Raymond said:
Also regarding that article, did you see the photo of Larry Garland? What is that big yellow thing he’s holding in his hand? Some kind of extra-fancy GPS receiver? It’s connected to the toilet plunger-shaped thing sticking up from his back? One of the guys I met last September on Black Nubble had one of those same things with him; they were mapping the road they thought they were going to be building up there.
The "toilet plunger" shaped thing is a choke ring antenna. It is better at rejecting multipath (reflected) signals than are the simpler (and smaller) antennas used on typical hiking GPSes. It is a common antenna for professional applications. Note the brand--Trimble is a manufacturer of professional GPS systems.
http://www.trimble.com/l1l2chokering.shtml
The yellow object in Garland's hand is a GPS receiver, likely also from Trimble. I do not know the Trimble line well enough to recognize the model, but it looks similar to the Trimble Ranger:
http://www.trimble.com/ranger.shtml
Mohamed Elozy said:
Probably the extra receiver for differential GPS. The first AMC maps with GPS trails were punlished (in 1988) before SA was turned off (May 1st, 2000), so DGPS was essential for reasonable accuracy.
The yellow object is unlikly to be a DGPS receiver (as noted above, it is most likely the GPS receiver itself). If you want high accuracy, a better method is post-processing. (You record data in the field and combine it with corrected satellite data to get accurate locations. Navigation requires real-time locations, but trail mapping does not so post-processing would be appropriate.)
As you noted, DGPS was used to remove the SA (selective availability)*, but it requires a nearby reference station which broadcasts the reference info. (DGPS was a Coast Guard program to improve accuracy near ports. DGPS reference stations may or may not be available for inland locations.)
Unfortunately, "DGPS" is both the name of the afore mentioned Coast Guard program and the name of a general technique. DGPS (differential GPS) in general is a technique for obtaining high accuracy relative locations between two nearby GPSes, even though the absolute location is only approximately known. (If the location of the reference GPS is known to high accuracy, then the location of the rover can also be found to high accuracy.) This can be done in real-time or by post processing. There are many forms of DGPS in use--WAAS (an FAA program for aircraft use) is one.
* SA intentionally introduced timing errors to degrade accuracy. It was turned off by presidential order on May 1, 2000. It is
very unlikely that it will ever be turned on again. Accuracy with SA was on the order of 40 meters, without on the order of 10 meters for the L1 C/A mode used in most hiking GPSes.
Doug
