A continuous monitoring of sea level changes is important for human society since more than 50% of the world's population live within 60 km of the coast. Sea level is traditionally observed with tide gauges that give measurements relative to the Earth's crust. To improve the understanding of sea level changes it is necessary to perform measurements with respect to the Earth's center of gravity. This can be done with satellite techniques, and thus a GNSS-based tide gauge is proposed that makes use of both GNSS-signals that are directly received and that are reflected on the sea surface. A test installation at the Onsala Space Observatory shows that the reflected GNSS-signals have only about 3 dB less signal-to-noise-ratio than the directly received GNSS-signals. A comparison of relative sea level observations from the GNSS-based tide gauge to traditional tide gauges gives an RMS-agreement on the order of 4 cm

Haas, Rüdiger

Johansson, Jan M.

Löfgren, Johan

A continuous monitoring of sea level changes is important for human society since more than 50% of the world\u27s population live within 60 km of the coast. Sea level is traditionally observed with tide gauges that give measurements relative to the Earth\u27s crust. To improve the understanding of sea level changes it is necessary to perform measurements with respect to the Earth\u27s center of gravity. This can be done with satellite techniques, and thus a GNSS-based tide gauge is proposed that makes use of both GNSS-signals that are directly received and that are reflected on the sea surface. A test installation at the Onsala Space Observatory shows that the reflected GNSS-signals have only about 3 dB less signal-to-noise-ratio than the directly received GNSS-signals. A comparison of relative sea level observations from the GNSS-based tide gauge to traditional tide gauges gives an RMS-agreement on the order of 4 cm

L\uf6fgren, Johan

Haas, R\ufcdiger

Johansson, Jan

Chalmers Research

Sea Level Monitoring Using a GNSS-Based Tide Gauge

Global climate change is believed to result in the melting of large masses of ice in Polar Regions, bringing freshwater into the ocean, and changing the sea level. The traditional way to measure the sea level, by tide gauges, results in measurements relative to the Earth’s crust. However, in order to fully understand the sea level changes, absolute measurements (change in sea level in relation to the Earth’s center of gravity) are necessary, in particular in regions affected by post-glacial uplift, e.g., Fennoscandia. Satellite techniques, e.g., GNSS can be used to determine the motion of the Earth’s crust in relation to the center of gravity. By measuring reflected GNSS-signals from the sea surface, information of the sea level change can be obtained. Therefore a GNSS-based tide gauge is proposed.

The proposed GNSS-based tide gauge installation consists of two antennas, one zenith looking right hand circular polarized (RHCP) and one nadir looking left hand circular polarized (LHCP), mounted back-to-back on a beam over the ocean. The RHCP antenna receives the GNSS-signals directly, whereas the LHCP antenna receives the signals reflected from the sea surface. Because of the additional path delay of the reflected signal, the LHCP antenna will appear to be a virtual GNSS-antenna located below the sea surface. When the sea level changes, the path delay of the reflected signal changes, thus the LHCP antenna will appear to be in a new position. The vertical position change corresponds to twice the sea level change, and therefore monitors sea level changes.

Multiple satellites with different elevation and azimuth angles are observed each epoch and will give rise to reflected signals with different incidence angles from different directions. This means that the estimated sea level change can not be considered to originate from one specific point on the surface, but rather represents the change of an average surface formed by the reflection points.

An experimental setup was installed in December 2008 over the ocean at Onsala Space Observatory (OSO) at the west coast of Sweden. Data was collected during three days using two Leica GRX1200+ receivers (one for the direct and one for the reflected signal). The receivers recorded 40 hours of continuous 20Hz data. The signal-to-noise ratio (SNR) as determined by the two receivers was used as a first data quality check. On average the SNR difference between the directly received and the reflected signals was less than 3dB.

The data was analyzed using an in-house developed software in MATLAB. Solutions were made using L1 phase delays for relative positioning. Two approaches to estimate the vertical difference between the RHCP and the LHCP antenna were tested: hourly estimates of the vertical difference, and high-rate estimates of the vertical difference. 

