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An Introduction to GPS
By Shaun Keenan, May. 08, 2009
Nobody likes to get lost. Let alone when you're miles from anywhere – way out in the boonies with a minivan full of tired and hungry kids yelling every eight seconds “Are we there yet? How much farther is it? Are we there yet? I have to go p...” Strike a nerve, have I?

Face it, long have you dreamed of the day when you can (after safely pulling over to the shoulder and parking your vehicle) turn around and say... “Actually, we're exactly 11.2 miles from our destination with an estimated time of arrival in 15 minutes at our current velocity. Barring unforeseen circumstances such as urinating on your brother, we will be at grandma and grandpa's house soon.”

Well, you can!

WHAT IS GPS?

Modern GPS technology has roots dating back to Cold War times. In fact, the U.S. Department of Defense (D.O.D.) began its development in 1973 for military purposes, launching its first Navigation Signal Timing and Ranging Global Positioning System (NAVSTAR GPS) satellite in 1978.

Before becoming operational in 1993, U.S. and allied troops relied heavily on experimental GPS technologies to navigate the vast barren Arabian Desert during the first Gulf War. And, after more than two decades of research, development and execution, the NAVSTAR GPS network constellation met its “fully operational” requirements in the spring of 1995. It continues to play a significant role in delivering real-time military intelligence for forces operating anywhere in the air, on the ground and on the water.

Master control falls to the U.S. Air Force's 50th Space Wing, which is responsible for monitoring and controlling two dozen satellites that speed around the Earth at over 12,550 mph on six different orbits every 12 hours. The massive amount of data is transmitted, collected, filtered and distributed around the clock by five ground-based monitoring stations and four antennas located around the world.

The prime directive for the NAVSTAR GPS network is to provide positioning, navigation, timing and velocity information worldwide. On top of countless homeland security and international defense applications, there are many commercial and civilian uses for GPS navigation ranging from search and rescue, surveying, exploration and cartography to resource tracking, vehicle location systems, for example.

HOW GPS WORKS

Though there are countless uses for GPS technology, the fundamentals are pretty much the same. Before understanding how it all works, it's important to distinguish between the two levels of service GPS provides.

The first is an encoded Precise Positioning Service (PPS) that's intended for D.O.D. use and available to authorized users only. There is also a Standard Positioning Service (SPS) for general public use that is free for all peaceful civil, commercial and scientific use on a perpetual worldwide basis.

Coverage is global and both services run 24 hours a day, 365 days a year. The main difference is that SPS signal accuracy is intentionally degraded to protect U.S. national security interests through a process called selective availability that “controls the availability of the system's full capabilities.”

As a result, SPS is only accurate to within 100 meters horizontally, 156 meters vertically and 340 nanoseconds. That is, SPS is 95 percent accurate with a position dilution of precision (PDOP) of six or less whereas the PPS service is more like 99.9 percent accurate.

CIVILIAN GPS DEVICES

The GPS network is designed to provide positioning, navigation, timing and velocity information for users at any time, any place.

To take advantage of this free worldwide navigational aid, you'll need a device with the ability to receive, decode and display GPS information so it can be put to use. The only other prerequisite in determining your location and other data (accurately), is to have an unobstructed wide open view of the sky.

The reason for this is because GPS devices need to be able to acquire signals from at least four of these satellites in order to give an accurate 3D fix complete with average speed, directional heading and elevation. Fortunately, the NAVSTAR GPS network is designed to ensure there are no fewer than six birds in the sky that can be reached at any given point in time with four more on standby.

GPS receivers constantly recalculate this data to tell you precisely where on Earth you are relative to the position of the satellites above, allowing you track your current whereabouts, heading speed and even altitude in real-time.

The GPS device compares this latitude, longitude and other precise information to known map coordinates, which either came preloaded on the device itself or are stored/accessed via some other kind of storage media such as CD, DVD or HDD (hard disk drive).

Most consumer GPS devices communicate with these satellites using a 12-channel receiver and antenna. An onboard CPU is necessary to process and display this data. The speed of the processor along with the quality of the receiver and quality of the signal itself all have an impact on how long it takes to acquire a solid fix.

Every time you turn on a GPS device it goes though a process called cold-starting, which can take up to several minutes to run through startup tasks and acquire a strong signal with the satellites. In the event the signal is lost or interrupted, it usually takes just a few seconds to reestablish a connection with the satellites after that. This can happen when you drive through a city with many tall buildings, heavily-wooded areas and/or through a tunnel, for example.

Good quality receivers should be able to reconnect almost instantaneously while weaker units may need more time to reacquire a signal. A clear view of the sky is important when it comes to GPS navigation. That said, the location of the antenna has a big influence on how well it works.

Ideally, the antenna should be as close to the front of the vehicle as possible with as much of the sky in view as possible. Most factory-installed in-dash navigation systems will use either a dash- or roof-mounted antenna.

Mounting consideration must be given to portable units so as to ensure their effectiveness. Suction cup type mounting brackets are fairly common, but any type of mounting solution that will allow a wide sky view through the front windshield is ideal. Some units may have a built-in antenna while others may use external antennas that may need special care and/or further installations.

In general, users should avoid mounting a GPS device down low on the center console since the signal quality will be significantly lower here in the bowels of the interior than up near the windows.

We've just finished covering the basics of GPS navigation here, so let's move on to the different types of systems that are available and also look at some of the important features.