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

Q. How does the GPS determine what Magnetic North is? Do the satellites transmit a database, or some kind of lookup chart to determine the declination for your given lat/long? - N.G. Fall River, Wis.

A. Chris Dietsch, Trimble Navigation: To answer the first question, GPS does not determine Magnetic North. GPS positions are determined in a 3D cartesian coordinate system and have no information regarding the declination at a given point. The satellites have no information, and therefore do not transmit any information regarding declination. Some GPS receivers take the positions and use look up tables to calculate the magnetic declination for a given location. Accuracy depends on the accuracy of the look up table used and the interpolation method used to determine the declination at any given point.

William Martin, Ashtech:
A GPS receiver capable of determining Magnetic North does so through the use of a table stored in the memory of the receiver itself. Using the current computed position, last computed position, and a declination value from the table, the GPS receiver can supply the user with a Course Over Ground (COG) based on Magnetic North versus True North.
      Most GPS receivers that utilize a declination table to supply COG based on Magnetic North do so for rough navigation purposes only. Therefore, declination tables found in these receivers do not support precision determination of direction. In most instances, declination values are good to 0.1 - 0.2 degrees. This is not to say that any GPS receiver is capable of determining COG based on Magnetic North to 0.2 degrees. Since Magnetic COG is computed by applying a declination value to True COG, the accuracy of the computed Magnetic COG is dependent on the accuracy of the GPS determined True COG value as well as the precision of the declination table.

Naush Ladha, NovAtel GPS: Magnetic North refers to the location of the Earth's Magnetic North Pole. Its position is constantly changing in various cycles over centuries, years, and days. These rates of change vary and are not well understood. However, we are able to monitor these changes.
      True North refers to the Earth's celestial pole, i.e., at 90 degrees north latitude or the location where the lines of longitude converge. This position is always the same and does not vary.
      The locations of these two poles do not coincide. Thus, a relationship is required between these two values for users to relate GPS bearings to their compass bearings. This value is called the magnetic variation correction or declination.
      GPS does not determine where Magnetic North is nor do the satellites provide magnetic correction or declination values. However, some GPS receivers store this information internally in lookup tables so that when users specify they want to navigate with respect to Magnetic North, this internal information is used. These values are also available from various information sources such as the United States Geological Survey (USGS). The USGS produces maps and have software which enable users to determine these correction values. By identifying your location (latitude and longitude), you can obtain the correction value. Contact USGS at 303-273-8475 for more information on their products.

Q. How long do I need to sit on a 10 km baseline? - C.R. Lexington, Ky.

A. Dietsch: The answer to this question varies from as little as a few epochs to an hour or more. The reason that the answer varies is due to factors such as the number of satellites in view and the PDOP, the method of data collection being employed, the type of receiver being used and the desired accuracy of the results. If a topographic survey was being conducted 10 km from the base receiver using a post processed kinematic technique, the surveyor would only have to occupy each point for a few epochs, assuming that the system was already initialized. For static surveys, the occupation time depends mostly on the number of satellites in view. With six or more satellites in view, 8-10 minute occupation times are sufficient, assuming that a dual frequency receiver is used. Static surveys on a 10km baseline would generally be 30 minutes or longer. Since there is no single answer to the question, my best recommendation is to read and use your manufacturer's guidelines for occupation times. For precise geodetic work, observations of one hour or more would generally be recommended.

Martin: Unfortunately, there is not an easy answer to this question. Occupation time for a 10 km baseline will vary greatly depending on the type of GPS equipment used and the conditions under which the data is collected. At one end, a user with an L1 only GPS receiver who will post process the data should occupy this baseline for 20-30 minutes. In most conditions, this will result in a favorable observation. At the other end, a user with a dual frequency real time kinematic system can determine a 10 km baseline with just seconds of occupation time.

Ladha: How long you need to occupy stations for a 10 km baseline depends on the system you are using and what type of accuracies you require. There are three major categories we can look at:
      For a DGPS system using only L1 C/A-code data, in theory all you will require is a single epoch of common data. Typically, you would log a few minutes worth of data. The type of accuracy you can expect out of this system would be in the meter range.
      For a DGPS system using L1 C/A-code and carrier data, you will require approximately 3 minutes of data including the initialization procedure under optimal conditions. This type of system will provide you with accuracies in the decimeter range. If centimeter level accuracy is desired, you will require approximately 10 to 15 minutes of data, again under optimal conditions.
      For a DGPS system using L1 C/A-code and carrier data along with L2 P-code and carrier data, you will require approximately 2 to 3 minutes of data under optimal conditions. This type of system will provide you with accuracies in the centimeter range.
      The term "optimal conditions" refers to five or more healthy satellites being tracked with a geometric dilution of precision - GDOP value of less than five and relatively low multipath.

Note: The above situations apply to both real-time and post-processed solutions with minor differences.

About the participants:
Chris Dietsch is a product test engineer for Trimble Navigation Ltd. in Sunnyvale, Calif. He may be reached at 408-481-8000 (phone), 408-481-8699 (fax), or e-mail: [email protected]
William Martin is marketing manager, survey products at Ashtech in Sunnyvale, Calif. He may be reached at 408-524-1400 (phone), 408-524-1500 (fax), or e-mail: [email protected] Naush Ladha is an applications engineer in NovAtel GPS's Sales and Marketing Division in Calgary, Alberta, Canada. Ladha may be reached at 403-295-4564 (phone), 403-295-4901 (fax), or e-mail: [email protected]

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