Home and Book Chapters

Previously: Navigation

GPS, Global Positioning System
GPS units are incredibly powerful navigation tools. Not only do they know and show you exactly where you are, but some have topo maps of the entire United States, databases of locations of springs and towns, ocean tide predictions, pressure altimeters, compasses and many other things of interest to hikers. They can record points and routes, and these points and routes can be uploaded and displayed on other GPS units. Extra maps can be loaded. Carrying a topo map of the entire US (and much more) in just a few ounces is a sweet deal for a long distance hiker.

Cheaper GPS units and smart phones may do a lot less, and you may have to spend time and money loading maps whenever you go someplace new, but they are still quite handy.

But GPS has downsides too. In spite of decades of development, operating systems are still very difficult to figure out. Models from different companies and even different models from one company work so differently that just knowing all about one model barely helps you to figure out another model. Batteries go dead, electronics fail, and satellites cannot be read from deep canyons. The whole system is so complex that many hikers cannot figure out what the current problem is, let alone how to fix it.

I once ran into a guy hiking the PCT across the Sierra Nevada using only his smart phone for navigation. He slipped and fell into the South Fork San Joaquin River near Muir Trail Ranch. His smart phone died and he had no maps or anything else to fall back on. This just seems dumb and dangerous to me.

GPS as a whole is so unreliable that long distance hikers should not rely on it. Being lost can kill you. So long as you have books or maps, etcetera, and you know how to use them, supplementing with GPS is fine, but counting on it is foolish.

Handheld GPS Receivers
Depending on their cost and intended functions, GPS receivers may come loaded with maps and locations of water, mountains, towns, and other things of interest to hikers. With a lower cost device, you might have to buy and load this type of information for each new area you visit. Or, cheapest of all, the device may just give you direction and distance information, with no ability to load maps at all.

In spite of decades of development, devices have not become simple to use. They contain large menus of choices with little information on what it is that you're choosing. Many choices preclude your doing something else later. Choices you have carefully and deliberately made are changed without notice. To understand why the device is not doing what you want it to, you must understand all the arcane settings, and deduce which is causing your problem. It can be very frustrating when you just want to take a reading to have to dig through a bunch of menus to get it working properly again.

Before you take a gps device on a long distance hike, you should take it on many day hikes and learn all of its functions, and all of its faults.

Receiving Satellite Signals
The GPS satellite orbits are designed so that with the design complement of 24 satellites in orbit, at least 6 will be in the line of sight, above the horizon, from almost anywhere on earth at all times. As of late 2013, 31 satellites were in orbit, giving some extra coverage / redundancy. Satellite signals are blocked by the earth. So if a mountain or canyon wall is between you and a satellite, you can't receive that satellite.

GPS determines your position by calculating the distance from several satellites to you.

Since the satellite's position is known, and your distance from the satellite is known, with only one satellite, your position would be known to be somewhere on the surface of a sphere of the known radius, centered on the satellite. If you assumed you were at sea level, you would be located somewhere on the circle where the sphere of the earth intersects the sphere around the satellite. Since not all of the earth is at sea level, but rather has mountains, canyons, etcetera, the circumference of the circle would be wiggly rather than smooth. So at best, with one satellite, we're down to being somewhere along a long line; not good enough.

With two satellites, you would know you were somewhere along the circle where the two spheres from the satellites intersect. If you were at sea level, that circle would intersect the earth's surface at two places. So with two satellites, there is still not enough information to tell you where you are. And given that hikers are often well above sea level, we don't even know the position of the two intersections with the earth's surface.

Minimum of 3 Satellites to Calculate Position
With three satellites, the three spheres intersect at two points. One is near the earth's surface and one way out in space. Except for receivers made for spacecraft, the assumption can be made that your position is the point closer to the earth's surface. This is where the idea that at least three satellites are needed to derive a position comes from. Many receivers don't make the earthbound assumption and require 4 satellites to lock a position. Experimenting with and reading up on with your receiver is the only way to understand its various limitations.

In practice, more satellites allow your receiver to calculate your position with more confidence. Where satellites are very close together, only one of the cluster is useful in the calculations. Furthermore, the satellites must not all be along the same line, but widely separated across the sky, to be useful in finding position. So more than 3 or 4 satellites may need to be visible to the device to calculate position.

Deep Canyons and Terrain
So if we're behind a mountain, or deep in a canyon, and we can't lock on our position, it may be useful to look at the satellite display on the GPS receiver. Blinking satellites, or those of a different color, etcetera, are satellites the receiver believes are above the horizon, but that the device is not receiving. Likely, a canyon wall or mountain is blocking the signal. Other satellites, displayed differently, are streaming data to the unit. The display usually has an outer circle, which represents the horizon if you're standing atop the highest mountain in the area. If you're in a deep canyon, satellites shown near the horizon may be behind the canyon walls, and therefore unavailable. Satellites shown in the center of the display are straight above you. An intermediate circle represents the cone 45° off the vertical axis, or half way between level and straight up. North, south, east, and west are shown, as are the relative signal strengths of the satellites.

