Geophysical Satellites and Instruments LR4-75

Transcription

1 Delft Institute for Earth-Oriented Space Research Delft University of Technology Kluyverweg 1, 2629 HS Delft The Netherlands Geophysical Satellites and Instruments LR4-75 P.N.A.M. Visser Delft, August, 1998

2 This document was typeset with L A TEX 2ε.

3 Contents 1 Introduction Space-geodetic observation techniques Geophysical satellites Altimeter satellites Geodetic/navigation satellites Gravity field satellites ERS 1 Calibration and Validation The ERS instruments Calibration and Validation Instrument calibration Data product validation Tracking systems One-way tracking systems Two-way tracking systems Optical and microwave systems Tracking coverage Satellite Laser Ranging (SLR) Measurement principle

4 iv Contents 3.2 A typical SLR station Meteorological station System controller Satellites equipped with laser reflectors Applications Microwave Tracking Systems Range-rate Measurement Principle Ionospheric path delay Tropospheric delay DORIS System description Orbit determination Positioning Precise Range and Range-rate Equipment (PRARE) Main Characteristics Measurement Principle PRARE Data Flow Applications The Tracking and Data Relay Satellite System Introduction Space segment Tracking and Data Relay Satellite TDRSS Coverage Ground segment

5 Contents v White Sands Complex The Bilateration Ranging Transponder System User segment TDRSS signal structure Precise Orbit Determination with TDRSS Introduction Orbit determination of TDRSs Orbit determination of user satellites Gravity field model adjustment with TDRSS The Global Positioning System System description Space Segment Control Segment User Segment GPS positioning services Precise Positioning Service (PPS) Standard Positioning Service (SPS) GPS Satellite Signals GPS Data Position, and Time from GPS Pseudo-Range Navigation Carrier Phase Tracking (Surveying) GPS Error Sources Differential GPS (DGPS) Techniques GPS Techniques and Project Costs

6 vi Contents 6.10 Mathematical description of the GPS observables The GPS observables Introduction into atmospheric refraction effects Signal multipath and scattering Linear combinations of GPS measurements Concluding remarks Scientific applications of GPS Precise positioning Gravity field determination Atmospheric profiling A Russian equivalent: GLONASS Space segment Ground segment User segment Satellite Radar Altimetry The Altimeter Measurements The principle of an altimeter range measurement Beam-limited and pulse-limited altimetry Reflection from a flat sea surface Reflections from a rough sea surface Example: The ERS Altimeter Pulse compression The full-deramp technique The tracking loop Heritage

7 Contents vii Skylab GEOS SeaSat Geosat ERS 1 and ERS TOPEX/POSEIDON and Jason EnviSat The satellite altimeter measurement Temporal and spatial resolution Altimeter measurement differences Applications of satellite radar altimetry Gravity field Oceanography Orbit determination Satellite Gravity Gradiometry Satellite Gravity Gradiometry vs. other measurement techniques Introduction Conventional tracking techniques Low-low satellite-to-satellite tracking Accelerometers Satellite gravity gradiometry Summary Gradiometer measurements Effect of satellite angular motion

8 viii Contents Effect of orbit error Other effects Gradiometer instruments Introduction Superconducting gradiometer Capacitive gradiometer Performance A Questions - Introduction 183 B Questions - TDRSS 185 C Questions - GPS 187 D Questions - Satellite Radar Altimetry 189 E Questions - Gradiometry 191 F Some notes on orbit determination 193 F.1 Introduction F.2 Example: ranges F.3 Example: range-rates G An introduction into superconductivity 199 G.1 Introduction G.2 Historical background G.3 Fundamentals of superconductors Bibliography 213

9 Chapter 1 Introduction This introduction serves as an overview of the material that will be taught in the framework of the lecture series lr4-75 Geophysical satellites and instruments. The purpose of these lectures is to give an introduction into earth observing satellites and their instruments together with support systems on the ground. Earth oriented space research focuses on the use of space techniques to the advancement of geodesy and satellite orbit dynamics as scientific disciplines and to the contribution of geodesy, modern tracking concepts, and advanced satellite orbit computations to geophysics, geodynamics, oceanography, and global change studies. It is therefore of major importance not only in studying the gravity field and figure of the earth and to satellite orbit determination, but also in studying the driving forces and processes behind solid-earth dynamics, e.g.mantle convection which drives plate tectonics, for earthquake hazard alleviation and volcanic surveillance, and in solving environmental problems like global warming, sea level changes, and other aspects of climate change. Using satellites to observe the Earth enables global coverage. Areas that are difficult to access by conventional means can be observed relatively cheaply. Global coverage can in general be achieved in relatively short time spans, ranging from a couple of hours to a few months, depending on the required spatial resolution. An important condition for earth oriented space research is the acquisition of high-quality, high-accuracy satellite-based measurements. The focus of these lectures will be laid on describing several space-geodetic observation techniques. These observation techniques will be discussed in detail. In addition, attention will be paid to how measurements obtained with these techniques have to be interpreted, corrected, and examples will be given of applications in the following research areas: satellite orbital mechanics; positioning;

