GPS receivers are usually smart enough to choose the location nearest to the Earth's surface. At a minimum, three satellites are required for a two-dimensional horizontal fix. Four ranges are needed for a three-dimensional fix horizontal and elevation. The process of acquiring a two-dimensional fix is illustrated in Figure 5. Satellite ranging is similar to an older technique called trilateration, which surveyors use to determine a horizontal location based on three known distances.
Surveying and trilateration are discussed more fully in section 5. Skip to main content. Figure 5. The Global Positioning System is based on the interoperation of three distinct segments. Adapted from Hurn Click for a text description of Figure 5.
Three graphics of Satellite Ranging: One satellite: position is anywhere on surface of sphere. Two satellites: position is anywhere on circle where spheres intersect. Three satellites: Position is one of two locations where all three spheres intersect. The GPS satellites circle the Earth at an altitude of about 20, km 13, miles and complete two full orbits every day.
The GPS satellites are not in a geostationary orbit, but rise and set two times per day. Each satellite broadcasts radio waves towards Earth that contain information regarding its position and time. We can receive this information by using special receivers, called GPS receivers , which can detect and decode this information.
By combining signals transmitted by several satellites and received simultaneously, a GPS receiver can calculate its position on the Earth i. There are more sophisticated receivers that can be used to determine position with an accuracy of a few millimeters. What does is mean to determine our "position" on the Earth? The usual method is to refer to a terrestrial position i.
Therefore, most GPS receivers will displaying their current latitude and longitiude. The usual format for displaying this information is in degrees and minutes. There are degrees in a complete circle, and 60 minutes in one degree.
The minutes are usually displayed as a decimal number, like The latitude is measured relative to the equator. If a location is in the Northern Hemisphere, the latitude will be followed or preceded by the letter N. If a location is in the Southern Hemisphere, the latitude will be followed or preceded by the letter S. Sometimes no letter is given, and the latitudes in the Southern Hemisphere will be expressed as a negative number.
By historical convention, longitude is measured relative to the "Greenwich" or "Prime" Meridian. If we measure an angle east of the Prime Meridian, we write the letter E preceding or following the longitude. These days, many people take Global Positioning System devices for granted.
Most people are familiar with the concept of using a GPS system either as a stand-alone device or in a phone to find one's way around. GPS can also identify a user's location, which is useful for orientation and even for fun activities such as geocaching finding hidden caches with the use of GPS devices. But there are other uses as well, besides location and navigation.
GPS can also be used for tracking, such as adding a tag to an animal to see the extent of its territory. The technology also assists with mapping, such as helping to better define the outline of a country.
There are likely thousands of applications for GPS systems, ranging from everything to helping hikers navigate in remote areas, to assisting farmers with precisely seeding their fields, to assisting drones with finding their targets. The U. Navy developed the first operational satellite navigation system — called Transit — in the s. These "spin-stabilized" spacecraft — meaning that their spin kept the spacecraft pointing in the same direction — were first used for navigation in by Polaris submarines.
Even in those early days, however, the people who built the system could see scientific uses for GPS. In particular, we did not predict the incredible extent to which size and cost would be reduced for everyday applications for the mass market, e.
There were other early systems as well, such as the Navy's Timation satellite that tested the use of accurate clocks in space. This was an important predecessor to Navstar, as that GPS system relies on timing to keep the satellites synchronized.
Meanwhile, other branches of the U. Satellites, for instance, are becoming critical for everything from internet connectivity and precision agriculture, to border security and archaeological study. Right now, there are nearly 6, satellites circling our tiny planet. As highlighted in the chart above, The Union of Concerned Scientists UCS , determined that 2, operational satellites circled the globe in April of This means that by , there could be 15, satellites in orbit.
Humans have long used space for navigation. While sailors once relied on the stars, today we use satellites for GPS, navigation, and various other applications. Commercial satellites, however, can serve multiple purposes. It could later be tasked to monitor the reclamation of a mining site or even the aftermath of a natural disaster. Following its series of summer launches, SpaceX announced that it had deployed enough satellites to support the beta version of its satellite-based internet service, Starlink.
Even Amazon is preparing for space. It may be no surprise that the United States, China, and Russia top the list of countries with operational satellites. The U. Both nations are found in the top three of current satellite operators, with the U.
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