11.1 Introduction

Shortly after the predictions of quantum mechanics for an optical maser had been demonstrated experimentally and with the advent of ruby pulse lasers capable of generating short laser pulses with a width of about 5 ns, the concept of laser ranging quickly became a widely used technology. Since time represents the SI unit that can be measured with the highest resolution, it was convenient to measure distance as the time of flight of short optical laser pulses. Using the velocity of light, a fundamental physical constant, time intervals can be converted to range with equally high resolution. Including a proper treatment of atmospheric refraction, an accuracy of several millimeters for the range measurement between a ground-based observatory and a satellite target is routinely achieved.

11.2 History of Satellite Laser Ranging

In 1960 the first laser operation was successfully demonstrated by Th. Maiman and, interestingly enough, the instrument was a pulsed ruby laser. It took almost another half year to develop a continuous wave (CW) laser system, based on the noble gases helium and neon. With the availability of bright laser sources with a short pulse duration came a major step forward for global geodesy. Since range measurements to satellites provide much higher resolution for orbit determination than the angle measurements, pulse lasers became an important tool and the technology of satellite laser ranging (SLR) developed into one of the three major techniques in the emerging field of space geodesy. Laser ranging to satellites equipped with cube corner reflectors started as early as 1964, delivering a range resolution of 3 m and was also adopted for the Apollo lunar exploration program, with the first retro reflector array deployed on the moon during the Apollo 11 lunar sortie mission. Within the next decade several dedicated SLR satellites, among them the LAser GEOdynamics Satellite (LAGEOS), were introduced and technical progress was improving the laser stations. As a result, new science models for continental plate motions, improvements for long wavelength parts of the gravity fields became available, and effects like lunar free libration and lunar tidal deceleration were measured. The technology of SLR improved by about one order of magnitude per decade between the early days of laser ranging and the change of the millennium, that is from 3 m of range accuracy to approximately 3 mm with increasing importance for the terrestrial reference frame in geodesy, Earth observation and global change monitoring. More and more satellite missions make use of SLR, so the number of tracked targets has increased from only three in the mid-1980s to more than 40 targets today. Laser-supported precise orbit determination plays an important role in many non-geodetic satellite missions too, which has progressively led to more complex observation patterns for the ranging stations.

Because cameras and other equipment on board of some active multi-sensor satellites are vulnerable to the exposure of bright laser light the International Laser Ranging Service (ILRS) has developed methods to avoid sensor blinding effects or damage. Furthermore, it has become more and more important to exclude interference of air-traffic and SLR activities. With new active transponder missions on the International Space Station, the necessity to avoid eye exposure to laser radiation from ranging ground stations becomes even more important.

11.3 Concept of SLR Technology

Satellite laser ranging is a technology that is based on the measurement of the time of flight of short laser pulses. Figure 11.1 shows a block diagram of the underlying measurement concept. A mode-locked pulse laser generates short laser pulses with a pulse width of about 100 ps or less. These laser pulses are guided to a transmitting telescope, which transmits the signal towards the satellite target. During this process an event timing unit is triggered to latch the current epoch . Cube corner reflectors with a low dispersion backscatter angle reflect the laser pulse back to the direction of origin and a very small portion of the transmitted beam power is collected with the receiving telescope. A photomultiplier or solid state detector unit converts the returning residual laser pulse to an electrical pulse, which in turn triggers the event timing unit again to obtain . From these two epochs the range to the satellite is computed. The range equation in its simplified form is

(1)

where is the modeled delay caused by the signal propagation through the atmosphere and accounts for the constant delay in time, which the measurement signal requires to pass through the instrumentation. The factor 1/2 accounts for the fact that the signal passes through the distance twice and is the vacuum velocity of light. Most SLR systems have a separate telescope for the transmit and receive functions (bi-static design), which simplifies the design of a station. A few stations with particular strength in observing remote satellites or even lunar targets, share the same telescope for transmit and receive (mono-static design). The latter systems require a fast switching unit between transmit and receive functions. Since SLR provides ranges as a function of time at a level below 1 cm, both the start location of the range measurement and the endpoint need to be defined with great accuracy.

