OFFSHORE RESOURCE ASSESSMENT - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 211

of comparison, researchers have developed semiempirical relationships between mea-
sured wind speeds and microwave observations of various types and frequency bands.
As a result of this research, over the last two decades, a large amount of offshore
surface wind speed data from a variety of instruments have become available (6).
A key advantage of using microwaves instead of other parts of the electromagnetic
spectrum is that they are indifferent to day and night and, depending on the fre-
quency, can pass through most clouds. Also, unlike cloud-tracking satellites, satellite-
based microwave sensors measure winds near the surface. However, the areal cov-
erage, spatial resolution, and revisit frequency 3 of the satellite-based sensors vary
greatly. Radiometers and SCATs tend to cover large areas with greater frequency
than SAR, but their spatial resolution is coarse. SAR achieves a much higher spa-
tial resolution, but with fewer SAR images available, its temporal coverage is more
limited.
Because they do not measure speed directly but rather infer it from properties of
the sea surface, it is unlikely that satellite-based microwave observations will achieve
the accuracy needed to replace on-site towers, lidar, or sodar for resource assessment.
One important limitation is that the microwave signals are calibrated to buoy mea-
surements taken just a few meters off the surface, which are adjusted to a nominal
10 m height. The resulting speed estimates must then be projected to the hub height
of wind turbines. Each stage of this process involves approximations and assump-
tions that can affect accuracy. Nevertheless, in somewhat the manner of numerical
atmospheric models, satellite-based microwave data can be an excellent tool for site
prospecting and, in conjunction with high quality surface-based wind measurements,
for characterizing the variation of the wind resource within project areas.
14.4.1 Types of Sensors
Table 14-1 lists many of the wide variety of satellite-based microwave sensors that
have been deployed. The list changes often as new satellites with improved sensors are
launched almost every year. The three main types of instrument, namely radiometers,
SCATs, and SARs, are described in the following sections.
Passive Microwave Radiometers. Passive microwave radiometers, such as
SSM/I, TMI, AMRS-E, and WindSat, record emissions from the earth's surface in
a number of frequency channels. (They are called passive because, unlike radar,
they do not bounce signals off the earth's surface but only passively observe its
emissions.) From these measurements, daily global wind maps are produced at a
horizontal resolution of approximately 25 km. These maps, in turn, can be compiled
into maps of monthly, seasonal, and annual mean wind speeds. Because the accuracy
of the speed estimates is affected by the presence of land within the sensor's field of
view, valid observations are unfortunately limited within about 25 km of coastlines,
where most offshore wind projects are located.
3 The revisit frequency is the number of times per day a satellite passes within range of the same
location.