The Evolution of GPS

In the GPS arena, a new Civil Frequency (L5) is in the planning stages for future satellites. Although not scheduled to be deployed for five years, this advancement will greatly increase the accuracy of GPS and decrease the time required for high-accuracy applications. High-accuracy positioning on moving platforms (auto, plane, boat) will be possible in real time, without the need to post process data through a computer to obtain solutions.

Also, the former Soviet Union and European Space Agencies will launch their own Global Satellite Navigation Systems which can be integrated with our current GPS system. As the number of space vehicles (satellites) increase, so to will the accuracies that can be obtained.

Evolving Technologies

LIDAR (Light Detection and Ranging) and INSAR (Interferometric Synthetic Aperture Radar): these and other interferometric imaging techniques will play a major role in future subsidence detection and tracking as sensors and science improve.

The first of thirteen (13) deep borehole extensometers (designed and installed by the United States Geological Survey – USGS – in the early 1960s) were used in preparation for the soon-to-be-built manned spacecraft center. Of the thirteen in operation today, six (6) of those are “subsidence” or “total depth” monitors (meaning their bottom is below the aquifers from which we draw water), and the other seven (7) are less than total depth, or “compaction” monitors.

What are they and how do they work? Borehole extensometers are deeply anchored benchmarks. To construct each, a hole is drilled to a depth at which the strata are stable. The hole is then lined with a steel casing with slip-joints to prevent crumpling as subsidence occurs. An inner pipe rests on a concrete plug at the bottom of the borehole and extends to the top. This inner pipe then transfers the stable elevation below to the surface. A measurement of the distance from the inner pipe to the surrounding land surface provides us with the amount of compaction that has occurred.

Although the accuracy of this measurement method is impressive, there is one drawback. The high cost to construct and install the equipment prohibits their use in sufficient numbers, resulting in a lack of adequate information for the entire Harris-Galveston and Fort Bend areas. And, as was stated in the overview, the sufficient amount of and wide distribution of data is extremely important. Over time, as technologies have evolved, we have moved toward more cost-efficient and equally accurate forms of measurement…but borehole extensometers are playing an important role in this new era. Three of our existing extensometers have been outfitted with GPS (Global Positioning System) antennas, and are now the only stable GPS points within the greater Houston area.