This paper describes time-series backscatter measurements using BorealScat-2, which is a multi-polarization, multi-frequency tomographic radar. It is installed on a 50-m tower at a boreal forest site in northern Sweden. We describe the system and show measurements related to forest water dynamics during summer conditions. The measurements include data at P-, UHF- and L-band. Results show periodic diurnal variations for all frequency bands but with different phases and amplitudes depending on frequency band and polarization. The results indicate the potential of estimating forest water dynamics from radar data.

The BorealScat tower radar experiment, in Remningstorp, Sweden, acquired tomographic L-band backscatter time series over a boreal forest stand consisting of Norway spruce (Picea abies). Data from summer 2018 is analysed in this study, where diurnal cycles with variations up to more than 1 dB are identified for HH and VV polarizations. The HH cycle has its minimum at night, its maximum at noon and mainly occurs in the middle layers of the forest. Conversely, the VV cycle has its maximum at night, its minimum at noon and is most apparent in the upper and lower forest layers, especially during June and July. The cycles are compared with vapour-pressure deficit (VPD), an indicator of favourable transpiration conditions, and they are seen to co-vary closely. The VV cycle compares well to that seen for P-band in a previous study and is speculated to originate from stem water content variations. The HH cycle cannot be directly connected to stem water content, but is indicated to follow a currently unknown transpiration related phenomena.

In this study, spaceborne SAR observations were compared to the tower-based radar observations of a hemiboreal forest site in southern Sweden. BorealScat radar tower observations at L- and C-band are verified using Sentinel-1 and ALOS-2 PALSAR-2 products. For L-band, a 14-month backscatter time series was produced. For C-band, a 3.5 year backscatter time series that includes tree deaths due to a bark beetle infestation was produced. Tomographic capabilities of the tower-based BorealScat allowed both canopy-only and full forest backscatter to be analyzed. For a healthy forest, the agreement of tower-based and spaceborne SAR observations was within 0.9 dB for C-band and within 0.6 dB for L-band. Sentinel-1 backscatter was found to be insensitive to tree deaths, whereas variations were clearly visible in the tomographic tower-based backscatter. These results show that tower-based radar observations agree with those of spaceborne SARs and can be used to understand their shortcomings and identify new mission needs.

This paper describes the development and initial evaluation of the BorealScat-2 radar. It is installed on a 50 m tall tower at a boreal forest site in northern Sweden and acquires tomographic and multi-polarization measurements at P- and L-band every 5 min. It is of the same design as its predecessor BorealScat except that the antenna frame can be moved along a 4 m horizontal track. A moveable antenna position is motivated by the need to acquire independent samples to reduce backscatter fluctuations. It also enables focused 3D imaging by combing tomographic and SAR processing when the backscatter fluctuations are temporally stable. We show results from data acquired in 2022, which demonstrates 3D imaging of a forest as well as the response from a trihedral.

Cross-polarized temporal coherence observations of a boreal forest, acquired using a tower-based radar, are presented in this article. Temporal coherence is analyzed with respect to frequency, temporal baseline, time of day of observation, season, meteorological variables, and biophysical variables. During the summer, P- and L-band temporal coherence exhibited diurnal cycles, which appeared to be due to high rates of transpiration and convective winds during the day. During the winter, freeze-thaw cycles and precipitation resulted in decorrelation. At temporal baselines of seconds to hours, a high temporal coherence was observed even at C-band. The best observation times of the day were midnight and dawn. Temporal coherence is the main limitation of accuracy in interferometric and tomographic forest applications. The observations from this experiment will allow for better spaceborne SAR mission designs for forest applications, better temporal decorrelation modeling, and more accurate forest parameter estimation algorithms using interferometric and tomographic SAR data.

In 2016, the BorealScat radar tower experiment was established in a forest site in southern Sweden. The system, based on a 20-channel vector network analyzer, acquires a dense, multi-annual, multi-frequency, multi-polarization tomographic dataset of a forest scene. In this paper we discuss how the system design relates to two performance metrics: backscatter precision and measurement duration. Speckle variance in the time domain is discussed and an approach for avoiding coherent integration loss is presented. These concepts are relevant for any ground-based imaging radar acquiring time series data of dynamic scenes.

