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Intensive Campaign to study the Earth using Quasars 2002/10/16


From 2002 October 16 to 31, the 26-m radio telescope at Hartebeesthoek will form part of a global network of eight such telescopes that will study the Earth continuously during that period.

CONT02 map
Location of the radio telescopes in experiment CONT02.

HartRAO as the only telescope in the network in the southern hemisphere. Its participation has been made possible as the MKIII VLBI terminal was recently upgraded to a MKIV, essential for this experiment. The long north-south baselines from the northern telescopes to Hartebeesthoek are of particular importance in improving the accuracy of the results in what is essentially a process of triangulation on the quasars.

Photographs taken with the experiment in progress are at the end of the text.

Why study the Earth?

The study of the Earth, its atmosphere and oceans, the solid ground beneath us, on down to the fluid iron outer core and solid iron inner core, is important for many reasons, ranging from the satisfaction of understanding the world we live in to the practical applications of the tools used for the observations. The better we understand "things", the better we can use them, whether that means better weather forecasting or better navigation of airliners, our private cars, or interplanetary spacecraft. All of these are examples of spinoffs from systems that were developed for the scientific study of the Earth's surface.

Using Space Geodesy to study the Earth

The three primary measurement systems for studying the shape of the Earth use objects outside the atmosphere to provide the signal that is detected. The three techniques are complementary in providing information about the size, shape, and rotation of the Earth, but each has its own strengths in terms of the specific property it can measure or in terms of the number of locations that can be measured.

While all systems measure the shape of the Earth and the changing locations on it due to plate tectonics or to earthquakes, VLBI is unique in determining the absolute value of Earth's time and the direction of Earth's axis of rotation relative the rest of the universe. SLR and GPS are used to determine very precisely where each site is relative to the center of mass of the Earth, and GPS systems are by far the most numerous, providing the locations of thousands of sites around the globe with accuracy comparable to VLBI, better than one centimeter uncertainty anywhere on the surface of the Earth.

HartRAO is unique in the southern hemisphere in having all three systems (VLBI, SLR, GPS) running together on the same site.

The CONT02 VLBI campaign

Over the next two weeks, October 16-31, an intensive campaign of continuous, 24 hours/day, measurements using VLBI, designated CONT02, will be conducted using eight large radio antennas spanning the hemisphere from Svalbard, Norway, at 79 degrees north to Johannesburg, South Africa, and from Europe to Alaska and Hawaii. These antennas are supported by the governments of seven different countries, all part of the International VLBI Service.

Outside of CONT02 measurements of 24-hour duration are conducted approximately three times per week involving six to eight antennas at a time selected from 30 antennas in 16 countries. While from a scientific and applications point of view it is desirable to have continuous measurements by all three techniques year-in and year-out, this is only possible, due to budgetary constraints, for the GPS systems. The CONT02 campaign will provide a set of VLBI data of the highest quality to study changes in the Earth's properties on a timescale from a few days down to a few hours that cannot be studied with the usual observation program.

The VLBI stations participating in CONT02 were selected on the basis of their overall performance level, their ability to operate continuously for 15 days, and their geographic location. The location of each station is important in determining the geometric strength of the network, where the distances between the stations should be as large as possible and the stations should be well distributed over the globe. It was also very important that the station personnel wish to be involved in these important scientific observations and are able to make the additional commitment required to complete the extended program.

The participating stations will acquire data 24 hours per day for a total of 360 hours, beginning on October 16 and ending on October 31. The stations operate under computer control, following a pre-determined schedule in which the telescopes point at one quasar and acquire data for a time span of one to several minutes, then quickly move to observe another quasar in a different part of the sky, and so on. During the observing, different sub-groups of the eight stations will observe sources simultaneously. The network will acquire a total of about 54,000 individual observations made by pairs of telescopes.

Science Goals

As might be expected of a system that uses radio waves from the distant reaches of the universe as its signal, VLBI contributes to the study of many phenomena, from the astronomical objects known as quasars, to General Relativity, to meteorology, to the properties of the innermost parts of the Earth.