10 2 Introduction ocean dynamics; gravity field of the earth. These research areas will be treated in more detail in the lecture series lr4-76 Earth Oriented Space Research. In addition, the material discussed in this lecture series will be helpful in conducting the (optional) Exercise Earth-Oriented Space Research (lr4-76pr). Some typical satellites that serve as the target or the platform for the measurements will be described in detail in conjunction with results obtained so far in the above specified research areas. 1.1 Space-geodetic observation techniques Over the years many methods have been developed and used to track satellites or observe (parts of) earth from space. Many of these methods aimed at the capability to precisely determine the position of satellites, a prerequisite for successfully using many types of satellite data. These systems are a driving force behind the advancement of the field of orbital mechanics. Satellites serve in many cases as the target or the platform for measurements, from a variety of tracking systems. Although it is not intended to cover the entire spectrum, the most important will be discussed, including (Figures 1.1): Satellite Laser Ranging (SLR); Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS); Precise Range and Range-rate Equipment (PRARE); Global Positioning System (GPS); Tracking and Data relay Satellite System (TDRSS). By closely analyzing Figure 1.1 already part of the measurement principle of these systems can be seen, together with a few typical satellites. For example, the European Remote Sensing Satellite ERS-2 is being tracked by both SLR and PRARE stations. It can be seen that an SLR measurement is basically a two-way range measurement from station to satellite to station. The above mentioned systems provide information of the orbital motion of the satellites, and thus of the driving forces behind this motion. The most important driving force is caused by the earth s gravity field. A huge data set of tracking measurements to many satellites has been collected leading to high-quality long-wavelength gravity field models. In addition, these data have led to a better understanding and modeling of non-conservative

11 1.1 Space-geodetic observation techniques 3 Figure 1.1 Satellite tracking systems forces acting on satellites, including solar radiation and (for low flying satellites) atmospheric drag. Thus tracking systems can be used for precise positioning of satellites. Vice versa, they can also be used for extremely accurate positioning, i.e.with sub-centimeter accuracy, of ground stations on the earth. Therefore, an important field of research that benefits significantly from satellite tracking is the study of plate tectonics (Figure 1.2). The earth s crust can be divided into many different plates, which are moving relatively to each other in a complex manner. Although it is not the objective to discuss the theory of plate tectonics, it will be clear that closely monitoring the motion of these plates will lead to a better understanding of the mechanism behind this motion, and possibly a better understanding of the causes of earthquakes. Many years of tracking data have been collected enabling the modeling of both short- and long-term plate tectonic motion. Finally, it has to be mentioned that systems like the U.S. GPS and TDRSS find their applications not only in the above specified research areas. GPS has many applications in the wide field of (real-time) navigation, with users ranging from pedestrians, cars, and ships to satellites. TDRSS was originally developed for operational tracking of Low Earth Orbiting (LEO) satellites and telemetry/telecommand. Besides the above mentioned tracking systems, two additional measurement concepts will be described in detail: Satellite Radar Altimetry plus supporting instruments ; Satellite Gravity Gradiometry.

12 4 Introduction Figure 1.2 Major tectonic plates Figure 1.3 Altimeter principle

13 1.1 Space-geodetic observation techniques 5 Figure 1.4 Global ocean currents The primary objective of satellite radar altimetry (Figure 1.3) is in general close observation and monitoring of the world s oceans (Figure 1.4), with possible extensions to ice sheet surveillance. Satellite altimetry plays a crucial role in the study of global climate change. In Figure 1.3 it can be seen that such a measurement is a result of the difference between the instantaneous positions of the satellite and sea surface. An altimeter measurement can thus be used as a pseudo tracking measurement. In principle, an altimeter observation is a measure of the distance between the satellite radar dish and the instantaneous surface of the earth (oceans/ice sheets). Emphasis will be laid on the ocean measurements. In a first approximation, the sea surface is the sum of an equipotential surface of the earth s gravity field, the so-called geoid, and elevation caused by ocean currents. As mentioned before, the most important driving force behind satellite motion, is also the earth s gravity field. It can thus be concluded that the earth s gravity field enters the altimeter measurement twice, at the earth s surface and at satellite altitude. Therefore, besides applications in the field of ocean dynamics and orbital mechanics, an important application of satellite altimetry lies also in the field of (marine) gravity field research. Many altimeter satellites are equipped with instruments that a.o. enable proper correction, modeling of the altimeter observations. Examples are the Along-Track Scanning Radiometer (ATSR), Advanced Very High- Resolution Radiometer (AVHRR), Active Microwave Instrument (AMI), etc. These instruments will be discussed together with the radar altimeter. Finally, attention will be paid to Satellite Gravity Gradiometry (SGG). The