For the hourly estimates 40 hours of continuous 1Hz data (reduced for faster processing using the TEQC software) were used. Each solution was made using 20 minutes of data every full hour, solving for differences in the local vertical components together with receiver clock and phase ambiguities differences for each epoch.

The solution for the high-rate vertical component was made in two steps. First, the phase ambiguity differences were determined. This was done using equally distributed short intervals of ~1 second (21 epochs) from ~20 minutes of 20Hz data, solving for difference in phase ambiguities and receiver clocks every epoch together with differences in vertical coordinate for each short interval. The processing was done based on the assumptions that the sea surface does not change significantly during ~1 second and that the satellite geometry changes considerably in ~20 minutes. Second, the differences in phase ambiguities were rounded to the nearest integer and inserted as known values for a reprocessing of the 20Hz data. In this reprocessing the receiver clock parameters were estimated every epoch and the vertical coordinate difference with different time resolutions (e.g. 0.05s, 1s, 30s).

The resulting time-series for the sea level change from the hourly solutions were compared to data from two traditional tide gauges operated by the Swedish Meteorological and Hydrological Institute at Ringhals and Göteborg, about 18 km south of and 33 km north of OSO, respectively. The GNSS-derived sea level change resembles reasonably well the independently observed sea level change. This indicates that the GNSS-tide gauge gives valuable results for sea level monitoring. Furthermore, the use of the high-rate GNSS-receivers additionally allows a flexible time resolution for sea level monitoring

