Maritime safety relies on navigation, detection, and radio communication technologies that function through electromagnetic radiation. Propagation of electromagnetic radiation can be impacted by a disruptive phenomenon known as ducting. Our four-week study using a X-band coastal radar and various meteorological and marine observations, including vertical profiles of temperature, humidity and wind, in early spring 2022 concluded that the combination of measurements at the Utö observatory provides a reliable means of detecting ducting in the Archipelago Sea. The modified refractivity calculated from the vertical profiles for the 22–59 m and 32–59 m altitude layers and coastal radar over-the-horizon observations agree 77% and 85% of the time, respectively. As such, the modified refractivity gradient can be considered a good indicator for over-the-horizon detection with the Utö coastal radar over the open sea. The horizontal wind profiles also revealed a low-level jet at the radar height that often coincided with the ducting observations. To quantify the results, we created an empirical ducting index which showed that ducting is spatially variable, showing the capabilities of the Utö observatory for research oriented towards monitoring and improving maritime safety and security.

The sea level retrievals from the latest generation of radar altimeters (the SAR altimeters) are still challenging in the coastal zone and areas covered by sea ice and require a dedicated fitting (retracking) strategy for the waveforms. In the framework of the European Space Agency’s Baltic + Sea Level (ESA Baltic SEAL) project, an empirical retracking strategy (ALES + SAR), including a dedicated sea state bias correction, has been designed to improve the sea level observations in the Baltic Sea, characterised by a jagged coastline and seasonal sea ice coverage, without compromising the quality of open ocean data. In this work, the performances of ALES + SAR are validated against in-situ data in the Baltic Sea. Moreover, variance, crossover differences and power spectral density of the open ocean data are evaluated on a global scale. The results show that ALES + SAR performances are of comparable quality to the ones obtained using physical-based retrackers, with relevant advantages in coastal and sea ice areas in terms of quality and quantity of the sea level data.

The designation of the Baltic Sea as a sulfur emission control area (SECA) in May 2006, with subsequent tightening of regulations in 2010 and 2015, has reduced the sulfuric emission from shipping traffic. This study assesses the impacts of SECA on observed SO₂ concentrations by providing a long-term analysis of 1 min time-resolution air quality data from 2006 to 2020 on Utö (an island in the Baltic Sea) supported by the predictions from the Ship Traffic Emissions Assessment Model (STEAM). Additionally, 1 h resolution data from 2003 to 2005 are utilized to investigate changes due to the SECA limits set in 2006. The observed SO₂ concentrations on Utö have continuously decreased since 2003 due to an overall decrease in SO₂ emissions in northern Europe, combined with reduced emissions from shipping traffic due to SECA regulations. The 3-year average SO₂ concentration dropped from pre-SECA (2003–2005) to post-SECA (2007–2009, 2011–2013, 2016–2018) periods by 38 %, 39 %, and 67 %, respectively. No clear trends were observed in the concentrations of other measured pollutants. Furthermore, we investigated wind-direction-resolved SO₂ concentrations for 2 selected years (2014 and 2019), and the results showed a significant decrease in high-SO₂-concentration shipping plumes due to the implementation of SECA in 2015. Our study brings out the importance of long-term, high-time-resolution air quality observations at a remote marine research station by providing means for both quantitative and qualitative analyses of the impacts of regulatory environmental legislation.

Atmospheric ducting can cause a radar to detect over its standard horizon. Two years of X-band coastal surveillance radar imagery from the island of Utö in the Baltic Sea was processed and analyzed to study the statistics of visibility over the standard horizon of the radar. The results were compared to the modified refractivity profiles and weather observations measured at Utö. In total, 275 ducting events were observed with the Utö coastal surveillance radar and the median duration of an event was 4 h. Over-the-horizon observation statistics from the Utö coastal surveillance radar show that ducting occurs most frequently during spring (MAM) and summer (JJA) months, 37% and 42% of the time, respectively. A peak is observed in the evening toward the night during both seasons. Ducting was significantly less frequent during the autumn (SON) and winter (DJF) months, 1% and 2% of the time, respectively. The corresponding frequencies from the modified refractivity profiles were 31% for spring, 56% for summer, 40% for autumn, and 16% for winter months. During the winter and autumn months, a shallow surface duct was often present in the profiles. The presence of this weak duct did not always result in over-the-horizon observations with the Utö coastal surveillance radar but could affect surveillance radars that are closer to the sea surface or radar systems operating at a different frequency to that of the Utö X-band radar. Significance Statement: In our article, we explore how an X-band coastal surveillance radar can be used to study atmospheric ducting, a phenomenon where radar signals can travel farther than usual. This is important for understanding how radars work in different weather conditions and how often the conditions occur that allow radars to see farther. Our study takes place on Utö, an island at the southern edge of the Archipelago Sea in the Baltic Sea. This location allows us to study ducting over both open sea and complex island environments. To our knowledge, this study presents long-term ducting statistics for the first time based on an X-band coastal surveillance radar.

Automatic Identification System (AIS) is a wireless communication system used by vessels to exchange real-time information with each other and with coastal authorities, enhancing situational awareness and maritime safety. Consequently, safety at sea depends on reliable signal transmission, which can be disrupted by anomalous signal propagation. In particular, tropospheric ducting can extend the AIS antenna horizon, allowing messages to be received over greater distances than under standard conditions. To study the behaviour of the AIS signal under standard and anomalous propagation conditions, 1-year of AIS-observations were collected from two antennae at 7 m and 30 m heights above the mean sea level on the Utö Island in the Baltic Sea. The AIS antennae were co-located with mast-mounted measurements of temperature and humidity. This allows for studying the AIS signal propagation alongside observed refractivity profiles. The AIS over-the-horizon observations occurred 59 % of the time for the 30 m antenna and 34 % of the time for the 7 m antenna, mainly during the spring and summer months. A strong diurnal cycle was observed in the Archipelago Sea, north of Utö, while no diurnal cycle was observed in the open sea region south of Utö. During periods of anomalous signal propagation, the AIS messages were received from farther away, from up to 600 km from Utö and the observed received signal strength decayed slower with distance, indicating reductions in propagation losses due to ducting. The anomalous AIS observations were also found to coincide with the stronger and higher observed ducts.