Most of the SLR systems in the world employ alt./az. mounts and the intersection point between the horizontal and the vertical axis defines the origin of the range measurement (Figure 11.2a). Calibration measurements at the station define this point technically. The end of the range measurement is usually given by the center of mass of the satellite. For a typical geodetic satellite, as shown in Figure 11.2b, this corresponds to the center of a spherical structure. Since the measurement reaches the surface only, it is derived numerically by a pre-launch determined survey value. For Earth-observing satellites such as ERS1 or ERS2, this requires a much more complex attitude dependent model. Despite the fact that SLR inherently follows a simple and robust concept, the final station layout is complex and demanding, because the telescope pointing must be precise to a few seconds of arc, the range prediction already good to about 100 m or less and the timer must have a resolution of about 1 ps. The laser link budget varies a lot from one satellite target to the other due to the overall signal loss, mostly because of the distance and the number and size of the cube corners involved. In order to determine if a laser ranging facility can successfully track a given target, laser link calculations have to be performed. The number of detected photo-electrons can be calculated for the ranging conditions and the given system parameters of the observing station. According to Degnan (1993) one can write the laser link equation as

(2)

where is the number of detected photo-electrons. The parameter is the quantum efficiency of the detector, is the transmit optics efficiency and is the receive optics efficiency. then is the energy of the laser pulse [J], λ the wavelength of the laser [m], h Planck’s constant and c the velocity of the light in vacuum. The transmitter gain essentially describes the divergence of the transmitted laser beam, σ is the effective lidar cross section of the target (i.e. the efficiency of the reflector array) in million square meters and the effective area of the telescope receive aperture. While is the one-way atmospheric transmission, is the one-way transmission of high cirrus clouds if present and R represents the slant range to the target. It is important to note that the signal strength reduces with , and with θ, being the half angle of the laser beam divergence. For the LAGEOS orbit, this means that a total of 50 mJ of frequency doubled Nd:YAG laser energy transmitted from the telescope, reduces to a small signal of something like 1–100 photo-electrons for an average SLR system, depending on the instantaneous transmission of the atmosphere. It is important to note that this corresponds to approximately 12 orders of magnitude of overall loss in signal strength. Unlike all other methods of range finding SLR can cope with an enormous dynamic range of the measurement signal.

11.4 International Laser Ranging Service and Mission Safety

SLR is one of the four important techniques of the International Association of Geodesy (IAG). The major objective of the IAG is advancement of geodesy, which becomes mostly visible by efforts related to the precise representation of the figure, the gravity field, the rate of rotation and the orientation of the rotational axis of the Earth, as well as the temporal variation of these quantities. This comprises the establishment of reference systems, monitoring the gravity field and rotation of the Earth, the deformation of the Earth surface including ocean and ice, and positioning for interdisciplinary use. The International Laser Ranging Service (ILRS) is one of several services under the umbrella of the IAG. The ILRS coordinates the laser ranging observations of about 30 globally distributed SLR stations. Several working groups within the ILRS deal with matters such as technological development, standardization of data products, new missions, topical measurement campaigns and ranging network performance.

One of the emerging areas of interest within the ILRS is the precision optical time transfer such as the T2L2 project (Guillemot et al., 2006; Samain et al., 2009). For this project the Jason 2 satellite was used. Future projects, such as the Atomic Clock Ensemble in Space (ACES) on the International Space Station, require SLR activities to manned space vehicles. Apart from challenging technical demands, projects like this also have enhanced safety requirements. Over the years the ILRS has been dealing with tracking restrictions several times, in order to avoid sensor irritation from SLR beams on the payload of multifunctional satellites. As a consequence of such requirements several practices have been developed by the corresponding working groups of the ILRS. These are:

