Science Spotlight

Station PBAY

Researcher: Max Kaufman
University of Alaska

Everywhere you look in Alaska, there is evidence of the dynamic and active processes that shape the land, build the mountains, and tear them down again.

GPS station PBAY photographed at low and high tide. (Credit: Clara Chew)

State: AK
Country: United States
Elevation: 19.5 m
Lat/Long:  59.5727 / -151.2722

Kachemak Bay

The relationship between vertical motion of the land and sea level rise needs to be better understood in order to provide useful information to coastal decision makers on a variety of issues such as land use and community planning. In Kachemak Bay, Alaska, across from the town of Homer, coastal uplift due to tectonic and postseismic deformation (following the great earthquake in 1964) and rapid ice-mass loss from immense ice fields has influenced coastal habitat and affected coastal communities in many ways that have largely gone undocumented. For these reasons, the University of Alaska installed a GPS receiver at Peterson Bay (PBAY) in 2011. They will be using the GPS vertical positions to estimate how fast PBAY is moving. These rates are likely to be small, as little as 1-2 mm/yr. While these small ground motions are going on, the effects of very large tidal forcings can also be observed in the GPS data. The reflections group at the University of Colorado have used the GPS data to measure sea level. In this spotlight, these sea level reflections are highlighted.

Figure 1. Location of PBAY, Homer, and Kachemak Bay. The GPS sea level estimates are compared to traditional sea level measurements made at Seldovia.

Figure 3. The island is in the middle of Peterson Bay (green arrow).

Figure 5. A GPS receiver measures the strength of the signal, in the Signal to Noise Ratio (SNR) data. For geophysics, the direct signal (red) is important. For reflections, the reflected signal (the black trace with the red trace removed) is important. The frequencies in the data shown for the rising and setting arcs (shown at the bottom) are used to estimate sea level.


Figure 2. PBAY is located on a small island.

Figure 4. At low tide you can walk to PBAY's island.

Figure 6. A comparison of two weeks of GPS sea level measurements from Peterson Bay and the Seldovia tide gauge located 30 km away. The tide gauge provides measurements every 6 minutes, while the GPS measurements are only made when a satellite is rising or setting over the ocean.

Spotlight Questions

This website describes the harmonics that you see in a typical tide gauge time series.
  • The biggest harmonics are M2 and S2. Why are they called this?
  • The tides at Peterson Bay are very large. How do they compare with the famous tides from the Bay of Fundy?
  • Can you name some practical reasons why it is important to be able to accurately predict tides?

Last modified: 2019-12-26  16:24:51  America/Denver  


Please send comments and corrections to

Copyright © 2012 - 2024 UNAVCO and the GPS Reflections Research Group.
All Rights Reserved.

Funding and Acknowledgements.