Science Spotlight

Station SC02

Researcher: Eric Leuliette

SC02 is located on San Juan Island, Washington State. The NOAA tide gauge is approximately 300 meters away.

Name: SC02 PNGA
State: WA
Country: United States
Elevation: -15.0 m
Lat/Long:  48.5462 / -123.0076

Friday Harbor GPS Tide Gauge

Satellites, like Jason-2 (Figure 1), carry altimeters that bounce radar beams off the ocean thousands of times each second. The ocean acts like a mirror and reflects some of the radar beam back to Jason-2. Since the radar beam travels nearly at the speed of light, the time that it takes the beam to get back to Jason-2 tells us the height of the ocean (Figure 2). To make sure that we are measuring it correctly, we compare the sea level we see from Jason-2 against sea level at tide gauges around the world (Figure 3). One problem with this is that the land the tide gauges are attached to moves up or down (Figure 4). To fix this problem, we use GPS to measure the motion of the land near the gauges.

In addition to telling us about ground motion, the GPS data from SC02 can also tell us about sea level. Part of the GPS signal reflects from the sea surface. If you focus on those reflections, you can determine sea level (Figure 5).

To find out more about sea level measurements, please see Eric Leuliette's short video on youtube.

Figure 1. The Jason-2 satellite altimeter.

Figure 3. Location of the tide gauges that are used to validate the altimeter data.

Figure 5. GPS-derived sea level (blue circles) compared with the nearby NOAA tide gauge.


Figure 2. Global sea level measured by altimeters over the past twenty years.

Figure 4. Position changes for SC02 in a North America fixed reference frame. (For help interpreting the graphs, see the GPS Data page.)

Figure 6. A ten year record of GPS data has been analyzed by Richard Ray, Simon Williams, and Kristine Larson. This figure shows the excellent agreement found between the monthly averages for sea level using GPS (blue) vs. the NOAA tide gauge (red).

Spotlight Questions

  • There is a recent dip (between 2010 and 2012) in the global sea level curve. Can you find out online what causes this dip?
  • How do the position changes for SC02 compare with nearby sites like ALBH and NEAH?

Last modified: 2019-12-26  16:24:50  America/Denver  


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