While the applications of VLBI (for example climate monitoring and the measurement of sea level change) will benefit most from the long-term program, the objectives of the continuous campaign are driven primarily by trying to improve our understanding of the excitation of the solid Earth by the tides and currents in the atmosphere and the oceans, which in turn are driven by the gravity of the Sun and Moon and by the heating of the Earth by the Sun. The length of each day varies by micro-seconds, and over a week or longer the accumulated changes are associated primarily with variations of the winds. For example there is a strong correlation of length-of-day with the effects of El Nino. A particular goal of CONT02 is to obtain better measurements of changes in the Earth's rotation rate within a day and to relate these changes to the tides of the oceans and to atmospheric winds.

Since VLBI also measures the effect of water vapor in the atmosphere on the radio waves as they pass through it, an important result of the campaign will be to provide data to improve the model for the effect of both the water vapor and the dry part of the atmosphere. In the long term this will help both the measurements of the changes in the shape of the Earth and the products of the long term program used by the meteorologists.

Technology

The measurements by VLBI require the acquisition of very large amounts of data from the quasars. Each antenna will record data at a rate of 256 million bits per second for approximately one-third of the time, giving a total number of bits for the eight antennas in the fifteen days of approximately one million billion. When recorded on the specialized high performance tape recorders, developed in cooperation with industry, the total length of tape for each station is about (5.5km/tape*2 tapes/day*15 days = ) 165 km.

A recent advance in the technology of VLBI is the development of disk recording to replace the tape recorders. This will facilitate taking advantage of the consumer market for computer disks to reduce the cost and to improve the reliability of the recording systems. This has become possible only because of the tremendous increase in disk storage capacity in the last couple of years. This campaign will see the first operational use of the new disk-based recording system. The record rate is equivalent to filling a 40 gigabyte hard disk every fifty minutes.

All of the tapes and disks for one day must be shipped to one of three locations for processing on custom designed digital correlators. These correlators are located near Boston, Massachusetts; Washington, D. C.; and Bonn, Germany. The results of the correlation are analyzed primarily at seven universities and government facilities around the world, although many other research institutions will analyze selected subsets of the VLBI data.

The next important improvement, which is in the initial stages of development, will be the ability to transmit the recorded data from each station to one of the correlators over high speed optical fibers, in much the same way that the Internet is used today. Initial tests of such a system have been conducted between antennas in Japan and the East Coast of the United States with correlation in both countries.

How accurate will the measurements be?

The precision expected for the UT1 measurements is about 2 microsec for the CONT02 network. Length of day variations (more detail) are computed as differences between daily measurements. Precision for station position measurements are expected to be 1-3 mm in east and north, 4-8 mm in height. We don't differentiate as to what effect causes what part of any changes. Analysts plan to study day-to-day variations in the three dimensional station positions to see if there are short-term effects. Comparisons with the day-to-day GPS station positions will be made to study any differences or offsets. Figure 1 indicates the geophysical effects on UT1 and measurement accuracies. UT1 is the time of the earth clock, determined astronomically, which has approximately 24 hours in a day. Length of day (LOD), is the difference between TAI (atomic time scale) and UT1. On January 1, 1958, UT1 equalled TAI. TAI is more stable than UT1, as the instabilities of TAI are approximately 6 orders of magnitude smaller than those of UT1.

UT1 effects

How do the measurements compare to previous data?

The most precise measurements came from CONT94 which had truly continuous sessions. The CONT96 campaign had measurements made on non-adjacent days with the main goal being to measure frequency components of Earth orientation. There are two main differences in CONT02: We expect that the results will get even better in the future as models are refined and improved. This "retroactive improvement" is an important feature of VLBI being able to re-analyze the entire data set when new insights, new models, and new processing methods are developed. This is the basis for attempting to acquire the best possible data today, anticipating that we will be able to take advantage of scientific progress in the future.

How will the HartRAO SLR data be used?

The HartRAO/NASA satellite laser ranger will be operating during this experiment to provide comparison data.

Analysts will compare the SLR baselines measured during the campaign, but the most useful input will be for comparisons of the SLR reference frame with the VLBA Terrestrial Reference Frame. Because SLR and VLBI have complementary error sources, for example SLR is almost immune to water vapor effects, the comparisons help to separate geophysical effects from systematic errors.

