Electronic Design

What's All This GPS Stuff, Anyhow?

The thing that just got me turned on to GPS----the Global Positioning System----was a recent advertisement I saw in an electronics magazine. "You'll never get lost when you carry the Model X GPS global positioning system from our company." The advertisement also showed a nice map of Trinity Lake and Weaverville, Calif., 200 miles north of San Francisco. I know this because I own an exact duplicate of the map1, a very nice map.

Yet in the advertisement, the Model X GPS receiver indicates on its LCD display that the position is 35º00´ North, and 117º52.3´ West.

That just happens to be 250 miles southeast of San Francisco, down by Muroc Dry Lake and Edwards Air Force Base----about 425 miles from Trinity Lake. So I was a little piqued at why these guys were bragging so much about their particular GPS receiver.  ".... never get lost...." ---???

I've been interested in Global Positioning System machines for a couple years. My son Benjamin is interested in building trails in the hills around here, and I'm interested in building trails, and hiking on them, too. After he and his colleagues have built a good trail, he likes to make a map so people can see where the trail goes. I suggested that a GPS machine might be helpful when he's gathering the basic information for the map, rather than the old method of using compass and measuring wheel. He was skeptical, and properly so.

Recently, the prices AND sizes of Global Positioning System receivers have dropped rapidly. A machine with capabilities that would cost several thousands of dollars and fill up 8 inches of rack space just a few years ago now weighs 10 or 20 ounces and costs somewhere around $500. The computer revolution really is applicable here. That's because the receiver not only has to include a compact high-gain receiver at 1227.6 MHz and at 1575.42 MHz with sub-microsecond resolution in real time, it also must include a versatile computer to compute, by spherical geometry and triangulation, exactly where you are on the surface of the earth (or above it). And the elevation, too, if you give it a little extra time.

It's pretty impressive to watch a little computer receive coded information from three or more of the 24 GPS satellites that are always whizzing by at 27,000 ft. per second at an elevation of 15,000 miles, and then compute for you exactly where you are located.

A year ago, a group of us were hiking up a trail in Yosemite Park. After a few hours, we deduced that we were about 1/4 mile east of Edson Lake, where we wanted to camp for the night. But we could not tell where it was, exactly----we could not see it, and we could not be sure if it was WSW or W or WNW of where we stood. And there was no trail to use. So we sent out three scouts. After half an hour, all three scouts returned, and admitted they had not found the lake.

We sent out more scouts, and within an hour we found the lake. What a silly waste of time! And it's a wonder that nobody got hurt, thrashing through the underbrush just to find where we were, with respect to a lake that was not lost at all. It had never been any place but right where it always had been....

 So as the time for our annual backpack trip approached, I decided to buy a GPS receiver to help us with any difficult navigational tasks. I went to West Marineand checked out the two lightest, smallest receivers. The Garmin GPS-45 was the lightest----about 9 oz.----but the Magellan Meridian seemed to have the right features, and at 14 oz., it cost $100 less than the GPS-45 (about $411 including tax). Now, I'm often willing to pay a reasonable premium for lighter backpacking equipment. But paying $20 per ounce saved seemed a bit steep. So I bought the heavier but more reasonably priced Meridian. I slapped some batteries into it, and started on our backpacking trip.

First of all, let me explain a new deal to those of you who haven't worked in this field. Of course, every USGS topographical map carries full information in degrees, minutes, and seconds. By using some interpolation with a scale or piece of folded paper, you can tell where the map says you are. Standard navigation. BUT, the user's manual on the GPS receiver explained the new UTM (Universal Transverse Mercator) grid. All recent USGS maps show where you're located on an arbitrary grid----a 10,000-meter grid----which is drawn on the maps.

It's a LOT easier to interpolate on this grid, because the grid lines are only an inch or two away from any location. So I've done most of my computations and estimates on the UTM scale. It does seem to be quite convenient, and I'll recommend it. But if you're using old maps, you lose this convenience.

Obviously, with any new system, you try to calibrate it. So, at the trail-head, I sat down at a well-defined location and turned on the receiver. After a couple minutes, the receiver said that I was at a location, which grew better-defined after another minute. But this location was up on a rocky hillside, 100 yards north of where the map said we were.

A few hours later, we sat on the north shore of Golden Trout Lake. I turned on the GPS receiver----and it said we were 100 yards away, on the south shore of the lake. About 10 times during the hike, I tried to get a correlation between the map's location and the GPS's location. Every time, the GPS had a disagreement of 100 yards north (or south) of where the map said we were (one time, it was 100 yards east).

