GPS 101

Hi again,

This is the second of five pages on Intro to Geocaching.

Here are the other four.

Geocaching 101

Geocaching with Smart Phones

Geocaching with Handheld GPS

Geocaching Software

I thought it would be useful to give you some background on the GPS system which we’ve all attached our lives to. All GPS devices operate under this system – smart phones, geocaching devices and Predator drones to name a few. Understanding a little bit about the GPS system itself will help you get the most out of your devices.

GPS is an American system that has been around since the late 1980’s. It consists of 24 satellites (called a constellation) in Medium Earth Orbit between 10,000 and 11,000 miles up. In contrast, the International Space Station is only about 230 miles up. The GPS satellites (and most other communications birds) are that high because 1) they get a direct line of sight (LOS) with a bigger portion of the earth’s surface and 2) gravity doesn’t suck them down to burn up in the atmosphere, making them steady and predictable. Most of the satellites are in an equatorial orbit between 55 degrees north latitude and 55 degrees south latitude. Except for the polar regions, coverage is worldwide but is always subject to the whims and restrictions of the local natural environment.

GPS Satellite Constellation

A depiction of the GPS constellation and its six planes. They run between zero and 55 degrees north and south latitude. If you are in the northern hemisphere, you get the best satellite signal by orienting your device to the south.

The Russians have an equivalent satellite system called GLONASS. It isn’t compatible with most of our handheld devices just yet but that day is coming. High end GPS devices are already GLONASS-compatible. The Russian system works the same as ours but has fewer satellites and has fallen into disrepair over the years. However, they are said to be working hard to correct that and may already have.

Other countries such as China, Israel, India and Japan have got systems in the works, as do several middle eastern countries. It’s getting real crowded up there.

Your position is determined with GPS by a process called triangulation. GPS satellites are active systems. They are constantly broadcasting their position, time and identification. The GPS receiver, on the other hand, is a passive system. All it does is listen for those satellite signals. When it picks up a signal, it basically draws a line from it to the satellite. That’s called a vector. It takes at least three vectors with three different satellites to establish a position on the ground. Your position is where the three vectors meet. A fourth one is needed for altitude. Once that initial position is determined, the process becomes an endless loop of time hacks, vectors and converting results to position, speed, direction, etc. That’s what all GPS receivers do – some better than others. All GPS-enabled devices use this system and work this way.

Ground stations are also part of the system. Called WAAS Stations for Wide Area Augmentation System, they help remove the distortion from the GPS signals caused by interference and degradation, both natural and man-made. They also amplify the signal. They were originally implemented for aircraft navigation but many technologies have integrated WAAS. Although we don’t need them to get a navigational fix, it’s the WAAS Stations that give us our pinpoint accuracy. One of the things you should look for in a GPS device is WAAS integration. It will be faster, more accurate and more expensive.

The single most important factor in GPS reception is the aforementioned “Line of Sight (LOS)”. For the GPS receiver to pick up and maintain a signal, it must have an unobstructed 11,000 mile path to the satellite. The single biggest obstacle to this is the curvature of the earth, but that is pretty much taken care of with the satellites up so high. Buildings, terrain, heavy overhead cover, atmospheric conditions and a host of other factors can influence GPS reception. Likewise, hardware and software inside the device can materially affect the results. As I said in the first post, there is definitely a quality spread among GPS devices, especially phones. In fact, given all that can go wrong, it’s amazing that the system works as well as it does – and it does work extremely well. It has proven reliable and resilient beyond all original expectations and it keeps getting better.

GPS Triangulation

A graphic showing triangulation. At any given time, any point on earth (except for the polar regions) can “see” at least four satellites. This will give you a position with latitude, longitude and altitude.

When GPS first came on line in the late 1980’s, it was restricted to the military and other government functions, such as air traffic control. Its first big test was Desert Storm and it passed with flying colors, enabling the famous Hail Mary end run that caught the Iraqi Army flat footed. Moving at night through the Empty Quarter of the Saudi desert would have been impossible without GPS.

During those early years, there were civilian GPS receivers but by design, they were not allowed to receive the unfiltered GPS signal. They threw in a fudge factor that essentially made the system useless. That changed on May 2, 2000, when the fully functional GPS system was opened up to everybody. The first geocache was placed the next day as a test of the new system by a GPS enthusiast and here we are 12 years and 2 million geocaches later.

GPS Orbits

This shows the pattern of orbits by the U.S. GPS System. There are 24 satellites placed in six different orbital geometric planes – four per orbit. Each satellite circles the earth twice a day and they all transmit their signals together, synchronized by on-board atomic clocks. The six colors in this picture represent the six orbits. The only regions not thoroughly covered are the polar ice caps.

So now that you know a little about GPS, here’s a few tips to help you in the field.

Keep the device flat as much as possible. The antennae are oriented to receive signals from satellites, so you want maximum surface facing the sky. If you point it up in the air, most of the antennae surface is useless.

To steady up a compass, you stand still. GPS is just the opposite. To steady up a reading, keep moving around. GPS corrects itself by detecting changes in speed and direction – even small ones. If you stand still, the GPS device essentially takes a break. Keep up some sort of motion, even if it’s just moving the device around a little bit.

If you use the device at one location, then travel over 100 miles before you turn it on again, use the reset function in the menu to tell the device where you are. Unless you tell it otherwise, it assumes you’re still at the place where it got turned off. If you are some distance away, it can take quite a while for the GPS fix to come up. Resetting the device can speed that up considerably.

Many people starting out have an expectation that the GPS will take them straight to the cache and when it gets to zero, there it is. Not so. GPS on its best day will get you within about 10 feet of the coordinates. You’ll probably never see zero on the display. If you do, it won’t stay there long. If you get a steady single digit reading, you’ve arrived. Then you put on your Sherlock Holmes and Indiana Jones hats and start looking. A circle with a 10 foot radius is an area of over 300 square feet. If you’re fighting bugs and bushes, it can seem like 300 thousand. That’s the fun of geocaching.

Learn to be flexible and use your equipment to its strengths. You’ll probably end up doing what we do. Depending on the circumstances, we use both our Droids and our Garmins, sometimes both at once. For local caching and targets of opportunity, we do a search on the Droid and go hunting, knowing that the GPS function isn’t quite as robust as the Garmin. On the other hand, if we’re planning a day of geocaching, we’ll do a search, filter the results then load up the Garmin. This is especially true if we’re on the road or in the boondocks and won’t be back soon.

There are lots of hardware and software options for your geocaching device. Those are addressed on other pages that are posted along with this one.

Cheers … Boris and Natasha

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