GPS appears to be the wave of the future in electronic navigation. Prices are falling fast, and there are now GPS units for under $400. Since Loran units cost over $300 (typically), the $400 GPS sounds like a pretty good deal. Loran has excellent repeatability (i.e., you can get back to the same spot, within about 100 yards), but GPS has greater accuracy (the LAT/LON reading is likely to be closer to where you are than that of a LORAN). (jh)
As an example, an Apelco DXL6350 ( I have a 6300) is available regularly at under $250. It functions very well but lacks route capability. It is not like the reallly low priced units that lack ASF and other needed features. No other apologies needed. I believe I saw it on sale for $224 from E&B. (1994 prices) (cp)
If my Loran gave out on me, I would, at this point, probably replace it with a GPS. If I were looking for a cheap way to navigate electronically, I'd look for some folks who just got GPS and offer to buy their Loran unit cheap. It's worked fine for a very long time, and there's nothing wrong with it. (jh)
Here's a summary of how GPS works, contrinuted by Craig Haggart:
HOW GPS WORKS: AN INTRODUCTION
Amazingly precise satellite navigation receivers are now widely available and reasonably priced, thanks to the Global Positioning System (GPS). How do these little wonders figure out exactly where you are?
The basic principle behind GPS is simple, and it's one that you may have used many times while doing coastal navigation: if you know where a landmark is located, and you know how far you are from it, you can plot a line of position. (In reality, it's a circle or sphere of position, but it can be treated as a line if the circle is very large.) If you can plot two or more lines of position, you know that you are at the point where the lines cross. With GPS, the landmarks are a couple of dozen satellites flying about 12,000 miles above the earth. Although they are moving very rapidly, their positions and orbits are known with great precision at all times.
Part of every GPS receiver is a radio listening for the signals being broadcast by these satellites. Each spacecraft continuously sends a data stream that contains orbit information, equipment status, and the exact time. All of the information is useful, but the exact time is crucial. GPS receivers have computers that can calculate the difference between the time a satellite sends a signal and the time it is received. The computer multiplies this time of signal travel by the speed of travel (almost a billion feet per second!) to get the distance between the GPS receiver and the satellite (TIME x SPEED = DISTANCE); it then works out a line of position based on the satellite's known location in space.
Even with two lines of position, though, the resulting fix may not be very good due to receiver clock error. The orbiting satellites have extremely accurate (and expensive!) clocks that use the vibrations of an atom as the fundamental unit of time, but it would cost far too much to put similar atomic clocks in GPS receivers as well. Since precise measurement of time is critical to the system - a clock error of only one thousandth of a second would create a position error of almost 200 miles - the system designers were faced with a dilemma.
Geometry to the rescue! It turns out that GPS receivers can use inexpensive quartz clocks (like the ones used in wristwatches) and still come up with extremely accurate position fixes as long as one extra line of position is calculated. How does this work? First, imagine two earthbound landmarks with known positions - for example, Honolulu and Los Angeles. If we measure the travel time of radio waves from each of these cities to San Francisco, we can use the known speed of the radio waves to compute two lines of position that cross. If our clock is a little fast, our position lines will show us to be closer to both cities than we really are; the lines will cross, but that crossing point might be somewhere out in the ocean southwest of San Francisco. On the other hand, if our clock is too slow, we will appear to be farther away from the chosen landmarks than we really are, and our position lines might cross to the northeast of us, near Sacramento.
Now, if we get just one more position line - from Seattle, let's say - the three lines would form a triangle, and the center of the area in this triangle is our REAL position. The clock error is the same for all three lines, just in different directions, so moving them together until they converge on a point eliminates the error. Therefore, it's OK if our GPS receiver's clock is a little off, as long as the clocks on the satellites are keeping exact time and we have a computer that can pinpoint the center of a triangular area.
For accurate two-dimensional (latitude and longitude) position fixes, then, we always need to get signals from at least three satellites. There are now enough GPS satellites orbiting the earth to allow even three-dimensional position determination (latitude, longitude, and altitude, which requires signals from at least FOUR satellites) anytime, from anywhere in the world. The more satellites your receiver can "see" at one time, the more accurate your position fix will be, up to the system's standard accuracy limit of a few hundred feet.
The U.S. Department of Defense is responsible for the GPS system, and they reserve increased accuracy for military users. For this reason, the satellites broadcast a coded signal ("encrypted P-code") that only special military receivers can use, providing positions that are about ten times more accurate than those available with standard receivers. In addition, random errors are put into the satellite clock signals that the civilian GPS receivers use. Not everybody is happy with this intentional degradation of accuracy, though, including the U.S. Coast Guard.
To get around the DoD-imposed accuracy limitation, the Coast Guard is setting up "differential beacons" around the U.S. A differential beacon picks up GPS satellite signals, determines the difference between the computed position from the satellite and the beacon's own exactly-known location, then broadcasts the error information over a radio channel for all nearby differential-equipped receivers to use. With this method, inexpensive GPS receivers can produce position information accurate to within a few inches using the standard, uncoded civilian signal. GPS receivers that can take advantage of this differential broadcast are becoming quite common, although a separate differential beacon receiver usually must be purchased.
The way GPS receivers pick up the satellite signals is pretty interesting: all of the satellites broadcast their messages on the same frequency, but they each include a unique identification number. The receiver determines which message is from which satellite by matching the identification number with the ones stored in its memory. This is sort of like standing in a room with many people speaking at the same time - you can listen to what just one person is saying among all of the conversations taking place simultaneously, and you can identify a person's voice by its particular sound. In the same way, a GPS receiver picks up signals from all of the satellites in view and matches them with patterns in memory until it figures out which ones are "talking" and what they are saying. This technique allows GPS receivers without backyard-sized dish antennas to reliably use the extremely weak signals that the satellites transmit towards the earth.
Ten years ago, it would have been hard to believe that you could buy a device capable of providing your precise location anywhere on the globe, much less that it would be smaller than a frozen waffle and cost less than a new winch. In just a few years, I suspect that these technological marvels will be just about everywhere, and much cheaper - at this writing (May 1994), there are terrific handheld units with basic course plotters selling for under $500, and the prices keep going down.