14 6 Introduction Figure 1.5 Summary of requirements for gravity measurement accuracy as a function of the spatial resolution compared to gravity model accuracy (Gravity Workshop,1987) primary objective is to obtain a global gravity field model with high accuracy and high resolution. Such a model find its application in many research areas, including geophysics, geodynamics, oceanography and orbital mechanics (Figure 1.5). An SGG observation is obtained by measuring the difference between the acceleration of two proof masses along a certain axis (Figure 1.6). The distance between these proof masses will be of the order of m and are located in a very low flying satellite, i.e km. The accelerations of the proof masses will be almost entirely caused by the earth s gravity field. The difference between these accelerations is a good approximation of the gradient of the gravity acceleration in the measurement direction. The gravity acceleration is obtained by taking the first derivative of the earth s gravity potential, thus an SGG observation can be modeled by taking the second derivative of this potential. It will be shown in the course of these lectures that by taking higher derivatives of the earth s gravity potential, relatively small-scale features are enhanced. Two types of satellite gravity gradiometer instruments are currently being development, one referred to as superconducting or cryogenic, and the other electrostatically suspended or capacitive. Both concepts will be discussed in the framework of these lectures. 1.2 Geophysical satellites Many satellites have served, serve, or will serve as target for the tracking systems or as carrier of instruments mentioned in the previous chapter.

15 1.2 Geophysical satellites 7 Figure 1.6 Satellite Gravity Gradiometry For the purpose of these lectures a distinction will be made into three categories: altimeter satellites; geodetic/navigation satellites gravity field satellites. In this chapter a short description of some past, current, and future satellites (partly) belonging to these categories will be given Altimeter satellites A data set of satellite radar altimeter observations has been collected covering already more than a decade. Past altimeter satellites include the U.S. Navy GEOSAT, and the European Remote Sensing ERS satellites of the European Space Agency (ESA). The latter was put into hibernation in June 1996 and serves as a possible backup for ERS-2, which is currently in operation. The ERS-1 and ERS-2 satellites have been categorized as altimeter satellites. However, these satellites actually perform a multi-disciplinary mission, carrying a whole suite of Earth observing instruments of which the altimeter, microwave radiometer and infrared radiometer are discussed in these lectures. A satellite especially optimized for monitoring ocean currents and tides is the U.S./French TOPEX/POSEIDON satellite. With this satellite sea surface height profiles can be determined with accuracies better than 5 cm. This mission revolutionized altimeter based ocean dynamics research and in conjunction global climate change studies. It also enabled realtime monitoring of important oceanographic phenomena like El Niño, a

16 8 Introduction Satellite ERS-1 ERS-2 TOPEX/POSEIDON Operation ? ? Weight (kg) Altitude (km) Inclination (deg) Table 1.1 Altimeter satellites predominantly two year feature occurring in the Pacific area with major implications on the local and global climate. A series of future altimeter carrying satellites is already envisaged to secure a continuous data stream and ocean monitoring capability. This series include the ESA ENVISAT, a Geosat Follow-On (GFO) and JASON. Certain characteristics and features of the satellites mentioned so far are listed in Tables 1.1, 1.2,and 1.3. In Table 1.3 it can be seen that the the radar altimeter forms a very small part of the entire ENVISAT mission. In fact, ENVISAT will be a very large satellite and will serve as a platform for carrying a huge amount of different sensors and instruments to observe many aspects of the system earth, from local through regional to global scales Geodetic/navigation satellites A number of satellites have been launched that were designed to serve as targets for SLR stations. These satellites include the NASA LAGEOS-I (Table 1.4) and the Italian/NASA LAGEOS-II satellites, launched in respectively 1976 and These satellites fly at relatively high altitudes and are only sensitive to relatively long-wavelength gravity field perturbations, and (almost) insensitive to atmospheric drag perturbations. They serve as an ideal target for SLR ground stations enabling very high accuracy positioning of these stations. A long history of SLR tracking data to these satellites exists and in conjunction to this a long time series of global plate tectonic motion. Many satellite configurations have been built up in the past enabling global communication with and navigation or real-time positioning of

17 1.2 Geophysical satellites 9 Satellite GEOSAT GFO JASON-1 Operation ? ? Weight (kg) Altitude (km) Inclination (deg) Table 1.2 Altimeter satellites (continued) many users, including satellites. Two such systems are the U.S. TDRSS and GPS systems. TDRSS currently exists of six geostationary operational satellites. It has been developed by NASA for operational tracking and communications support of LEO satellites. The TDRSS configuration provides 100% visibility of satellites at ,000 km altitude, decreasing to 85% visibility at 300 km. GPS is a constellation consisting of 24 satellites flying at about 20,000 km altitude homogeneously distributed around the earth. Although originally developed for the military, a wide range of civil applications have emerged, including tracking os user satellites. At any time and at any place on of near the earth more than 5 GPS satellites are visible (assuming an unobstructed field of view), enabling very high accuracy navigation and positioning. A few details of typical TDRSS and GPS satellites can be found in Table Gravity field satellites Finally, several missions are planned or under investigation that will lead to a major advancement in the modeling of the earth s gravity field, both in resolution and in accuracy. These missions include the German CHAMP ( CHallenging Minisatellite Payload ), the ESA GOCE ( Gravity field and steady-state Ocean Circulation Explorer ), and

18 10 Introduction Satellite ENVISAT Operation ? Weight (kg) 8000 Altitude (km) 800 Inclination (deg) 98.5 Table 1.3 Altimeter satellites (continued) Satellite LAGEOS-I GPS BLOCK II TDRS Operation Weight (kg) Altitude (km) ,940 (geostat.) Inclination (deg) Table 1.4 Geodetic/navigation satellites

