Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 20

parallel with data from the ship's meteorological sensors. With the increasing move
towards large interdisciplinary projects, the availability of berths for numerous
scientists is vital.

1.5.6 Remote sensing of ocean properties

A limitation of the measurements made by floats, AUVs, and indeed research vessels
is that they are not synoptic, i.e. the data are not sampled simultaneously at different
locations, as is the case for an array of current meter moorings. Thus, it is not
possible to construct true 'snapshot' pictures of the velocity and other parameters.
The complex sampling scheme in space and time which results from the movement
of the measurement vehicle presents a rather different perspective on the evolution of
the flow and water column structure which, in many applications, presents a chal-
lenge in analysis.
Since the 1970s, the remote sensing of ocean properties has become an increasingly
valuable supplement to direct observations by the techniques described in the pre-
ceding sections. The new information comes mainly from sensors carried on orbiting
satellites, but aircraft-borne sensors are also used for the survey of surface properties.
The extensive spatial coverage available through remote sensing has proved
extremely valuable in interpolating and extrapolating limited data sets from in situ
measurements.
Remote sensing can also be very cost-effective in comparison with ship observa-
tions, which are increasingly expensive. Depending on size, costs for a research vessel
capable of 24-hour continuous operations could be somewhere between US$7000
and US$50 000 per day, and that is without allowance for days lost due to bad
weather. So there is a strong motivation for oceanographers to use remote sensing
data from satellites which is made available by agencies like the National Oceanic
and Atmospheric Administration (NOAA) at modest cost.
An early success in remote sensing was the measurement of surface temperature
by infra-red sensors on polar orbiting satellites which have been operated by NOAA
since the mid-1970s. Temperature is now routinely measured with an accuracy of a
few tenths of a degree Centigrade and the data coverage is global apart from small
regions around the poles. The principles behind this technique will be explained in
Chapter 2 . There are two limitations: (i) infra-red sensing of temperature is only
possible in cloud-free conditions when the satellite can 'see' the ocean surface, and
(ii) the temperature measured is that of a very thin (

1mm) near surface layer which
may differ significantly from the average temperature of the surface water.
Differences in the colour of the surface ocean, as observed by sensors operating at
optical wavelengths, have been used to deduce the concentration of the plant pigment
chlorophyll and hence of phytoplankton. The interpretation of the radiation data
here is not as straightforward as for infra-red measurements, but the distribution of
chlorophyll has been successfully mapped for the deep ocean by optical scanners on
satellites. In the shelf seas, the situation is complicated by the presence of other
substances, notably suspended sediments and gelbstoff (coloured dissolved organic

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