Point of Beginning

GIAA Mailbag

September 1, 2004
Tool tips.

Q: We have been told that GPS antenna phase center error is one of the causes of uncertainties in GPS measurements. What is this?

A: When surveying with optical instrumentation, a physical point in a total station is plumbed using a plumb bob or an optical plummet at a known height over a point on the ground. This height is measured to the intersection of the horizontal and vertical axes of the instrument. This is often referred to as the instrument's measuring reference point. If properly adjusted, the optical plummet or plumb bob is used to represent the vertical line passing through the instrument's measuring point and the point on the ground. GPS antennae housings also have marks to enable measurement of the height of a physical point or plane in them over the ground. However, the physical point in a GPS antenna only represents a "best-fit" of the many points at which the phase of all satellite signals are measured; this "best-fit" is determined by the manufacturer. Depending on antenna design and satellite elevation and azimuth, the electromagnetic field node at which the receiver does measurements can vary over time in horizontal and vertical position by up to a few millimeters. Some antennae have a smaller spread in these positions than others. Many antennae have been evaluated by their manufacturers or third parties to characterize their phase center shifts as a function of the elevation and azimuth of the satellite signal. This characterization information may be available through the manufacturer and from the National Geodetic Survey (NGS). It may be that your GPS processing software integrates this information into the solutions it generates to minimize uncertainties from this cause.

Reduction in apparent target width due to phase.
Q: What is "phase error"?

A: "Phase error" existed before "phase center error" (see above question) was coined. Phase error refers to errors in GPS measurements contributed by uncertainty in the position of the measuring point in a GPS antenna. Phase error generally refers to a theodolite pointing error occurring from uneven lighting of the observed target. Its cause can arise from the shape, color and lighting of the target, clarity, brightness and ability to accurately transmit colors to the eye of the telescope's optics as well as the acuity of the observer's eye. When observing a target with a theodolite, it is possible that the cross hair will not be centered on the target (either horizontally or vertically or both) due to a change in the appearance of the target. This is best illustrated in the case of a target that has a circular or square cross section such as a range pole. If the sun is shining brightly and its rays strike the pole from a direction that is approximately 45º from the direction that the observer views the target (i.e. over the observer's right or left shoulder), the pole will be brightly illuminated along a vertical line on the pole that is on a radial projection from the center of the pole towards the sun. As you proceed right or left from this principal axis of the sun's rays and observe which parts of the pole the sun lights, this lit area will end at 90º to the left and right from this principal axis. As a viewer proceeds from this demarcation line and around the pole, the pole will be in shadow. Given the brilliantly lit part and the part in shadow, an observer may only "see" part of the pole, depending on the conditions noted above. Thus, an error in pointing due to phase is created, if the cross hairs are placed only on the part that appears to exist, which is a fraction of the entire width of the target.