Introduction
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 from 1993 onwards the system started to be used (in a degraded mode) by the general public.There is also a Russian GPS system called GLONASS with similar capabilities.
The US NAVSTAR GPS consists of a constellation of 24 satellites orbiting the Earth, broadcasting data that allows a GPS receiver to calculate its spatial position.
Satellite Position
Positions are determined through the traditional ranging technique. The satellites orbit the Earth (at an altitude of 20,200 km) in such a manner that several are always visible at any location on the Earth's surface. A GPS receiver with line of site to a GPS satellite can determine how long the signal broadcast by the satellite has taken to reach its location, and therefore can determine the distance to the satellite. Thus, if the GPS receiver can see three or more satellites and determine the distance to each, the GPS receiver can calculate its own position based on the known positions of the satellites (that is, the intersection of the spheres of distance from the satellite locations). Theoretically, only three satellites should be required to find the 3D position of the receiver, but various inaccuracies (largely based on the quality of the clock within the GPS receiver that is used to time the arrival of the signal) mean that at least four satellites are generally required to determine athree-dimensional (3D) x, y, z position.
The explanation above is an over-simplification of the technique used, but does show the concept behind the use of the GPS system for determining position. The accuracy of that position is affected by several factors, including the number of satellites that can be seen by a receiver, but especially for commercial users by Selective Availability.
Each satellite actually sends two signals at different frequencies. One is for civilian use and one for military use. The signal used for
commercial receivers has an error introduced to it called Selective Availability. Selective Availability introduces a positional inaccuracy of up to 100m to commercial GPS receivers. This is mainly intended to limit the use of highly accurate GPS positioning to hostile users, but the errors can be ameliorated through various techniques, such as keeping the GPS receiver stationary; thereby allowing it to average out the errors, or through more advanced techniques discussed in the following sections.
Differential Correction
Differential Correction (or Differential GPS - DGPS) can be used to remove the majority of the effects of Selective Availability. The technique works by using a second GPS unit (or base station) that is stationary at a precisely known position. As this GPS knows where it actually is, it can compare this location with the position it calculates from GPS satellites at any particular time and calculate an error vector for that time (that is, the distance and direction that the GPS reading is in error from the real position). A log of such error vectors can then be compared with GPS readings taken from the first, mobile unit (the field unit that is actually taking GPS location readings of features). Under the assumption that the field unit had line of site to the same GPS satellites to acquire its position as the base station, each field-read position (with an appropriate time stamp) can be compared to the error vector for that time and the position corrected using the inverse of the vector. This is generally performed using specialist differential correction software.Real Time Differential GPS (RDGPS) takes this technique one step further by having the base station communicate the error vector via radio to the field unit in real time. The field unit can then automatically updates its own location in real time. The main disadvantage of this technique is that the range that a GPS base station can broadcast over is generally limited, thereby restricting the range the mobile unit can be used away from the base station. One of the biggest uses of this technique is for ocean navigation in coastal areas, where base stations have been set up along coastlines and around ports so that the GPS systems on board ships can get accurate real time positional
information to help in shallow-water navigation.
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, usually via 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, the 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 the use of 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. The 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.
Figure 34 shows some additional uses for GPS coordinates.
Figure 34: Common Uses of GPS Data
Source: Leick, 1990