GEOG 862
GPS and GNSS for Geospatial Professionals

Optical Systems

Night sky and the International Space Station (ISS)
Photograph of the International Space Station (ISS)

Some of the earliest extraterrestrial positioning was done with optical systems. Optical tracking of satellites is a logical extension of astronomy. The astronomic determination of a position on the earth’s surface from star observations, certainly the oldest method, is actually very similar to extrapolating the position of a satellite from a photograph of it crossing the night sky. In fact, the astronomical coordinates, right ascension α and declination δ, of such a satellite image are calculated from the background of fixed stars.

Here's a picture of a track of the International Space Station against the so-called fixed stars. Of course, it is possible to determine the position of a satellite in this way because the stars have known coordinates in the right ascension and declination system. And so, if one knows the time and knows the stars in the background, it is quite possible to learn the ephemeris—or the position—of the satellite based upon that control. This is an extension of astronomy that has been used—and to some extent continues to be used—in terms of positioning the satellites. For example, it's worth noting that some of the major remote sensing satellites have star trackers on board that help determine their orientation and position.

A ballistic camera
A ballistic camera

Photographic images that combine reflective satellites and fixed stars are taken with ballistic cameras whose chopping shutters open and close very fast. The technique causes both the satellites, illuminated by sunlight or earth-based beacons, and the fixed stars to appear on the plate as a series of dots. Comparative analysis of photographs provides data to calculate the orbit of the satellite. Photographs of the same satellite made by cameras thousands of kilometers apart can thus be used to determine the camera’s positions by triangulation. The accuracy of such networks has been estimated as high as ±5 meters.

Of course, by implication, if one knows the position of a satellite relative to a sensor on the Earth's surface, then, obviously, it is possible to extrapolate the position of the sensor on the Earth's surface. The result is a positioning system that uses extraterrestrial objects to determine positions on the Earth's surface. The idea of using satellites to get positions on the Earth has a history of many decades.

Photograph showing a satellite laser ranging station
Satellite Laser Ranging (SLR)

Other optical systems are much more accurate. One called SLR (Satellite Laser Ranging) is similar to measuring the distance to a satellite using a sophisticated EDM. A laser aimed from the earth to satellites equipped with retro reflectors yields the range. It is instructive that two current GPS satellites carry onboard corner cube reflectors for exactly this purpose. The GPS space vehicles numbered SVN 35 (PRN 05) and SVN 36 (PRN 06) have been equipped with Laser Retro-reflector Arrays (LRA), thereby allowing ground stations to separate the effect of errors attributable to satellite clocks from errors in the satellite’s ephemerides. The second of these was launched in 1994 and is still in service.

The same technique, called LLR (Lunar Laser Ranging), is used to measure distances to the moon using corner cube reflector arrays left there during manned missions. There are four available arrays -  three of them set during Apollo missions and one during the Soviet Lunokhod 2 mission. These techniques can achieve positions of centimeter precision when information is gathered from several stations. However, one drawback is that the observations must be spread over long periods, up to a month, and they, of course, depend on two-way measurement.

We discussed EDMs earlier, and you know that it is possible to use EDMs as an optical system to determine distances. Satellite Laser Ranging, SLR, can measure distances using an electronic distance measuring device, an EDM, and a laser that points from the Earth to the satellite. The reflectors on the satellite can return that signal to the terrestrial sensor. This a method of measurement by phase comparison with a reference wave at the EDM. It works well for satellites orbiting. This system is also used for Lunar Laser Ranging (LLR). There are banks of reflecting retro prisms on the lunar surface that were left by the manned missions, both Russian and American. We can now determine the position of the moon relative to the Earth using electronic distance measuring to centimeter precision. This is useful for many positioning needs.

Optical drawbacks

Optical system in use on a cloudy day
Cloudy Skies are an Impediment to Optical Systems
Source: CP24 Article

While some optical methods, like SLR, can achieve extraordinary accuracies, they can at the same time be subject to some chronic difficulties. Some methods require skies to be clear simultaneously at widely spaced sites. Even then, local atmospheric turbulence causes the images of the satellites to scintillate. The bulky equipment is expensive, and optical refraction is difficult to model. Still, optical tracking remains a significant part of the satellite management programs of NASA and other agencies.

Optical systems can be restricted by clouds and other atmospheric troubles—fog, and so on. Clear skies are sometimes hard to come by. Turbulence can also be a problem. The troposphere and the ionosphere can be very uncooperative. The necessity of broadcasting a signal revealing your position in this way and the need to have expensive—and sometimes bulky—equipment are some of the drawbacks to the optical method.