Making light of distance

Getting a bearing on an object ashore is an easy task for a coastal navigator. “Hockey puck” handbearing compasses and newer handheld fluxgate types, like the KVH Datascope, make the task fast and accurate.

Obtaining the range to an object is not quite so easy, however. Up to recently, the only sufficiently accurate direct methods for determining range to an object have been with radar or an optical rangefinder. (There are other accurate methods for determining distance off – horizontal and vertical sextant angles, doubling the angle on the bow, etc. – but these are more involved processes.)

Now there is a new way to directly measure the range to an object: via a laser (technically, laser rangefinders have been around for some time in the surveying field). One instrument now being marketed in the marine market is a unit called the Lasertape FG21, by Riegl Laser Measurement Systems. This device, about the size of a compact pair of binoculars (6 x 4.5 x 2 inches, and weighing about 2.5 pounds), uses a pulsed laser to measure ranges with half-yard accuracy up to 3,850 yards (1.9 nm). The Lasertape FG21 was originally manufactured for sale to hunters in Europe and is now being offered in the North American marine market.

This type of ranging capability means that when a navigator is entering a harbor, for example, he or she can go on deck with one of these units and measure the range to, say, a lighthouse, a water tank, and a daybeacon; plot the three ranges on a chart and get a fix where the range arcs intersect. This is the same procedure one would use when getting a fix using radar ranges. And while it does duplicate a radar’s ranging capability, a laser device like this does provide a navigator with another source of data, plus a backup system should the vessel’s electrical system go down. It also can reduce a navigator’s sole reliance on interpreting radar images of an unfamiliar harbor.

Using the Lasertape involves putting the 6 x 30 sighting telescope to one eye and placing the cross hairs on an object or feature of interest. Pressing a button on top of the unit sends pulses of laser light at the target. (The “light” is in the infrared part of the spectrum, at a wavelength of 940 nanometers; “visible light” is between 750 to 400 nanometers). Some of the infrared (IR) bounces off the target and returns to be detected by the unit. Like a radar, the unit notes the time between sending a pulse and first echo, divides by two then multiplies this time by the speed of light to get distance. About two seconds after pressing the button, the Lasertape FG21’s red LED readout gives the distance in yards to the target. By holding the button down, the Lasertape can also track a moving target, giving a changing readout as the range changes. Racers might use this feature for checking the range to their opponents on a race course.

While all this does sound great, there is one salient drawback to the Lasertape: it is expected to sell for $7,500. Clearly, most mariners are not going to head down to their marine electronics dealer and purchase one. Still, only eight or nine years ago, GPS receivers cost $20,000, so there is no telling what this type of device might cost in a few years.

Lasers, of course, have come a long way in the past 30 years. From bulky, liquid-cooled devices that routed light in precise geometric paths around a lab table, lasers have shrunk to semiconducting chips that are standard equipment in every CD player. The principle of operation is still the same, however: a material is “pumped” with energy and it gives off light. Of course, that definition works perfectly well for describing how the tungsten filament of an incandescent bulb works – a flow of current heats the filament, and it gives off light.

In the case of the laser, however, the chosen material has the property of resonating at optical frequencies. When energy is supplied, in the form of white light, microwaves, or electrical current, the atoms in the material absorb the energy, become “excited,” and eventually emit photons. Rather than the incoherent, random frequencies of white light, however, these photons are moving in lockstep; they are all in phase, and are classified as “coherent.” These in-phase, single-frequency light waves flow back and forth in the material, further “pumping” up energy levels. Some of the coherent light waves escape from the laser, staying together in a tight beam that barely widens compared to a beam of their unruly, incoherent cousins.

The key to building a laser is to choose a material that acts as an optically resonant cavity, allowing standing waves to form. Early lasers used a ruby rod as the resonator. Later, other types of materials – argon, carbon dioxide, helium-neon mixtures, neodymium oxide – were discovered. While we are familiar with the red color of the laser at the supermarket checkout counter, not all lasers produce visible light, some operate in ultraviolet and others produce IR.

For the purposes of a laser rangefinder, however, the most important type of laser to be devised was the diode laser. Rather than an unwieldy liquid-cooled ruby rod, diode lasers use gallium arsenide (GaAs), a semiconductor material like silicon. A diode laser is like a sandwich, with two “pieces of bread” that have different electrical properties. Because of this difference, the middle of the sandwich acts as an electrical junction or gate between the two elements. If the two pieces of the GaAs sandwich are made properly, the area near the junction becomes a resonant cavity, producing coherent light when current flows through the diode.

Because they are solid state devices, diode lasers can be made very small and placed inside devices like portable CD players, for example. Individual units can also be assembled into an array of lasers, thus increasing total output power.

The subject of power is important when considering how a handheld unit like the Lasertape FG21 can measure distance out to 3,850 yards. A radar set can determine ranges out to tens of miles, but radars have transformers that take a vessel’s steady 12-volt power and step it up to several thousand volts. When running on its internal power supply, the Lasertape must rely on six AA batteries (the unit can operate off ship’s power via a connection box). In order to obtain up to 2,000 measurements using battery power, the Lasertape FG21 borrows a trick used in radar sets.

A microwave signal that travels 12 miles out and 12 miles back must use plenty of power. A radar set develops this power by dumping a high voltage signal into a microwave-resonant cavity called a magnetron. The more power put into a magnetron, however, the hotter it gets. To avoid overheating the magnetron and melting it, power is sent into the magnetron in short pulses, giving the cavity time to cool. (This resting time also saves the vessel’s batteries from being knocked flat in short order. It also allows the receiver to listen to the reflected signal.) The result of pulsing is a peak power measured in kilowatts but, since each pulse is of short duration, an average power of only a few watts.

The Lasertape FG21 also transmits its energy in pulses, packing laser energy into a short-lived burst that provides enough power for some energy to be reflected and make its way back to the unit.

An IR laser produces a tight beam that can be directed at specific targets (the beam widens to a square 1.1 yards on a side at 660 yards). However, it does have the disadvantage of operating in a part of the electromagnetic spectrum where it has some formidable competition – the sun. Solar IR sloshing around during the day increases the “background noise” that the unit must look through to “see” returns. Thus, in daylight, the unit’s working range drops from the 3,850 yards possible at dawn, dusk, and night, to 2,750 yards. Additionally, haze and fog can greatly reduce maximum range as some IR energy is scattered by airborne dust and moisture particles.

Even though a unit like the Lasertape FG21 has its limitations, it does represent an impressive, leading edge use of technology for the coastal navigator. Now, all one needs is a Lasertape, an electronic handbearing unit like the KVH Datascope, and a night vision unit (see page 00) and one can do “electronically assisted” traditional coastal navigation – on the other hand, it might be a good idea to keep the hockey puck and binoculars ready should the batteries run out.

By Ocean Navigator