Jaakko Seppänen

L-Band Radiometer Observations of Soil Processes in Boreal and Subarctic Environments

The launch of the European Space Agency (ESA)s Soil Moisture and Ocean Salinity (SMOS) satellite mission in November 2009 opened a new era of global passive monitoring at L-band (1.4-GHz band reserved for radio astronomy). The main objective of the mission is to measure soil moisture and sea surface salinity; the sole payload is the Microwave Imaging Radiometer using Aperture Synthesis. As part of comprehensive calibration and validation activities, several ground-based L-band radiometers, so-called ETH L-Band radiometers for soil moisture research (ELBARA-II), have been deployed. In this paper, we analyze a comprehensive set of measurements from one ELBARA-II deployment site in the northern boreal forest zone. The focus of this paper is in the detection of the evolution of soil frost (a relevant topic, e.g., for the study of carbon and methane cycles at high latitudes). We investigate the effects that soil freeze/thaw processes have on the L-band signature and present a simple modeling approach to analyze the relation between frost depth and the observed brightness temperature. Airborne observations are used to expand the analysis for different land cover types. Finally, the first SMOS observations from the same period are analyzed. Results show that soil freezing and thawing processes have an observable effect on the L-band signature of soil. Furthermore, the presented emission model is able to relate the observed dynamics in brightness temperature to the increase of soil frost.

The Effect of Boreal Forest Canopy in Satellite Snow Mapping—A Multisensor Analysis

Satellite-based snow-cover monitoring is performed using optical, synthetic aperture radar (SAR), and passivemicrowave sensors. Effects of forest canopy on the observed signal need to be considered with all of these sensor types. Various models describing the interaction of electromagnetic radiation with forest canopy have been developed, but many of these are overly complex with high computational and ancillary data requirements. However, for retrieval purposes, simple models are preferred. This work aims at increasing the understanding of the effect of forest canopy on remote sensing observations of snow-covered terrain for both microwave and optical regimes and at quantifying the capability of simple zeroth-order models in simulating these effects. To achieve these goals, a spatial analysis of optical, SAR, and passive-microwave remote sensing data in the northern boreal forest region was performed. Model parameters for vegetation transmissivity as well as the properties of the underlying surface were optimized by utilizing lidar-ranging- and Landsat-based simplified proxy parameters describing forest canopy closure and stem volume. The results demonstrated that despite using these relatively simple proxies, a zeroth-order model can accurately estimate the extinction of electromagnetic signals in a forest, particularly for passive microwave and optical data. The SAR model successfully estimated the median of the observations, but larger scatter of the observations was reflected by a higher root mean square error and lower correlation between models and observations. Due to both good estimation accuracy and simplicity, the presented models can be considered to be applicable in existing snow retrieval algorithms.

Observation and Modeling of the Microwave Brightness Temperature of Snow-Covered Frozen Lakes and Wetlands

Small-scale variability in land cover influences both the snow cover and the microwave response of a snow-covered surface. Since low microwave frequencies penetrate below the snowpack, the differing dielectric properties of soil and water have a significant effect on passive microwave observations and therefore cause errors in the interpretation of snow parameters from satellite data. Here, the brightness temperature of snow- and ice-covered lakes and wetlands is studied using airborne and spaceborne microwave radiometer observations and modeling of brightness temperature from in situ measurements. We aim at assessing the validity of the multilayer Helsinki University of Technology (HUT) snow emission model on lake- and wetland-rich areas and at examining the error from omission of water bodies in the forward modeling of brightness temperature. The results indicate that the model can estimate brightness temperatures of lakes and wetlands with rms errors of 12-28 K and 9-16 K, respectively. The inclusion of lakes in the satellite-scale simulations reduces the simulation error in 52%-100% of the simulated areas at 18.7 and 36.5 GHz. The inclusion of wetlands further improves simulations, resulting in an rms error of satellite scenes of 4-5 K at 18.7 and 36.5 GHz (5-10 K without lakes and wetlands). However, the natural variability of brightness temperature over water bodies is not entirely captured particularly at 10.65 GHz. The inclusion of lakes and wetlands can be used to reduce errors in the forward model and thus increase the accuracy of snow parameters derived from satellite data.

