remote sensing Article The SARSense Campaign: Air- and Space-Borne C- and L-Band SAR for the Analysis of Soil and Plant Parameters in Agriculture David Mengen 1,*, Carsten Montzka 1 , Thomas Jagdhuber 2,3 , Anke Fluhrer 2,3 , Cosimo Brogi 1 , Stephani Baum 4, Dirk Schüttemeyer 5, Bagher Bayat 1 , Heye Bogena 1 , Alex Coccia 6, Gerard Masalias 6, Verena Trinkel 4, Jannis Jakobi 1, François Jonard 1,7 , Yueling Ma 1, Francesco Mattia 8, Davide Palmisano 8 , Uwe Rascher 4 , Giuseppe Satalino 8, Maike Schumacher 9, Christian Koyama 10, Marius Schmidt 1 and Harry Vereecken 1 1 Forschungszentrum Jülich, Institute of Bio-and Geosciences: Agrosphere (IBG-3), 52428 Jülich, Germany; [email protected] (C.M.); [email protected] (C.B.); [email protected] (B.B.); [email protected] (H.B.); [email protected] (J.J.); [email protected] (F.J.); [email protected] (Y.M.); [email protected] (M.S.); [email protected] (H.V.) 2 German Aerospace Center, Microwaves and Radar Institute, 82234 Wessling, Germany; [email protected] (T.J.); [email protected] (A.F.) 3 Institute of Geography, University of Augsburg, 86135 Augsburg, Germany 4 Forschungszentrum Jülich, Institute of Bio- and Geosciences: Plant Sciences (IBG-2), 52428 Jülich, Germany; [email protected] (S.B.); [email protected] (V.T.); [email protected] (U.R.) 5 Mission Science Division, European Space Agency, 2201 Noordwijk, The Netherlands; [email protected] 6 Metasensing BV, 2201 Noordwijk, The Netherlands; [email protected] (A.C.); [email protected] (G.M.) 7 Earth and Life Institute, Université Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium Citation: Mengen, D.; Montzka, C.; 8 Consiglio Nazionale delle Ricerche (CNR), Institute for Electromagnetic Sensing of the Environment (IREA), Jagdhuber, T.; Fluhrer, A.; Brogi, C.; 70126 Bari, Italy; [email protected] (F.M.); [email protected] (D.P.); [email protected] (G.S.) Baum, S.; Schüttemeyer, D.; Bayat, B.; 9 Geodesy and Surveying, Aalborg University, 9220 Aalborg, Denmark; [email protected] Bogena, H.; Coccia, A.; et al. The 10 School of Science and Engineering, Tokyo Denki University, Tokyo 120-8551, Japan; [email protected] SARSense Campaign: Air- and * Correspondence: [email protected] Space-Borne C- and L-Band SAR for the Analysis of Soil and Plant Abstract: With the upcoming L-band Synthetic Aperture Radar (SAR) satellite mission Radar Ob- Parameters in Agriculture. Remote serving System for Europe L-band SAR (ROSE-L) and its integration into existing C-band satellite Sens. 2021, 13, 825. https://doi.org/ missions such as Sentinel-1, multi-frequency SAR observations with high temporal and spatial reso- 10.3390/rs13040825 lution will become available. The SARSense campaign was conducted between June and August Received: 25 January 2021 2019 to investigate the potential for estimating soil and plant parameters at the agricultural test site in Accepted: 18 February 2021 Selhausen (Germany). It included C- and L-band air- and space-borne observations accompanied by Published: 23 February 2021 extensive in situ soil and plant sampling as well as unmanned aerial system (UAS) based multispec- tral and thermal infrared measurements. In this regard, we introduce a new publicly available SAR Publisher’s Note: MDPI stays neutral data set and present the first analysis of C- and L-band co- and cross-polarized backscattering signals with regard to jurisdictional claims in regarding their sensitivity to soil and plant parameters. Results indicate that a multi-frequency published maps and institutional affil- approach is relevant to disentangle soil and plant contributions to the SAR signal and to identify iations. specific scattering mechanisms associated with the characteristics of different crop type, especially for root crops and cereals. Keywords: ROSE-L; soil moisture; plant parameters; L-band; C-band; SAR; airborne campaign Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and 1. Introduction conditions of the Creative Commons With the increasing impact of human activities and the effects of climate change on Attribution (CC BY) license (https:// hydrological systems worldwide, appropriate and adapted management and mitigation creativecommons.org/licenses/by/ concepts are required [1–4]. This is particularly true with regard to the goal of using 4.0/). Remote Sens. 2021, 13, 825. https://doi.org/10.3390/rs13040825 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 825 2 of 29 natural resources more effectively and sustainably in the future [5]. Since soil moisture and water-related vegetation conditions are key parameters in this context, they need to be assessed and monitored at both global and local scales. By providing global data with high temporal and spatial resolution, modern Earth Observation (EO) satellites have become a key technology in this field, whose importance will significantly increase in the future [6–8]. Radar Observing System for Europe L-band SAR (ROSE-L), as one of