Determining the range to a satellite in orbit around the earth utilising the technique of satellite laser ranging (SLR) was pioneered in the early 1960s. The first successful ranging experiment was reported in the 3 December 1964 issue of Flight International (Smith 1964). Dr. Henry H. Plotkin of Goddard Space Flight Centre led a NASA team to track the Beacon-B (also known as Explorer-22) satellite for ten successful sessions during the period 11 October to 13 November 1964. A team from General Electric Co. (Valley Forge, Pennsylvania) also participated from Phoenix, Arizona. Using a telescope mounted with a ruby laser, expected range accuracy was about 3 m. Current accuracy is at the level of 1–2 cm.

Satellite Laser Ranging

The extension of radio frequency techniques to the optical region of the spectrum has made possible many types of measurements which were not feasible before. One area in which important scientific contributions can be expected during the next few years is the use of lasers to measure long distances with high accuracy. Three types of distance measurements which have been discussed in the literature and which are now being investigated actively are: 1) interferometric measurements over distances of up to hundreds of meters through evacuated or sealed-off tubes, 2) measurements with modulated laser beams over distances of perhaps one to fifty kilometers with corrections made for the atmospheric index of refraction along the path, and 3) range measurements to artificial satellites and to the moon using laser radar. Some of the possible geophysical and geodetic applications of such measurements will also be discussed.

Laser measurements of long distances

Satellite tracking methods, geodetic procedures and results are described with the emphasis placed on the geophysical significance of the results. The discussion is limited mainly to the solid Earth. The use of the satellite techniques for the Moon and planets is also discussed, again with the emphasis placed on the geophysical consequences of these measurements.

https://iopscience.iop.org/article/10.1088/0034-4885/42/4/001/pdf

Methods and geophysical applications of satellite geodesy

Corrections for the errors that occur in pulsed optical ranging such as satellite ranging are generally analyzed. It is shown that corrections for range errors can be described as a linear combination of the atmospheric correction, the spatial correction, and the corrections for the errors in electrical circuits. A method for the calibration and the correction of observed distances by using a standard target is presented. Particularly in local ranging, corrected distances are easily calculated by two standard targets, even if the atmospheric refractive index and the accuracy of the time base of the counter are unknown. Ranging experiments on earth from 100 m to 10 km were made with the laser system. The corrected distances, using two standard targets, agree well with distances measured accurately by a Geodimeter within the pulse width of the laser, 50 nsec (7.5 m), and the resolution of the time interval counter, 10 nsec (1.5 m).

Correction for errors in optical pulsed range measurements.

Applications utilizing high peak-power Q-switched laser systems are specialized but include the determination of satellite orbits,’ the measurement of distant satellite ranges,”’ and the precise triggering of high voltage spark gaps.* The use of lasers for simulating nuclear detonation relates specifically to triggering million-volt spark gaps. The use of high peak-power oscillator amplifier Qswitched lasers at the Kirtland Air Force Base’s Weapons Laboratory has enabled investigators to reduce delay time, i.e., the time interval between activating the breakdown to complete gap breakdown, from 100 nsec to less than 2 nsec. For practical measurement purposes, this process has simultaneously eliminated jitter relating to this delay time by a unique enhancement of so-called streamer propagation. In simulating nuclear detonations, this minimum delay and reduction of jitter is important in (1) initiating multiple discharges and in (2) using multiple gaps in efforts to reduce switch impedances in order to create very fast rise times. For several years the task of data acquisition of satellite orbits has been accomplished by the Smithsonian Astrophysical Observatory at Cambridge, Mass. They have established a world wide tracking system equipped with Baker-Nunn cameras which allow very precise angular orbital calculations. High peak-power laser systems are now being used to obtain, simultaneously with the angular coordinate, an exact range measurement. Range measurements for these satellites have been carried out with satellites as distant as 3,000 km. The Smithsonian Astrophysical Observatory anticipates that ultimately, with all tracking stations equipped with laser systems and Baker-Nunn cameras, satellite orbits can be determined to within a few inches. The NASA Goddard Space Flight Center at Greenbelt, Md. will use a high peak-power laser to utilize an array of retroreflectors to be left on the moon by the first astronauts to land there in July, 1969. The retroreflectors are designed to permit the precise measurements of the distances between the earth and the moon and to determine more exactly the distance on the earth between a pair of pointing laser beams, In order to do this, the laser system has an output requirement of 1 Gw in a 5 nsec pulsed duration. Such precise measurement will allow calculations of continental drift, movement of the earth’s crust, etc. Each of these applications has in common a requirement for short pulse duration, extremely high peak power, and short rise times. In addition, the high voltage switching requires absolute minimum jitter in the pulse to pulse timing. This aspect relates. to a future application of “triggered pulse charging” up to 10 Mv.

Applications of high-power pulsed lasers.

The results from a series of experiments at the Smithsonian Astrophysical Observing Station, Organ Pass, N. Mex., indicate how lasers may be used to supplement the worldwide network of Baker-Nunn cameras in obtaining precise satellite orbits for geodesy and other purposes. A pulsed ruby laser and photoelectric receiver were located at the observing station. Laser returns were obtained from the three satellites equipped with retroreflectors. The problem of aiming the laser so that its narrow beam reaches the satellite is discussed for the cases of the satellite in sunlight and in shadow. The value of the limiting range obtained experimentally is compared with that calculated from the range equation and from measurements and estimates of the background noise. Range measurements made with the laser system are compared with values determined from the orbit computations based on observations from the Baker-Nunn network. The effects of the earth's atmosphere are discussed.

G. Snyder

Papers

2A10 - Laser experiments for determining satellite orbits

The use of lasers for the precise determination of satellite orbits