GPS 101

American nuclear submarines in the 1960’s were having a difficult time finding position, quickly and accurately. They knew target coordinates, ballistics, and missile trajectory but the essential element in the fire control solution was current submarine coordinates. Something had to be done. The solution was to create an orbiting satellite network that transmitted position information to a worldwide nuclear submarine fleet. In 1970, Congress authorized the U.S. Department of Defense to develop the Navstar GPS system for $13 billion. The GPS signal was degraded to deny enemy use.  Selective Availability (SA) or signal degradation was removed in May, 2000.  The current accuracy of an uncorrected GPS signal is about 25-30 feet.  Inaccuracy can be attributed to satellite and receiver timing errors, ionosphere interference, multipath, and system errors. Differential correction (DGPS) can reduce the uncorrected signal errors to submeter. The military or authorized civilian users can obtain five to fifteen meter accuracy or better from a decrypted GPS signal without differential correction.

It seems complicated to use 24 satellites orbiting at 12,600 miles, rising and setting each 12 hours, to find our geographic position. In fact, the solution is simple. Trilateration (the measurement of distance and location) is used to pinpoint our location. Imagine you are on a camping trip armed with compass and map. You want to find your location, so you take four bearings to map landmarks. The cross-bearing is your location. GPS works in a similar way.  For GPS trilateration, we need two essential information pieces about each satellite: (1) Orbit position, and (2) Distance from satellite to our position. An almanac transmitted to a GPS receiver during regular operations contains the orbit position. Distance is calculated by multiplying the speed of light (186,000 miles per second) by the lapsed time required for a GPS signal to arrive from a satellite. Accurate satellite and receiver clocks provide timing. Once we have orbit and distance information on four satellites, we have a trilaterated position on the face of the earth. Three satellite solutions are satisfactory on the sea or in the air but four satellites are essential on land. GPS accuracy varies by receiver quality and correction method (described later). Survey grade receivers can provide five millimeter accuracy. The normal range is 5mm to 2cm. Resource grade receivers (used in GIS applications) provide submeter accuracy with a range of 10cm to 5m. A linear relationship exists between accuracy and cost.  GPS receiver cost increases as accuracy improves.

GIS data acquisition and conversion are costly. GIS data transfer is 60% to 70% of total GIS system costs. GIS data acquisition is an ongoing task. After initial data gathering for a GIS system, Information must be continuously added and updated. It is important to reduce data acquisition and conversion costs. It is here that GPS shines.  GPS can automate and speed GIS data processing at a lower cost compared with other methods. Several studies have shown GPS data gathering can reduce GIS data collection costs by 50% or more. For example, the City of Ontario compared a fire hydrant inventory by GPS and conventional methods. GPS cost $515 and 41 man-hours for a 942 fire hydrant inventory.  The same inventory by conventional methods would have cost $4,575 and have taken two to four months. In addition, GPS data offers both spatial and tabular information. Digitizing can be labor intensive and subject to positional error. Scanning offers speed but lost detail and editing are disadvantages. Remote sensing and photogrammetry, unless high-resolution imagery is possible, may not meet GIS spatial detail requirements. GPS positions are collected in a digital form in the field. Spatial and tabular data are collected simultaneously.