The elevation mask is used for missions, where the laser beam of a tracking station is moving into the field of view of a nadir pointing camera or any other sensitive optical sensor on the satellite. Due to provisions at the ranging station, firing the laser above a given elevation angle of the tracking is disabled. Although the elevation angle dependence criterion is defined by the satellite mission, the actual compliance with this requirement is realized at the individual observatory. For that purpose the control program of the ranging facility evaluates the instantaneous pointing angle during the measurement process and switches the laser off when the requested cut-off angles are reached. When a ranging station has implemented this feature, the functions are verified by the corresponding working group of the ILRS by testing the procedure on known uncritical satellites and comparing the measurement diagnostic files with the preset requirements. In addition a precise description of the implemented modifications are evaluated for compliance by the working group members. This procedure is necessary, since most of the SLR systems are individually developed and there is no common hardware or software base in the community. Once a station has passed a number of such tests successfully, it obtains access to the otherwise not freely distributed orbit information in order to track the restricted target. Similar procedures apply for the restrictions in delivered laser beam powers. Furthermore, it is common practice that only a few SLR stations are tracking delicate satellites on demand. That means that they are scheduled for specific times and there are no valid orbital elements available to the station generally. This is an additional safety feature and depends mostly on the mission goals. The Go, NoGo flag provides additional control over SLR functions to the satellite mission. It works on very short notice and does not require any notification procedures for the ranging observatories. Critical missions that apply and require this function are setting this software flag either to “Go” or “NoGo” at a predefined server location at any time this is required. By default the value of this flag is “NoGo” and the mission actively has to set it to “Go” in order to allow tracking. Watchdog functionality resets this flag to “NoGo” on the server at a predefined period, usually after one minute and the mission has to reset it to “Go” periodically. The same applies to the SLR station. Within one minute it has to reload the flag from the mission server in order to continue tracking. In this way it is guaranteed for a received “Go” status, that both the link to the mission server is operating and the system has a valid permission to track the target. Again, each laser ranging station has to run through a qualification process, where the mission is toggling this flag without prior notice in order to test this safety measure.

11.5 In-Sky Laser Safety

Safety measures against laser hazards and radiation have also to be implemented at the SLR site. SLR technology today is mostly based on Nd:YAG lasers, most of which are flashlamp pumped and run under high voltages. Autonomously operating optical telescope structures and other moving mechanical parts provide additional hazards right at the observing station. Apart from these personal safety aspects, there is also the need to provide safety functions for the tracking hardware. For illustration purposes mentioned here, this requires features that avoid tracking a satellite too close to the sun, because excessive light exposure can cause damage to the telescope or the photomultiplier. These types of safety measures also have to work on a system that is not actively operated, because the sun moves along the sky over the course of a day. Most of the laser ranging facilities around the world have an operations room, well separated from the laser and mechanical subsystems of the SLR system. Safety switches on doors and on protective encasements and several emergency stop contacts throughout the facility ensure safe operations of the SLR system. Sun-avoidance routines, auxiliary sensors for the detection of sensitive system states, rain detectors, and in some places a infrared all-sky camera, are common system health-monitoring features today, paving the way to fully automated system operations.

Some SLR sites are operated in remote areas with next to no air traffic. In all other cases the requirement for ensured eye-safety for passing aircrafts, including hot air balloons and hang-gliders, is given with high priority. Since the laser beam from SLR systems is collimated, even distant aircrafts need to be detected and exposure to the laser beam avoided. Figure 11.3 shows the area filled by the laser beam as a function of distance. A collimated beam of the Wettzell Laser Ranging system, for example, fills less than 1 m² at a distance of 10 km. Under typical working conditions of 10 seconds of arc beam divergence, approximately 20 m² are illuminated. While this small beam area makes an illumination incidence very unlikely, a direct conclusion however is that the aircraft path has to be clear of any ranging activity at all times in order to avoid a hazard. A purpose-built non-scanning aircraft detection radar system from Honeywell Inc. is widely used throughout the ILRS. This pulsed radar is slaved to the SLR telescope and always points into the same direction. Because of the antenna gain, it detects aircrafts as far as 30 nautical miles away. At elevation angles below 20°, there is generally no tracking because of the problem of enhanced ground clutter and false alarms from the radar system. With a typical cruising altitude of aircrafts of around 33,000 ft. and a lower ranging cut-off angle of 20° a radar safety range of 30 nautical miles is sufficient to cover all air traffic. Some SLR stations, such as the Geodetic Observatory Wettzell, are also protected by a no-fly zone for air traffic below 3600 ft. and less than 5 km distance. Over the last few years additional redundant information about the actual air traffic around an SLR site is now available from receiver systems that directly receive transponder messages from cruising aircrafts. From this status information the trajectory of the aircraft can be compared to the track of the currently observed satellite. So the laser can be switched off under program control when close proximity of the laser beam with respect to the aircraft occurs. Since the existence of no-fly zones and the application of actively transmitting transponders in aircrafts does not automatically ensure eye-safety, only redundancy with several different concepts offers a higher level of safety. For this reason some SLR sites are cooperating with national air-traffic safety authorities in order to use tracking data available to these institutions for the evaluation of the safe sky condition for the tracking laser system.

