The role of satellite laser ranging in space geodesy: developments, applications and Indian perspective

Indian Space Research Organisation (ISRO) is reviving the Space Geodesy by opening a Space Geodesy Division in ISRO Telemetry, Tracking and Command Network (ISTRAC). ISRO is already operating a PRARE and GPS station as a part of GeoForschungsZentrum (GFZ) global network. There is a proposal of establishing a third generation Satellite Laser Ranging (SLR) station at the same site thereby elevating Bangalore as a fundamental reference station for Geodesy and Geodynamic studies. With the long experience ISRO has gained earlier, the data from the above techniques can be effectively used for geodetic purpose. As geodesy is a nascent field in India, ISRO proposes to conduct an international workshop on Space Geodesy, which will provide more exposure and bring the national and international experts on a common platform to evolve utilisation and research using these data for the next decade. ISRO will collaborate with academic and research institutes to evolve a strong and vibrant science group in India carrying out studies on different aspects of Space Geodesy such as orbital dynamics, geophysics, earth dynamics, ocean dynamics, plate tectonics etc. ISRO has round the clock operation at SCC which is more suited for this programme and a regional data centre can be established. The data from these systems can also be used for ISRO’s future satellites such as OCEANS AT, CARTOSAT and CLIMATSAT which requires stringent ground imagery accuracy of the order of 2.5 m.

The advances in geodesy that are continually occurring allow the science of investigating the Earth’s shape and size to be applied to the observation of its geodynamics as well. Space geodesy and remote sensing methods enable the desired phenomenon to be observed with high precision and at different scales, where the phenomenon can be global or local in nature. By tracking changes in the position of unique points on the Earth’s surface using space geodesy methods (GNSS – Global Navigation Satellite System, VLBI – Very-long Baseline Interferometry, SLR – Satellite Laser Ranging, DORIS – Doppler Orbitography and Radiopositioning Integrated by Satellite), it is possible to indirectly track the movement of the Earth system and describe changes, anomalies and their evolution over time. The main objective of the article is to define and describe the tectonic plates’ movement based on the selected approach and analysis, compare the results with conventional tectonic boundaries definition and support the anomalous outcomes for discrete locations by ongoing tectonic phenomena over the time period under study. By implementing space geodesy data into deformation analyses, the aim is to demonstrate the geodynamic contribution of space geodesy in studying and monitoring the geodynamics of the Earth system.

Much of the success of plate tectonics can be attributed to the near rigidity of tectonic plates and the availability of data that describe the rates and directions of motion across narrow plate boundaries \m=~\1 to 60 kilometers wide. Nonetheless, many plate boundaries in both continental and oceanic lithosphere are not narrow but are hundreds to thousands of kilometers wide. Wide plate boundary zones cover \m=~\15 percent of Earth's surface area. Space geodesy, which includes very long baseline radio interferometry, satellite laser ranging, and the global positioning system, is providing the accurate long-distance measurements needed to estimate the present motion across and within wide plate boundary zones. Space geodetic data show that plate velocities averaged over years are remarkably similar to velocities averaged over millions of years.

한국천문연구원은 인공위성 정밀궤도 결정, 우주측지 및 인공위성 자세역학 연구를 위해서 2kHz 반복율을 가지는 SLR 시스템을 운영하고 있다. 그러나 측지위성의 회전속도를 보다 정밀히 결정하고 거리 측정 정밀도 향상을 위해서 고반복율의 SLR 관측 데이터가 요구된다. 따라서 고반복율 시스템 구현을 위해 운영 소프트웨어 및 레인지 게이트 생성기를 개발하여 최대 10kHz 반복율로 레이저추적이 가능한 HSLR-10(High repetition-rate Satellite Laser Ranging-10kHz) 시스템으로 개선하였다. 본 연구에서는 10kHz 반복율을 가지는 HSLR-10 시스템의 운영 소프트웨어 개발 방법, 구성 및 검증 결과를 제시한다. Korea Astronomy and Space Science Institute (KASI) has been operating SLR (Satellite Laser Ranging) system with 2kHz repetition rate for satellite precise orbit and spin determination as well as space geodesy. But the SLR system was improved to be capable of laser ranging with high repetition rate, up to 10kHz by developing new operation software and novel range gate generator, called HSLR-10. The HSLR-10 will contribute to the accurate spin rate determination of geodetic satellites and geodetic research due to its largest repetition rate in the world. In this study, the development methodology and configuration of operation software are addressed, and its validation results are also presented.