Chalmers Publication Library

Johan Löfgren, Rüdiger Haas, Jan Johansson
Department of Radio and Space Science, Chalmers University of Technology, Göteborg, Sweden
Email: johan.lofgren@chalmers.se
Sea Level Monitoring Using a GNSS-Based Tide Gauge
Figure 1. The experimental setup of the 
GNSS-based tide gauge at the Onsala
Space Observatory (OSO).
Signal evaluation
• 40 hours of continuous 1 Hz data. 
• A data quality check was done using  the signal-to-
noise (SNR) ratio as determined by the two receivers 
for 12 hours (see Fig. 3A, 3B, 3C, and 3D). 
• The reflected signals are noisier than the direct 
signals, however the SNR of the reflected signals is 
always above 25 dB. 
• When comparing the SNR of the RHCP antenna to 
the SNR of the IGS station ONSA (see Fig. 3E and 3F) 
a similar decrease is seen for northern azimuth 
angles, but the IGS station has higher SNR for low up 
to intermediate elevations.
• The surrounding to the north of the GNSS-based tide 
gauge consisted of coastline and a boathouse, while 
all other directions were open sea. To avoid 
influences of unwanted signals we used an azimuth 
mask.  
• The SNR of the remaining data (Fig. 3G, 3H, 3I, and 
3J) resembles the IGS station SNR, meaning a 
significant increase in SNR above 20° elevation.
• A second order polynomial was fitted to the SNR of 
each antenna (see Fig. 3H and 3J). The average 
difference between the two SNR polynomials was 
between 1.0 and 3.4 dB.
• Global climate change is believed to 
cause melting of large masses of ice in 
the polar regions, and changing the 
sea level.  
• More than 50% of the world's 
population live within 60 km of the 
coast and are potentially endangered 
by sea level rise.
• Monitoring of sea level change is thus 
important. 
• Traditionally, sea level is observed by 
tide gauges, giving measurements 
relative to the Earth’s crust.
• In order to improve the understanding 
of sea level changes, absolute 
measurements are necessary, i.e., 
measurements relative to the Earth’s 
center of gravity.
• We thus propose a GNSS-based tide 
gauge that can give relative and 
absolute sea level measurements.
Introduction Concept
• The GNSS-based tide gauge consists of two 
antennas, one zenith-looking right hand 
circular polarized (RHCP) and one nadir-
looking left hand circular polarized (LHCP), 
mounted back-to-back on a beam over the 
ocean (see the experimental setup in Fig. 1). 
• The RHCP antenna receives the GNSS-signals 
directly, whereas the LHCP antenna receives 
the signals reflected from the sea surface. 
• The reflected signals change polarization from 
RHCP to LHCP.
• The reflected signals experience an additional 
path delay (a+b=c in Fig. 2), implying that the 
LHCP antenna can be regarded as a virtual 
antenna located below the sea surface.
• When the sea level changes, the path delay of 
the reflected signal changes, thus the LHCP 
antenna will appear to change position.
• The vertical position change corresponds to 
twice the sea level change.
Conclusions and outlook
• The reflected signals were noisier than the direct 
signals and the noise was higher at low elevations. 
• The maximum average reduction in SNR of the 
LHCP antenna as compared to the RHCP antenna 
was 3.4 dB. 
• The GNSS-derived sea level change resembles 
reasonably well the independently observed sea 
level changes from two traditional tide gauges.
• The RMS agreement is better than 4 cm, indicating 
that the GNSS-based tide gauge gives valuable 
results for sea level monitoring.
• We plan to install the GNSS-based tide gauge 
permanently at OSO and to develop strategies for 
real-time sea level monitoring. 
• Furthermore, our plan is to install additionally  a 
traditional tide gauge at the same site for 
comparison purposes.
• We want to analyze high-rate (20 Hz) observations 
in post-processing, which might allow us to derive 
parameters that describe the sea surface 
roughness, as well as sea water properties.
Data analysis
Figure 3. SNR versus azimuth (left column) and elevation (right column) during 12 
hours. Values are presented for all data received with the RHCP antenna (A and B) and 
LHCP antenna (C and D),  the IGS station ONSA (E and F) as a comparison, and for the 
RHCP antenna (G and H) and LHCP antenna (I and J) after the azimuth mask is applied. 
The polynomial fit to the SNR is also shown (H and J).
A B
C D
E F
G H
I J
• An elevation and azimuth mask was applied to the data 
(keeping data above 20° elevation and within 45° to 225°
azimuth) to avoid unwanted reflections (see Fig. 4A and 4B).
• The 1 Hz data were analyzed with an in-house developed 
software in MATLAB.
• The software used L1 phase delays for relative positioning to 
perform a least-squares estimation of
 differences in the local vertical components
 receiver clock differences
 phase ambiguity differences 
• Each solution was made using 20 minutes of data every full 
hour during 40 hours. 
• The resulting time series of the sea level change was 
compared to data from two traditional tide gauges operated 
by the Swedish Meteorological and Hydrological Institute at 
Ringhals and Göteborg, about 18 km south of and 33 km 
north of OSO, respectively (Fig. 5).
• The root-mean-square (RMS) agreement between the time 
series is better than 4 cm (see Tab. 1).
Figure 5. Sea level change from the GNSS-based tide gauge at OSO and from 
two traditional tide gauges at Ringhals and Göteborg. The mean of each time 
series is subtracted.
Table 1. RMS agreement between the time series.
Figure 2. Schematic drawing of the GNSS-based tide 
gauge installation and the signal geometry.
RHCP
LHCP
Figure 4. Reflection points on the sea 
surface during 12 hours for satellites above 
5° elevation (A) and after the azimuth and 
elevation mask was applied (B).
A
B


