Accurate, reliable, and up-to-date heights are essential for a wide range of economic activities in many professions, including: surveying, engineering, emergency managers, Earth scientists, and natural resource managers [Veilleux, 2013b]. Historically, accurate orthometric heights have been obtained by tying into the control benchmarks of a vertical datum. Spirit leveling and gravity readings are used to establish, maintain, and update the heights of the benchmarks, which is a costly and time consuming process. Contemporary heights can also be established with the Global Positioning System (GPS) and can be combined with a geoid model for a quick and cost effective method of obtaining the orthometric heights used in a vertical datum.
The National Height Modernization Program enables access to accurate, reliable, and consistent heights [Veilleux, 2013b]. This program is being employed by the National Geodetic Survey (NGS), with the goal of implementing a new vertical datum by computing the orthometric heights through the combination of GPS and gravimetric data. The expected result is a high accuracy vertical datum that will establish the orthometric heights with an accuracy that will be sufficient for a multitude of applications in science, engineering, mapping, etc. The accuracy of such a vertical datum is, therefore, dependent on the accuracy of the underlying GPS and geoid models and a better understanding of the error sources associated with the GPS ellipsoidal height and the geoid model may enable orthometric heights to be obtained with a high accuracy. This thesis will assume that an accurate geoid model exists and will focus on any inaccuracies in orthometric heights caused by the GPS-derived height. There are many error sources that may enter into the GPS observable, including: satellite and receiver clock errors, satellite orbit errors, atmospheric delays of the GPS signal caused by the ionosphere and troposphere, receiver bias, environmental multipath, and antenna phase center variation [Grejner-Brzezinska, 2011]. These error sources must be accounted for, if high accuracy heights are to be established through GPS.
This thesis principally examines the effects of station dependent error sources, including phase center variations (PCV), far-field multipath reflection, and near-field multipath reflection [Berglund, 2011]. The effects of neglecting the PCVs particular to an antenna with a radome will be examined to see how much height deviation is caused by not properly accounting for how the radome alters signal reception at the antenna. The effects of multipath caused by the following near-field error sources will be examined: GPS signal interference caused by high voltage power lines, multipath reflection from a snow-covered field, the effects of a robin sitting on an antenna, and the effects of a seagull sitting on an antenna that will be modeled through simulation. In addition, an investigation will be conducted to analyze the level of height variation caused by using different antenna models to determine the height of the same point.
The results of the near-field multipath experiments show that small changes in the snow depth of an area result in a consistent pattern of multipath, while drastic changes in the snow depth of the surrounding environment will alter the magnitude of the multipath reflection. Data collected around high voltage power lines suggests that major obstructions to a GPS signal could perhaps be avoided through site planning and using only those satellites less likely to be obstructed by the high voltage power lines. The test with the birds sitting on antennas showed that the amount of error a bird causes on GPS-derived heights depends on the size of the bird and that the error will eventually average out when the bird leaves the antenna, but will impact instantaneous height estimation in, for example, real-time kinematic (RTK) GPS applications.