The techniques of space geodesy, comprising the four techniques, Global Navigation Satellite Systems (GNSS), Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Doppler Orbitography and Ranging Integrated by Satellite (DORIS) are currently reaching a measurement resolution in the range of 1 part per billion for the terrestrial reference frame. However, a small set of discrepancies remain evident within each of the techniques as well as in the combination of different techniques. Systematic measurement errors are causing this and problems in the local ties between the reference points of the various measurement systems and biases in the atmospheric refraction correction have long been suspected as the main contributors. However, it turns out that errors in the internal delay compensation of the measurement systems are also a significant contributor. They are extremely hard to detect, since they are small and come with different characteristics. It is understood that the experienced delay variation is related to a complex pattern of ambient temperature variation inside of the electronic devices. These changes relate to the micro-climate of the electronic signal path and can both be slow and highly variable. With the advent of high bandwidth mode-locked lasers and active delay compensation in the optical domain, it is now possible to utilize coherent time as an independent probe for instrumental signal delays. The research unit FOR5456 of the German National Science Foundation (DFG) has been formed in 2022 in order to apply and investigate active delay compensation to the techniques of space geodesy. This talk introduces the application of coherent time in space geodesy and its potential to act as a novel tie in fundamental stations.

Until now, time itself is not an observable in space geodesy. The major reason for this fact is the considerable difficulty to keep track of the phase of the clock oscillation between the point of origin and the point of the measurement. However, if geodesy will attempt to provide a reference frame fully based on general relativity, a proper treatment of time is mandatory. The Geodetic Observatory Wettzell is currently in the process to modernize the timing system such that the phase of the master clock can be established at all times. The ultra-short pulses of an optical frequency comb are transporting both time and frequency from the master clock of the observatory to the individual space geodetic techniques, namely Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Navigation Satellite System (GNSS), using a two-way approach. In order to verify the functionality of this system not only in sense of delay stability but also accuracy, we have developed a new TWTT system based on the exchange of timing signal via standard optical telecommunications Small Form-factor Pluggable (SFP) transceivers to transfer timing information between two or more terminals with the accuracy below 1 ps via optical fibers of a length of up to several tens of kilometers. The heart of the measurement device is an event timing module using surface acoustic wave filters as a time interpolator, which allows the registration of the times-of-arrival of electrical pulses with sub-picosecond timing resolution, linearity and stability. These pulses are derived from the optical signal, which is used for the communication between the terminals. Great care was taken in order to minimize terminal internal delays instability, which can be the result of temperature changes inside terminals. The design, applications and the first experiments at GO Wettzell will be discussed.

Geodetic control on recent tectonic movements in the central Mediterranean area

We use new space geodetic data from very long baseline interferometry and satellite laser ranging combined with other geodetic and geologic data to study contemporary deformation in the Basin and Range province of the western United States. Northwest motion of the central Sierra Nevada block relative to stable North America, a measure of integrated Basin and Range deformation, is 12.1±1.2 mm/yr oriented N38°W±5° (one standard error), in agreement with previous geological estimates within uncertainties. This velocity reflects both east‐west extension concentrated in the eastern Basin and Range and north‐northwest directed right lateral shear concentrated in the western Basin and Range. Ely, Nevada is moving west at 4.9±1.3 mm/yr relative to stable North America, consistent with dip‐slip motion on the north striking Wasatch fault and other north striking normal faults. Comparison with ground‐based geodetic data suggests that most of this motion is accommodated within ∼50 km of the Wasatch fault zone. Paleoseismic data for the Wasatch fault zone and slip rates based on seismic energy release in the region both suggest much lower slip rates. The discrepancy may be explained by some combination of additional deformation away from the Wasatch fault itself, aseismic slip, or a seismic rate that is anomalously low with respect to longer time averages. Deformation in the western Basin and Range province is also largely confined to a relatively narrow boundary zone and in our study area is partitioned into the eastern California shear zone, accommodating 10.7±1.6 mm/yr of north‐northwest directed right‐lateral shear, and a small component (∼1 mm/yr) of west‐southwest ‐ east‐northeast extension. A slip rate budget for major strike‐slip faults in our study area based on a combination of local geodetic or late Quaternary geologic data and the regional space geodetic data suggests the following rates of right‐lateral slip: Owens Valley fault zone, 3.9±1.1 mm/yr; Death Valley‐Furnace Creek fault zone, 3.3±2.2 mm/yr; White Mountains fault zone in northern Owens Valley, 3.4±1.2 mm/yr; Fish Lake Valley fault zone, 6.2±2.3 mm/yr. In the last few million years the locus of right‐lateral shear in the region has shifted west and become more north trending as slip on the northwest striking Death Valley‐Furnace Creek fault zone has decreased and is increasingly accommodated on the north‐northwest striking Owens Valley fault zone.

