all this. For a start, the system is capable of measuring the distance to the satellite to within 2 mm. This implies the ability to measure the time to within femtoseconds (it takes light 6.6 x 10-12 seconds to cross 2 mm). Some very clever electronics, developed by the maker of the telescope, Electro-Optical Systems, is employed to do this.
Secondly, the process works because, in theory, the satellite is in orbit about the centre of mass of the Earth, with its orbit determined precisely by gravity. In practice, this is only partially true. Residual atmosphere causes drag that can decay the orbit measurably over time. Naturally, this effect can be minimised by moving the satellite further away from the earth. Also, it can be minimised by reducing the size and increasing the mass of the satellite, as well as by making the satellite symmetrical in shape so that whatever the drag, it is at least constant.
Thus specialist geodetic satellites such as NASA's LAser GEOlogical Satellite (LAGEOS-1) or the Italian LAGEOS-2, are small round spheres, typically only half a metre or so in diameter. Their outsides are coated with retro-reflectors.
Satellites are also affected by solar pressure, which causes a small but finite drift away from the sun at all times. On asymmetric spacecraft, such as the European Space Agency's ERS-1, this force varies with the attitude of the spacecraft; on symmetrical spacecraft it is more nearly constant, but it is always there, and needs to be allowed for in the calculations.
A third consideration is that the laser radiation is subject to bending and distortion in the atmosphere, which results in a given ray of light taking a longer path than is necessary. 'Atmospheric distortion is probably the most significant cause of error left in the system,' says John Luck. Currently, moves are afoot to use a dual frequency laser to see whether this can improve things. In exactly the same way as GPS satellites use dual frequencies to reduce the effect of the ionosphere on radio signals, so scientists hope that two frequencies, each of which will be refracted differently as it passes through the atmosphere, will allow for more precise measurement.
There is a certain scientific interest in understanding something of the nature of the Earth's dynamics, but results from the SLR system also affect day-to-day positioning very closely.
The observatory ranges to the GPS satellites as they come into view of Mt Stromlo, allowing an independent determination of their distance. There is a GPS receiver on a pole just outside the observatory, so the results from the two systems can be compared very precisely, and used to refine the position of the Australian Fiducial Network, and the Australian National Network.
Both these are systems of first order base stations that have their positions determined by long-term monitoring of the GPS. All legal position fixes in Australia have to be traceable back to these stations, usually via state networks.
Currently, a number of programs are under way to improve the facility, or to extend its usefulness. One plan is to increase the power of the laser up to 100 watts. This would allow it to be used for mapping space debris, the junk left over from rocket launches and discarded satellites. Objects range in size from leftover rocket boosters, through astronauts' fumbled Hasselblad cameras, to flecks of paint.
A second project is to fully automate the operation of the site. Currently, the station is manned from 9pm to 4am the next morning. Luck says he would like to institute full-time operation, but only at an affordable price.
