Geology Reference
In-Depth Information
7
Remote Sensing Principles Relevant to Sea Ice
Satellite remote sensing refers to the recording of
reflected, scattered, and emitted radiation by the Earth's
surface and atmosphere using sensors onboard satellite
platforms. The observations are usually presented in the
form of imagery data, although a few sensors provide only
profile data along the satellite track line. Processing the
data leads to the retrieval of the geophysical properties and
geometrical structure of the imaged area. This is different
from remote sounding, which measures the properties of
the atmosphere between the sensor and the ground target.
Satellite remote sensing is commonly referred to in the lit-
erature as “Earth observations from space” or simply “Earth
observation” (EO). More than 100 EO satellites, developed
by a wide range of national and commercial space agen-
cies, are orbiting the Earth at this time. Sea ice applications
use data from only a few satellites, especially those carrying
microwave sensors. The preferred spatial resolution for
operational ice monitoring would be a few tens of meters
(fine resolution) and hundred meters (medium resolution)
for tactical navigation or a few kilometers (coarse resolu-
tion) for synoptic‐scale observations. Operational centers
usually assess the trade‐off between spatial resolution and
the swath in order to preserve the information at the finest
possible scale while minimizing the data volume to be
processed within the allowable turnaround time.
Remote sensing is the primary tool for monitoring and
retrieving information about sea ice because the major
volume of ice is located in remote areas at high latitudes,
particularly in the polar regions. Therefore, it was not
unexpected to envision sea ice as a leading application of
EO data since its inception in the 1970s. In fact, one of the
driving forces behind the development of the Canadian
Radarsat program was to monitor sea ice especially in the
Arctic. Initially, this was important to support marine
traffic as well as the safety of marine structures for gas
and mineral exploration. Later on, it became even more
important as the decrease in ice extent in the Arctic
became alarming since the late 1990s. Sea ice has been
identified as an important indicator of global warming.
The sea ice data from satellite microwave sensors repre-
sent one of the longest records of EO. It started in the
early 1970s with observations from a few microwave and
optical sensors and has expanded significantly in the past
decade to include a wide variety of sensors. Today, it is
impossible to imagine an operational ice monitoring and
analysis program that does not depend heavily on satellite
remote sensing.
Sea ice can be easily discriminated from the surround-
ing open water in remote sensing observations by utiliz-
ing their contrasting radiometric signatures, which are
triggered by their different physical properties. The
properties include physical temperature, salinity, reflec-
tivity, and surface roughness. This generates contrast in
albedo, emissivity, and dielectric constant between ice
and water or ice of different types. The challenge arises
when the contrast is not sharp enough. Remote sensing
observations are strictly triggered by the optical, ther-
mal, and electrical properties within the penetration
depth of the signal. This depth is usually limited to a
few millimeters or centimeters, depending on the wave-
length of the signal, and can be extended to tens of cen-
timeters for microwave penetration into saline‐free ice.
Information about ice composition below that depth
(e.g., thickness‐based ice types) can only be inferred
using ancillary data such as meteorological conditions,
regional climatic history, and recent history of the ice
field, which have proven to be useful for this task. A
familiar example is the use of radiometric signature and
its texture along with climatological data to infer the
thickness‐based ice types.
