Journal of Surveying Engineering

The University of Nottingham has been involved with GPS (global positioning system) research since 1983 and has developed a package of computer programs capable of processing GPS data from a variety of different receivers. Current research covers a wide range of applications, including navigation, precise relative positioning, kinematic surveying, leveling, orbit determination, and refraction modeling. The GPS processing algorithms and the related software are described in this paper, together with a review of past and current GPS research at Nottingham.

Faced with diminished survey forces and a sparse geodetic reference system, the Tennessee Department of Transportation reduced its geodetic survey workload to a minimum. The introduction of Global Positioning System (GPS) technology provided a possible solution to the manpower problem but the need for a consistent statewide reference system remained. A cooperative effort between state, federal, and private sectors resulted in a highly accurate reference network of three‐dimensional points on a moderate budget. The network, designed specifically for GPS use, is tied to the National Geodetic Reference System using the National Geodetic Survey's very long baseline interferometry (VLBI) points as fiducial stations. Similar projects by other states would result in a national network of control points suitable for accuracies available through GPS and provide a framework for a national cadastre.

Conventional surveying operations in the vicinity of Boston's Central Artery/Third Harbor Tunnel Project are hampered by large buildings, severe traffic congestion, pedestrians, and access difficulties. However, the use of the Global Positioning System (GPS) made it possible to meet tight scheduling requirements and at the same time produced accuracies that would have been extremely difficult to achieve using terrestrial geodetic techniques. The projectwide primary network comprises 33 stations and 100 quasi‐independent baselines, which vary in length from 150 m to 11,000 m. Trimble 4000SX receivers were used for the measurements. Double‐difference baseline solutions were computed using the broadcast ephemeris. The minimally constrained adjustment of the baseline components yielded an “average” relative horizontal positional accuracy of 3.8 mm plus 1.7 ppm at the 95% level of confidence. A corresponding relative vertical accuracy of 6.1 mm plus 2.1 ppm of the station separation was obtained for the ellipsoidal height differences. A simple model of geoidal height differences was produced and used for the estimation of orthometric heights.

In the fall of 1987, the National Geodetic Survey (NGS) conducted the first phase of a global positioning system (GPS) survey in Dane County, Wisconsin, in cooperation with the University of Wisconsin‐Madison, the CONSOIL Land Records Modernization Project, and Dane County. The primary objective was to provide the basis for a consistent, accurate, positional reference framework, integrated with the U.S. Public Land Survey System (PLSS), to support a land information system (LIS) currently being developed. Substantial local level efforts, for planning, reconnaissance, and other related activities, were required to support the NGS survey. A county wide network of 141 GPS stations was planned, with a higher density subnetwork, in a nine‐section area, designed to serve as an experimental platform for continuing research. Many PLSS corners originally proposed as GPS receiver sites proved to be incompatible with observation requirements, necessitating placement of eccentric stations and thereby increasing conventional surveying requirements. Results of the first phase are presented, including the network adjustments performed by NGS. Benefits include the opportunities provided for continuing and future research related to LIS development and implementation.

Rough error estimations of positioning are considerably simplified by taking continuous satellite observation distributions into account. Forming normal equations by integration allows explicit calculations of the effects of the measurement errors on position. Application of this method to tropospheric delay shows a very good correspondence with real measurements. An error in refractivity determination of 1% causes a 6‐mm height error per 1‐km height difference. It is shown that the tropospheric ranging errors contaminate the height errors with an amplification factor of nearly three. Baseline determinations can be treated exactly in the same way. As a further example, the effect of phase center variation will be discussed.

This paper examines the techniques of relative static and relative kinematic surveying and discusses the benefits of each. Examples of applications in which both techniques could prove beneficial to the Corps surveyors, such as dam monitoring, are presented. In August 1986, the U.S. Army Engineer Topographic Laboratories (USAETL) began a three‐year program to introduce the use of GPS technology to all U.S. Army Corps of Engineers Districts as a means of performing geodetic surveys. During demonstrations, GPS receivers are used in a relative static mode with observation sessions lasting from 1/2 to 1 hour, depending on the precision required. Although GPS is a much more efficient tool for performing geodetic control surveys, a means of reducing the amount of time spent at each monument would make GPS more attractive for applications such as dam monitoring. The technique of relative kinematic GPS provides the surveyor with the capability to significantly reduce observation time compared to static GPS measurements.

GIS (Geographic Information Systems) will become increasingly important for engineers and planners because of their inherent ability to provide both positional and attribute information about the land. In the past, the expense involved in tying land‐related information to a common, accurate geographic base was largely prohibitive. With the advent of the GPS (global positioning system), a new avenue has been opened to surveyors and others to measure relative geographic positions quickly and accurately. A high‐accuracy GPS reference network provides the means of resolving differences in horizontal datums, which is the most serious obstacle to relating data from disparate sources within a GIS. Initiatives are being taken in New Mexico to establish such a network as a public resource. The New Mexico State Highway and Transportation Department will use the network as the basis for developing a statewide parcel‐level highway data base. This will be part of a statewide effort to promote the improvement and sharing of geographic information.

A new portable dual‐frequency Global Positioning System (GPS) receiver is being delivered to customers by Aero Service. The MINI‐MAC Model 2816 receiver is lightweight, portable, and designed for operation by one person in field environments. The receiver is packaged to withstand harsh environmental conditions and deliver dual‐frequency data of the highest quality. Decoded broadcast ephemerides allow for field data. Time and pseudoranges obtained via the unit's course/acquisition (C/A) code L1 capability are used to obtain precise time synchronization. The receivers record on canonical times, so it is not necessary to start different receivers at the same time in order to maintain synchronous phase observations. The MINI‐MAC 2816 Surveyor is capable of tracking up to eight satellites simultaneously using C/A code on L1 and Aero's proven proprietary codeless methods on L2. An L1‐only version, Model 1816, is also available. Phase lock can be maintained on up to eight satellites in kinematic operations. The receiver is designed for future implementation of kinematic software without any required hardware changes. Various aspects of the receiver are discussed and details of a number of field results are provided.