Monitoring Water Levels and Space Weather Using Global Position Satellite (GPS) Receivers
Location of Data
Homer, Alaska: Trial data are being finalized prior to being made public.
Utqiaġvik, Alaska: Siting and permitting for this installation is in process 2020
Accurate water level observations are fundamental for storm-surge forecasting, informed emergency response, ecosystem management, safe navigation, and efficient mapping/charting. Portions of Alaska’s remote coastline are among the nation’s most vulnerable to geohazards such as coastal sea level rise, tsunami, extra-tropical storm surge, and erosion. Observations in Alaska are severely lacking to support water level warnings and ice forecasts despite these vulnerabilities. Gaps in active observations of water level variations are limiting our state’s ability to provide useful marine forecasts and endangering coastal populations and infrastructure. This is in-part because of obstacles including seasonal ice, lack of coastal infrastructure and rapid coastal erosion, which make using conventional water level sensing technologies difficult if not impossible for year-round observing.
Approved NOAA Center for Operational Oceanographic Products (CO-OPS) National Water Level Observation Network (NWLON) technologies primarily consist of in-water sensors in stilling wells or down-looking microwave systems, and station siting is heavily reliant on ice-free conditions and the existence of local infrastructure. Systems are duplicative, meaning there are two water level measurements being made at each NWLON station for redundancy. NWLONs are expensive and remote installations and annual operations and maintenance of a more widespread series of NWLONs are cost prohibitive for most of the low infrastructure coastline in Alaska. Currently, the entire west and north coasts of Alaska – a span of over 1600 miles by air – have only five NWLON tide gauges. A comparable span of coastline along the east coast has 85 NWLONs. Though NWLON installations are always desirable, a tiered water level data policy that designates how various quality levels of data can be applied for coastal navigation and hazards applications, allows for observations with lesser accuracies and redundancy. Within this policy, water level data quality tiers A (e.g., NWLON being the most stringent and accurate), B and C data designations are required to match data accuracy requirements to the use of the measurements.
AOOS and the National Weather Service (NWS) have been researching the efficacy of a land-based, GPS/Global Navigation Satellite System (GNSS) reflectometry water level measurement approach as an alternative to provide a Tier B water level measurement (within 11 to 30 cm of an NWLON measurement for example). The GPS/GNSS systems do not rely on coastal infrastructure and can be operated and maintained long-term in real-time year-round for a fraction of the cost of an NWLON installation. They also provide the quality level data necessary to improve storm surge forecasts while providing adequate real time water level conditions for communities.
This project was initiated to assess the ASTRA, LLC commercial GPS receiver system currently used for space weather observing, and evaluate its potential for remote deployments along low-infrastructure regions across the state for making water level observations. After three years of successful trial deployments and evaluation, AOOS is now working with ASTRA on moving this system into an operational application for water level observing.
Larson et al. [2012; 2013;2017] demonstrated that reflected GPS signals can be utilized to monitor changes in the height of the reflecting surface, thereby allowing GPS receivers located near the shoreline to be used as ‘tidal gauges.’ ASTRA, LLC has developed and commercialized dual-frequency GPS receivers that are designed for space weather monitoring activities, but also provide other observations, including soil moisture, snow depth measurement, and ocean water level. In April 2017, AOOS contracted with ASTRA to install two discrete dual-frequency GPS receivers in Seward, Alaska to test this technology near an existing NWLON station 9455090 for comparison. These installations were deployed for one year, then one was relocated to Homer, AK to test the method on a beach with a long tidal excursion, to mimic similar topography in western Alaska. These trial deployments informed on installation requirements and limitations, endurance, and data processing methods and requirements to maximize data quality. ASTRA continues to work on automating and operationalizing both the data processing and data telemetry components of their system to provide the capability to report converted water level observations directly from the installation site. In 2020-21, AOOS is working to deploy a permanent, real time operational ASTRA GPS installation in Utqiaġvik, Alaska.
The basic approach for using GPS for water level observing uses reflected GPS satellite signals to determine the height of a reflecting surface, such as the ocean, relative to a stable GPS antenna of fixed local height. The total received GPS signal measured by the antenna is the sum of the direct signal and the reflected signal. The interference between these two signals depends on the satellite altitude in the sky and on the receiver height above the ground. Given the satellite altitude is known, the observed interference pattern as the satellite rises/sets is used to extract the receiver height, after which the antenna height is subtracted to determine the true water level.