Use your compass, the north star, etcetera to know north. Look at the canyon walls, mountains, etcetera. The goal is to find a place where the receiver can see 3 or 4 or more widely separated satellites with strong signals. If a cliff wall or mountain rises just to the south of us, and the receiver shows unreceived satellites in that direction, moving a little to the north may bring them into view. If the cliff or hill is short enough, maybe ascending it would be a reasonable way to get the signals. We're looking for a place where at least 3 satellites, preferably more, not in a cluster, and not along one line, but spread throughout the sky, and with strong signals, are being received by the unit.

If your device can receive from GLONASS or some other satellites, in deep terrain it might be helpful to turn this feature on, as more satellites would probably be visible. GLONASS is discussed more fully later.

Almanac and Knowing Satellite Locations
Over the course of about 12½ minutes, an almanac is transmitted which allows the receiver to predict where all of the satellites are, which are in service, error correction data, etcetera. This allows the display above to be created, and allows the receiver to start listening to the satellites most likely to indicate position well right away. If the unit has not been turned on recently, or was last turned on very far away, the display will start out wrong and the unit will have to figure out that it's listening to the wrong satellites and start over. If you want your device to boot up and provide useful results quickly, turn it on often enough to keep the almanac up to date.

Calculating Locations
Handheld receivers have a cheap clock which is continuously updated by the very accurate atomic clocks aboard the satellites. The clock is not accurate enough until it has been listening to satellites and calculating for a while. If the receiver does not know its location, it does not know how long the signal travelled at the speed of light from the satellite. The signal is somewhat out of date by the time it arrives. Each satellite continuously transmits its position and time. A complex calculation is made with various assumptions, and a rough position and time are calculated. These results replace the assumptions, and the calculations are made again, and again, and again. Therefore, when the receiver first locks a position, the accuracy may be quite poor, but with succeeding calculations, the accuracy improves. If you need the best available accuracy, watch the display and wait until the position and accuracy settle.

Sources of Error
The receiver clocks themselves and the way they measure incoming signals cause errors. Signals do not necessarily travel in a straight line: atmospheric effects and earth may bend them or reflect them (echoes). This results in longer travel times, and if the signal traveled multiple paths, garbled signals. Natural and artificial signals may also garble the signals. Incorrect or out of date ephemeris / almanac data cause errors. Since a position can be calculated from 3 or 4 satellites, using more satellites allows multiple possible positions to be compared. If these are widely separated, the device can try to calculate errors and correct for them. There's little hikers can do to limit the effects of atmospheric disturbances. However, if it seems likely that large cliffs are causing echoes, or if there are powerful transmitters in some nearby antenna farm, or if you are near some other electric facility, try moving away and seeing if that improves performance.

In any case, using your GPS near powerful electric fields is a bad idea: It may permanently fry the electronics. Don't turn it on near mountaintop transmitters, substations, etcetera.

GLONASS
GLONASS is a similar system operated by Russia. As of late 2013, 28 satellites were in orbit. If your receiver can receive GLONASS, turning that ability on will about double the number of satellites available to your receiver. In a deep canyon where you can not see enough GPS satellites, this would have an obvious advantage. In the open, where you are accessing enough GPS satellites, some receivers become much slower and show degraded accuracy when GLONASS receiving is on. More power may also be drawn from your battery. Experimentation with your device is the best way of knowing when to turn GLONASS on or off.

Galileo and Other Systems
Galileo is a system being developed by the Europeans. The satellites should come on line between 2014 and 2020. I expect my comments on whether to receive GPS and Galileo simultaneously will be about the same as for GLONASS.

Various other systems are being developed and deployed worldwide. Comments on their usefulness, etcetera, will have to wait until they have been deployed and used.

WAAS, Wide Area Augmentation System
Part of the position error of GPS is caused by the receiver, and part by the satellites and ground equipment that support the satellites. WAAS is a method of subtracting out the error inherent in the satellites and supporting equipment. It does nothing to correct receiver errors.

It is expected that within a certain area, and at any given time, most of the error inherent in the satellite system is the same distance and the same direction. Therefore, for example, if you knew that in a certain area, the signals indicated you were 20 feet northwest of your actual location, you could compensate for that error by knowing where the signal said you were, and knowing that you were 20 feet southeast of that spot.

