1996 June           updated 97 Jan 7

Free GPS for Ohio Kentucky Indiana

In June 1995 the Department of Defense completed deployment of 24 Satellite Vehicles (SVs) in orbit 20,200km above Earth representing the space-based component of the Global Positioning System (GPS). At that time it became the mission of NCAD to explore our role in innovating GPS technology in the Ohio-Kentucky-Indiana region based in Cincinnati. This is a natural extension of our mission to serve private and government professionals in Surveying, Civil Engineering, Architecture, Planning, and Geographic Information Systems.

With billions of our tax dollars, DoD has established a positioning system that while serving a variety of military and defense purposes is also globally available for civilian use. With GPS, we can achieve highly accurate geodetic positions and baseline measurements accurate to 1mm horizontal and 2mm vertical. Because GPS technology is critical, not widely understood, highly technical, and based on microprocessor and software technology, NCAD is responding to the need for understanding, clarification, simplification and application to our community interests.

The most familiar name in GPS is Trimble. Thus as Michael S. Reed joined NCAD to assist in GPS research, his first assignment was attendance at the first Trimble Surveying & Mapping Users Conference & Expo in Santa Clara 95Aug9-11. We supplemented this exposure to Trimble with a one-day training session at the Illinois regional office outside of Chicago and grateful attendance at the riverboat hospitality party in Nashville at GIS/LIS 95Nov12.

Other primary manufacturers of high-end GPS technology include Texas Instruments (TI), Motorola, Allen Osborne Associates (AOA), Magnavox, Rockwell, Novatel, and Ashtech. In as much as GPS represents a tool that may be used for measuring land-based distances, companies that provide tools for land-based surveying are buying GPS technology made by primary GPS manufacturers. For example, Leica, who had already acquired Wild Heerbrugg of Germany, acquired the GPS division of Magnavox. Topcon America buys OEM from AOA, while Topcon Japan buys from Ashtech. Sokkia, which had previously acquired Leitz, buys OEM from Ashtech; and Geotronics buys OEM from Ashtech.

We constructed a spreadsheet to catalog receivers (columns) with features and capabilities (rows), using vendor-provided spec sheets and other, less complete, published product matrices. While this was fruitful for our continuing education, after 78 columns and 118 rows, no clear differences were evident in our quest to find the best producer to represent as application engineers.

Early in 1996, I began to reason as an electrical engineer: (1) the primary technology in GPS is signal processing; (2) a basic parameter applicable to signal processing is Signal-to-Noise Ratio (SNR). We began a phone query of each receiver manufacturer to assess SNR specs. Only Ashtech responded to this query. James Stowell called to begin the story of SNR in GPS and sent textbook excerpts for background. Another item sent was a 144-page document obscurely titled, obscurely written, multi-authored, not dated (but published in November 95), produced by UNAVCO (University NAVstar Consortium) and entitled: UNAVCO Academic Research Infrastructure (ARI) Receiver and Antenna Test Report.

It turns out this is the most important report since the beginning of GPS because it represents a current, rigorous, comparative testing of the five leading (in terms of capability) GPS receivers, purportedly impartial based on public funding from the National Science Foundation. UNAVCO, based at the U. Colorado in Boulder, represents a consortium of University-based researchers funded by the NSF but receives products from manufacturers--especially Trimble. Stowell, an Associate Professor and honorary doctorate at the U. Colorado, co-founded UNAVCO with Dr. Randolph ("Stick") Ware in July 1986. Based on what is evident in the report, Stowell left academia and joined Ashtech in April 1995.

Since we quickly perceived the Report was more compelling than all the specs tabulated in our prior work, we directed the same spreadsheet strategy to tabulating the Report. The result is a five-page spreadsheet copyrighted by NCAD, entitled: Normalized Summary of the UNAVCO GPS Receiver/Antenna Test Report, November 1995. The strategy was to normalize the scoring for each test result; for each of the five receivers, the score on each test is either 0 for average, -1 for less than average, or +1 for better than average. This scoring method was applied to real numeric results as well as to graphs requiring informed visual interpretation.

This document was made widely available and freely passed to attendees of the ASPRS/ACSM Annual Convention & Exhibition in Baltimore 96Apr22-25, some informally providing critical review of the initial document dated 22 Apr 96 resulting in the 2nd Draft dated 2 May 96. On 97 Jan 7 the 3rd Draft is released featuring a thorough cell-by-cell review, correction, and application of more automated scoring. The UNAVCO Report may be obtained from www.unavco.ucar.edu/docs/science (The file ARI_test.ps may be output to a postscript printer.), or we can provide a bound document for $10 (copy expense).


CORS: "Erla"

The National Geodetic Survey (NGS) coordinates the current nationwide implementation plan of Continuously Operating Reference Stations (CORS). CORS have been implemented on the coasts by the US Coast Guard, on inland waterways by the US Corps of Army Engineers (one planned for Louisville), and (in Phase 1) at 29 selected airports by the Federal Aviation Administration (one planned in Dayton). At each CORS, GPS range distances are computed in the GPS receiver upto 20 times per second and recorded at 5 or 30 second intervals in synchrony with other CORS sites and with the 24 SVs whose positions are known instantaneously based on atomic clocks with nanosecond accuracy.

Since the actual geodetic location of each CORS site has been determined by high accuracy survey on the International Terrestrial Reference Frame (ITRF-94), its 3D coordinate position is fixed and known. When it computes a range distance from each visible SV (subject to distortion by the military), the GPS solution will be incorrect. But the correct range distance is known since the SV position at each instant is known. The initial GPS range solution is then differenced with the known range and the result is a Correction for each SV.

The GPS user in the field sets the antenna over a point to be located and, by way of the attached "rover" receiver, computes a GPS range distance to the same set of at least 4 SVs that is also incorrect. But since the GPS solution was computed from the same satellites, by the same method, and at the same time, the Correction from the Reference may be applied to the rover to achieve the accurate result. This is differential GPS and is applicable to rover positions as far as 150 miles from the Reference site, although accuracy improves as this distance is reduced.

If the Correction is transmitted from the Reference station (by radio), then the accurate result may be obtained in real time. Generally, the term DGPS implies the use of this radio link. Otherwise, the incorrect GPS solutions are recorded in the rover and downloaded to a desktop PC in the form of a file that is post-processed with the corresponding file obtained (usually by Internet) from the Reference Station to achieve the accurate result after the fact. An even more accurate result is obtainable one week later when data describing SV orbital positions more precisely is made available by NGS, Scripps Oceanographic Institute, or the Jet Propulsion Lab (JPL).

A significant aspect of the CORS program is that the data recorded at each CORS site is available for the public at no charge. In view of the CORS program, it becomes desirable to plan a regional strategy for using GPS, exploiting the public resource of the CORS network and thus minimizing dependence on multiple, costly field receivers deployed as network nodes and base stations in lieu of the CORS.

NCAD has established the new CORS site for Ohio-Indiana-Kentucky. "ERLA" is located in Erlanger, KY, at an elevation of about 950’ above sea-level (775’ above ellipsoid) and features the Ashtech choke-ring antenna and Z-12 dual frequency receiver. The internet service of NCAD.net provides free access to the highly-precise range data enabling lower-cost and more efficient positioning activities by public and private practitioners within a 150-mile radius. The same data is also available from NGS in RINEX format at ftp.cors.ngs.noaa.gov. We welcome your enquiry and look forward to assisting your implementation of this new utility.