   Global Positioning Systems - October 16, 2002 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County I am writing this column while attending a NASA remote sensing workshop in Salt Lake City with other Extension Agents from around the country. In case you were wondering, remote sensing is about acquiring information about our surroundings without necessarily going there. The workshop is also teaching us about global positioning systems (GPS). I'm sure many readers have used GPS while recreating and/or driving. Global Positioning Systems (GPS) are space-based radio navigation systems initiated by the US Department of Defense. The GPS satellite system first became operational in 1993. The 24 satellites, called a constellation, orbit the Earth at an altitude of approximately 12,500 miles. The satellites are moving around the earth in a more or less equally spaced pattern and are updated and adjusted from ground-based stations. GPS provides users with accurate information on their global location, velocity, and time anywhere in the world and in all weather conditions. Many cars are equipped with GPS as optional navigation equipment. GPS works through trilateration. Each satellite emits a radio signal at the speed of light that is transmitted in all directions creating a sphere. Where two satellite's spheres intersect, it creates a circle. Where three satellite's spheres intersect, it creates two points in space (one of which is on the earth's surface). The fourth satellite gives a precise location and time. The satellites emit unique radio signals and carry extremely precise clocks. The GPS receiver (the device that you carry to determine locations) can measure the distance to each satellite by using the radio signal, time the signal was sent, and the speed the signal traveled (the speed of light). Today, GPS receivers are available for under $100. However, the more you spend, the better the performance, data storage capacity, and accuracy. GPS receivers used by surveyors can cost upwards of $40,000. Initially, the US government broadcast the GPS signals using selective availability (SA). SA created interference to intentionally lower the accuracy of GPS receivers. During this time, government agencies and members of the public could obtain correction information to increase accuracy of their data through a yearly subscription. On May 1, 2000, President Clinton announced that SA was to be discontinued. This increased accuracy by at least ten times. Most GPS receivers available to consumers today will get you within 10-25 feet of any point on the earth's surface. The accuracy of GPS receivers is still variable depending on atmospheric conditions, satellite clocks, orbits of satellites, interference from obstacles (trees, buildings, clouds, etc.), quality of receiver. There is also a new technology called Wide Area Augmentation System (WAAS) which uses additional satellites and ground stations to correct GPS information. Many newer GPS receivers are "WAAS enabled". This technology has been developed to assist in handling the ever increasing commercial air traffic. The thought is that planes could actually be flying closer together. Inexpensive WAAS enabled GPS receivers will potentially have errors of less than 10 feet. By now, if you have not given up on this "non-gardening" topic, you are probably wondering why Cooperative Extension is interested in GPS. Across the world, satellite navigation is helping farmers and ranchers to become more productive and efficient in their farming practices. Today, many farmers use satellite navigation to conduct precision farming operations such as chemical and fertilizer application. Satellite navigation also provides location information that enables farmers to plow, harvest, map their fields, and mark areas of disease or weed infestation - any time of day or night. GPS is also very useful in wildlife management, forestry, navigation, mapping, and much more. Personally, I use GPS to locate and map rangeland monitoring sites, noxious weed infestations, insect outbreaks, and in navigating around on the back roads of Yavapai County. GPS is a great technology and simple to learn. Let me know if you need information about GPS and maybe Cooperative Extension could offer a workshop. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |