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The Global Positioning System (GPS) project started 50 years ago, in 1973. I was fortunate to be part of incorporating GPS into the National Spatial Reference System (NSRS) when I worked for the National Geodetic Survey (NGS). GPS was not considered operational until 1993, but NGS started performing GPS surveys in 1983. Geodetic control surveys that formerly took six to 12 months to perform using classical methods could be performed with GPS in a few weeks using fewer personnel and resources. It changed the way NGS and others performed their surveying operations.
While one group in NGS was developing programs to evaluate and compute coordinates using GPS, another NGS group was completing the readjustment of the North American Datum of 1983 [NAD83 (1986)]. The analysis of GPS indicated that some of the latitude and longitude values estimated using GPS did not agree with the published NAD83 coordinates. The classical techniques used a triangulateration process (involving angles and distances) that required several triangles to connect two stations that were not intervisible. GPS, on the other hand, could directly measure the distance between the two stations, resulting in more accurate coordinate differences.
To support surveyors, NGS, working with other federal agencies under the auspices of the Federal Geodetic Control Subcommittee (FGCS), developed a GPS test network in the Washington, D.C., area to demonstrate whether a specific manufacturer’s GPS receiver and associated geodetic post-processing software was an accurate relative positioning satellite survey system. This facilitated the use of GPS for incorporating geodetic control in the NSRS. As mentioned above, GPS surveys exposed many inconsistencies between existing NAD83 (1986) control. Organizations such as NGS and state transportation departments that performed control surveys used GPS as soon as equipment met the federal testing requirements because it was more efficient and cost-effective than classical techniques. This led individual states to perform statewide geodetic network projects to upgrade their NAD 83 (1986) coordinates. These surveys were ultimately designated as High Accuracy Reference Networks (HARN).
In the beginning, the attitude of the individual surveyor accepting GPS was one of “trust after verifying.” Many surveyors considered it to be a “black box” that could not be trusted. Surveyors were accustomed to having angles and distances they could write down and check the results. Also, there were some key challenges and limitations of using GPS for surveying in the early days. This included the cost and size of the equipment, the peripheral devices required, the power requirements (including 12v car batteries and generators), “black box” computer processing software, obstructions near monuments, and limited visibility of GPS satellites.
Prior to GPS becoming fully operational, some surveys had to be performed in the middle of the night to have four or more satellites visible during the observing session. This required a significant amount of technical planning, which sometimes required complicated logistics for coordinating observing sessions. Also, at that time, most private surveyors did not perform control projects, so even though GPS may be more accurate, it was not more cost-effective than classical techniques for their typical projects.
Over time, after GPS became operational, more surveyors (and other professionals) embraced using GPS after the cost of receivers decreased, user-friendly processing software became available (e.g., NGS OPUS), Continuously Operating Reference Stations were densified (e.g., NOAA CORS), and statewide Real-Time Networks (RTN) were established (e.g., North Carolina RTN). GPS technology now underpins many sciences, large areas of engineering (such as driverless vehicles and UAVs), navigation, and precision agriculture. GPS (today GNSS) and its applications have changed the way surveyors and geospatial users perform their work, and the world has seen the development of applications that were not ever imagined 50 years ago.