BorealScat is a tower-based radar which measures multipolarization and tomographic backscatter time series data at a forest site in Southern Sweden. In this paper, we describe a methodology for absolute radiometric calibration of BorealScat and apply it to a set of backscatter measurements at P-band (420-450 MHz). We present radiometric calibration corrections which take into account antenna gain and range variations over the tomographic resolution cell. We also use data acquired by the LORA airborne UHF-band SAR over the BorealScat site to determine the absolute radiometric calibration factor for P-band and HH- polarization. It is shown that the methodology produces consistent results for multiple SAR images.

SAR tomography at P-band allows the separation of scatterers throughout the vertical extent of a forest canopy, offering a SAR observable that can be used for biomass estimation. But the vertical backscattering distribution of forests are sensitive to changes in the weather and seasons, effects which are poorly understood. In this study, a tower-based radar is used to produce fully-polarimetric tomographic images of a boreal forest at P-band which are analysed over a period of one year. The largest variations seen were due to sub-zero temperatures, causing a drop in the effective scattering height. Seasonal changes in soil moisture and temperature caused a drop in ground-level backscatter at HH-polarisation and a drop in canopy-level cross-polarised backscatter during the summer.

We describe a P-, L- and C-band radar, BorealScat, designed for polarimetric time-series measurements of forests. Radar tomography is implemented with a vertical antenna array, which provides measurements of the vertical scattering distribution. To minimize temporal decorrelation, the radar performs simultaneous measurements of the reflected signals using all array elements. The system is implemented using a 20-port vector network analyzer (VNA) and a stepped-frequency waveform. It has two 20-element arrays: one array optimized for P- and L-bands and one for C-band. The arrays are installed on a 50-m high tower and radar measurements are collected over a hemiboreal forest stand. We discuss several design issues and demonstrate some tomographic imaging capabilities. The multiport VNA tomography results are compared with results from the system operating in the slower 2-port VNA measurement scheme.

This paper describes results from BorealScat, which is a tower-based P/L/C-band radar being developed by Chalmers in the forest site Remningstorp, Sweden. The objective is to investigate the radar signatures in varying environmental conditions. The 50-m tower has been built adjacent to a forest stand, with above-ground biomass of 300 tons/ha and dominated by Norway spruce (Picea abies (L.) Karst.). The radar consists of an antenna array at the top of the tower connected to a 20-port vector network analyser (VNA) with low-loss cables. The first version of BorealScat performs polarimetric and tomographic measurements in the frequency range 420-1375 MHz. Meteorological and soil moisture measurements are also available. We present first results from a time series collected 16-27 December 2016 during which air temperature varied in the range -4° to 8°C.

A first evaluation of ALOS-2 PALSAR-2 data for forest stem volume estimation has been performed at a coniferous dominated test site in southern Sweden. Both the Fine Beam Dual (FBD) polarization and the Quad-polarimetric mode were investigated. Forest plots with stem volume reaching up to a maximum of about 620 m3ha−1(corresponding to 370 tons ha−1) were analyzed by relating backscatter intensity to field data using an exponential model derived from the Water Cloud Model. The estimation accuracy of stem volume at plot level (0.5 ha) was calculated in terms of Root Mean Square Error (RMSE). For the best case investigated an RMSE of 39.8% was obtained using one of the FBD HV-polarized images. The corresponding RMSE for the FBD HH-polarized images was 43.9%. In the Quad-polarimetric mode the lowest RMSE at HV- and HH-polarization was found to be 43.1% and 66.1%, respectively.

This paper describes BorealScat, a tower-based radar campaign for acquiring multitemporal polarimetric, tomographic and Doppler radar measurements at P-, L- and C-band over a hemi-boreal forest site in Remningstorp, Sweden. The facility consists of a 50-m high tower equipped with a radar system including an array of 30 antennas. The site will also be equipped with meteorological instruments and moisture sensors. The aim of the experiment is the temporal survey of radar signatures over time scales ranging from sub-seconds to years in varying environmental conditions. This experiment will provide fundamental information about the electromagnetic scattering mechanisms in boreal forests at P-, L- and C-band.