Using GNSS Signals to Measure 
Water Levels in ITRF 
Kristine M. Larson | Makan Karegar | Simon Williams 
1 GNSS Instruments and Water Reflections 
GNSS instrumentation has been used in land surveying and geodetic position- 
ing for many decades. More recently it was demonstrated that these same GNSS 
instruments can be used to accurately measure water levels (Larson et al. 2013, 
Larson et al. 2017). An instrument that can simultaneously measure three-di- 
mensional position and relative sea level means that it is possible to measure 
water levels in a well-defined terrestrial reference frame. The method of using 
GNSS signals to measure water levels is often called GNSS interferometric reflec- 
tometry (GNSS-IR) because it uses the interference pattern created by the direct 
and reflected GNSS signals. The interference pattern is related to the extra path 
The frequency of the interference pattern 
created by the 
direct and reflected 
GNSS signals depends on H 
Hu{mm} 
MM = 
ih 
a 
GNSS 
antenn;- 
Hacted Sıc 
Fig. 1: Each rising or setting GNSS signal is used to estimate H —- and thus the GNSS site 
serves as a tide gauge defined in ITRF. Data can be automatically analysed at https://gnss- 
reflections.org. 
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