On the global geodetic observing system: Africa's preparedness and challenges

Using global positioning system, very long baseline interferometry, satellite laser ranging and Doppler Orbitography and Radiopositioning Integrated by Satellite observations, including the Canadian Base Network and Fennoscandian BIFROST array, we constrain, in models of postglacial rebound, the thickness of the ice sheets as a function of position and time and the viscosity of the mantle as a function of depth. We test model ICE-5G VM2 T90 Rot, which well fits many hundred Holocene relative sea level histories in North America, Europe and worldwide. ICE-5G is the deglaciation history having more ice in western Canada than ICE-4G; VM2 is the mantle viscosity profile having a mean upper mantle viscosity of 0.5 × 1021 Pa s and a mean uppermost-lower mantle viscosity of 1.6 × 1021 Pa s; T90 is an elastic lithosphere thickness of 90 km; and Rot designates that the model includes (rotational feedback) Earth's response to the wander of the North Pole of Earth's spin axis towards Canada at a speed of ≈1° Myr−1. The vertical observations in North America show that, relative to ICE-5G, the Laurentide ice sheet at last glacial maximum (LGM) at ≈26 ka was (1) much thinner in southern Manitoba, (2) thinner near Yellowknife (Northwest Territories), (3) thicker in eastern and southern Quebec and (4) thicker along the northern British Columbia–Alberta border, or that ice was unloaded from these areas later (thicker) or earlier (thinner) than in ICE-5G. The data indicate that the western Laurentide ice sheet was intermediate in mass between ICE-5G and ICE-4G. The vertical observations and GRACE gravity data together suggest that the western Laurentide ice sheet was nearly as massive as that in ICE-5G but distributed more broadly across northwestern Canada. VM2 poorly fits the horizontal observations in North America, predicting places along the margins of the Laurentide ice sheet to be moving laterally away from the ice centre at 2 mm yr−1 in ICE-4G and 3 mm yr−1 in ICE-5G, in disagreement with the observation that the interior of the North American Plate is deforming more slowly than 1 mm yr−1. Substituting VM5a T60 for VM2 T90, that is, introducing into the lithosphere at its base a layer with a high viscosity of 10 × 1021 Pa s, greatly improves the fit of the horizontal observations in North America. ICE-4G VM5a T60 Rot predicts most of the North American Plate to be moving horizontally more slowly than ≈1 mm yr−1, in agreement with the data. ICE-5G VM5a T60 Rot well fits both the vertical and horizontal observations in Europe. The space geodetic data cannot distinguish between models with and without rotational feedback, in the vertical because the velocity of Earth' centre is uncertain, and in the horizontal because the areas of the plate interiors having geodetic sites is not large enough to detect the small differences in the predictions of rotational feedback going across the plate interiors.

Many measurements to satellites, whether of direct scientific interest or merely for tracking purposes, require a knowledge of the atmospheric propagation delay which affects the observation, The delay has to be precisely determined for such purposes as geodetic or navigation positioning using the TRANSIT system or GPS (Global Positioning system) and gravity-field determination by precise orbit determination using satellite laser ranging. In particular, the high accuracy of satellite altimetry can be severely degraded by propagation delays not only in the altimeter measurement itself but also in the tracking-system observations. This paper describes the effects of the ionosphere and troposphere on both optical and microwave observations, and discusses both the instrumental and mathematic modelling approaches to the determination of the propagation delay, giving examples of the solution adopted for geodetic, surveying and navigation satellite observations.