In the United States, Canada, and Mexico, there were 38 Wide Area Reference Stations (WRS) as of 2007. These have permanent antennae at known locations. Since the location is known and permanent, when the GPS receiver there receives a signal that says the station is a little bit away from the actual location, the station knows the error. Any data arriving at these locations which would cause a calculation to show a position error are noted. Correction factors are sent to geostationary satellites, and retransmitted to earth.

Since the Wide Area Reference Stations are observing the errors in North America and Hawaii, and the satellites are above the equator in the longitudes of the Americas, the system should work in North America. In South America, you could receive the satellite signals, but without nearby reference stations, the signals would be useless. Elsewhere in the world, there would be no satellite signal and no useful data. If you were at the base of a north facing cliff, you could not receive a signal from a satellite above the equator. In far northern latitudes, the line of sight to geostationary satellites is low, close to the southern horizon. In hilly northern terrain, WAAS reception may not be possible.

Without WAAS, the typical error of GPS, not including errors due to the receiver, is about 8' (2.5m) sideways and 15' (4.7m) vertically. With WAAS, again not including errors due to the receiver, it is about 3' (0.9m) sideways and 4' (1.3m) vertically.

So, if your GPS unit can receive WAAS, should you turn it on? Usually, if a hiker knows where he is within a few hundred feet, he can see what he's looking for and that's good enough. As discussed in the navigation chapter, only once in 28,000 miles of hiking did I need to be within 5 feet to see a water source. I easily found that by walking around a few minutes. Likely, the position error caused by your cheap GPS receiver is far worse than the WAAS correction. But more accuracy is better. The question is whether turning it on causes problems. The answer can be found by playing with your receiver with WAAS on and off. If turning it on slows the GPS down, eats the batteries, or causes other problems, maybe it's not worth it. If you're not in North America, it does no good.

EGNOS, European Geostationary Navigation Overlay Service
EGNOS is similar to WAAS, except that the reference stations are spread throughout Europe and the geostationary relay satellites are south of Europe. If you're in Europe, maybe it will improve your accuracy a bit.

Other similar systems will probably eventually be deployed elsewhere in the world, and show up as options on receivers.

Loading Files in GPS Receivers
There are many types of files and maps available commercially, online, and from other hikers. Various mapping programs and utilities like GPSBabel help users load files into GPS units, or convert the files to other formats. Each of these programs has a limited set of functions. If you want to do something they don't do, there's still the possibility of doing it yourself.

First, try the canned programs. If you're satisfied with the results, there's no need to dive in deep with the stuff below.

Now you'll need to look at the data you want to convert. Many files can be viewed in text editors and spreadsheets. If you see a bunch of weird symbols, the file is probably compressed (.gpi, .kmz, etcetera). Use some decompression software, and the resulting files will probably be readable. You'll need some familiarity with data hacking. Assuming that you can get columns of latitude and longitude in a spreadsheet, you could make a row looking something like this:

You would fill all the rows with lat and lon with the text and formula. The spreadsheet may take out the qoute " marks as you paste in the xml tags, so you may need to put them back in. Then, when the point rows have been calculated (sixth column), you can copy and paste them to a text editor, add your various headers and footers, and save your finished file as text, and with the appropriate file suffix. The concatenating formula in the sixth column was necessary for me because copying and pasting the first five columns resulted in tabs in the files, which my GPS did not like. The result of the formula in the sixth column would be one typical row of a track file:

<trkpt lat="32.589" lon="-116.466"></trkpt>

As stated previously, with some devices, the files must be in a specific directory to work. With files you can make on the device, like points or tracks, just make a file and find it in the device to reverse engineer it. But if you have a map file, etcetera, and you can't figure out what directory it goes in, research on line may help. And you may need to create and name the directory if it does not already exist in the device. To store files on removable memory cards, recreating the same directory structure on the card as is in the receiver may be necessary.

If the data seems screwy, try loading it into a mapping or GIS program. There may be wild points or discontinuous line segments. Or maybe you've swapped lat and lon. Seeing the data over a map may illustrate the problem.

Conclusions:
• GPS devices are powerful tools and fun toys for Long Distance Hikers.
• GPS has all kinds of screwy flaws that make it unreliable.
•• Therefore, always be navigating by some other (reliable) method.
• Non TechnoGeeks will have a hard time figuring out what screwy thing has disabled their GPS.

The Future of GPS:
• I hope GPS devices become much simpler to use.
• I hope that hiker GPS units come preloaded with more types of maps - Like satellite photos, land ownership, etcetera.
• I hope the ancient, out of date trails in the topo maps will be updated to their current locations, status, etcetera.
• I hope that like with road routing in gps for cars, hikers will be able to select, highlight, and measure routes on the GPS using trails already in the topo maps.

Next: Public Land Survey System

Home and Book Chapters