James P. Reilly Ph.D.

Jan Van Sickles’ third edition of “GPS for Land Surveyors” is written like a textbook with questions and answers in a review format at the end of each chapter. I commend him on this format because it makes the concepts in the book easy to learn. ...

We constantly hear about the speed of technological innovation in today’s society. But changes in GPS satellite technology come slowly. Unlike the auto industry, it’s not necessary to come out with a different model each year.

Although I don’t normally write about National Geodetic Survey (NGS) software programs, I’m making an exception in this column. OPUS-RS (Online Positioning User Service-Rapid Static) is an operational program available to GPS surveyors at www.ngs.noaa.gov. The inspiration for writing on this topic came from an old friend. Charles R. Schwarz was an office mate of mine when we were graduate students at The Ohio State University.

It’s been nearly 26 years since the former Soviet Union launched the first Glonass spacecraft on Oct. 2, 1982. Since that time, there have been regular launches with as many as three satellites put into orbit simultaneously. The constellation is in three orbital planes inclined 64.8° to the equator.

On Feb. 15, 2008, an Associated Press article appeared in major newspapers across the country announcing that the Pentagon was going to fire a Navy missile to destroy a broken U.S. spy satellite before it re-entered the atmosphere.¹

As many readers know, the horizontal control stations in the NAD 83 system were readjusted and the results published in February 2007. It’s a national readjustment; all readjustment stations are in the same coordinate system.

The first two articles in this series (POB August 2007 and October 2007) explained how to solve for geoid heights, N, using Stokes’ equation. The gravity anomaly in that equation, Δg, has many different forms.

The previous article in this series, “Physical Geodesy 101,” (POB August 2007) described the methods of collecting gravity measurements.

Geodesy is not a dead science as some people think. GPS satellites are geodetic satellites. Their orbits have origins at the center of mass of the earth, and are tracked from stations located on a geodetic datum or reference frame.