One of the most important advantages to GPS is the ability to have continuous data. Using dual-frequency, full-wavelength GPS instruments (with geodetic antennas), data is collected at 30-second intervals and averaged over 24 hours. This method of data collection means that specific sites can be monitored and assessed on a daily basis. And just as important, the measurements are more reliable and handled at a fraction of the cost than conventional surveying. Improved GPS techniques and processing have reduced the cost of two centimeters. Now that’s progress!
The use of GPS to monitor subsidence has continued to evolve. In the mid-1990s, the HGSD in cooperation with the NGS developed the use of GPS PAMs to provide land-surface elevation or subsidence measurements. Originally, seven portable trailers were constructed to store and secure GPS receivers, geodetic antennas, and associated equipment such as batteries, solar panels, and recording equipment). The trailers were moved weekly to different locations in order to record a week’s worth of GPS data on each PAM every month.
Portable trailer stores the receiver (the yellow box) and the configuration of the geodetic antenna (the circular disk on top of the tripod) used to collect GPS data. Source: HGSD
The HGSD first started installation of permanent GPS monitoring stations in 2000. As of 2019, the HGSD has installed 85 GPS stations and also collects GPS data from over 220 GPS stations across the region.
During the period of monitoring, GPS data are collected every 30 seconds, and then processed and converted to a stable reference frame to eliminate tectonic plate movements. The GPS data include a three-component displacement time series involving the horizontal (East-West), vertical (North-South), and the ellipsoidal height (i.e., elevation above or below the reference ellipsoid) components. The ellipsoidal height is used to estimate land surface elevation and is how the HGSD measures subsidence.
The majority of GPS stations operated by the HGSD collect data periodically and some stations collect data continuously. The periodic monitoring stations collect GPS data for approximately seven days every two months. The continuous monitoring stations, also known as a Continuously Operating Reference Station (CORS), collect GPS data 24 hours a day every day of the year. The HGSD primarily uses periodic monitoring for the GPS stations because less equipment is needed since it is rotated to multiple GPS stations and therefore is less expensive than operating a CORS site.
Periodic monitoring GPS stations are designed as a 2.5-in pipe drilled approximately 35 feet below ground surface with an 8-foot extension pipe above the ground. The extension pipe is mounted with a geodetic antenna. A separate pipe, which holds an enclosure box to store a battery and GPS receiver as well as a mount for a solar panel, is installed within a few feet from the antenna pipe. Both pipes are surrounded by four bollards to provide additional protection and all pipes are secured in a 5’ x 5’ concrete pad.
Example of a permanent GPS station, P023 located in Bayou Vista, that collects GPS data periodically. Source: HGSD
CORS are designed in two ways: 1) the same manner as the permanent GPS stations previously described or 2) mounted on pre-existing structures. As of 2020, the HGSD operates seven CORS that are constructed in the same manner as the periodic monitoring stations. The pre-existing structures include buildings and extensometers. For the building type, GPS equipment is usually mounted to the roof or high along an exterior wall. Additionally, buildings are selected with deep foundations as well as clear sky views to limit interference. The building mount CORS design is used by the UH who operate 71 permanent CORS as of 2020. For the extensometer type, the District operates five CORS (ADKS, LKHU, NETP, TXEX, and P096) installed on top of extensometers as of 2020.