GPSQ&A
ByKeith Kirkby

Q.What advantages do dual frequency GPS receivers offer over single frequency? I.H. Boston, Mass

A.Dual frequency GPS receivers offer two major advantages over single frequency equipment:
     1. Ionospheric errors that are inherent in all GPS observations can be modeled and significantly reduced by combining satellite observations made on two different frequencies, and
     2. Observations on two frequencies allow for faster ambiguity resolution times and On-The-Fly (OTF) technology.
     The first feature will improve differential GPS accuracy over long baselines (typically 10 kilometers or more). The effects of the ionosphere at two receivers located a short distance apart is essentially the same at each location. As a result, these errors are eliminated in the differential processing of the data. In fact, an "iono-free" L1/L2 solution is often discouraged over short baseline observations since combining the data from each frequency will introduce extra noise into the solution that exceeds any ionospheric errors that are eliminated in the process.
     As the baseline distance between receivers increases, this correlation weakens and the ionospheric errors can become very significant. This happens because the atmosphere that the GPS signal passes through is different at each respective station. By taking measurements on two frequencies, the different effects of the ionosphere on the GPS signal at each station can be modeled, and the errors reduced. This allows for more accurate baseline measurements over long distances.
     The second feature, OTF technology, is useful for kinematic surveys and can provide rapid-static capabilities. In order to provide geodetic level accuracy (1-3 cm), the GPS receiver must be able to resolve the cycle ambiguities of the carrier frequency. With conventional single frequency receivers, this process can take anywhere from 30 to 60 minutes depending upon many factors, not the least of which is the length of the baseline.
     Dual frequency equipment allows for an operation called wide-laning which can significantly reduce the amount of time required for ambiguity resolution. Static surveys can be carried out much faster in a "rapid-static" mode where station occupation times are, again, significantly reduced. In addition, in the event of loss of satellite lock in the middle of a kinematic survey, carrier ambiguities can be solved while in motion or "On-The-Fly."
     In general, dual frequency GPS receivers will provide a faster, more accurate, and more reliable solution than single frequency equipment. They do, however, cost significantly more to purchase, thus it is important for potential GPS buyers to carefully consider their current and future needs.

Q.When using a stand-alone receiver for navigation, will SA degrade velocity and direction information to the same extent as position (i.e. 40-100m)? P.C. Thornwood, N.Y.

A. Selective availability (SA) consists of intentional biases or errors placed in the GPS signal by the United States Department of Defense. These errors can be mitigated by applying differential processing techniques. It is not possible, however, to avoid the impact of SA on stand-alone receivers, and a position accuracy of plus or minus 40 to 100 meters must be accepted as long as SA is on.
     Velocity vectors (speed and direction) are affected by SA also, but because these calculations involve a difference operation between successive satellite measurement epochs, the error is reduced. As a result you can expect velocity accuracy approaching plus or minus 0.25 m/s, 0.6 mph, or 0.5 knots assuming phase measurement capability and a relatively high measurement rate (i.e. 1 Hz or better) by the GPS receiver.
     Direction accuracy is derived as a function of the vehicle speed. A simple approach would be to assume a worst case 0.25 m/s cross-track velocity that would yield a direction error function something.
     For example, if you are flying in an airplane at a speed of 120 knots, or 62 m/s, the approximate directional error will be degrees.
     Another example applicable to hiking would be to consider an average walking speed of 3 knots or 1.5 m/s. Using the same error function yields a direction error of about 9.5 degrees. It is obvious, then, that a faster vehicle speed will allow for a more accurate heading indication. As the vehicle slows down, the velocity information will become less and less accurate. If the vehicle is stopped, a GPS receiver will still indicate some kind of movement at speeds between 0 and 0.5 m/s in random and changing directions. This is a direct result of SA and represents the random variation of the static position.
     In a navigation capacity, the velocity information provided by your GPS receiver will be as or more accurate than that indicated by conventional instruments as long as the vehicle is moving at a reasonable rate of speed. It is important to set the GPS measurement rate fast enough to keep up with all major changes of the vehicle's speed and direction. It is important to keep in mind that although the velocity vector is quite accurate in terms of heading and speed, the actual track of the vehicle might be skewed or offset from the true track by plus or minus 0 to 100 meters as per the standard positional error caused by SA.

QUESTIONS ABOUT GPS? Send your GPS questions to us at: EOM, 4901 E. Dry Creek Rd., Suite 170, Littleton, CO 80112, fax to 303-713-9500, or send via e-mail: [email protected]

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