SMOS calibration and validation activities with airborne interferometric radiometer HUT-2D during spring 2010

In this paper we present calibration and validation activities of European Space Agencys SMOS mission, which utilize airborne interferomentric L-band radiometer system HUT-2D of the Aalto University. During spring 2010 the instrument was used to measure three SMOS validation target areas, one in Denmark and two in Germany. We present these areas shortly, and describe the airborne activities. We show some exemplary measurements of the radiometer system and demonstrate the studies using the data.

Studies of radio frequency interference at L-band using an airborne 2-D interferometric radiometer

Potential radio frequency interference (RFI) sources at L-band include L-band radars; mobile, navigation and other satellite services; and various land services. We have collected data using our airborne L-band interferometric HUT-2D radiometer in order to support the ESA SMOS mission. We participated in April-May 2008 in ESAs rehearsal campaign for SMOS satellite validation activities in Germany and Spain. Additional data have been collected in Finland. Two basic categories of RFI have been observed: (1) Point-wise weak sources that do not saturate the HUT-2D instrument, and (2) strong sources that totally saturate the sensor over a large area.

Boreal forest height estimation with SAR interferometry and laser measurements

In this paper we summarize the results of FINSAR campaign, which was arranged to evaluate X- and L-band SAR interferometric and polarimetric SAR techniques for Boreal forest. The main emphasis of the work was on L-band polarimetric interferometry and forest height estimation. Also X-band interferometry and coherence tomography for X- and L-band, phase center height, extinction coefficient of forest and several other aspects of polarimetric interferometry were studied with help of ancillary measurements. Our results show that L-band polarimetric SAR interferometry can estimate well Boreal forest height. Also X-band interferometry shows good potential in height estimation. When accurate ground model is available, tree height can be estimated even by using one polarization interferometry. SAR appears to be more accurate in forest height measurement than forest inventory database, but not as accurate as laser measurement.

Multifrequency microwave radiometry of snow on lake ice: Observations and simulations

We have conducted airborne multi-frequency radiometer measurements over two lakes and adjacent land areas in southern Finland over a period of several winters using a frequency range of 1.4 to 36.5 GHz. Data have been collected under a variety of snow, ice, and weather conditions in order to determine the behavior of the snow-ice-water system. This paper presents an overview of the airborne campaigns and results confirming that the brightness temperature behavior of the snow/lake ice/water system is different from that of the snow/terrain system. This needs to be taken into account in algorithms for retrieval of snow characteristics from space-borne radiometer data for northern lake-rich areas. Comparisons between experimental brightness temperatures and theoretical results show that the HUT snow emission model performs well for lake ice.

Brightness temperature behavior of snow on lake ice over a wide frequency range

We have conducted airborne microwave radiometer campaigns over a test site consisting of two lakes and their immediate surroundings in Southern Finland since 2004 in order to (1) determine the effect of snow cover to the brightness temperature of lake ice, and to (2) compare the brightness temperatures for snow-covered lake ice and snow-covered terrain (both open and forested areas). Measurements have been conducted under a variety of weather and snow/ice conditions covering situations from early winter to melting season. Using the HUTRAD and HUT-2D radiometer systems a frequency range of 1.4 to 36.5 GHz is covered. Airborne measurements are always accompanied with in-situ data collection including relevant snow, ice, and terrain parameters.

Multifrequency microwave radiometer measurements of snow on lake ice

Airborne microwave radiometer measurements of lake ice have been performed in 2004, 2007, 2011, and 2012 over two lakes in southern Finland using radiometer systems that cover frequencies from 1.4 to 36.5 GHz. Airborne and surface data have been collected under circumstances ranging from early winter dry snow to late winter dry and wet snow conditions. Water and slush on top of ice and dry snow grain size have been determined to be the two most important parameters affecting brightness temperature.

Microwave emission signature of snow-covered lake ice

Airborne microwave radiometer measurements of lake ice have been performed in 2004, 2007, and 2011 in southern Finland. The HUTRAD radiometer system provided data in the 6.9 to 36.5 GHz range using an incidence angle of 50 degrees off nadir. In 2011 also the interferometric HUT-2D radiometer was used to provide 1.4 GHz imagery of lake ice.