19 1.3 ERS 1 Calibration and Validation 11 Satellite Champ GRACE (2X) GOCE Operation Weight (kg) Altitude (km) 470 (BOL) Inclination (deg) 83 near-polar 96 Sat-Sat Distance (km) N.A N.A. Table 1.5 Future gravity missions the U.S. GRACE ( Gravity Recovery and Climate Experiment ). A short overview is presented in Table ERS 1 Calibration and Validation Introduction Video The First and Second European Remote-Sensing Satellites (ERS 1 and ERS 2) are developed by the European Space Agency as a family of multidisciplinary Earth Observation Satellites (Figure 1.7). They orbit the Earth in about 100 minutes and in 35 days have covered nearly every corner of the globe at least once. Both satellites are still in good health and provide a wealth of observations related to the global climate. On 17 July 1991, ERS 1, the first European satellite to carry a radar altimeter, was launched into an 800 kilometer altitude and 98.5 inclination orbit. During the first few months, the Commissioning Phase, all instruments were calibrated and validated. Since then ERS 1 has been flying two Ice Phases (in which the repeat period was 3 days), a Multi-Disciplinary Phase (a 35-day repeat orbit lasting from April 1992 till December 1994), the Geodetic Phase, (two interleaved repeat cycles of 168 days), and the Tandem Phase (back in the 35-day repeat since March 1995), operating simultaneously with its successor: ERS 2. Satellite dimensions Total mass at launch: 2516 kg (ERS 2) Overall height: 11.8 m Solar generator: m

20 12 Introduction Figure 1.7 ERS 1 in the radar testing facility at ESTEC and instrument locations SAR antenna: 10 1 m For more information on the ERS satellites, read ESA bulletin, no. 65, february 1991 and no. 83, august The ERS instruments ERS 1 and ERS 2 carry on top of a SPOT satellite bus a payload of remote sensing instruments, all aimed at the monitoring of the Earth s climate and climate change. The exploded view in Figure 1.8 shows the various payload components. Synthetic Aperture Radar (SAR) The SAR provides cloud-free radar images of mainly the Earth s surfaces, monitoring and vegetation and growth, flooding, cultivation of land. When two consecutive images are merged though the technique of interferometry (INSAR), the instrument can even detect landslides and depressions over an approximately km area.

21 1.3 ERS 1 Calibration and Validation 13 Figure 1.8 The ERS 1 payload Wind Scatterometer (SCAT) This instrument (together with the SAR forming the Active Microwave Instrument, AMI) maps the wind speed and wind direction over ocean surfaces. Radar Altimeter (RA) The Radar Altimeter is a purely nadir looking instrument with a footprint of a few kilometers. It sends radar signals to the earth and ocean surface and collects the return pulse. The returned power as a function of travel time is called the wave form. Processing of the waveform provides information on: the wave height and wind speed (over oceans), the surface backscatter, and the height of the satellite above the surface. Together with a precisely computed orbital altitude, the latter gives the height of the surface above a well-defined geocentric reference frame. This provides the possibility to monitor the global ocean circulation, but also regional current systems, and study the marine gravity field.

22 14 Introduction Along-Track Scanning Radiometer (ATSR) A sweeping mirror detects and maps infrared radiation in various wavelengths along the satellite track. When the sky is cloud-free, these measurements can be converted to land and sea surface temperature. Microwave Sounder Operating together with the ATSR, the Microwave Sounder provides a measurement of the total water vapor content in the Earth s atmosphere vertically below the satellite orbit. This measurement is essential for the correction of the RA height measurement. Global Ozone Monitoring Experiment (GOME) The GOME is a new instrument on ERS 2 that was not available on ERS 1. At is intended to map the ozone content in the upper atmosphere and provide more conclusive statistics of the development of the Ozone Hole and the effect of pollution on this. Precise Range and Range-Rate Equipment (PRARE) PRARE is an active satellite tracking equipment. It sends signals to transponders positioned on Earth (currently some 20 are deployed). After reception of the return signal the relative range and range-rate (velocity) of the satellite to up to 4 transponders can be determined simultaneously. This provides a perfect means to determine the satellite position (latitude, longitude, and altitude) all around the orbit. The PRARE instrument on ERS 1 unfortunately failed soon after launch. Laser Retro-Reflector (LRR) The Laser Retro-Reflector is a purely passive instrument and can be compared to reflectors on cars and bicycles. It reflects laser pulses transmitted from Satellite Laser Ranging (SLR) systems on Earth back to the systems. The total travel time of the laser pulse (once received back by the laser system) is a direct measure for the distance between the satellite and the station. For ERS 1 this was the only means to compute a precise satellite orbit; for ERS 2 it is a helpful addition to be used in the orbit determination and for calibrating the PRARE instrument Calibration and Validation The ERS 1 instruments and their data products were extensively calibrated and validated during the Commissioning Phase, the first three