GPS data collection begins with the creation of a data dictionary. In GPS software, the tabular information to be associated with spatial data is defined. The data dictionary creates the point, line and area features. Attributes and values associated with each feature are described. For example, a well point feature or road line feature is described by road name, well depth, and well type attributes. Attribute values can be defined by menu selection, numeric, or text entries. Well type values may be a menu selection: domestic, industrial, or agriculture. The road’s name will be a text entry. Upon completion, the data dictionary is transferred to a GPS data logger.  GPS data collection can be around the clock. The collection process begins by occupying the defined point, line, and area. Point features (tree, well, lamppost, manhole cover, blast holes or fire hydrant) are stationary GPS activities requiring several positions. A line and area are kinematic activities with spatial data collected on the move. As a result, point features have better spatial accuracy than line or area features. During data gathering, the data dictionary attributes and values are entered and logged to spatial features.  A road is a typical line feature. The data dictionary feature might include road name, road condition, and road surface (concrete, asphalt, dirt, gravel). Data collection begins on the road centerline. The GPS operator enters the road name and condition and proceeds to gather line (road) data. Some GPS systems can segment the road feature to mark changes in surface values (concrete, asphalt etc.). The data interval is normally five seconds. GPS receiver has been placed on SAR (search and rescue) dogs to track search patterns in rescue situations. The dog’s tracking path (line feature) is evaluated to provide further search and rescue directions.  An area is similar to a line feature except it closes to form a polygon.  A lake, pond, tennis court or parking lot are typical area features. In addition, area features may mark vegetation, archeological sites or contaminate areas. GPS data gathering begins on any area location. The operator walks the area, returning to the point of beginning or allows the software to close the area feature automatically by joining the first position with the last position. The data interval is normally five seconds. Data Dictionary features can describe the area feature (pond, tennis court) and its condition (fresh water or brackish water). The area and perimeter measurements will appear in the GPS or GIS software.

While GPS data collection has improved in ease and speed, some obstacles remain. Solid or dense objects can block GPS signals. Wet trees with heavy branches and leaves can mask or attenuate GPS signals. Mountains and buildings can block satellite transmission. Multipath signals can corrupt GPS data. Multipath is a reflected signal from some nearby fence or surface. The resulting propagation delay can spoil measurement accuracy. GPS electronics advancements have reduced the multipath threat but GPS field operators should avoid obvious multipath environments.  Signal blocking can be reduced by careful mission planning. GPS mission planning software can model terrain to display satellite availability. As a result, data collection can be done during the best satellite hours.

Differential Correction

Differential Correction  removes signal errors caused by satellite and receiver timing errors, and atmospheric interference.   An uncorrected signal is accurate to 7-8 meters or or about 25-30 feet.  There are two differential correction methods: postprocessing (above) and real-time (below.)  The concept is the same.  A base station is surveyed to 1-3cm accuracy.  When the satellite signals reaches a base station,  the base station recognizes the difference between its accurate surveyed position and the error position sent by the satellites.  The correction is either saved to a hard disk for postprocessing or communicated real-time to the GPS receiver. 

Real-time is valuable for navigation.  Postprocessing is more accurate.  Consider this information when purchasing a Trimble GPS receiver. 

There are over 200 Trimble base stations world-wide for postprocessing.  Navigation beacons and satellite systems are available for real-time differential correction.

A few years ago GPS data conversion to GIS could be difficult and time consuming.  Today the process is easy and straightforward. GPS manufacturers provide utilities for data conversion to popular GIS software. The differential corrected GPS rover files is converted automatically to the selected GIS format. Data format customization is possible. For example, many conversions do not transfer height since most GIS software works in a horizontal X, Y coordinate system. Customization allows GPS height transfer to GIS for contouring or surface treatments.

Most GPS data collection is ground-based. Major exceptions are fire management and wildlife inventory aerial data collection. The Bureau of Land Management and the U.S. Forest Service use GPS in helicopters to locate fire perimeters and calculate acreage. In several major fires, aerial and ground GPS data collection has proven invaluable for rapidly learning structure damage, environmental damage and fire extent.  GIS is used to analyze and map the fire area.  A GPS equipped fast moving airplane or helicopter can quickly locate and inventory wildlife in hard to reach locations or over large areas.

GPS Advantages and Disadvantages

The experienced GPS users learn to optimize data gathering by understanding system strengths and weaknesses. Over the past few years GPS maturity has eliminated many system disadvantages. For example, before 1993 the full 24 GPS satellite constellation was not in place. As a result, periods existed where four satellites were not available for accurate positioning. This happens only rarely today. Technology has reduced the effect of multipath and GPS data gathering capabilities are being strengthened. The user must be informed, however, about GPS advantages and disadvantages.