GPS is incorporated in many of our daily objects and serve many purposes in our daily navigation. Drivers use GPS when navigating streets, pedestrians use GPS on mobile devices to navigate walking paths, and GPS devices are being used to monitor human and animal movement. In addition, many recreational uses, such as hiking, use GPS to track locations.
GPS is defined as “A system of radio-emitting and -receiving satellites used for determining positions on the earth. The orbiting satellites transmit signals that allow a GPS receiver anywhere on earth to calculate its own location through trilateration. Developed and operated by the U.S. Department of Defense, the system is used in navigation, mapping, surveying, and other applications in which precise positioning is necessary.” (ESRI, GIS Dictionary, GPS)
For GPS units to properly locate your position, this requires the three components: space segment, control section and user segment to work in sync with one another. It is important for the user to be moving to properly identify locations.
Space Segment: GPS satellites (a minimum of 24 in constellation) send signals to earth with satellite position information, including time the signal is received.
Control Segment: There are ground stations on the earth that are receiving the information from the GPS satellites, and sending data to the GPS satellite to correct position and relay information.
User Segment: User holding a GPS unit using the data from the satellite to locate position on Earth.
Trilateration: This is the process of using three points of reference to determine location.
3D trilateration: This is the process of determining location based on three satellites.
Multiple factors can play a role in the accurate determination of the GPS location seen by the user using a handheld device, and/or other GPS devices. Some brief factors related to GPS accuracy are listed below.
Atmosphere effects: The ionsphere and troposphere can impact the GPS signals by delaying the signal.
Multipath effect: This is the reflection of the GPS signal off od other surfaces, such as buildings, before reaching the GPS receiver.
Differential GPS: Corrects the GPS position determination by using a series of base stations. Differential GPS is used to overcome GPS errors.
Wide Area Augmentation System (WAAS):Additional augmentation is possible from satellites to improve accuracy, an example of a satellite based augmentation system (SBAS) is the Wide Area Augmentation System (WAAS).From Garmin, “A WAAS-capable receiver can give you a position accuracy of better than 3 m, 95 percent of the time.”
GNSS:refers to the satellite position system with global coverage, which is used to improve GPS accuracy. Examples of GNSS include the USA’s NAVSTAR, andRussia’s GLONASS.
Waypoints: How many waypoints can the unit store? How does the number of waypoints stored relate to your recreational and fieldwork needs?
Electronic Compass: Reminder the GPS receiver does not determine the direction or location of a user is headed, if the user is not moving. Electronic compass aid in the real-time navigation by letting the user know cardinal directions (North, South, East or West).
Display: Are you seeking specific display features, such as widescreen or touch screen, and/or other viewing characteristics?
Memory: Will you be loading additional content, basemaps, or auxiliary data to your GPS? Some GPS Units have the ability to add additional memory through an SD card.
Receiver Characteristics: How well does the receiver receive satellite signals? Does the receiver support WAAS capability?
Accessories: Are additional accessories available to improve accuracy, such as an antenna? Is additional battery power available for the unit? Are the required cables available for transfer of GPS data from the unit to computer and/or other devices? Is there a carrying case available for the unit?
Price: How much are you looking spend on a GPS unit? Are there tradeoffs if going with a more/less expensive model?
Three general classes of GPS units
Navigation/Recreational grade: These are gps units used in vehicles and for recreational purposes, which can range in accuracy from 5 to 15 meters. Typically, these range in cost from $200-$500.
Mapping grade: These include a range of positional accuracy; however, with WAAS enabled this can improve to under 3 meters. Accuracy improves with the use of differential correction and the use of higher quality antennas. These can range from $500 to thousands of dollars. Additional features may be present to identify different map features from the GPS unit, use a stylus for GPS unit navigation, and additional accessories to increase functionality. With the use of differential GPS, accuracy can increase to be less a meter.
Survey grade: These include GPS receivers which can receive accuracy levels in the 1 meter range or better in terms of less than a foot, centimeter, and millimeter. Based on the USGS Global Positioning Application and Practice site, for a GPS receiver to be consideredsurvey grade, the receiver must record the full range of signal strengths and frequencies (dual-frequencies), and simultaneously track eight satellites.
Some comparisons: Garmin and Trimble Products
Garmin (Recreational to Mapping Grade)
Trimble (Mapping to Survey Grade)
Mobile Device GPS considerations: Data usage, using in a location without a signal, some apps have ability to save map for offline usage.
Mobile devices use three mechanisms to determine GPS location: Assisted GPS (A-GPS), WiFI positioning, and cellular network position. The Assisted GPS (A-GPS) uses the cellular network to obtain GPS satellite information.