the Copernicus High Priority Candidate satellite missions is foreseen to be able to target the abovemen- tioned objectives. The mission was first agreed on at the European Space Agency (ESA) ministerial conference Space19+ in Seville in November 2019 and was contractually signed by ESA and Thales Alenia Space later in December 2020 as part of the Fourth ESA Coper- nicus Space Component Program. With a scheduled launch in 2028, the two satellites, carrying a quad-polarimetric L-band SAR, are designed for collecting valuable data, espe- cially for various research and applications in the field of soil moisture, land cover mapping, maritime surveillance, and natural and anthropogenic hazards [9]. A third add-on satellite is currently under discussion for bi-static records (ROSE-L+). In synergy with the existing Sentinel-1 A/B SAR mission, ROSE-L will enhance the European radar imaging capacity by increasing the frequency of successive radar data collections. In this regard, it will also enhance the possibilities for using soil and plant parameter retrieval based on change detection methods (e.g., alpha approximation and interferometry methods). Since L-band wavelength is able to penetrate through various media like vegetation or dry snow, it additionally provides unique information that cannot be obtained using higher frequency bands like the Sentinel-1 C-band and vice versa [10–12]. In combination, a quasi multi- band space-borne radar product can be obtained, which is currently only available using individual airborne flight campaigns [9]. The joint NASA-ISRO SAR (NISAR) satellite mission planned by NASA and Indian Space Research Organization (ISRO) for 2022 can be seen as a potential precursor, carrying both an L- and S-band SAR [13]. In the course of the planning phase of the ROSE-L satellite mission, the potential of L-band SAR data for the proposed applications and the synergy effects from combining L- and C-band SAR data need to be explored. Such information will help to optimize ROSE-L regarding its synergies with the Sentinel-1 mission as well as with other radar satellite missions, e.g., RADARSAT Constellation Mission (RCM), NISAR, Advanced Land Observing Satellite (ALOS-2/4), Satélite Argentino de Observación con Microondas (SAOCOM), TerraSAR-X/TanDEM-X, Paz, and optical satellite missions, e.g., Sentinel-2 and the Landsat series. Various flight campaigns were conducted in the past to unlock the information content of SAR data, particularly for measuring environmental parameters over agricultural and forested areas as • The AgriSAR 2006 campaign was conducted over the Durable Environmental Multidis- ciplinary Monitoring Information Network (DEMMIN) agricultural site in Germany recorded C- and L-band SAR observations and multispectral images in preparation of Sentinel-1 and Sentinel-2 satellite missions [14]. • TropiSAR 2009 campaign was conducted over Nouragues, Paracou in French Guiana, with simultaneous P- and L-band SAR data recording, evaluating the potential of SAR for estimation of biomass over tropical forests [15]. • The Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) flight campaign was conducted between 2012 and 2015 using P-band SAR for polarimetric measurements over major North American biomes, especially focusing on root-zone soil moisture [16]. • The NASA-ISRO Airborne Synthetic Aperture Radar (ASAR) flight campaign in 2019 was conducted over different biomes in North America, investigating the potential of L- and S-band for environmental monitoring in the context of the upcoming NISAR satellite mission [17]. • The UAVSAR AM-PM campaign in 2019 was conducted over different biomes in the Southeastern United States in preparation for the upcoming NISAR satellite mission, using L-band SAR with alternating morning and evening acquisition times [18]. Remote Sens. 2021, 13, 825 3 of 29 Soil moisture, being one of the key parameters within the hydrological cycle, is of high interest for a wide range of research, e.g., for weather and climate research, hydrological modeling, and water resources management [19–21]. In addition, as soil moisture directly affects agricultural production, e.g., by water stress and irrigation demand, it is a crucial parameter for agricultural management decisions and practices at a local scale [22]. As polarimetric SAR data is capable of estimating soil moisture for various environmental and vegetation conditions, the potential of this technology has already been assessed, and various methods are currently employed for soil moisture retrieval [22–24]. The SAR backscatter coefficient sigma nought (s0) is directly proportional to the effective scattering area of an illuminated surface, and is affected both by surface parameters, e.g., soil moisture content (mv), soil texture, surface roughness, and vegetation cover, as well as observation system (instrument) parameters, e.g., frequency, polarization, and incidence angle (θ)[25–30].