Global climate change is believed to result in the melting of large masses of ice in Polar Regions, bringing freshwater into the ocean, and changing the sea level. The traditional way to measure the sea level, by tide gauges, results in measurements relative to the Earth’s crust. However, in order to fully understand the sea level changes, absolute measurements (change in sea level in relation to the Earth’s center of gravity) are necessary, in particular in regions affected by post-glacial uplift, e.g., Fennoscandia. Satellite techniques, e.g., GNSS can be used to determine the motion of the Earth’s crust in relation to the center of gravity. By measuring reflected GNSS-signals from the sea surface, information of the sea level change can be obtained. Therefore a GNSS-based tide gauge is proposed.The proposed GNSS-based tide gauge installation consists of two antennas, one zenith looking right hand circular polarized (RHCP) and one nadir looking left hand circular polarized (LHCP), mounted back-to-back on a beam over the ocean. The RHCP antenna receives the GNSS-signals directly, whereas the LHCP antenna receives the signals reflected from the sea surface. Because of the additional path delay of the reflected signal, the LHCP antenna will appear to be a virtual GNSS-antenna located below the sea surface. When the sea level changes, the path delay of the reflected signal changes, thus the LHCP antenna will appear to be in a new position. The vertical position change corresponds to twice the sea level change, and therefore monitors sea level changes.Multiple satellites with different elevation and azimuth angles are observed each epoch and will give rise to reflected signals with different incidence angles from different directions. This means that the estimated sea level change can not be considered to originate from one specific point on the surface, but rather represents the change of an average surface formed by the reflection points.An experimental setup was installed in December 2008 over the ocean at Onsala Space Observatory (OSO) at the west coast of Sweden. Data was collected during three days using two Leica GRX1200+ receivers (one for the direct and one for the reflected signal). The receivers recorded 40 hours of continuous 20Hz data. The signal-to-noise ratio (SNR) as determined by the two receivers was used as a first data quality check. On average the SNR difference between the directly received and the reflected signals was less than 3dB.The data was analyzed using an in-house developed software in MATLAB. Solutions were made using L1 phase delays for relative positioning. Two approaches to estimate the vertical difference between the RHCP and the LHCP antenna were tested: hourly estimates of the vertical difference, and high-rate estimates of the vertical difference. For the hourly estimates 40 hours of continuous 1Hz data (reduced for faster processing using the TEQC software) were used. Each solution was made using 20 minutes of data every full hour, solving for differences in the local vertical components together with receiver clock and phase ambiguities differences for each epoch.The solution for the high-rate vertical component was made in two steps. First, the phase ambiguity differences were determined. This was done using equally distributed short intervals of ~1 second (21 epochs) from ~20 minutes of 20Hz data, solving for difference in phase ambiguities and receiver clocks every epoch together with differences in vertical coordinate for each short interval. The processing was done based on the assumptions that the sea surface does not change significantly during ~1 second and that the satellite geometry changes considerably in ~20 minutes. Second, the differences in phase ambiguities were rounded to the nearest integer and inserted as known values for a reprocessing of the 20Hz data. In this reprocessing the receiver clock parameters were estimated every epoch and the vertical coordinate difference with different time resolutions (e.g. 0.05s, 1s, 30s).The resulting time-series for the sea level change from the hourly solutions were compared to data from two traditional tide gauges operated by the Swedish Meteorological and Hydrological Institute at Ringhals and G\uf6teborg, about 18 km south of and 33 km north of OSO, respectively. The GNSS-derived sea level change resembles reasonably well the independently observed sea level change. This indicates that the GNSS-tide gauge gives valuable results for sea level monitoring. Furthermore, the use of the high-rate GNSS-receivers additionally allows a flexible time resolution for sea level monitoring

