Global Positioning System (GPS) data has been in existence since the launch of the first satellite in the US Navigation System with Time and Ranging (NAVSTAR) system on February 22, 1978, and the availability of a full constellation of satellites since 1994. Initially, the system was available to US military personnel only, but in 1993 the system started to be used (in a degraded mode) by the general public. During the 1990s, GPS employed Selective Availability that degraded civilian accuracy for national security reasons. In May 2000, the U.S. government ended its use of Selective Availability in order to make GPS more accessible to worldwide civil and commercial users. The civilian service is freely available to all users on a continuous, worldwide basis.
The United States GPS system is one of many global navigation satellite systems (GNSS) throughout the world. The United States holds a cooperation policy with many nations including Australia, China, European Union members, India, Japan, Russia, and other international organizations.
The International Committee of Global Navigation Satellite Systems (ICG) was founded in 2005 to promote cooperation, compatibility, and interoperability among the GNSS systems.
GPS is a U.S.-owned utility consisting of three segments:
- Space Segment
- Control Segment
- User Segment
Space Segment consists of a minimum constellation of 24 satellites orbiting the Earth, that transmit one-way signals to calculate current position. This constellation of satellites is managed by the U.S. Air Force to make sure at least 24 satellites are available at least 95% of the time.
Control Segment consists of ground facilities in a global network that track the satellites, monitor transmissions, and send commands to the satellites. The daily command and control of the constellation is performed at Shriever U.S. Air Force base in Colorado. There are 16 monitoring stations throughout the world and four dedicated GPS ground antenna sites co-located with some of the monitoring stations.
User Segment consists of the GPS receiver equipment, which receives the GPS signals from the satellites and uses these signals to calculate the user’s position and time. GPS receiver equipment is manufactured and offered by many commercial companies.
Positions are determined through the traditional ranging technique. The satellites orbit the Earth at an altitude of 20,350 km in such a manner that several are always visible at any location on the Earth's surface.
GPS satellites transmit radio signals providing their locations and precise time.
A GPS receiver collects the radio signals noting their exact time of arrival. The receiver calculates the receiver’s distance from each satellite in view.
Once a GPS receiver knows its distance from at least four satellites, the GPS receiver uses geometry to calculate a three-dimensional (3D) x, y, z position.
The figure below shows an artist rendering of GPS Block IIF series of GPS satellites
The F in IIF stands for follow-on, which is the next generation of Block II series of satellites including Block IIA, Block IIR, and Block IIR (M). The IIF series includes a total of 12 satellites. The first IIF satellite was launched in May 2010.
GPS Block IIF Satellites
Source: National Coordination Office, 2013d.
Applications of GPS Data
GPS data finds many uses in remote sensing and GIS applications, such as:
- Collection of ground truth data, even spectral properties of real-world conditions at known geographic positions, for use in image classification and validation. The user in the field identifies a homogeneous area of identifiable land cover or use on the ground and records its location using the GPS receiver. These locations can then be plotted over an image to either train a supervised classifier or to test the validity of a classification.
- Moving map applications take the concept of relating the GPS positional information to your geographic data layers one step further by having the GPS position displayed in real time over the geographical data layers. Thus you take a computer out into the field and connect the GPS receiver to the computer, typically by the serial port. Remote sensing and GIS data layers are then displayed on the computer and the positional signal from the GPS receiver is plotted on top of them.
- GPS receivers can be used for the collection of positional information for known point features on the ground. If these can be identified in an image, positional data can be used as Ground Control Points (GCPs) for geocorrecting the imagery to a map projection system. If the imagery is of high resolution, this generally requires differential correction of the positional data.
- DGPS data can be used to directly capture GIS data and survey data for direct use in a GIS or CAD system. In this regard the GPS receiver can be compared to using a digitizing tablet to collect data, but instead of pointing and clicking at features on a paper document, you are pointing and clicking on the real features to capture the information.
- Precision agriculture uses GPS extensively in conjunction with Variable Rate Technology (VRT). VRT relies on using a VRT controller box connected to a GPS and the pumping mechanism for a tank full of fertilizers, pesticides, seeds, water, and so forth. A digital polygon map (often derived from remotely sensed data) in the controller specifies a predefined amount to dispense for each polygonal region. As the tractor pulls the tank around the field the GPS logs the position that is compared to the map position in memory. The correct amount is then dispensed at that location. The aim of this process is to maximize yields without causing any environmental damage.
- GPS is often used in conjunction with airborne surveys. Aircraft, as well as carrying a camera or scanner, has on board one or more GPS receivers tied to an inertial navigation system. As each frame is exposed precise information is captured (or calculated in post processing) on the x, y, z and roll, pitch, yaw of the aircraft. Each image in the aerial survey block thus has initial exterior orientation parameters which therefore minimizes the need for control in a block triangulation process.
Source: Leick, 1990.