Carrying out measurements at the UL observatory. Photo: Liene Kaibe, UL Foundation scholarship recipient.

The measurements performed at the Institute of Astronomy of the University of Latvia (UL) are at the forefront on the global scale in the field of space geodesy. A project for the modernization of the event timer RTS 2006 was implemented with the financial support of the patron “Mikrotīkls” and administered by the UL Foundation. Currently, at the time of the writing. the UL Institute of Astronomy ranks among the Top 10 of the world's best measurement systems in terms of measurement accuracy to out of 40 stations of the International Laser Ranging Service ILRS.

The UL Institute of Astronomy is a scientific institute of the University of Latvia, which conducts international-level fundamental and applied research in astronomy, related interdisciplinary fields, as well as conducts research related to laser measurements of Earth's artificial satellites and related technologies. “The institute has two observatories – the Astrophysics Observatory in Baldone, Riekstukalns, and the Fundamental Geodynamic Station in Riga, in the territory of the UL Botanical Garden. The observatories are supplied with modern equipment and successfully participate in the implementation of international projects,” adds Kalvis Salmiņš, director and project manager at the UL Institute of Astronomy.

Donation promotes achievements

In the framework of the project, the main goal of the UL Institute of Astronomy was to develop a new, improved signal processing scheme to increase the accuracy of measurements. The result is excellent – the measurement accuracy can be doubled compared to the previous version of RTS 2006.

“Looking at the development within the last years of Latvia's second independence, I am glad that our University of Latvia graduates have become wealthy and retain a great sense of responsibility towards their alma mater. In the case of the UL Foundation, this means that the UL graduates donate to various excellent UL researcher projects,” says Laila Kundziņa, Executive Director of the UL Foundation, and continues: “within the last four years, our greatest donor is our patron “Mikrotīkls”, which at present has already donated 2 million euro to the University of Latvia researchers. One of such positive examples of the support result is the growth of research implemented by the UL Institute of Astronomy on a global scale”.

Carrying out measurements at the UL observatory. Photo: Liene Kaibe, UL Foundation scholarship recipient.

Satellite laser ranging – the basis for important measurements

Artificial satellites of the Earth or Satellite Laser Ranging (SLR) is a method of determining the distance to a satellite by measuring the time interval in which a very short laser pulse travels from a specially designed telescope to the satellite and back. By knowing the speed of light and the time at which the laser pulse travels back and forth, the distance to the satellite can be determined with very high accuracy – up to 1 cm or even less.

Satellite Laser Ranging is one of the three main methods of space geodesy. It is used to determine coordinates on the Earth's surface, to adjust satellite orbits, to test theories of fundamental physics, and to monitor space. Distance measurements are performed with very high accuracy using time interval meters. One measurement error is in the range of about 1 cm, but by grouping measurements and processing them in short series, an accuracy of up to 1 mm can be achieved. Determining the coordinates on the Earth and adjusting the satellites’ orbits with such accuracy allows to detect and measure processes such as tectonic plate drift, changes in ocean levels, and alterations in the Earth's gravitational field over time.

The beginnings of space geodesy

The hypothesis of a continental drift was first formulated by the German meteorologist and polar explorer Alfred Wegener in 1912, but it was recognized only after the Second World War. Some of the first measurements of tectonic plate motion took place in Western California, USA, in 1973, when the researchers succeeded in measuring tectonic plate motion in the San Andreas fault line using laser ranging and demonstrating its potential. As a result, in 1976, the first Laser Geodynamics Satellite – Laegos – was launched into space. One of its tasks was to detect tectonic plate motion using SLR. Today, the speed and direction of tectonic plate drifts can be measured with an accuracy of 1 mm/year. For example, Latvia is located on the Eurasian tectonic plate, which moves in the north-eastern direction at a speed of about 2.4 cm / year. Plate tectonics at present is an active line of research. One of the reasons is that the contact points of tectonic plates are usually also seismically active. The high accuracy of laser ranging in orbit detection also serves to calibrate the radars of ocean level and glacier measurement satellites Jason-2, Jason-3, Cryosat-2.

Student explores a stellar map. Photo: Liene Kaibe, UL Foundation scholarship recipient.

Satellites prove Einstein's theory of general relativity

In order to be able to interpret radar measurements correctly, it is necessary to know the orbits of these satellites with high accuracy; this is ensured by regular laser ranging measurements. Additionally, laser ranging can be used in fundamental physics. The high measurement accuracy allows to measure the differences in satellite motion from the classical Newton's theory.

One of the forecasts of Einstein's general theory of relativity, the so-called Lense-Thirring effect, was tested in 2004 with 10 % accuracy using the satellites Lageos and Lageos-2. For further experiments with even higher accuracy, the satellite LARES (LAser RElativity Satellite) was launched in 2012. The core of LARES is made of tungsten alloy, and therefore this satellite is the densest object known to date in the Solar system. Its mass is 386.8 kg, the radius – 182 mm and the density – 15.3 g / cm3. Another fundamental physics experiment GREAT (Galileo gravitational Redshift Experiment with eccentric sATellites) was performed in 2017/2018. In framework of this experiment, two Galileo Global Navigation System satellites, Galileo-201 and Galileo-202 were measured for a whole year. Due to a launch vehicle failure, these satellites ended up in the wrong orbit with a greater eccentricity than originally planned. Therefore, the distance of these satellites to the Earth periodically varies between 17 000 km and 26 000 km. Each of these satellites is equipped with a very accurate atomic clock – a hydrogen maser. Together, these two factors made it possible to carry out this experiment, because, according to the theory of general relativity, the course of clocks must change as the distance to Earth changes.

