GPS Trivia Quiz

Answer: Global Positioning System

To the US Department of Defense, which is responsible for maintaining the system of GPS satellites, GPS is known as NAVSTAR, for NAVigation Satellite Timing And Ranging.

Answer: 1978

GPS was originally developed for military use. An executive decree in the 1980's made it available to the general population. The development of small, cheap microprocessors in the 1990's led to the small, cheap GPS units you can buy today!

Answer: 24

There were 21 active satellites and 3 operating spares in the original system set up by the US Department of Defense. The GPS satellites orbit the earth at an altitude of 12,000 miles. They travel in one of six orbits, all inclined 55 degrees relative to the equator, and are spaced 60 degrees apart.

Their orbital period is 12 hours. The full set of satellites became operational in 1994.

Answer: 3

A signal from one GPS satellite will just tell you your distance from that particular satellite. A signal from two GPS satellites gives a circle of possible places you could be located. Three gives you two points which differ greatly in latitude and longitude. If you know your approximate latitude and longitude, you can figure out which point you're at. Four satellites are necessary to accurately determine altitude.

In general, the more GPS satellites your receiver can get a fix on, the more accurate your location will be.

Answer: All of these

The time the message was sent and the location of that particular GPS satellite allow the GPS receiver to determine its location. The orbital information for all the other GPS satellites in the fleet is known as "Almanac information," and it helps the GPS satellite get a fix on the GPS satellites more quickly the next time it is turned on.

Answer: True

This is how any GPS you would buy off the shelf would probably work. If it uses DGPS, things are a bit more complicated (see the next question). There is actually more than one way to measure the difference in time. The first is by using broadcast signals. Coarse acquisition (C/A) code is the least accurate, but is available to all users. Precise or protected code is more accurate, but available to registered users only. Y code is for military use only.

The other is by using carrier cycles.

This is more precise, but more difficult to compute. The GPS satellite transmits in two different frequencies (which, naturally, have two different wavelengths.) The receiver compares the difference in phase of the two frequencies to arrive at a location that is good to within a fraction of the largest wavelength, or 1 or 2 mm.

Answer: It uses two receivers, one at a known location and one at an unknown location

DGPS uses two receivers. One is set up at a known location, and gathers information from the GPS satellites constantly. From this, the receiver calculates the error in the GPS satellite signal, and then transmits it to the receiver located at an unknown position. That receiver is then able to correct for the error in the GPS satellite signal, and arrive at an even more accurate and precise location.

During the Gulf War, the US government introduced a random error into the GPS satellite signal. This made GPS too inaccurate to use in airline navigation. But, engineers being the clever people they are, set up one stationary GPS unit at the airport, and used this to calculate and transmit the error to incoming airplanes. After all, if you're trying to land an airplane, you only need to know the correct relative position of the airplane to the airport. I'm not sure if this was the first use of DGPS or not, but it certainly does seen a smart way of solving a problem.

Answer: Signal multi-pathing

DGPS corrects for orbital errors (error in the position of the satellites), receiver clock errors (the clock in your receiver won't be as accurate as an atomic clock), and refraction of the signal from the satellites in the ionosphere and troposphere.

The biggest source of error is signal multi-pathing-- receiving two copies of the signal from the same satellite because one copy happens to be reflected off a building.

Answer: It improves the accuracy and precision of location measurements

WAAS stands for Wide Area Augmentation System. It consists of a system of 25 stationary receivers that monitor GPS data, and 2 geosynchronous satellites that transmit a correction message. It is only available in North America, but any GPS receiver that is WAAS-enabled can use the signal to come up with a more precise and accurate location. Europe and Japan are developing (or is it already done?) a similar network.

Answer: Meters horizontally, a few tens of meters vertically

Some GPS units can indeed get sub-centimeter accuracy, but those are seriously expensive, and not the sort of thing you'd bring along with you on a camping trip! There is a sport called "geocaching," which involves hunting down locations with a GPS unit! Once you find the "geocache," you can take out some of the goodies as a prize, but then you have to leave some yourself. Sounds fun!