23 1.3 ERS 1 Calibration and Validation 15 months of operation. Calibration and validation aim at ascertaining the accuracy of the basic measurements and the quality of the derived products, and if needed adjust the measurements or the algorithms to improve the data such that a high quality can be ensured. Where calibration is aimed at the instruments and its basic measurement quantities, validation is the assessment of the quality of the derived (geophysical) data product Instrument calibration The calibration of remote sensing instruments can be done either on ground, in a laboratory environment, or in flight, by special calibration devices on the satellite or by a dedicated calibration campaign. The video shown during the lectures gives some examples. The radar altimeter range calibration The range between the satellite and the (sea) surface as measured by the radar altimeter, includes a path delay from the antenna dish to the instrument, the delay in the instrument itself, and offsets of the measurement detection. Because, at the time before the launch, an end-to-end calibration of the altimeter range was not feasible up to the required 10-cm level, a dedicated in-flight calibration campaign was set up to determined the excess path delay of the altimeter range. The outcome would be two values: the delay itself and a confidence level. The basic approach was to determine the altimeter range h alt by independent means: Measuring the satellite altitude h orb accurately by lasers over the dense network of european laser tracking systems (See Lecture notes lr4-75 #1) Determine the sea surface elevation h sea at an off-shore tower in the Adriatic Sea by means of a tide gauge, linked to the laser network coordinated by GPS (See Lecture notes lr4-75 #3). Measure atmospheric path delay by in-situ equipment h atm. With this the altimeter range bias h follows simply from: The radiometer calibration h orb = h sea + h alt + h atm + h The infrared radiometer was calibrated on ground and is continuously calibrated in-flight by exposing the instrument to two black bodies of welldefined temperatures, thus determining the offset and scale to convert the basic measurements to temperatures.

24 16 Introduction Data product validation In validation we are not dealing primarily with the basic measurements of range, radar backscatter, temperature, etc., but the overall analysis of the quality of the data products at various conditions, regions, operating modes, etc. It thus covers the end-to-end line of instrument, data collection, auxiliary data, algorithms, and product control. An example: Altimeter and scatterometer wind speed and direction Neither the altimeter, not the scatterometer measure wind speed or direction in a direct, analogue way, such as a speedometer would measure a vehicle s speed. For the altimeter it requires a conversion of the sea surface backscatter at the satellite nadir (at itself to be calibrated) to wind speed by means of an empirical algorithm. More complicated is the wind scatterometer that combines spatial patterns of backscatter measured in three directions and converts that into imagettes of wind speed and direction. The final reading of the wind speed and direction can be validated against alternative observations (air-borne scatterometers or air-minus-ground speed) or data (atmospheric models).

25 Chapter 2 Tracking systems Five lectures of the lecture series lr4-75 Geophysical Satellites and Instruments are devoted to satellite tracking systems, one of the key elements to many Geophysical and Remote Sensing Missions because these systems provide the necessary information for the determination of the position of the satellite. But also, satellite tracking is a means to determine the position of the tracking system in a global reference frame and to follow the movement of the global reference frame in inertial space. The five tracking systems discussed during these lectures (Table 2.1) measure in various ways either range or range-rate: Range: the distance between the satellite and the ground station. This is essentially a measurement of the travel time of an optical or electromagnetic signal multiplied by the speed of light. Range-rate: the time derivative of the distance between the satellite and the ground station. Generally, this is based on the Doppler shift of an electro-magnetic signal due to the relative velocity of the two points. But also combinations of the two measurements into one tracking system is possible (e.g., PRARE). The details of each of the measurement types, their advantages and disadvantages, will be discussed in the following Chapters. The tracking systems can also be characterised by producing either one-way or two-way signals. 2.1 One-way tracking systems One-way tracking systems have either ground beacons sending an omnidirectional signal, which is than received by the satellite. This is called a satellite-ground configuration, such as DORIS. But also the opposite is feasible where the satellite itself sends a wide-beam signal down, which can be received by many receivers, either on the ground, or air- or spaceborne. Such a satellite-ground or satellite-user system is GPS.

26 18 Tracking systems Tracking system Spectrum Observation type Precision Path Satellite Laser Ranging (SLR) Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) Precise Range and Range Rate Equipment (PRARE) Global Positioning System (GPS) Telemetry Data Relay Satellite System (TDRSS) nm Range 1-20 cm Ground- Satellite-Ground GHz Range-rate 0.4 mm/s Ground-Satellite Down: GHz Up: 7.2 GHz Range and Range-rate GHz Range and Range-rate 2.2 GHz Range and Range-rate 7 cm 0.4 mm/s Satellite- Ground-Satellite Satellite-User Ground- Satellite-User- Satellite-Ground Table 2.1 Overview of various satellite tracking systems.