The year-long pilot study conducted in Seward, Alaska enabled the performance of the ASTRA GPS receiver to be evaluated against water level measurements from a NOAA-operated NWLON station 2 km away. The water level data comparisons between the two methods showed acceptable agreement with the GNSS-R major tidal constituents (M2, S2, N2, K1, and O1), measuring within 5 cm of the NWLON estimates (Janzen et al., 2018). The Seward sea trials using GNSS-R for water level observing have provided valuable logistical installation information for future remote deployments. For example, this installation indicated that deployment near mountainous regions can block satellite coverage and reduce the data rate during certain times of the day. GPS receivers provide high frequency data rates for water level observing, and fortunately, the reduced data rate in Seward did not prevent tidal harmonic and subtidal analyses of the data. However, other obstacles also posed problems. During a period of time when a ship was moored near the GPS receiver installed on the Seward Marine Pier, the ship caused interference in the data quality, causing the loss of about 30 days of data. This trial installation was on the pier as it had the best exposure to the ocean, but this trial illustrated the importance of locating installations away from areas frequented by boats.
|Tidal Constituent||ASLC ASTRA 85.6%||NWLON 96.9%||NOAA Predicted Constituents|
This table shows the Alaska Sea Life Center ASTRA installation astronomical tidal amplitudes (meters) with 95% confidence estimates and the same for the nearby (2 km) Seward Marina NWLON observations for period August 1 –November 1, 2017 (92 days). The reported ratio is the percent variance predicted to the variance of the original. Accuracies are shown for the M2 and S2 dominant tidal constituents. The ASTRA observations produced an M2 tidal amplitude estimated within 5-6 cm of the NWLON, within 4%. Water level accuracy must be within +/- 30 cm on the tidal datum. Preliminary error analysis on the ASTRA Seward raw water level data outputs (not filtered for noise, though outliers removed) indicates an error of about 35 cm, nearly qualifying for Tier B water level data quality applications. Part of the next phase of operationalizing these systems will be to determine filtering techniques to further reduce erroneous data and bring results into Tier B level accuracy.
In 2018, one ASTRA GPS receiver was installed on the Homer Spit in Homer, Alaska. The Homer installation provided a unique but challenging opportunity to demonstrate the ASTRA GPS receiver system for measuring water levels in a large inter-tidal zone, which is common to most of Western Alaska. Results from this trial are currently being drafted in report.
In 2019, AOOS and ASTRA started planning to deploy a permanent water level GPS station in Utqiaġvik. ASTRA will provide their modified GPS system that provides real time water level outputs directly from the installation for preliminary data reporting (as with all real time data systems). These data will be made available for analysis by the community and post processing techniques refined to bring the data accuracy to Tier B.
Publications and Professional Presentations
Andrew Gisler, Irfan Azeem, Erik Stromberg, Adam Reynolds, Geoffrey Crowley, Carol Janzen, and Molly McCammon “Monitoring ocean water level in remote shoreline locations using GPS reflectometry”, Proc. SPIE 10631, Ocean Sensing and Monitoring X, 1063111 (25 May 2018); https://doi.org/10.1117/12.2305157
Janzen, C. D., Thompson, G., Gisler, A., Reynolds, A., Azeem, I., Crowley, G., et al. (2018). “Using space physics technologies for accurate land-based water level measurements,” in Proceedings of the Conference on AGU Ocean Sciences Meeting, Portland, OR.
Larson, K. M., Löfgren, J. S., and Haas, R. (2012). Coastal sea level measurements using a single geodetic GPS receiver. Adv. Space Res. 51, 1301–1310. doi: 10. 1016/j.asr.2012.04.017
Larson, K. M., Ray, R. D., Nievinski, F. G., and Freymueller, J. T. (2013). The accidental tide gauge: a GPS reflection case study from Kachemak Bay, Alaska. IEEE Geosci. Remote Sens. Lett. 10, 1200–1204. doi: 10.1109/LGRS.2012 2236075
Larson, K. M., Ray, R. D., and Williams, S. D. P. (2017). A 10-year comparison of water levels measured with a geodetic GPS receiver versus a conventional tide gauge. J. Atmos. Oceanic Technol. 34, 295–307. doi: 10.1175/JTECH-D-16-0101.1
Funding Sources & Partners
> NOAA National Weather Service through AOOS’ Cooperative Agreement with NOAA IOOS
> Nic Kinsman, NOAA
> Nathan Wardwell, JOA (John Oswald, Associates)
Special thanks to Konna Raub from Homer, who allowed us to install a GPS receiver on her business on Homer Spit.