Geocaching: The High-Tech Scavenger Hunt

When the U.S. government turned off GPS Selective Availability (SA) in May 2000, it was like magic. Suddenly civilian GPS receivers that were formerly accurate to about 300 feet were accurate to 30 feet. This level of accuracy offered some creative possibilities. Three days after SA was turned off, a message appeared in the sci.geo.satellite-nav USENET newsgroup proposing a worldwide stash hunt, where people would post GPS waypoints on the Internet to lead searchers to hidden goodies.

Starting with a humble little bucket full of goodies in Oregon, the game took off like wildfire. Within weeks, caches were hidden in Washington, Kansas, California, New Zealand, Australia, and Chile. A newsgroup and Web site that hosted the coordinates of the stashes soon popped up as the word started to get around.

Today, geocaching has grown popular, and the rules are still pretty much the same: Take some stuff, leave some stuff, record it in the logbook, and have fun! Relatively cheap and accurate GPS receivers and widespread Internet access have helped the sport flourish. As of November 2003, the http://www.geocaching.com (currently the largest geocaching site on the Net) had over 72,500 active caches in 188 countries listed in its database. That's a lot of caches out there to find!

Sounds pretty intriguing, doesn't it? But before you can try it out, you need a few things. You probably already have many pieces of the required gear, and your biggest investment will be a GPS receiver if you don't already have one. With that in mind, here's a list of basic things you need.

• Cache location: Obviously, you need to know where to look for a cache: a set of latitude and longitude or UTM (Universal Transverse Mercator) coordinates. You'll find tens of thousands of caches freely listed on the Internet.

• Geocaching alias: Most people who geocache use a registered handle (alias) instead of their real name when they sign cache logs or make Internet posts. The aliases are cool-sounding names like Navdog, Wiley Cacher, or Moun10Bike. Be imaginative and come up with an alias that fits your personality. The aliases are all unique: If you try to register an alias on one of the popular geocaching Web sites and someone else already has registered the alias, you need to select another name.

• GPS receiver: You can certainly find caches by using only a map and compass, but it's sure a lot easier when using a GPS receiver. You don't need an expensive GPS unit with lots of whistles and bells to geocache; a basic model around or under $100 will work just fine; receivers that support WAAS (Wide Area Augmentation Service) usually are more accurate than those that don't.

Don't forget to bring the GPS receiver user manual, especially if you just purchased your receiver and are still trying to figure out how to use it.

A few other things can make your outing a little more enjoyable:

• Map and compass: A fair number of geocachers use only their GPS receiver to get them to a cache, but a good local map of the area can be very helpful. Although a receiver can lead you directly in a straight line to cache, it's probably not going to tell you about the river, deep canyon, or cliffs between you and the cache. Even GPS receivers that display topographic maps often won't show enough detail that can help or hinder you on your way to a cache. Additionally, a map and compass serve as a backup just in case something goes wrong with your GPS. (Just make sure you know how to use them.)

• Pen or pencil and paper: Carry a small pad of paper and a pen or pencil for taking notes about your route or things that you see on the way. Some geocachers keep an ongoing journal of their adventures, and you never know — you might turn into a geocaching Hemingway.

• Something to leave in the cache: When you locate a cache, you'll find all sorts of swags, which are treasures other people have left. Don't expect diamonds, gold bullion, or Super Bowl tickets. (You're far more likely to find baseball cards, costume jewelry, or corporate marketing giveaways.) Just remember that one man's trash is another man's treasure. The best things to leave in a cache are unique, out-of-the-ordinary items (such as foreign coins, fossils, exotic matchbooks, or anything that has a high cool factor). And, please, avoid leaving McToys, geocaching lingo for junk that you reasonably expect to find with fast-food kid's meals.

• Appropriate clothes and footwear: There are no geocaching fashion police, so wear clothes that are comfortable, weather appropriate, and suitable for getting dirty. Even if it's the middle of summer, it's not a bad idea to bring along a jacket in case of an unexpected rain shower or drop in the temperature. Also, make sure you're wearing sturdy and comfortable footwear if the cache is outside an urban area. High heels and silk pants generally aren't recommended.

• Food and water: Some caches take all day to find, so be prepared with enough food and water to get you through your search; you can even plan a picnic lunch or dinner around your outing.

• Walking stick/trekking poles: If the terrain is really rough, a good walking stick or set of trekking poles can make life much easier when going downhill and negotiating uneven surfaces. A stick or a pole is also useful for poking around in rock cracks looking for a cache, just in case there's a creepy-crawly inside.

• Digital camera: Although definitely not a required piece of geocaching gear, a number of cachers tote along a digital camera to record their adventures or to post pictures on the Web.

• Small pack: It's much easier to put all your geocaching gear in a small daypack rather than stuffing your pockets full of stuff.

Don't forget a few safety-related items. As the Boy Scouts say, be prepared:

• Flashlight: This is a must-have for looking in cracks and crevices where a cache might be hidden — and also in case you run out of daylight. If you're smart, your flashlight uses the same type of batteries as your GPS receiver, giving you even more spare batteries.

• Cellphone: You probably have a cellphone, so bring it along (preferably with the battery fully charged). But don't fall into the trap of assuming cellphone as an absolute insurance policy against trouble. They can fail, cellphone batteries go dead, and you might have really bad cell coverage out in the middle of nowhere. So although a cellphone is great to have along, be prepared to take care of yourself!