SEA LEVEL MONITORING USING A GNSS-BASED TIDE GAUGE
Johan S. Lo¨fgren, Ru¨diger Haas, Jan M. Johansson
Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory,
SE-439 92 Onsala (Sweden), Email: johan.lofgren@chalmers.se, rudiger.haas@chalmers.se, jmj@chalmers.se
ABSTRACT
A continuous monitoring of sea level changes is im-
portant for human society since more than 50% of the
world’s population live within 60 km of the coast. Sea
level is traditionally observed with tide gauges that give
measurements relative to the Earth’s crust. To improve
the understanding of sea level changes it is necessary to
perform measurements with respect to the Earth’s cen-
ter of gravity. This can be done with satellite techniques,
and thus a GNSS-based tide gauge is proposed that makes
use of both GNSS-signals that are directly received and
that are reflected on the sea surface. A test installation
at the Onsala Space Observatory shows that the reflected
GNSS-signals have only about 3 dB less signal-to-noise-
ratio than the directly received GNSS-signals. A com-
parison of relative sea level observations from the GNSS-
based tide gauge to traditional tide gauges gives an RMS-
agreement on the order of 4 cm.
Key words: GNSS; reflection signals; sea level monitor-
ing; tide gauge.
1. INTRODUCTION
Global climate change is believed to result in the melting
of large masses of ice in Polar Regions, bringing fresh-
water into the ocean [1], and changing the sea level [2].
This can have important consequences for human soci-
ety since more than 50% of the world’s population live
within 60 km of the coast [3]. The traditional way to
measure the sea level, by tide gauges, results in mea-
surements relative to the Earth’s crust [4]. However, in
order to fully understand sea level change processes, ab-
solute measurements, i.e., change in sea level in relation
to the Earth’s center of gravity, are necessary. This is in
particular important in regions that are affected by post-
glacial uplift, e.g., Fennoscandia. Satellite techniques,
e.g., Global Navigation Satellite Systems (GNSS), can be
used to determine the motion of the Earth’s crust in rela-
tion to the center of gravity [5]. By observing reflected
GNSS-signals from the sea surface, information of both
relative and absolute sea level change can be obtained.
Therefore, a GNSS-based tide gauge is proposed.
2. INSTALLATION
The proposed GNSS-based tide gauge installation con-
sists of two antennas, one zenith-looking right hand cir-
cular polarized (RHCP) and one nadir-looking left hand
circular polarized (LHCP), mounted back-to-back on a
beam over the ocean (see Fig. 1). The RHCP antenna
receives the GNSS-signals directly, whereas the LHCP
antenna receives the signals that are reflected from the
sea surface. When the signals are reflected, they change
polarization from RHCP to LHCP. The reflected signals
experience an additional path delay, as compared to the
directly received signals. This means that the LHCP an-
tenna can be regarded as a virtual antenna located below
the sea surface. This virtual antenna is depicted in blue
in Fig. 1. When the sea level changes, the path delay of
the reflected signal changes, thus the LHCP antenna will
Figure 1. Schematic drawing of the GNSS-based tide
gauge installation.
appear to change position. The vertical position change
corresponds to twice the sea level change, which can be
derived from the geometry in Fig. 1. The height of the
LHCP antenna over the sea surface corresponds to
h =
(a+ b)/ sin ε− d
2
(1)
where a + b = c is the additional path delay of the re-
flected signal, ε is the elevation of the transmitting satel-
lite, and d is the vertical separation between the phase
centers of the RHCP and the LHCP antennas. A change
in the sea level corresponds to a change in height for the
LHCP antenna, thus the installation monitors sea level
changes.
Multiple satellites with different elevation and azimuth
angles are observed each epoch and will give rise to re-
flected signals with different incidence angles from dif-
ferent directions. This means that the estimated sea level
change can not be considered to originate from one spe-
cific point on the sea surface, but rather represents the
change of an average sea surface formed by the reflec-
tion points. The distribution of these points is limited by
the placement of the antenna (height of the antenna, the
landmass on which the antenna is positioned, and also
obstacles in the water) and the antenna geometry.