27 2.2 Two-way tracking systems 19 Because of the wide beam and high power of the signal, receivers do not need to be pointed can can receive signals from various beacons simultaneously. So can one GPS receiver simultaneously collect signals from about 5 satellites. The current DORIS space-borne receivers, however, can only handle signals from one beacon simultaneously, which means that time is shared between the various links. Also, with the global distribution of the beacons minimum overlap and maximum coverage is ensured. However, signals from any beacon can be received and utilised by more than one satellite simultaneously. Future space-segments of the DORIS system, to be launched on Jason-1 and EnviSat-1 around 2000 will have a two-channel receiver, handling two beacons at a time. An important deficit of one-way tracking systems is that high-precision ranging is impossible. Centimeter-level ranging requires a measurement of travel time to be accurate to as good as 33 picosecond. This means that the relative precision of the clocks of the transmitter and receiver ought to be accurate to this level, which is impossible to achieve with independent clocks. Instead, one-way systems are generally measuring range-rate (Doppler shift) or combine this with a low-precision range measurement. 2.2 Two-way tracking systems Two-way tracking systems need only one clock to measure travel time. With a precision of 66 picosecond over the total travel time of 5-10 milliseconds, or 6-12 ppb, a 1-cm ranging precision is achieved. Were lasers indeed measure in this kind of stop-watch scenario, microwave systems actually rather measure a phase shift between the transmitted and received pulse, which provides also a measure of the delay time, up to an integer number of periods of the signal. Also two-way tracking systems are there in two configurations. A groundsatellite-ground configuration like SLR requires a very simple, completely passive, array of reflectors on the satellite which reflect the laser pulse. The laser station itself is much more complicated and expensive than a oneway ground station because it requires both a transmitting and receiving part and accurate detection and timing mechanisms. The radio-frequency PRARE system acts in a satellite-ground-satellite configuration, and needs ground-based transponders to reflect and amplify the signal transmitted to the ground. Because the transponder is to be pointed, it can only handle one satellite at the time (there are currently only two satellites carrying a functioning PRARE system). This makes the transponder clearly more expensive than a simple laser reflector, but also provides the possibility to send information along with the down-link signal, like information on the space-segment health, satellite position, clock,

28 20 Tracking systems etc. The space-segment has a limitation of handling signals returned by four ground stations simultaneously. 2.3 Optical and microwave systems Optical and radio signals travel at the speed of light in vacuum, but in the atmosphere the travel time of laser and microwave signals is delayed by refraction. The ionospheric delay is proportional to the total electron content (TEC) along the signal path and also depends highly on signal frequency, so signals with different frequencies travelling along the same path at the same time are delayed differently. This relative delay is still proportional to the TEC so a measure of this determines the delay on both signals. Dual-frequency tracking systems, like DORIS, PRARE, and GPS, use this trick to measure their ionospheric delays. Similarly, the ionosphere affects range-rate measurements proportional to the TEC. Laser, however, is unaffected by ionosphere. The neutral troposphere (dry and wet) delays both optical and microwave signals. The delay is usually modelled based on measurements of temperature, pressure, and humidity at the ground station. But because of the uncertainty in the distribution of humidity away from the station it is more accurate to measure the delay. Again, because of the dependence of the delay on the laser wavelength (colour), it can be measured from the delay difference of two laser pulses of different colour. Multi-colour lasers are currently being implemented in some of the laser stations. 2.4 Tracking coverage An observer can only follow a satellite when it is above the horizon. But because local geometry (trees, buildings, mountains, etc.) usually prevents tracking at very low elevations a minimum elevation of 10 is common. Also, below this elevation the uncertainty in the path delay corrections is very large, because the tracking signal travels a very long path through the atmosphere. So how long can a tracking station actually follow any satellite? And how many tracking stations would you need to ensure continuous tracking? Consider a low-altitude satellite S (h << R) in Figure 2.1 and observer P. When the satellite is viewed at an elevation E and distance d, the distance of the sub-satellite point S to the observer is s and equals Rα. Furthermore we have, assuming a near-circular orbit and a spherical earth, d cos(α + E) = R sin α (2.1)

29 2.4 Tracking coverage 21 Observer P r E s d S h Satellite S α R O Figure 2.1 The visibility circle radius s depends on the satellite altitude h and the cut-off elevation E below which the satellite is no longer visible. R cos α + d sin(α + E) = R + h (2.2) Because s << R we have α << 1, these equations reduce to and yield d cos(α + E) = Rα (2.3) d sin(α + E) = h (2.4) s = Rα = h tan(α + E) (2.5) When we substitute for E the minimum (cut-off) elevation (e.g., 10 ) and the satellite altitude h, we can compute α and s, the so-called radius of the visibility circle, iteratively. Figure 2.1 shows the dependency of the visibility circle radius upon satellite altitude for various cut-off elevations. If the sub-satellite point is inside this circle it will be visible above this cut-off elevation. So, how many stations do we actually need to have a full global coverage for a satellites like ERS 2 and TOPEX/POSEIDON? This is simply estimated by dividing the Earth s surface area by the area covered by one range circle, so numberofstations = 4πR2 πs 2 = ( 2R s ) 2 (2.6) Assuming a 10 cut-off, this means we need 30 stations to track TOPEX/POSEIDON continuously, and 56 for ERS 2. Clearly, such large number of stations is a costly business. Therefore, it is most efficient to have several satellites using the same tracking system and low-cost ground stations. These virtues are both implemented in the DORIS system.