Well, I knew that the basic military system accuracy of the GPS was very good, but for commercial uses, there were errors added by "Selective Availability," so it could not be used at full accuracy. I hadn't expected the error to have a constant magnitude and a random angle. But in retrospect, it made perfect sense. I mean, if the error was random in magnitude, one could take a large number of readings, and average them out, so as to get low errors. If the error had a constant magnitude, but a random direction, as I was seeing, it would be hard to average or cancel out the error. I also learned a few more interesting things about GPS....

One day, the "Battery Low" flag displayed on the LCD display. I decided to leave the battery pack disconnected, so I could not turn on the receiver by accident. When I turned the machine ON a couple days later, it had forgotten all of the info I had keyed into it. I thought there must be some EEPROM to save all of the info while you change your battery. Wrong.

I guess if you bought the most expensive machine in sight, you might get that kind of feature. I presume it will remember the data for a few minutes while you change batteries. It probably has a 0.1-farad capacitor to retain power to the CMOS RAM----and the instrument's main timebase.

On the way back from our trip, I tuned in the receiver and we got rolling on the road. With 55 mph indicated on the speedometer, the GPS said we were going 53.5 mph, which was exactly the calibration error I had expected on my speedometer. The receiver is rated to be able to tell your velocity at any speed from 2.0 mph to 951 mph.

Let me also say that I'm not a sailor, so I'm slow to appreciate all of the nice navigational features that this machine can do. I DO, however, appreciate that when you're out on the ocean, and there are no landmarks, a GPS receiver would be very helpful to get you where you're going.

I ALSO appreciate that one should not rely solely on the GPS receiver for navigation----if you were counting on the GPS machine, and you ran your last battery down, and you left your sextant home, you might be in REAL TROUBLE. So Magellan Corp. was quite correct in cautioning you not to rely solely on the Global Positioning System.

After all, when we had been hiking four days, my little receiver said that we were just 7.8 miles from our trail-head, due east. But in actuality, we had to go 18 miles to get back to the trail-head, because of the wall of cliffs that stood in the way. Our maps were quite valuable in showing us that we could not reasonably or safely get up those cliffs. A straight line may be the shortest distance between two points, but it's not always the best way to get there.

Another time, we had been debating----which of those mountains over there is Mount Starr King? I thought it was THIS one, and other people insisted it was THAT one. If I had my GPS machine, I could have keyed in the location for three mountains, and the machine could have given me the compass bearing of all three. We could have proved which of the mountains was the one we were looking at. So that's a useful kind of feature.

I'm going to start petitioning my Senators and Representatives to convince the Air Force to turn off the Selective Availability----the code that will let only military users of GPS achieve full accuracy. Since the Russians and our military people aren't targeting missiles at each other any more, it's silly to crank a purposeful error into the system, especially during peace time. Besides, the Russians are already claiming that their GPS system has better accuracy than our system, so long as we refuse to turn off the error-causing code....

While my GPS receiver isn't yet a precision machine, and while I haven't yet found it highly useful, I think it will become useful in the future----NOT a bad investment. After all, if I aim for a lake that is 200 yards long, I'm pretty sure to be able to find it with the help of a GPS machine, even if its accuracy is just +/-100 yards.

All for now. / Comments invited!  RAP / Robert A. Pease / Engineer

Address:

Mail Stop D2597A

National Semiconductor

P.O. Box 58090

Santa Clara, CA 95052-8090

References:

1. Northern California Atlas and Gazetteer, by DeLorme Mapping Company, Freeport, Maine; phone: (207) 865-4171. This map covers all of Northern California at a scale of 2 miles per inch, with nice resolution. Also available are maps for Florida, Maine, Michigan, Minnesota, New Hampshire, New York, Ohio, Pennsylvania, Southern California, Tennessee, Vermont, Virginia, Washington, and Wisconsin. Coming soon: Illinois. The price is about $13.00. The only minor drawback is that the elevations and contours are listed in meters ....

2. West Marine is located at 850 San Antonio Rd., Mountain View, Calif., just off Bayshore. Also at 45 other locations in the U.S. You can order by phone from 1-(800)-538-0775, or (408) 728-4430. Ask them for a catalog, which has much useful technical info. (No, you cannot buy GPS machines at Fry's.)

 

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