GPS Q&A: Industry experts answer reader's GPS questions

Q. I've heard that GPS receivers can compute either two-dimensional or three-dimensional position data. Is this correct? Are they the same in terms of accuracy?
- R.C. Calgary, Alberta Canada

A.John C. Bohlke, Sokkia Corp.: GPS receivers are capable of computing either two-dimensional or three-dimensional position data. The accuracy of the two-dimensional position relies on the accuracy of the local altitude. A two-dimensional position could have equal or better accuracy than a three-dimensional position if an accurate local altitude is known. If the altitude is unknown, a two-dimensional position would be less accurate than a three-dimensional position.

Carl Carter, Allen Osborne Associates: This is correct. Some receivers will offer the option of two-dimensional positions. This is done because there may be limited resources in the receiver (one or only a few channels available for tracking the satellite signals), limited availability of satellites to track, or similar limitations. In general, it takes observations from four satellites to compute a three-dimensional position since the mathematical solution involves solving not only for x, y and z coordinates, but also the time error at the receiver. This implies four unknowns, and requires four data points for the equations. If only three (or with some receivers, two) satellites can be tracked, a solution for position can still be estimated, but some compromises must be made. Generally, this involves assuming the local height above the reference surface (called the ellipsoid) is zero. For receivers located on ships at sea, this is generally a pretty good approximation, but on land it might result in considerable error. The availability of less observation information, along with forcing an assumption that could in itself have considerable error, results in the 2-D fix generally being much less accurate.

Wendy Corcoran, NovAtel Communications: The majority of GPS receivers are designed to compute three-dimensional positions - latitude, longitude and height. During the earlier days of GPS when there were fewer satellites, two-dimensional positioning was used because of satellite availability. Since four satellites with good geometry are required to compute 3-D positions, if there were less than four satellites or the four were poorly positioned, some GPS receivers would allow you to hold the height component fixed so the latitude and longitude would continue to be computed. The accuracy of the horizontal position is acceptable in 2-D if the height is known exactly. Any error in the height would be translated into the computed latitude and longitude values. Again, 2-D was used as an interim measure and once more satellites were available, most users operated in 3-D positioning. This technique could still be used today but with the full constellation, it is not really necessary.

Arthur Lange, Trimble Navigation: To compute a 3-D position, data from at least four satellites is required. When a 2-D position is computed, it is assumed that a satellite is at the center of the Earth and it is at a given distance. This is done by asking the user to enter the altitude. However, if the user enters the wrong altitude in the GPS receiver and a 2-D position is then computed, there will be an error in the 2-D position. This error in 2-D position is usually within 2 to 5 times the error in the vertical position. For example, a 10 meter error in altitude may result in a 50 meter error in 2-D position. For serious mapping and GIS data collection 3-D manual positioning mode must be used to avoid additional sources of error, since seldom is the GPS altitude known to sufficient accuracy.

Dr. Frank van Diggelen, Ashtech, Inc.: It is true that most GPS receivers can compute either two-dimensional or three-dimensional positions. Usually 2-D positions are computed because the receiver is not tracking enough satellites to compute a 3-D position.
      The 2-D position is computed in the horizontal plane. To compute this position the receiver must be told the altitude. If the altitude is well known then the 2-D position will be more accurate than the horizontal component of the 3-D position. If the altitude is not well known then the 2-D position will be worse than the horizontal component of the 3-D position. What does "well known" mean? The altitude is well known if it is more accurate than the vertical accuracy that would have been achieved from a 3-D position.
      Knowing your geoidal or ellipsoidal altitude is easier said than done, even on apparently flat land or at sea. If at all possible, use a receiver in 3-D mode.

Q. Can GPS receivers be connected to other external devices such as video cameras or depth sounders? How? -T.R. Boca Raton, Fla.

A. Bohlke: Some GPS receivers can be connected directly to various devices such as cameras, depth finders and bar code readers for the synchronization of data collection. Additionally, GPS receivers and other devices can be connected to a single data collection unit. GPS connections usually require some form of serial communication. This enables a user to collect unique forms of attribute data for any feature.

Charles Branch, Ashtech, Inc.: Yes, using RS-232 cables and the appropriate logging software. GPS receivers have been used for some years now as sensors that position-stamp whatever the external sensing imagery, seismic surveys, etc. is commonplace. Just coming on line now is the reverse scenario: where the GPS receiver is the recording device and the external device is used as the sensor providing measurements that are attached to the GPS positions. Examples of this type of application include geophysical field surveys (sensor = depth sounder, magnetometer, gravimeter, etc.) and pollution monitoring (sensor = pH meter, gas sniffer, Geiger counter, etc.)