The LAser RElativity Satellite (LASER) was launched in 2012 by ASI for studying general relativity. A new mission, LARES 2, will be launched in 2019/2020 on a higher orbit and using a different design of the optical payload. LARES is a target for Satellite Laser Ranging; the passive satellite carries 92 cube corner reflectors that allow to measure with high accuracy the position of the centre of mass of the satellite with respect to the ground stations. Aside from the principal scientific goal, the measurement of the relativistic frame-dragging effect, data from LARES (and in future, from LARES 2) are used also in other research related to the study of gravity, to space geodesy, and to Earth science. The accuracy of the laser ranging depends both on the characteristics of the ground station that is tracking the satellite, on the environmental conditions of the atmosphere during the pass of the satellite, and on the design of the target. Each laser ranging station provides the data collected for a particular satellite in a specific format, called Normal Points (NP) that are released daily. By comparing the quantity of NPs produced by a satellite, and the quality of those datasets, with the NPs produced by other satellites having a similar design (passive, spherical targets), it is possible to assess the quality of the optical design of that satellite. For its main scientific mission, the data of LARES are combined with the data of LAGEOS and LAGEOS 2 satellites, orbiting on much higher orbits and considered two benchmarks for laser ranging. The data of geodetic satellites such as LARES, LAGEOS, Stella and Starlette satellite are used by the scientific community for measurements of space geodesy, for the study of the effect of global climate changes on the rotation of the Earth, and for other research related to Earth science. The availability of high-quality targets for laser ranging will improve the accuracy of the results of the research in all the above scientific fields. In this paper the quality of the satellite laser ranging data from LARES is compared with the data of the LAGEOS satellites, with the data of the Stella/Starlette satellites (on lower orbits) and with the data of AJISAI (orbiting at a similar altitude). This information will provide important information for the design of the new LARES 2 satellite.

For over 40 years, NASA's global network of satellite laser ranging (SLR) stations has provided a significant percentage of the global orbital data used to define the International Terrestrial Reference Frame (ITRF). The current NASA legacy network is reaching its end-of-life and a new generation of systems must be ready to take its place. Scientific demands of sub-millimeter precision ranging and the ever-increasing number of tracking targets give aggressive performance requirements to this new generation of systems. Using lessons learned from the legacy systems and the successful development of a prototype station, a new network of SLR stations, called the Space Geodesy Satellite Laser Ranging (SGSLR) systems, is being developed. These will be the state-of-the-art SLR component of NASA's Space Geodesy Project (SGP). Each of SGSLR's nine subsystems has been designed to produce a robust, kilohertz laser ranging system with 24/7 operational capability and with minimal human intervention. SGSLR's data must support the aggressive goals of the Global Geodetic Observing System (GGOS), which are 1 millimeter (mm) position accuracy and 0.1 mm per year stability of the ITRF. This paper will describe the major requirements and accompanying design of the new SGSLR systems, how the systems will be tested, and the expected system performance.

Crustal deformations in the mediterranean area computed from SLR and GPS observations

Satellite laser ranging (SLR), a cornerstone technology in space geodesy, plays a critical role in satellite orbit determination and Earth gravity field inversion. Here, we developed a compact single-photon LiDAR system for SLR operating at 1550 nm. The system features a bistatic configuration for backscattering noise suppression, an enhanced scan-tracking technique to improve dynamic target detection probability, and an absolute ranging method utilizing chaotic pulse position modulation (CPPM) and the Hough transform. Experimental results demonstrate a static target absolute ranging of 8.56 km, and dynamic ranging capabilities of up to 953.89 km with a ranging RMSE of 0.41 m. The theoretical normal point precision at high pulse repetition frequencies is estimated to be within a few millimeters. Trajectory and full-waveform analysis further validate the system's ability to detect radial velocity (-6.53 ∼ -2.05 km/s ) and attitude changes of targets. This work proves the feasibility of single-photon LiDAR for SLR applications and enables what we believe to be new solutions for satellite orbit determination, space target identification, attitude sensing and debris monitoring.