Geology Reference
In-Depth Information
the observations from a calibration target against mod-
eled (simulated) observation to establish an empirical
calibration equation. This is useful to account for any
drift in the sensor's electronics. Stability of calibrated
data is an issue that should be examined routinely. A nat-
ural target that has been used frequently to examine the
calibration stability of the Canadian satellite Radarsat‐1
was an area in the Amazon rainforest region. The calibra-
tion accuracy was established for the ascending and
descending orbits of this satellite (S. Srivastava, Canadian
Space Agency, personal communication).
Relative calibration should be satisfactory in sea ice
applications such as delineation of sea ice boundaries,
identification of ice surface features that contrast the
background image tones, or tracing trajectory of ice floe
motion in sequential images. Most of the image classifi-
cation techniques employ procedures that required rela-
tive calibration only. If, on the other hand, the purpose is
to retrieve geophysical parameters such as surface tem-
perature, ice concentration, or thickness, then absolute
calibration becomes necessary. This is particularly true
for approaches that involve combining data from differ-
ent sensors. Producing the operational ice charts based
on visual analysis of remote sensing images does not
require calibrated data at all.
Figure 7.6
First image captured and transmitted by TIROS‐1
satellite on 2 April 1960 showing sea ice cover in the Gulf of
St. Lawrence (the gray area in the middle) (photo library, U.S.
National Oceanic and Atmospheric Administration).
The Nimbus satellite series started with the launch of
Nimbus‐1 on 28 August 1964 and ended 20 years later
with Nimbus‐7. These were Sun‐synchronous near‐polar
orbit satellites. The program effectively marked the begin-
ning of the Earth observations era. The first four satel-
lites carried visible and infrared sensors. They collected
orbital data on the ice extent in the polar regions for the
first time in the mid‐1960s. However, later in that decade
it became clear that frequent synoptic observations of
polar ice caps required microwave sensors because of the
limitations of the VIS and IR sensors during the long
dark winter and the frequent cloudy conditions in the
remaining months of the year.
The first passive microwave satellite sensors were
launched on the Russian Kosmos‐243 and Losmos‐384 on
23 September, 1968 and 10 December, 1970, respectively.
In the United States, the first passive microwave sensor,
the Electrically Scanning Microwave Radiometer (ESMR),
was launched on Nimbus‐5 on 10 December, 1972. This
was a cross‐scan instrument, which had one horizontally
polarized radiometer operating at a frequency of
19.35 GHz (1.55 cm wavelength). This channel discrimi-
nates well between sea ice and open water but data have to
be integrated over large footprints. The spatial resolution
of ESMR (a cross‐track scanner) was 25 km × 25 km near
nadir, degrading to 160 km × 45 km at the end of the scan.
This sensor allowed the first estimates of ice concentration
during its lifetime from 1973 to 1976. Daily and monthly
averaged sea ice concentration maps from ESMR were
calculated later from reprocessed data for the Arctic and
the Antarctic from 12 December 1972 through 31
December 1976. The maps are available in 25 km gridded
resolution from the National Snow and Ice Centre
(NSIDC) via the link
http://nsidc.org/data/nsidc‐0009.
html.
The data are processed to reduce weather and coastal
7.2. Historical synoPsis of satellite
remote sensinG for sea ice
While sea ice observations from coastal stations and
ships have a history of more than 100 years, observations
from satellite sensors are relatively new. This section pre-
sents a few historical synopsis of satellite remote sensing
of sea ice with more focus on microwave sensing (passive
and active) because of their wider use in sea ice monitor-
ing and parameter retrieval.
The first satellite programs that collected data on sea
ice were NASA's Television and Infrared Observations
Satellite (TIROS), Nimbus (a meteorological research
satellite), and the Earth Resources Technology Satellite
(ERTS) programs. The latter was renamed later to
Landsat. The first satellite image of sea ice was captured
by the television camera onboard TIROS‐1 on its second
day of operation, 2 April 1960. It was an image of the
Gulf of St. Lawrence in Canada (Figure 7.6). It showed
dark areas west of New Brunswick and north of Prince
Edward Island, which could be interpreted as cloud cover.
Upon comparison against aerial photographs from the
Canadian Meteorological Service at that time, it was con-
firmed that the observed tone represents sea ice. That,
indeed, was the first indication that satellite ice reconnais-
sance would be a valuable tool to add to traditional ice
observation wards at that time, namely ships, aircrafts,
and ground meteorological stations.