How will the HartRAO GPS data be used?

The HartRAO/NASA Global Positioning System receiver operates continuously, 24 hours per day. HartRAO has similar GPS systems in Lusaka (Zambia), Richardsbay, Sutherland and Simonstown. These GPS receivers are part of a global tracking network (the International GPS Service, IGS), providing continuous monitoring of station position as well as atmospheric and ionospheric information. The results of the GPS data will be compared with those of SLR and CONT02 to perform independent checks of the different measuring techniques' performance.

Summary

Although the CONT02 program has as its primary goal the study of the Earth's dynamics on relatively short time scales, the results will contribute to the long-term objectives of the geodetic program. Among the more amazing and curious features of the study of the properties of the Earth is that the only means by which we currently are able to learn about the magnetic properties of the Earth's core, where the magnetic field of the Earth is most likely generated, is by using the VLBI technique to study signals from the edge of the Universe to determine the variations in the direction of the rotation axis over days, weeks, months, and years.

Links

Images

These images were taken at 0900 to 0915 on the morning of October 18. The radio telescope was tracking the distant radio source 1124-186 (this exotic name refers to its position in the sky). The Satellite Laser Ranger (SLR) was simultaneously tracking the satellite Lageos 1, several thousand kilometres distant in its orbit around the earth.

Tel + SLR
Radio Telescope and SLR.
Click on the picture for the full size image.

The laser transmitter and telescope the receive the laser pulses back from the satellite are on the far right of the van. The big black disk is actually where light enters the telescope. the white "drum" is the body of the telescope.

Tel + SLR
Radio Telescope and SLR.
Click on the picture for the full size image.

Tel + SLR
Radio Telescope and SLR.
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Tel + SLR +
GPS
Radio Telescope, SLR and GPS basestation antenna.
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VLBI
operator
Marisa Nickola checks the VLBI schedule, as the radio telescope tracks 1124-186 under computer control.
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VLBI
MK IV terminal
Marisa Nickola checks the tape on the MK IV VLBI tape recorder.
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SLR operator
Wilhelm Haupt tracks the Lageos 1 satellite with the SLR, seen here in a flash picture.
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SLR operator
Wilhelm Haupt tracks the Lageos 1 satellite with the SLR, seen here in the subdued light in which the operators work.
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radio
telescope
Close-up of the radio telescope.
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On top of the central conical structure are the two angled panels ("dichroic reflector") that split up the incoming radio waves so that they can be recorded simultaneously at wavelengths of 13 cm (2300 MHz) and 3.5 cm (8500 MHz), a critical part of this technique.
The 26m diameter main surface of the telescope is being replaced, and this upgrade is largely complete. The new panels are bright white. The few remaining old panels near the centre appear grey.

slr closeup
Wilhelm Haupt re-adjusts the SLR after calibration using a ground target.
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LC closeup
Space Geodesy Programme leader Dr. Ludwig Combrinck in front of the SLR and radio telescope.
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LC closeup
Space Geodesy Programme leader Dr. Ludwig Combrinck in front of the radio telescope.
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LC closeup
Space Geodesy Programme leader Dr. Ludwig Combrinck in front of the SLR.
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LC closeup
Space Geodesy Programme leader Dr. Ludwig Combrinck in front of the radio telescope.
Click on the picture for the full size image.

WM at slr
Flash photo of William Moralo tracking the Lageos 1 satellite several thousand km away with the Satellite Laser Ranger on the morning of Monday October 21. The oscilloscope with a blue screen in front of William shows the return signal of the laser pulse as a "spike" within the range gate.
Click on the picture for the full size image.

WM at slr
Natural light photo of William Moralo tracking the Lageos 1 satellite with the Satellite Laser Ranger on the morning of Monday October 21. The weak return pulse of laser light is much easier to see in low light conditions.
Click on the picture for the full size image.

Acknowledgements: Thanks to Arthur Niell (MIT, Haystack Observatory) and Nancy Vandenberg (NASA) for providing information on CONT02.