3. EXPERIMENTAL SETUP AND EVALUATION
An experimental setup was installed in December 2008
over the ocean at the Onsala Space Observatory (OSO)
at the west coast of Sweden, see Fig. 2. The antenna
installation was mounted on the coastal bedrock so that
there was open sea water in a southward direction. This
was done in order to maximize the potential reflective sur-
face. The downward looking LHCP antenna was approx-
imately 1 m over the sea surface and both antennas were
protected by hemispherical radomes.
Data were collected during three days using two Le-
ica GRX1200+ receivers (one for the direct and one for
the reflected signal) connected to one RHCP antenna
(Leica AT504 GG choke-ring) and one LHCP antenna
(Leica AR25 multi-GNSS choke-ring). The receivers
recorded 40 hours of continuous Receiver Independent
Exchange Format (RINEX) data [7] with 20 Hz sam-
pling. The signal-to-noise ratio (SNR) as determined
by the two receivers was used as a first data quality
check. The receiver connected to the upward looking
RHCP antenna received GNSS-signals from all direction
except the north direction between -16◦ and +6◦ azimuth.
The downward looking LHCP antenna received GNSS-
signals from all directions except the north direction be-
tween -16◦ and +22◦ azimuth. The surrounding to the
north of the GNSS-based tide gauge installation, between
-135◦ and +45◦ azimuth, was dominated by bedrock from
the coastline and a boathouse, while all other directions
were open sea. The received signals in the north direction
had low SNR compared to the rest of the signals, indicat-
ing influence of multipath reflections from the bedrock
Figure 2. The experimental setup of the GNSS-based tide
gauge at Onsala Space Observatory.
Figure 3. Signal-to-noise ratio, as determined by the two
receivers, for the direct and reflected signals from 5 satel-
lites during 4 hours as a function of azimuth (upper two
panels) and elevation (lower two panels).
coastline and the boat house. To avoid influences of mul-
tipath we thus used an azimuth mask to excluded this re-
gion, and additionally applied an elevation mask of 20◦.
As an example Fig. 3 shows the SNR from both receivers
for the remaining 5 satellites after the azimuth mask was
applied. The satellites were continuously visible in the
south during 4 hours. In Fig. 3 it can be seen that the
reflected signals are noisier than the direct signals (espe-
cially below 20◦ elevation). However, the SNR for the
reflected signals is always above 25 dB and it increases
with elevation.
In order to evaluate the average difference in SNR be-
tween the direct and the reflected signals, a second or-
der polynomial was fitted to the SNR of each satellite
arc. The differences between the SNR polynomials of the
direct and the reflected signals were calculated for each
satellite arc. In general, the SNR differences were larger
for low elevations than for high elevations. The maxi-
mum SNR difference was 3.5 dB and this occurred for
only one satellite at low elevations (less than 20◦ eleva-
tion). For all other satellites the SNR differences were
less than 2.3 dB during the 4 hours.
4. DATA ANALYSIS
The strategy to estimate the vertical difference between
the RHCP and the LHCP antenna was to produce hourly
estimates that could be compared to other data sets. First,
to enable faster processing the 20 Hz data were decimated
to 1 Hz data using the Translation, Editing, and Quality
Check (TEQC) software [6]. Second, an elevation and
azimuth mask for the installation site was applied, remov-
ing data below 20◦ elevation and outside the south direc-
tion -135◦ to +45◦ azimuth. This was done in order to
remove unwanted signals, e.g., multipath from the coast-
line and to remove signals at low elevations. Reflected
signals from low elevations have low SNR since they are
highly elliptical polarized and at the Brewster angle even
purely horizontal polarized [8].
Thereafter an in-house developed software in MATLAB
was used to analyze the data. Solutions were made us-
ing L1 phase delays for relative positioning. Each solu-
tion used 20 minutes of data every full hour, solving for
the difference in the local vertical component for this in-
terval, together with receiver clock and phase ambiguity
differences for each epoch (every second). The differ-
ence in local vertical component corresponds to 2h + d,
see Fig. 1. These results were used to calculate a time
series of sea level h, and are presented in Fig. 4. The
Figure 4. Sea level from the GNSS-based tide gauge dur-