• Spare batteries: Bring them along for anything that uses them. (Your GPS receiver and flashlight — and if you're really safety conscious, your cellphone, too.)

That's the basic gear you need for geocaching. The whole key with gear lists is to find out what works best for you. You'll probably end up carrying too much stuff in your pack at first. After you've geocached for a while, check your pack and see what you're not using so you can lighten your load.

Most geocaches are located in pretty tame, civilized areas (usually 100 feet or so off a main trail or road), but still let someone know where you're going, when you'll be back, and what to do if you're late. Twisted ankles and broken-down cars seem to happen a lot in areas without cellphone service. If you haven't spent much time in the great outdoors, check out a list of The Ten Essentials (which has been expanded to 14 over the years) at Backpacking.net.

Getting into GPS, How GPS Works

GPS stands for Global Positioning System. A special radio receiver measures the distance from your location to satellites that orbit the earth broadcasting radio signals. GPS can pinpoint your position anywhere in the world. Pretty cool, huh? Aside from buying the receiver, the system is free for anyone.

You can purchase an inexpensive GPS receiver, pop some batteries in it, turn it on, and presto! Your location appears on the screen. No map, compass, sextant, nor sundial is required. Just like magic. It's not really magic, though, but has evolved from some great practical applications of science that have come together over the last 50 years.

How GPS works

The intricacies of GPS are steeped in mathematics, physics, and engineering, but you don't need to be a rocket scientist to understand how GPS works. GPS is composed of three parts:

• Satellites

• Ground stations

• Receivers

Eyeing satellites

In GPS jargon, a satellite is the space segment. A constellation of 24 GPS satellites (21 operational and 3 spares) orbits about 12,000 miles above the Earth. The satellites zoom through the heavens at around 7,000 miles per hour. It takes about 12 hours for a satellite to completely orbit the Earth, passing over the exact same spot approximately every 24 hours. The satellites are positioned where a GPS receiver can receive signals from at least six of the satellites at any time, at any location on the Earth (if nothing obstructs the signals).

A satellite has three key pieces of hardware:

• Computer: This onboard computer controls its flight and other functions.

• Atomic clock: This keeps accurate time within three nanoseconds (around three-billionths of a second).

• Radio transmitter: This sends signals to Earth.

GPS satellites don't just help you stay found. All GPS satellites since 1980 carry NUDET sensors. No, this isn't some high-tech pornography-detection system. NUDET is an acronym for NUclear DETonation; GPS satellites have sensors to detect nuclear-weapon explosions, assess the threat of nuclear attack, and help evaluate nuclear strike damage.

The solar-powered GPS satellites have a limited life span (around 10 years). When they start to fail, spares are activated or new satellites are sent into orbit to replace the old ones. This gives the government a chance to upgrade the GPS system by putting hardware with new features into space.

GPS satellites transmit two types of radio signals: C/A-code and P-code. Coarse Acquisition (C/A-code) is the type of signal that consumer GPS units receive. Precision Code (P-code) provides highly precise location information. P-code is difficult to jam and spoof. The U.S. military is the primary user of P-code transmissions, and it uses an encrypted form of the data so only special receivers can access the information.

Covering ground stations

Ground stations are the control segment of GPS. Five unmanned ground stations around the Earth monitor the satellites. Information from the stations is sent to a master control station — the Consolidated Space Operations Center (CSOC) at Schriever Air Force Base in Colorado — where the data is processed to determine each satellite's ephemeris and timing errors.

An ephemeris is a list of the predicted positions of astronomical bodies such as the planets or the Moon. Ephemerides (the plural of ephemeris) have been around for thousands of years because of their importance in celestial navigation. Ephemerides are compiled to track the positions of the numerous satellites orbiting the earth.

The processed data is sent to the satellites once daily with ground antennas located around the world. This is kind of like syncing a personal digital assistant (PDA) with your personal computer to ensure that all the data is in sync between the two devices. Because the satellites have small built-in rockets, the CSOC can control them to ensure that they stay in a correct orbit.

GPS receivers

Anyone who has a GPS receiver can receive the satellite signals to determine where he or she is located. Consumers can buy practical GPS receivers in sporting goods stores. They're easy to use and are mostly targeted for recreational and other uses that don't require a high level of location precision. The Big Three manufacturers in the consumer GPS market are Garmin, Magellan, and Lowrance. Consumer GPS receivers are reasonably priced, from less than $100 to $400 in the U.S.

How accurate is a GPS receiver?

According to the government and GPS receiver manufacturers, expect your GPS unit to be accurate within 49 feet (that's 15 meters for metric-savvy folks). If your GPS reports that you're at a certain location, you can be reasonably sure that you're within 49 feet of that exact set of coordinates.

GPS receivers tell you how accurate your position is. Based on the quality of the satellite signals that the unit receives, the screen displays the estimated accuracy in feet or meters. Accuracy depends on

• Receiver location

Obstructions that block satellite signals

Although survey-grade GPS receivers can provide accuracy of less than two centimeters, they are very specialized and expensive, require a lot of training, and aren't very portable. Their accuracy is achieved with DGPS and post-processing collected data to reduce location errors. The average GPS user doesn't need this level of precision.

Clouds, rain, snow, and weather don't reduce the strength of GPS signals enough to reduce accuracy. The only way that weather can weaken signals is when a significant amount of rain or snow accumulates on the GPS receiver antenna or on an overhead tree canopy.