Ten Things You Need to Know About GPS

June 18, 2012

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1) Definition: Global Positioning System

The Global Positioning System refers to a constellation of 24 satellites (plus some “hot spares” for when one fails) put into space by the U.S. Department of Defense to help locate stationary and moving features on or near the Earth’s surface. The satellites send out radio signals that receivers use to determine position, velocity and time.

2) The Generic Term for Such a System is GNSS

GPS, while used generically, refers only the U.S. constellation of satellites. The generic term for these systems is Global Navigation Satellite System, GNSS. Among those are existing systems such as Russia’s GLONASS, and in-development solutions including the European Union’s Galileo and China’s COMPASS. India, France and Japan are building regional systems.

  • GLONASS - Wikipedia, the free encyclopedia
  • Galileo (satellite navigation) - Wikipedia, the free encyclopedia
  • Beidou navigation system - Wikipedia, the free encyclopedia

3) How GNSS Works

All of these systems work in roughly the same way. The satellites orbit around earth and broadcast information. That information is basically a time stamp. A receiver (a computer built to listen for signals and do math on the data received) collects the time stamps from four satellites and compares them with the current time. By knowing where each satellite is in space and how long each signal took to travel to earth, the receiver can determine its location on the earth’s surface.

4) GNSS Receivers Receive

GNSS receivers are designed to receive GNSS signals and process them into location information. The devices fall into three categories for different uses and price points. As you move from recreational to survey grade accuracy goes up, as does price:

  • recreational/consumer grade
  • mapping grade
  • survey grade

GNSS receivers may also have functions that can turn a location into a point on a map stored on the device, determine speed, direction and other derived measurements. However, unless the receiver also has a transmitter of some kind (like a cell phone does), the data remain on the device. In particular, GNSS receivers do not have two-way communication with the navigation satellites; the satellites are broadcast-only.

5) GNSS is not Great at Elevation

Horizontal (location) accuracy of GNSS devices is still far higher than vertical (elevation) accuracy. Horizontal accuracy for recreational/consumer devices and many cell phones is about +/- 10 meters (within a tennis court) while vertical accuracy can be two or three times that, about +/- 20 or 30 meters.

The reason vertical accuracy is relatively poor relates to geometry. When you look up at the sky, if you had Superman eyesight you would see GNSS satellites all around.  That supports accurate horizontal positioning.  However, when you look out horizontally, out at the horizon, you can only see satellites in front of and above you, not below, since the earth gets in the way. It’s the lack of signals coming from beneath you that results in the lower quality of altitude measures.

If you need to capture accurate vertical data, you may want to look at other measurement tools to complement your GNSS-captured horizontal data.

6) GNSS Systems Work Together and with Other Positioning Devices

With multiple GNSSs online, or coming online, and a variety of non-GPS locating technologies in use (wi-fi, cell tower, etc.), there’s an effort to bring them together for enhanced solutions. Hardware developers are building chips that receive signals from two or more satellite constellations, while software developers are finding new ways to tap into the variety of locating technologies for use in different environments (indoor/outdoor/underground) and situations.

7) GPS Interference

Because GPS signals from satellites are very weak radio signals, they are susceptible to interference by natural and man-made signals. The former include signals resulting from solar storms and the latter, from other radio frequency-based projects including GPS “jammers” (illegal in the United States). In 2011/2012 concerns rose regarding GPS interference by a proposed wireless service from a company called LightSquared. The FCC has decided not to let this company move forward with its network at this time.

8) Government Control of GPS

While the U.S. Department of Defense developed and operates GPS, it serves a wide range of civilian users from surveyors, to mappers, to hikers, to vacationers geotagging photographs on their cell phones. To date, the Department of Defense has never “shut off” GPS in any part of the world, for any reason.

9) Navigating by GNSS

GNSS receivers are sometimes referred to as satnavs, that is, as satellite navigation devices. The GNSS only provides information to a receiver to determine its current location, velocity and time. Any other information, such as the direction of motion, a suggested route to a destination, a map, is provided by software algorithms and map data stored on a hand-held device or accessible on a server. Said another way, GNSS satellites do not provide directions or maps.

10) The GPS is Wrong!

Most of the time a GPS receiver is correct in determining its position, within its tolerance. If a navigation system fails to provide an accurate route, other things are more likely to be the culprit, for example:

  • poor algorithm
  • out-of-date street network
  • wrong destination address

The U.S. government received so many complaints on this matter it compiled an article to explain to users how to “fix” such errors.

Special thanks to Michelle Frye at Trimble for reviewing this article for accuracy.


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