Summary of Workshop Clues
History of GPS | How GPS Works | Who Uses GPS Receivers
GPS Receivers | Geocaching
History of GPS
- The Global Positioning System is a $12 billion system of 24 satellites (plus a few spares) deployed and maintained by the U.S. Department of Defense (DOD).
- Selective Availability, which made the civil use of GPS less accurate than the military GPS, was turned off on May 1, 2000. Thus, the inexpensive GPS receivers available to civilians are as accurate as those used by the military today.
- Deployment of the satellites began in 1978, and the system became fully operational (uninterrupted global coverage) in 1995.
- Currently, there are millions of civilian users of GPS and GPS receivers worldwide.
- Although the cost of maintaining the system is approximately US $400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good.
- In late 2005, the first in a series of next-generation GPS satellites was added to the constellation, offering several new capabilities, including a second civilian GPS signal called L2C for enhanced accuracy and reliability.
How GPS Works
- GPS provides satellite signals that can be processed in a GPS receiver, enabling the receiver to compute one's position on the face of the earth (often indicated in terms of latitude and longitude).
- The GPS device receives data from the closest satellites, triangulating data to determine the unit's exact location (typically in latitude and longitude), elevation, speed, and time.
- Each GPS satellite passes around the earth twice in a 24-hour period at an altitude of about 12,500 miles. The satellites continuously broadcast position and time data to users throughout the world.
- Four GPS satellite signals are used to compute positions in three dimensions, so your altitude is also indicated on your GPS receiver.
- Each satellite contains a computer, an atomic clock, and a radio. The satellite continually broadcasts its changing position and time. Once a day, each satellite checks its own sense of time and position and makes minor corrections as needed.
- The location accuracy of a GPS signal as received by a GPS receiver is anywhere from 1 to 100 meters depending on the type of GPS receiver used.
Who Uses GPS and GPS Receivers
- Global Positioning System (GPS) receivers are used in cars, boats, airplanes, and even in cellular phones. Most airlines allow passenger use of GPS units on their flights, except during landing and take-off when other electronic devices are also restricted.
- Handheld GPS receivers are carried by hikers, land surveyors, scientists, emergency personnel, medical evacuation helicopters, mapmakers, and others who need to know where they are.
- The US Military is a primary user of GPS. GPS allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions. US Military personnel also use GPS in ground operations.
- Handheld GPS receivers are carried mountain climbers and hang glider pilots. The system can be used to automate harvesters, mine trucks, tractors and other large agricultural machines, and other vehicles.
- Examples of GPS-based services are MapQuest Mobile and TomTom digital maps.
- GPS equipment is available for the visually impaired.
- Hand-held GPS receivers cost $90 - $1000 and vary according to number of features and sophistication of features.
- Major Brands of Handheld GPS Receivers
- Major Brands of Auto GPS Receivers
- What GPS Receivers Tell You
- How far you've traveled (odometer)
- How long you've been traveling
- Your current speed (speedometer)
- Your average speed
- A "bread crumb" trail showing you exactly where you have traveled on the map
- The estimated time of arrival at your destination at your current speed
- Most GPS receivers are easy to setup and operate. There are different ways to input numbers and letters into your GPS unit. Most units use arrows or buttons to move between functions and screens. Others have actual alphanumeric keys, like your phone, for ease in entering names and numbers.
- GPS Receiver Features: Satellite Status, Menu, Position, Set Up Options, Compass, Navigation, Waypoint (marks a location), Distance to Location, Time, Sun and Moon Phases, Interface with maps and computer software, and more.
- Recreational applications include location-based games like Geocaching. Geocaching involves using a hand-held GPS unit to travel to a specific longitude and latitude to search for objects hidden by other geocachers.
- A geocacher places a waterproof container, containing a log book (with pen or pencil) and trinkets or some sort of treasures, then note the cache's coordinates. These coordinates, along with other details of the location, are posted on a website for other geocachers.
- Geocachers obtain cache coordinates from the Internet and seek out a cache using their GPS handheld receivers. The finding geocachers record their exploits in the logbook and online.
- Geocachers are free to take objects from the cache in exchange for leaving something of similar or higher value, so there is treasure for the next person to find. Typical cache treasures aren't high in monetary value but may hold intrinsic value to the finder.
- Geocaches can range in size from "microcaches", too small to hold anything more than a tiny paper log, to those placed in five-gallon buckets or even larger containers. Many are in Tupperware containers or ammo boxes.
- Geocaching rules are very simple:
- Take something from the cache
- Leave something in the cache
- Place nothing dangerous in a cach
- record your visit in the logbook and online
- Respect the environment and others