30 22 Tracking systems Figure 2.2 Distribution of SLR (top) and DORIS (bottom) tracking stations.

31 Chapter 3 Satellite Laser Ranging (SLR) Satellite Laser Ranging (SLR) is a technique to measure the range (distance) between a laser station on the Earth, to an orbiting satellite equipped with special laser retro-reflectors to a precision of less than 1 cm. When multiple laser stations are globally distributed and collect data from the same satellite, the precise position of that satellite can be determined along with the positions of the laser stations. This technique is called Precision Orbit Determination. By tracking satellites for several years, not only can the distance between the laser stations be calculated to a few millimeters, but also the rate of motion between the lasers can be monitored to the same precise level. SLR data is also used in numerous other scientific applications including studies of the Earth-atmosphere-ocean system, physics, and intercontinental time transfer. Figure 3.1 Three laser tracking stations tracking the geodetic satellite LAGEOS. This provides information on the orbit of the satellite and about the relative positions of the tracking stations.

32 24 Satellite Laser Ranging (SLR) 3.1 Measurement principle The technique uses time-of-flight measurements with short pulses of light to determine the range to a satellite as it moves in orbit around the Earth. The light pulses come from a pulsed laser and are beamed at the satellite. The retro-reflectors work like cat s eyes and reflect the light they receive back to the receiving telescope. The monochromatic lasers make it particularly suitable for ranging at long distances and filtering out background noise. Because a narrow pass-band filter is used to cut out the light from the sky, most systems can work both by day and night, provided that the skies are clear. The principle for determining distance is the same as that of radar except that light, rather than radio waves, is used. The time t taken for the pulse to travel to the satellite and to return is a direct measure for the distance R between the satellite and the ground station: R = 1 c t + R (3.1) 2 with being the c the velocity of light in vacuum. Since neutral particles in the atmosphere refract light and slow it down, the formula requires some corrections, R. A correction for the tropospheric delay is based on measurements of temperature, pressure, and humidity, measured at the laser site. Assuming a homogeneously layered atmosphere, the Marini- Murray model predicts the correction to the (one-way) laser ranges: with R = where f(λ) f(φ, H) A + B sin E + B/(A+B) sin E+0.01 (in meter) (3.2) A = P e 0 (3.3) B = KP 0 T P 0 T 0 (3 K 1 ) (3.4) K = sin 2 φ T P 0 (3.5) f(λ) = ( λ 2 )λ 2 (3.6) f(φ, H) = sin 2 φ H (3.7) (3.8) P 0 = atmospheric pressure at the observing site (millibar) T 0 = atmospheric temperature at the observing site (Kelvin) e 0 = water vapour at the observing site (millibar) E = elevation angle of the satellite

33 3.2 A typical SLR station 25 φ, H = geodetic latitude and elevation (meter) of the observing site λ = laser wavelength (micrometer) This correction amounts to about 2 metres at the zenith and is modelled by the above equations to a precision of about 1 cm. However, the atmosphere is not homogeneous, and local deviation of pressure, temperature, or humidity (or errors in this measurements) can render this model to be inaccurate for a 1-cm precision. Especially at low elevations the travel path through the atmosphere is long (hundreds of kilometers) compared to the approximately 20 km for the zenith, making the modelled path delay quite inaccurate. Thus, multi-colour laser systems are now being developed, which can measure directly the tropospheric delay along the path. The multi-coloured lasers take advantage of the fact that (according to equation (3.2) the correction is a function of the laser s wavelength. Measuring at two distinct wavelengths, λ 1 and λ 2, we have: R 1 = f(λ 1 )Q (3.9) R 2 = f(λ 2 )Q (3.10) where Q is a function of the atmospheric conditions and site location only, like it is modelled in (3.2). Measuring the difference in travel time of the differently coloured pulses, we have the measure for the difference in the tropospheric delays: R 1 R 2 = (f(λ 1 ) f(λ 2 ))Q (3.11) From which follows the tropospheric delay path of either pulse of wavelength λ i, as R i = f(λ i ) R 1 R 2 f(λ 1 ) f(λ 2 ) (3.12) In contrast to radio waves, light is only insignificantly affected by iononised particles, so no additional ionospheric delay correction is required. 3.2 A typical SLR station Although SLR stations differ considerably in their design and capabilities they all contain the following basic subsystems (3.2): Pulse laser Telescope optics Detection package

34 26 Satellite Laser Ranging (SLR) Figure 3.2 A typical SLR station configuration. Telescope mount Time reference unit Meteorological station System controller In the following we will discuss the instrumentation of the most common high-precision lasers, like the Delft University of Technology Modular Transportable Laser Ranging System (MTLRS-2). Pulse laser Various colours of lasers are used, but the Neodymium-YAG lasers are most commonly used. These produce radiation in the infrared (1064 nm), which is converted in the optics to green (532 nm). The energy of the laser is so high, that it can never operate continuously, but is firing 10 pulses per second with a length of only 30 ps (9 mm), each carrying an energy of 30 mj. This amounts to an eye-safe level of only 0.3 W. (Would it produce a continuous beam, this would be 100 MW!) Telescope optics A block diagram of an SLR system s telescope optics is presented in Figure 3.3. A 10-Hz pulse train from the laser (21) is transmitted through a beam splitter (16) passing the infrared radiation produced by the laser and reflecting the returning green radiation. The light is focused by a lens (14) into a doubling crystal (15) and the resulting green radiation at 532 nm is magnified by a second lens. After passing through a second beam splitter (10), the laser beam is directed into the base of the telescope mount

