Publishers version - DTU Orbit

15 SAR for wind energy
Charlotte B. Hasager and Merete Badger
DTU Wind Energy, Risø Campus, Roskilde, Denmark
15.1 Introduction
Satellite Synthetic Aperture Radar (SAR) can provide a wide range of information about the
surface of the Earth. SAR instruments have been flown on several satellite platforms since the
early1990s.OneofthepurposesofthesatelliteSARsisresearch.Thesuccessfulinvestigations
have led to a variety of (near-) operational products including sea ice mapping, oil spill
detection, ship movement, mapping of flooded land surfaces, earth quake, land slide and
subsidence mapping, digital elevation mapping, urban development, vegetation and biomass
changes, glacier coverage, ocean wave and ocean current information, and last but not least
ocean surface wind mapping.
One advantage of SAR compared to optical remote sensing is that the radar carries its
own illumination source, and is thus independent of daylight. This is particularly useful near
the Arctic and Antarctic where daylight is limited for several months per year. SAR operates
in the microwave bands. Microwave radiation is able to penetrate clouds and precipitation.
SARs are all-weatherinstruments and so not limited by cloud cover. This is particularly useful
in cloudy and rainy scenarios including hurricanes.
Ocean surface wind mapping from SAR has been described in numerous articles. A recent
state-of-the-art white-paper on ‘Wind retrieval from Synthetic Aperture Radar - an overview’
from the SEASAR 2012 workshop ‘Advances in SAR Oceanography’ by the European Space
Agency(ESA) (Dagestadet al.,2013)summarizesthetechnicalfundamentalsofsatelliteSAR
ocean surface wind retrieval. A wide range of applications are also presented including ocean
wind mapping for weather prediction, wind farming, tropical cyclones, polar lows, katabatic
winds, gap winds, vortex streets, boundary layer rolls and atmospheric gravity waves. See this
paper for references (157 in total).
SAR measurements are high-resolutionobservations of the Earth surface. Although no SAR
sensor has been designed specifically for wind mapping, it has become clear that SAR data is
very suitable for high-resolutionwind retrievals over the ocean includingnear-shore areas. The
spatial resolution of SAR makes it particularly useful for resolving mesoscale wind variability.
Planning of offshore wind farms has emphasized the need for reliable ocean wind observations.
Ocean wind observations are generally costly to obtain from meteorological masts or
ground-based remote sensing instruments. Furthermore, such data is only valid near the local
point at which it is measured. In contrast, satellite SAR can provide spatially resolved ocean
wind information. Most potential offshore wind farm sites are covered by archived SAR data.
Wind resource mapping can thus be performed without any delay whereas it takes time to
plan and conduct a ground based observational campaign.
15.2 SAR technical description
SAR is an active microwave sensorwhich transmits coherent microwaves.The imagesshowing
the normalized radar cross section (NRCS) are made from advanced signal processing of the
originalobservations.NRCSistherecordedbackscatteredsignalperunitarea.Themicrowaves
transmitted and received are either vertically (V) or horizontally (H) polarized. Co-polarized
(VV or HH) images are made if the same polarization is used for both transmitting and
receiving. Cross-polarized (VH or HV) images are made otherwise.
The co-polarized NRCS has traditionally been used for ocean surface wind retrieval at
spatial pixel scales finer than 1 km. Cross-polarized NRCS has been tested for wind retrieval.
276 DTU Wind Energy-E-Report-0029(EN)