UL Institute of Astronomy among world leaders

The time dilation in the gravitational field is called gravitational redshift. Clocks in gravitational fields run more slowly than clocks in free space. International Laser Ranging Service (ILRS) stations also participated in the GREAT experiment, ensuring accurate orbit detection. The results obtained at the end of 2018 were published in the journal Physical Review Letters. The postulate of Einstein's theory of relativity was confirmed and tested with 5.4 times more accuracy than in the previous experiment with a special satellite Gravity Probe A in 1976. As a member of ILRS, the Observatory of the UL Institute of Astronomy also participated in the GREAT experiment, and its measurements also were used in this experiment. The Observatory of the UL Institute of Astronomy is located in the territory of the UL Botanical Garden and conducts satellite-related studies since the launch of the first artificial Earth satellite on February 4, 1957. The laser ranging telescope LS-105, in the construction of which took part the UL Institute of Astronomy specialists, commenced regular functioning in September of 1987. In the autumn of 1991, a collocation with the German transportable laser ranging system MTLRS took place, and as a result, the SLR system of the UL Observatory successfully became part of the international circulation. In 1991, it became a member of the European laser ranging network EUROLAS, and immediately after the establishment of the International Laser Ranging Service (ILRS), the UL Observatory became its member. Today, ILRS combines more than 40 laser ranging systems from 16 countries around the world. One of the activities of UL IA is the development of new SLR-related technologies. UL IA cooperates with the Institute of Electronics and Computer Science (IECS) in the development and testing of very accurate event timers used in laser ranging.

To achieve an accuracy of 1 cm in distance measurements, the measurement error of the time interval must not exceed 10 picoseconds (ps) (1 ps = 10-12 seconds).

Director of the UL Institute of Astronomy Kalvis Salmiņš. Photo: Liene Kaibe, UL Foundation scholarship recipient.

Global triumph of event timers manufactured in Latvia

Today, event timers manufactured in Latvia are among the most widely used in the ILRS network. They are also used in other scientific and technical fields that require very high accuracy in measuring time intervals, such as absolute gravimeters, nuclear physics and laser altimeters. The event timers of the very first generation were first tested at the observatory of the UL Institute of Astronomy. At the same time, work was done to increase the accuracy of the entire system, because even if the time interval can be measured with the required accuracy, it does not yet guarantee a good result. There are other factors that affect the overall accuracy of the system. One such factor is that the intensity or amplitude of the signal received from the satellite can vary over a very wide range, and the effect of this variation, converted into distance units of measurement, can constitute tens of centimetres. This widely known effect, time-walk, can be compensated for by using several methods.

One solution is to use a special electronic signal processing device or constant fraction discriminator (CFD). The disadvantage of this method is that the compensation implemented by CFD works well only in a relatively small range of signal amplitudes. An additional difficulty is presented by the adjustment of the CFD, which is a non-trivial task to achieve the highest possible accuracy.

The result of a donation opens new horizons

In 2006, IECS in cooperation with UL IA developed the event timer RTS 2006, which in addition to the time interval measurement function also implemented a hybrid time-walk compensation scheme, which extended the CFD operating range using a parallel signal amplitude measuring device, calibration procedure and signal processing software. Furthermore, the device had a specific feature – the ability to measure the signal amplitude in conditional time units using the RTS 2006 event timer. The project for modernization of “RTS 2006” was implemented in 2017–2018 with the financial support of the patron “Mikrotīkls” and administration of the UL Foundation. The main goal was to develop a new, improved signal processing scheme using new electronic components that had recently appeared on the market in order to increase the accuracy of measurements and to achieve its operation also with the latest models of event timers. The TS/ATIC (Time Selector/Amplitude Time Interval Convertor) device developed in the framework of the project not only gained a significant increase in measurement accuracy, which can reach up to 30 % and more compared to RTS 2006, but also provides an opportunity to abandon the hybrid scheme with CFD, thus realizing a fully digital solution and simplifying the measuring equipment. The simplification of the measurement system also permits to reduce the overall measurement error, as each electronic component or device in the measurement scheme is also an additional source of error. As with the previous version, which was built into RTS 2006, the newly created equipment must be calibrated for the time-walk compensation software to work properly.

Telescope in action. Photo: Thibaud L. Mourlon.

Improvements provide twice as accurate measurements

Laboratory tests with a signal generator and photon detector emulator signals show that the developed solution allows to compensate for the time-walk effect of the event timer signal with an error of 3σ = ± 8 ps, which is more than twice less than the compensation accuracy of the best CFD currently available on the market. This is also confirmed by calibration tests with SLR system in real operating conditions at the UL IA observatory – the accuracy can even be doubled compared to the previous version of RTS 2006 in the cost effective way. The calibration improvements created in the working process have already been implemented in the regular operation of the UL IA event timer system. In the near future, it is planned to upgrade the RTS 2006 software and develop a new calibration scheme.

About the University of Latvia Foundation

Since 2004, the University of Latvia Foundation has provided an opportunity for patrons and cooperation partners to support the University of Latvia and other leading Latvian higher education institutions, thus investing in the future of Latvia. The priorities of the UL Foundation are the support to the best students and researchers, promoting creation of a modern study environment, as well as construction and redevelopment of university buildings.