HYDROGARPHY

Abstract
This review of remote sensing aims to provide a basic understanding of the different remote sensing technologies that can be applied to the marine environment. It is focused towards the coastal zone and coastal water bodies and examples, from around the UK, are shown. The listed WWW sites and publications in the reference list provide links to further information.

INTRODUCTION
Remote sensing may be broadly defined as “the collection of information about an object without being in physical contact” and commonly is restricted to methods that employ electromagnetic energy as the means of detecting and measuring target characteristics (Sabins, 1978). It can produce maps of physical properties, but they have inherent biases/errors that must be appreciated if the data is to be used in an appropriate way. Increasingly, remotely sensed products are made available to a wider audience through electronic media such as the World-Wide-Web (WWW), but the background information is often lost. In applications, such as Hydrographic Surveying, accuracy as well as data availability is a key issue. This paper describes the technology, and hence data, that is or will be available in the near future. Remote sensing can be divided into different categories using several criteria, and this paper (focusing on satellite and airborne platforms) has divided it into active and passive systems.
ACTIVE SYSTEMS
Active systems emit and then detect a returning signal and mainly operate in the microwave region (as there are low natural levels of microwave radiation), but a notable exception is Light Detection and Ranging (LIDAR) which is discussed later. Radar (Radio Detection and Ranging) is a common operating mode for airborne and satellite systems and is also used by ships as a collision avoidance and navigational aid. As radar produces its own electromagnetic radiation and it is not dependent on solar illumination, so can operate during the night, and this is often combined (at the longer wavelengths of around 23cm, the L band) with an ability to penetrate cloud. However, radar is affected by wind speed and this can cause either a bias or unsuitable radar return signal.
SEA SURFACE HEIGHT
Radar altimeters transmit high frequency signals (over 1700 pulses per second) to the Earth and receive an echo from the sea surface. A precise measurement of the round-trip time is derived and, scaled by the speed of light, yields a range. By averaging the estimates over a second we get an accurate (within 2cm) measurement of the satellite-to-ocean range. However, the ultimate aim is to measure sea level relative to a terrestrial reference frame and so independent measurements of the satellite’s orbit are also made. Therefore, in simple terms, the sea surface height (SSH) is the range at a given instant from the sea surface to a reference ellipsoid. The wind speed, and hence sea state, influence SSH because the altimeter is sensitive to sea surface elements perpendicular to the target line (electromagnetic bias). These elements are more frequent in the wave trough than crest, so the altimeter-measured mean is increasingly shifted towards the wave trough as the wave height increases.
The US was the first country to launch satellite-borne altimeters: Seasat in 1978 and Geosat in 1985. Additional missions were launched by the European Space Agency (ESA) and US in the 1990s: ERS-1 (1991-1996), TOPEX/POSEIDON (since 1992) and ERS-2 (since 1995). These missions are part of international oceanographic and meteorological programmes, such as the World Ocean Circulation Experiment (WOCE) and Tropical Ocean and Global Atmosphere (TOGA) that in turn are linked to the World Climate Research Programme, WCRP. Unfortunately, the absence of a sufficiently accurate geoid model means that altimetry only provides information on the variable component of topography. Also, in coastal areas, the limitations in the antenna tracking often cause a loss of data when the satellite goes from the land to the sea. Altimeter data is available via the WWW (see “Further Reading via the WWW”) as global maps of, for example, changes in the SSH that are used for predicting tides and storm surges and oceanographic of events such as El Nino/La Nina.
LIDAR systems have lasers as the emitting source (normally a red wavelength of around 532nm) and receive the reflected optical signal. They are mounted on airborne (aircraft or helicopter) rather than satellite platforms and can provide topographic measurements, for example, inter-tidal or sub-surface bathymetry (bathymetric measurements are dependent on the water clarity). Operational systems are routinely flown by organisations such as the UK Environment Agency and US Army (see the SHOALS WWW site at http://www.sam.usace.army.mil/op/shoals/ ). LIDAR systems can also be used to stimulate florescence in natural or anthropogenic substances e.g. NASA’s Airborne Oceanographic Lidar (AOL) has been used to study phytoplankton, Coloured Dissolved Organic Matter (CDOM), thin oil films and tracer dyes (Hoge et al., 1986). One specific usage is the detection of oil slicks, which can be separated from natural fluorescing substances by the different fluorescence wavelengths (Fingas and Brown, 2000).
SEA SURFACE ROUGHNESS
Synthetic Aperture Radar (SAR) imagery contains a wealth of information on the processes within the water and atmospheric boundary layer. Its potential for oceanographic research was first demonstrated in 1978 (Seasat), but operational uses had to wait until 1991 when ERS-1 was launched by ESA.
SAR systems record the time (between the emitted and returned signal) and strength of the signal scattered back by the surface (radar return). Targets that backscatter significant amounts of radiation will have a high radar return and therefore be assigned a high pixel value in the image i.e. appear bright. The images are built up from a series of strips in the range direction (not vertically downwards, but facing to one side) by the forward movement of the platform, and the final product is a measure of sea surface roughness (at the capillary scale).
For a given target, as the platform moves, the distance from the radar to the target (i.e. the slant range) changes continuously, hence the phase of the reflected signal changes. The return signals can therefore be correctly phased, so that the net effect is equivalent to them all having been received simultaneously by an antenna whose length is equal to the total path length of the collected radar signals i.e. the synthetic aperture, which forms part of the name.
During the last thirty years, the pollution of the world’s oceans (particularly in coastal areas) has become a matter of increasing international concern. The monitoring of oil spills and illegal ballast discharges (which can also introduce foreign micro-organisms and bacteria) is thus an important component in ensuring compliance with marine protection legislation and the general protection of coastal environments. Airborne SAR imagery has been used to detect slicks and now fast delivery satellite SAR products (which can cover a large area at any one time) can be used to target or replace airborne systems. Major factors in oil slick detection are the wind speed, and hence sea state, and the separation of oil slicks from other factors that influence the sea surface roughness i.e. natural films and wind effects. The Tromsø Satellite Station (Norway) has an in-depth explanation and on-line papers on the detection of oil spills and natural oil