ing 40 hours. Values are relative to the RHCP antenna.
values shown in Fig. 4 are negative because they are rel-
ative to the upward looking RHCP antenna. Adding the
ellipsoidal height of the RHCP antenna to these values
will thus give absolute sea level heights in an ellipsoidal
system.
5. RESULTS
The resulting time series for the sea level change from the
hourly solutions were compared to data from two tradi-
tional tide gauges operated by the Swedish Meteorolog-
ical and Hydrological Institute (SMHI) at Ringhals and
Go¨teborg, about 18 km south of and 33 km north of OSO,
respectively (see Fig. 5). Since the sea level change from
the GNSS-based tide gauge is relative to the position of
the RHCP antenna, and the sea level changes of the tradi-
tional tide gauges refer to the mean sea level of the year,
mean values were removed from each time series. This
allows to compare the three series in a meaningful way
avoiding any biases. The results are presented in Fig. 6.
To investigate the agreement of the three time series,
the root-mean-square (RMS) differences were calculated.
The RMS values are given in Tab. 1 and show that the
agreement between the three independent time series is
better than 4 cm. The traditional tide gauges appear to
agree slightly better with respect to each other than the
GNSS-based tide gauge to each of them. This indicates
that there might still be some systematic effects that need
further investigation.
Figure 5. Map of parts of the Swedish west coast showing
the locations of the traditional tide gauges at Go¨teborg
and Ringhals, and the GNSS-based tide gauge at Onsala.
The distances are about 33 km between Go¨teborg and
Onsala, and 18 km between Onsala and Ringhals.
Figure 6. Sea level change from the GNSS-based tide
gauge at OSO and from two traditional tide gauges at
Ringhals and Go¨teborg. The mean is removed from each
time series
Table 1. RMS-differences in sea level between tide gauges
at Ringhals, Go¨teborg, and OSO.
Site 1 Site 2 RMS [cm]
Ringhals OSO 3.7
Go¨teborg OSO 3.7
Ringhals Go¨teborg 3.3
6. CONCLUSIONS AND OUTLOOK
The GNSS-based tide gauge installation shows that it is
possible to receive GNSS-signals reflected in the sea sur-
face. As expected the reflected signals were noisier than
the direct signals and the noise was higher at low eleva-
tions. The maximum reduction in SNR as compared to
the directly received signals was 3.5 dB. However, the
typically SNR difference was lower than 2.3 dB.
The GNSS-derived sea level change resembles reason-
ably well the independently observed sea level changes
from two traditional tide gauges, and the RMS-agreement
is better than 4 cm. The two traditional tide gauges agree
slightly better than the GNSS-based tide gauge with each
of them, indicating that there might be unaccounted sys-
tematic effects. One reason could be antenna phase cen-
ter variations. The upward-looking RHCP antenna and
the downward-looking LHCP antenna were two different
types of antennas and phase center corrections were not
applied to the data in the pre-processing. Another reason
could be hydrodynamic effect differences due to the dif-
ference in geometry of the coast line at Ringhals, Onsala,
and Go¨teborg.
The agreement between the GNSS-based tide gauge and
the traditional tide gauges indicates that the GNSS-based
tide gauge gives valuable results for sea level monitoring.
Therefore, our plans are to install the GNSS-based tide
gauge permanently at OSO during the fall of 2009. For
this installation we plan to exchange the upward look-
ing GNSS-antenna to an antenna of the same type as the
downward looking one, but RHCP, in order to minimize
systematic effects due to, e.g., phase center variation dif-
ferences. Our ambition is to develop strategies for a real-
time sea level monitoring with this installation.
Additionally, we plan to install a traditional tide gauge at
the same site in order to further evaluate the GNSS-based
tide gauge and to avoid any effects due to hydrodynami-
cal differences caused by different coastal geometry.
Furthermore, we plan to analyze the high-rate (20 Hz)
observations in post-processing. This high sampling rate
might allow us to derive parameters that describe the sea
surface roughness, as well as sea water properties.
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Sea Level Monitoring Using a GNSS-Based Tide Gauge

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Chalmers Research

Chalmers Publication Library