GPS & Sea Level Case Study

Tide gauges have been used to measure sea level for hundreds of years. These tide gauge data are useful in many different ways. Scientists use tide gauge data to validate the measurements from altimetry satellites that are used to estimate global sea level. Local fishing communities and other boat users need to be able to predict tides to safely use harbors. Most tide gauges consist of a float isolated from high-frequency wave motion by a stilling well, but acoustic, pressure, or microwave-based systems are also now in use. Recently the GPS community has started using reflected GPS signals to measure sea level. Here we describe how to measure sea level using a GPS receiver.

Figure 1.GPS receivers and antennas are optimized to receive the direct signal. If the surface is relatively smooth, a reflected signal can also be observed. The reflected signal interferes with the direct signal, and causes an interference pattern in the SNR data. Credit: Kristine Larson.

GPS receivers and antennas are optimized to receive the direct signal. If the surface is relatively smooth, a reflected signal can also be observed. The reflected signal interferes with the direct signal, and causes an interference pattern in the SNR data. Credit: Kristine Larson.

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Figure 2.Here we show a simulation for a site where the tides vary by 3 meters. High tide is then approximately 4 meters below the antenna; low tide is 7 meters. The SNR data we would expect to see for these scenarios is also shown. Credit: Kristine Larson.

Here we show a simulation for a site where the tides vary by 3 meters. High tide is then approximately 4 meters below the antenna; low tide is 7 meters. The SNR data we would expect to see for these scenarios is also shown. Credit: Kristine Larson.

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Figure 3.SNR data for four satellite tracks are shown for a single day of data. This green track has oscillations with the shortest period, and is farthest away from the antenna, which indicates the lowest tide level of the four signals. The satellite in red (longer periods) is closest. Each dataset extends from an elevation angle of 5 to 15 degrees (shown as sin(5) an sin(15)). Credit: Larson et al. 2013.

SNR data for four satellite tracks are shown for a single day of data. This green track has oscillations with the shortest period, and is farthest away from the antenna, which indicates the lowest tide level of the four signals. The satellite in red (longer periods) is closest. Each dataset extends from an elevation angle of 5 to 15 degrees (shown as sin(5) an sin(15)). Credit: Larson et al. 2013.

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Figure 4.Two weeks of GPS tide gauge results are shown for Friday Harbor, WA, compared to a stilling well tide gauge operated by NOAA. Credit: Larson et al. 2013.

Two weeks of GPS tide gauge results are shown for Friday Harbor, WA, compared to a stilling well tide gauge operated by NOAA. Credit: Larson et al. 2013.

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