11.6 Laser Safety in Space

Low Earth-orbiting satellites are characterized by a along-track velocity well in excess of 10 km/s. The tracking of such satellites with a laser ranging system is therefore a demanding task requiring precise pointing and smooth tracking of the telescope at the level of a few seconds of arc. Each laser station also needs precise orbit predictions in order to successfully acquire echoes from the cube corner reflectors on the satellite. From that point of view it is practically impossible that a SLR station can accidentally cause an unwanted illumination hazard to an space object. However, when a satellite is added to the catalog of tracking targets, a number of safety considerations apply. If the satellite is not equipped with optical sensors that can be irritated by laser beam illumination no further activities are necessary. If the satellite however carries a critical payload, exceptional treatment has to be implemented, and all tracking stations participating in the observations have to go through a compliance procedure as outlined in the section on the International Laser Ranging Service. Depending on the specific requirement of the satellite, either a tracking restriction by implementing an elevation mask, or an upper laser power density level must be defined and reliably demonstrated before access to the orbital parameters is given. Also the process of scheduling specific limited observation windows and the application of the Go, NoGo flag may be used. Additional safety considerations in excess of the previously described measures apply for tracking targets which are visited by astronauts, such as the International Space Station (ISS). Because laser radiation in excess of a maximum permissible energy (MPE) of 20 nJ/cm² for a frequency doubled Nd:YAG pulsed laser system at a pulsewidth of less than 100 picoseconds is potentially hazardous to the eye, it must be ensured that the safety limit for the beam energy level is not exceeded. Figure 11.4 shows a rather typical example for a SLR station with 10 mJ of generated laser power from a frequency doubled Nd:YAG laser system under typical ranging conditions. The horizontal line marks the MPE level as defined by relevant standard ANSI Z136.1. Since the energy level always stays well below the maximum permissible level, eye-safety is insured at all times, not only for the naked eye but also to the use of binoculars. Nevertheless, additional safety measures have to be taken at the observing station. These are:

Controlling the beam power involves a power measurement device, which constantly measures the energy of the pulse laser on a shot-by-shot basis. This analog measurement signal is inverted and an actuator actively moves a beam block out of the beam path, if the measured voltage is above the predefined threshold. This corresponds to the situation that the output beam satisfies the safety criterion. Evaluating the inverted signal ensures a blocked laser beam in the case of power failure on this electronic safety device. The laser fires under computer control. The fire command is utilized by generating a voltage pulse with transistor-transistor logic (TTL) level corresponding to voltages between 0 and 5 Volts. This command line is passing through the optical setup of the SLR station, where motorized lenses are setting the required laser beam divergence. Mechanical safety switches, which are sensing the actual position of the divergence lens by means of a physical contact are only passing the fire command if the lens is at a divergence setting equal or wider than the safety requirement. For inherently hazardous settings, there is no physical contact and the signal line is not connected to the laser.

11.7 Summary

Controlling the amount of laser radiation illuminating targets tracked by the ILRS stations has become progressively sophisticated to ensure protection and to secure the valuable payload on satellites. There are several very effective safety measures that have been developed and implemented by the ILRS and no hazardous incident has occurred up to now. However, because of human presence at the space station future projects involving SLR tracking of the ISS for precision time transfer warrants special considerations for laser eye safety. While all laser stations stay well below the maximum permissible energy level, additional safety measures at each observing site must be exercised by physically controlling the outgoing laser beam on a shot-by-shot basis to ensure safe power density levels at all times.

References

1. Degnan JJ. Millimeter Accuracy Satellite Laser Ranging: A Review. In: Smith DE, Turcotte DL, eds. AGU Publication 1993;133–181. Contributions of space geodesy to geodynamics: technology. Vol. 25 ISBN: 0-87590-526-9.

2. The International Association of Geodesy: www.iag-aig.org/.

3. The International Laser Ranging Service: http://ilrs.gsfc.nasa.gov/.

4. Guillemot P, Gasc K, Petitbon I, et al. Time Transfer by Laser Link: The T2L2 experiment on Jason 2. Proceedings of the IEEE International Frequency Control Symposium 2006; (pp. 771–778).

5. Samain E, Exertiere P, Guillemot P, et al. Time Transfer by Laser Link T2L2: First Results. Frequency Control Symposium, 2009 Joint with the 22nd European Frequency and Time forum IEEE International 2009;194–198. doi 10.1109/FREQ.2009.5168168.

6. Atomic Clock Ensemble in Space: http://www.esa.int/SPECIALS/HSF_Research/SEMJSK0YDUF_0.html.