This paper introduces a potential method for the remote sensing of sea surface salinity (SSS) using the measured propagation delay of low-frequency Loran-C signals transmitted over an all-seawater path between the Sylt station in Germany and an integrated Loran-C/GPS receiver located in Harwich, UK. The overall delay variations in Loran-C surface waves along the path may be explained by changes in sea surface properties (especially the temperature and salinity), as well as atmospheric properties that determine the refractive index of the atmosphere. After removing the atmospheric and sea surface temperature (SST) effects from the measured delay, the residual delay revealed a temporal variation similar to that of SSS data obtained by the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite.
Sea surface salinity (SSS) plays a fundamental role in the density-driven global ocean circulation and the water cycle. The latest remote-sensing SSS sources include the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite (Reul et al., 2013) and the NASA Aquarius satellite (Lagerloef et al., 2013). This study attempts to explore a different approach, by measuring the propagation delay of low-frequency ground waves transmitted over a path that consists entirely of seawater. For an all-seawater path, we expect sea surface salinity (SSS) to have a predominant effect on the delay variations.
The 100 kHz Loran-C transmitters in western Europe, primarily used for marine navigation in European and Arctic waters, have a long history of development. At the frequency used by these transmitters, the propagation of radio signals occurs in two ways. The surface-wave component follows the curvature of the Earth, while the sky-wave component propagates through multiple reflections between the ground and the ionosphere. As Loran is a pulsed system, the difference in path lengths between the ground and sky waves generally ensures that only the ground wave is tracked by the receiver, provided it is less than 2000 km from a transmitter (Pelgrum, 2006). However, under certain conditions a receiver may track a combined signal (ground- and sky-wave) as close as 800 km, and this can contribute to errors.
The propagation velocity of the Loran-C surface wave is influenced by the
refractive index of the atmosphere,
Loran-C propagation path between Sylt and Harwich (image from
Google
However, the PF and SF values are usually computed within a Loran receiver
under the assumption that
The time variations in ASF for the path (as displayed in Fig. 1) between the Sylt Loran-C station in Germany and Harwich, UK, were recorded by a fixed Reelektronika LORADD Differential eLoran reference station, operated by the General Lighthouse Authorities of the UK and Ireland (GLAs). This receiver calculates the variation in ASF by taking the difference between the measured TOA and expected TOA (using expected TOF and using distance to calculate PF, SF, nominal ASF and TOE). The temporal resolution of these data is 30 s, and they were measured from February 2010 to July 2011.
For this reference station, Safar et al. (2010) suggest that the variance
To determine the contribution of SSS, we must remove all factors from the
reference station measurements of the time-varying TOF delay that are not
related to salinity content. Thus we need to subtract the time-varying PF and
sea surface temperature (SST) dependent component. This is done using data,
including 2 m nominal altitude temperature (
Measured Loran-C delay variations (blue) and modelled SST (red) and atmospheric delays (green). Removing the atmospheric and SST delays from the measured delay will leave the delay due only to sea surface salinity (SSS).
A 24 h filter was applied to the measured ASF data to remove variations
which are unlikely to be due to SSS and to be consistent with the resolution
of the atmospheric data used. Following this, the static PF found under the
assumption, as used in the receiver, that the atmosphere refractive index was
constant (
Sea surface temperature (SST) was retrieved from the ECMWF Interim Reanalysis
at the same spatial and temporal resolution. The SST delay shown in Fig. 2 is
based on the assumption that across the 560 km path, a 1 K increase in SST
represents a 5.6 ns decrease in Loran-C delay (i.e. 1 ns
(100 km)
Comparison of the residual Loran-C delay (blue) with monthly SMOS SSS (red) on the practical salinity scale PSS (a dimensionless quantity corresponding roughly to parts per thousand).
This SST delay was removed from the measured Loran-C delay variations. This
leaves a residual delay, which shows a variation pattern similar to that in
SSS observed by SMOS (1
This paper describes a novel method which has the potential to provide SSS estimates. The idea is that, across an all-seawater path, the variations in the propagation delay of low-frequency signals can reflect changes in atmospheric and sea surface properties. When the effects of the atmosphere and SST were removed from the measured Loran-C delay variations, the residual delay shows good agreement with satellite SSS observations.
The authors would like to express thanks to Paul Williams and Chris Hargreaves of the General Lighthouse Authorities of the United Kingdom and Ireland (GLAs) for providing access to their measured Loran-C data. We would also like to thank the reviewers (Sherman Lo, Gregory Johnson and the anonymous reviewer) for their suggestions which have improved this technical note.
SMOS Ocean surface salinity was distributed in netCDF format by the
Integrated Climate Data Center (ICDC,