Corcoran: One of the strengths of GPS is that it can be integrated with other measuring devices. There are four ways that GPS can be interfaced to external devices: through RS232, 1 PPS, mark input and/or mark output.
      1. RS232 - most devices work with an RS232 output from the GPS receiver. For Marine applications, an ASCII standard called NMEA 183 has been established for interfacing purposes. Most high-end GPS receivers can support ASCII, Binary and NMEA 183 outputs to external devices.
      2. 1 PPS - this stands for one pulse per second. The GPS receiver will output this pulse on the even second to synchronize another measuring device to the same time. If both instruments are working on the same time, measurements can be combined with position. Most manufacturers have the ability to adjust the 1 PPS to allow for routing time in the cabling to make sure the measurements are on the second mark.
      3. Mark input - some GPS receivers have the ability to accept a pulse from an external device. This is used so that the GPS receiver will take a position when the external device takes a measurement. Once the pulse is received the GPS receiver will perform an action (specified by the user) such as mark or output the position and time of the pulse for comparison purposes.
      4. Mark output - This is similar to the 1 PPS, but the pulse timing can be varied such as 10 or 20 pulses per second. Mark output is best suited for high dynamic work or for external devices that sample at a rate greater than once per second.
      These techniques can be used separately or concurrently. An example of this would be photogrammetry. To obtain a position for the center of each photo taken, the GPS receiver needs to record the time the exact instance the camera shutter opens. To minimize the amount of time interpolation between the GPS receiver and camera, the GPS receiver's 1 PPS may be used to trigger the camera to take a photo. The camera would then send a pulse to the GPS receiver to mark the time. To allow for the time travel of the pulse (from the GPS receiver to the camera then back to the GPS receiver), the 1 PPS is adjusted to go prior to the 1 second mark so that the return pulse back to the GPS receiver is on the even second.

Craig Hudson, II Morrow, Inc.: The Apollo GIS 940 interfaces with external devices using a serial RS232 port. This port is programmable and allows for the transmission of simple ASCII text strings for direct serial interface. The Apollo GIS 940 uses the standard NMEA RS232 data protocol.

Lange: Yes, GPS receivers may be used with external devices such as video cameras or depth sounders. One way of using a GPS receiver with a video camera is to time-synchronize the video recorder with the GPS receiver using a SMPTE (Society of Motion Picture and Television Engineers) horizontal time code generator. Once synchronized, the video camera and recorder may then be operated independently of the data-logging GPS receiver. The video images are correlated with the GPS position by matching GPS time. Alternatively, a SMPTE code generator may be used to encode and write the GPS position in a machine readable form onto the video tape. For more details, refer to a 14 page document, titled "How to Use GPS Pathfinder Products With Video Recorders and Camcorders," available from the Trimble FAXBACK system at 408-481-7704. This document (FAXBACK document number 3040), has a detailed discussion of several different ways to use a GPS receiver with video equipment.
      To use a measurement device such as a depth sounder, Geiger counter or other measurement device with a GPS receiver usually requires the GPS receiver to have an external sensor port which can record the ASCII data generated by the measurement device along with the GPS position data. After differential correction (either real-time or post-processed) the GPS positions and the related measurement data are exported to an appropriate mapping or contouring software program.

About the participants:
John C. Bohlke serves as GPS support manager at Sokkia Corp. in Overland Park, Kan. He may be reached at 913-492-4900 or 800-4-SOKKIA in the U.S. (phone) or 913-492-0188 (fax).
Charles Branch is a GIS marketing manager at Ashtech in Sunnyvale, Calif. He may be reached at 408-824-1603 (phone), 408-524-1500 (fax) or e-mail: [email protected]
Carl Carter is a software systems engineer at Allen Osborne Associates, Inc. in Westlake Village, Calif. He may be reached at 805-495-8420 (phone) or 805-373-6067 (fax).
Wendy Corcoran is manager, survey and mapping products at NovAtel Communications, Ltd. in Calgary, Alberta, Canada. She may be reached at 403-295-4789.
Craig Hudson serves as portable products manager at II Morrow Inc. in Salem, Ore. He may be reached at 503-391-3411 or 800-742-0011 in the U.S., or 800-654-3415 in Canada.
Arthur Lange is the GIS product manager for Trimble Navigation in Sunnyvale, Calif. He may be reached at 408-481-2994 (phone), 408-481-6074 (fax) or e-mail: [email protected]
Frank van Diggelen is a marketing manager at Ashtech in Sunnyvale, Calif. He may be reached at 408-524-1508 (phone) or via e-mail: [email protected]

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