GPS Q&A
By Karen Steede-Terry

Q. Is it true that new GPS receivers can capture point, line, and area data for a GIS at the centimeter accuracy level? H.D. Torrance, Calif.

A. To answer your question, let's divide GPS receivers into two common categories, "survey grade" and "mapping grade."
     Previously, only survey grade receivers could be used to obtain high accuracy positions at the centimeter level. This was due to several factors, including the design of the survey-grade receiver, which allowed for collection of two different kinds of radio transmissions from the GPS satellites.
    To elaborate, the GPS satellites communicate with a GPS receiver on the ground by transmitting on two different radio frequencies, L1 and L2. Survey grade receivers are known as dual frequency receivers, due to the fact that they can receive transmissions from both the L1 and the L2 signals. The majority of receivers used for mapping, including GIS data collection, are typically single frequency receivers, receiving transmissions only from the L1 signal.
     The L1 and the L2 radio signals transmit different kinds of information to the GPS receivers on the ground. The L1 frequency from the satellites transmits a code superimposed over the signal.
     In addition, the mapping grade receivers generate their own pseudo-random code internally, at the same time as the satellite. This is the reason that single frequency mapping grade GPS receivers are also known as "code-phase" receivers. In addition to code, carrier phase data is another type of information transmitted by GPS satellites. Carrier information is transmitted in waves, instead of superimposed pseudo-random codes. GPS positioning using carrier phase data involves counting the number of waves sent between the satellite and the receiver. Each wavelength is a constant, and, as a result, carrier waves are a much finer measuring tool than code, yielding increased accuracy. Until recently, only survey grade receivers have been able to process carrier phase data.
     Using code-phase GPS receivers for positioning is fine if your application requires accuracies between .5m and 1 meter. However, if your application requires greater accuracies, from 10-100 cm, then only survey grade receivers can achieve that accuracy. Also, using a GPS for GIS data collection requires the GPS unit to capture data in the form of points, lines, and areas. Historically, only mapping grade units possessed the capability to record feature and attribute information. Until the recent introduction of "crossover" products, this has always been the dilemma of GPS.
     For GIS data collection purposes, everything in the world is defined as a point, line, or an area feature. For instance, points include trees, fire hydrants, and manhole covers. Lines include streets, roads, pipelines, and streams. Areas are parking lots, land parcels, and building perimeters. In order for these features and their attributes to be recorded by the GPS receiver, a database or "data dictionary" defining features and attributes must be created for use in the field. This data dictionary is usually set up within the software accompanying the mapping unit, and then downloaded into the GPS receiver. The features and attribute information are then recorded by the GPS user in the field. After the GPS data is collected in the field, the data is downloaded back into the computer software, and then exported into a GIS format.
     Recent advances in GPS technology have brought us mapping products that can process carrier waves for point features only. Processing carrier phase data requires a long occupation time (minimum 10 minutes) with a stationary antenna, therefore only point features on mapping grade products can be recorded using carrier waves. However, by connecting a mapping grade computer software interface to a survey grade GPS receiver, we are now able to collect point, line and area features using both the L1 and the L2 signals. The L2 signal on these GPS units can map point, line, and area features to 5 cm accuracy.
     As advances in technology take place in the GPS world, we are increasingly bridging the gap between the "survey world" and the "mapping world." It will be interesting to see what the future brings us.

Q. If we turned Selective Availability (S/A) off, would we still need differential correction (DGPS)? B.N. Rockleigh, N.J.

A. The short and simple answer is yes, we would.
     Selective Availability, or S/A, is intentional scrambling of the GPS signal by the Department of Defense. S/A exists to discourage hostile forces from using our own GPS technology against us. S/A is essentially a timing, or clock error, artificially introduced into the satellite constellation. S/A is the largest source of error in the GPS, reducing accuracies by up to 100 meters, however it can be corrected using differential correction (DGPS) techniques. Even if S/A were turned off, we would still need to use DGPS for correction, due to interference from other factors.
     If selective availability were turned off, accuracies from autonomous positioning (that is, no differential correction) would be approximately 15 meters. This is largely due to interference from atmospheric factors, including weather systems.
     Keep in mind that the GPS satellites are over 12,000 miles above us, orbiting in space. That means the signals must pass through the Earth's atmosphere (a considerable distance) before reaching a GPS receiver on the ground. The signals pass through the troposphere, which is the layer of the atmosphere where our weather occurs. The signals also pass through the ionosphere, which is a band of electrically charged particles encircling the Earth. GPS signals are delayed as they bounce off these charged particles and pass through weather systems, thereby altering the amount of time it takes to reach the Earth's surface, and increasing GPS error.
     There are other sources of error which can decrease the accuracy of the GPS signal. For now, when you are actively using a GPS unit, always assume that you will have some source of error which will degrade your position. Fortunately, most errors, even S/A, are correctable, using differential correction.

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