35 3.2 A typical SLR station 27 Figure 3.3 Block diagram of a SLR system s optics. where first the divergence of the beam is adjusted to the desired value. By controlling the position of the diverging (negative) lens (8) the divergence of the beam that leaves the telescope can be continuously adjusted from less than 0.05 mrad to 2 mrad. The rest of the transmitting optics consists typically of Cassegrain mirrors. The outgoing reflections of the transmit beam off the two beam splitters are used to monitor the infrared (23) and green (25) energies from the laser and doubling crystal respectively. Filter (24) blocks 1064 nm light from entering the green detector (25) which functions as the start trigger of the delay timing. The green radiation reflected from the satellite retraces the same optical path until it is reflected by the beam splitter (16) towards the detector (20). But first it passes through a chopper and narrow-band spectral filter (17) that only opens during when the return pulse is expected and blocks out ambient light, an imaging lens (18), and a variable spatial filter or diaphragm (19). The combination of beam splitter (16) and doubling crystal (15) provides a passive, wavelength-dependent transmit/receive switch which allows the entire telescope (2) to be used simultaneously by both the transmitter and receiver.

36 28 Satellite Laser Ranging (SLR) Detection package A stable Quartz oscillator with a short term variance of only (0.5 ps/s) is used as the reference for the measurement of the travel time. The start-stop timer is triggered to start when the 532 nm laser pulse first hits the detector (25) and stops when as little as a single photon is detected by the detector (20). From each pulse of photons emitted towards the satellite only about 0.2 photons are returned back to the SLR station. Despite quantisation, this still means that every 5 shots the energy of a single photon is returned. The detection package generally includes a photo-multiplier to amplify the returned radiation such that it can indeed detect at a single photo level. For a single pass of the LAGEOS satellite, lasting up to 40 minutes, this may yield still more than 5000 returns. To reduce the number of data points and the noise of the range measurements, Normal Points are created at 10, 15, or 20 second intervals, depending on the satellite. These Normal Points are each representative of a number of raw measurements, without loss of information. Telescope mount The tracking mount keeps the telescope pointed at the satellite during the overflight by rotating the optics in azimuth and elevation. The path followed by the laser along the sky is predetermined based on preliminary orbit information. The overall pointing precision of the mount is about 5 arcsec. Time reference unit UTC time is provided by using a GPS receiver. The receiver gives a 1-persecond timing pulse which is within 1 microsecond of the GPS on-time point. A frequency control system provides the intelligence necessary for drift correction of the Quartz Oscillator Meteorological station A meteorological station provides measurements of air pressure, temperature and relative humidity at the laser site. These values are needed to correct the laser ranges for the path delay due to interaction of the laser light with neutral particles in the troposphere (Section 3.1) System controller One or several PC s control all functions of the laser system:

37 3.3 Satellites equipped with laser reflectors 29 Preparations: Converting predicted satellite orbit information into azimuth and elevation for several pre-selected targets (satellites); based on priorities decide which one to track; etc. Operation: Ensure accurate pointing of the telescope; adjust field of view when needed; register travel time measurements. Processing: Computing Normal Point data from the raw ranging data and sending this data out to a central archive where it can be retrieved by other institutes. 3.3 Satellites equipped with laser reflectors Laser ranging to satellites is working two ways: it provides the possibility to compute the orbits of these satellites very accurately, and, once the orbits are known, the position of the tracking stations can be computed. Best results are obtained when the satellites fly in different orbital planes and at different altitudes. This is achieved by putting laser reflectors on a range of very different satellites: Geodetic satellites with the sole function of being a laser target. Remote-sensing satellites benefitting from a precise orbit determination as a reference for other instrumentation like altimetry. Navigation satellites that have other positioning equipment, like GPS, that can be cross-calibrated with the high precision laser ranges. Geodetic satellites The geodetic satellites are inert, massive spheres designed solely to reflect laser light back to the ranging systems. These objects resemble golf balls. The orbits of the geodetic satellites can be computed very accurately, because of the non-gravitational forces acting upon their orbital motion are minimized due to their shape and very high density. A monthly continuous arc on LAGEOS SLR data can be fit to an accuracy of several centimetres. The accuracy of the orbits are limited by the accuracy of the global SLR dataset, the global distribution of SLR stations, and the modeling of the forces acting upon the satellite. Over the past 3 decades, global geodetic satellite observations has evolved into a powerful source of data for studies of the solid Earth; its ocean, and atmospheric systems. Reflectors have been placed on the Moon by the Apollo astronauts and by unmanned Soviet probes. Laser ranging to the Moon has provided important information on the Moon s shape, structure and orbit and has verified Einstein s theory of relativity. Lunar Laser Ranging Data is also