SECTION 3
Remote sensing of the aquatic environment can be carried out from a variety of observation platforms. Depending on the distance between the sensor and the target, four categories of platforms can be identified: boats, balloons, aircraft and satellites.
Boats, buoys and submarines and other submersibles have been in use as remote sensing platforms for fifty years, primarily in conjunction with echo sounders and sonar. Sonar was developed in 1918 and was first used for fishery applications in the 1930's. Most modern fishing boats are equipped with echo sounders which utilize paper strip charts or cathode ray tubes (CRT) as display units. Now, however, sonar with audio systems is becoming popular as it is a quick and effective method of transmitting information.
The use of buoys and submarines for visual or echo detection of fish has been mainly experimental. Echo sounders or sonar have been installed in submersibles towed at a distance from the mother ship to minimize the noise interference of the ship's engine on the target species. Submersibles such as RUFAS (Remote Underwater Fishery Assessment System), equipped with underwater TV cameras, have been used successfully in assessing scallop resources.
Free floating or anchored balloons have been used to a limited extent for the aerial photography of water bodies such as bays and lakes to trace water circulation, sedimentation, etc.
Balloons are of limited use for the remote sensing of vast ocean surfaces due to their instability and slow speed.
Aircraft have been used extensively as remote sensing platforms for land and coastal mapping, oceanographic studies and the spotting of fish schools. This is one of the most efficient methods of remote sensing the earth's surface at larger scales. Aircraft have the advantage of optimizing data acquisition by providing operator access to the remote sensing instrumentation and by allowing a wide choice of acquisition parameters. A remote sensing mission can be performed over a particular area at a specified time (weather permitting) and may be repeated under controlled conditions. A suitable altitude can be chosen to optimize resolution and coverage area. Commercially available aircraft can reach an altitude of 15 km. Aircraft can be equipped with black and white, colour or colour infrared (CIR) photographic equipment, multispectral scanners or active sensors such as radar.
The main disadvantages of remote sensing from aircraft are the instability of the platform, the limited geographic coverage (due to the relatively low altitude of the aircraft), the high cost and the dependence on weather conditions. This method, therefore, is used mainly for time- critical missions. When the mission requires repetitive imaging of the same area, the significantly lower access cost of satellite data generally is preferred.
With the use of satellites as remote sensing platforms it has been possible to overcome some of the difficulties encountered in remote sensing with aircraft. Satellites can monitor the entire earth surface on a periodic basis, covering a sizeable section on each revolution. Satellites designed for remote sensing on an operational basis are generally unmanned. Nevertheless, some manned satellites have provided valuable information despite the short duration of their mission, e.g., SKYLAB, SOYUZ and space shuttles.
The theoretical orbit of a satellite is an ellipse. In the case of environmental satellites, however, this ellipse is generally considered as a circle having the earth as its centre. The orbits of satellites are described as follows (refer to Figure 3.1):
i) equatorial - having an orbit within the plane of the equator;
ii) polar - having an orbit within the plane of the earth's axis;
iii) near-polar - having an orbit oblique to the axis of the earth.
Most of the communication satellites have equatorial orbits, while the earth observation satellite series such as TIROS, NOAA and NIMBUS have polar or near-polar orbits. The polar orbits are preferred for viewing longitudinal zones in full daylight or during nighttime.
The orbital inclination “i” may be defined as the angle formed by the orbital plane and the equatorial plane (refer to Figure 3.1).
The near-polar orbiting satellites can be of two types:
i) prograde - which move in the same direction as the earth's rotation, i.e. “i” is less than 90°;
ii) retrograde - which move in the opposite direction to the earth's rotation, “i” is between 90° and 180°.
The track of the satellite crosses the equator at the nodal points. The ascending node is the nodal point at which the northbound track of the satellite crosses the equatorial plane and the descending node is the nodal point at which the southbound track crosses the equatorial plane. Two particular orbits are described by the near complete earth coverage of environmental satellites: the geosynchronous orbit and the sun- synchronous (heliosynchronous) orbit.
Geosynchronous orbits have altitudes up to 3600 km. Satellites in these orbits move in the same direction as the rotation of the earth (prograde) and their velocities are adjusted to maintain the satellites position over a designated point on the earth's surface. When a geosynchronous satellite “hovers” in the equatorial plane (i.e. the orbital inclination “i” equals zero), it is called a geostationary satellite, e.g., the GEOS/METEOSAT meteorological satellite series. Geostationary satellites cannot image the earth's surface at latitudes higher than 80°. They are generally able to image and read out their whole viewable area (1/3 of the earth's surface) every 30 minutes. The large viewable area and the repetitiveness of data acquisition have made this type of satellite popular for meteorological and oceanographic studies.
Figure 3.1 Orbits of satellites. (After E.C. Barrett and L.F. Curtis, 1982)
The main advantages of a geosynchronous satellite are:
i) the most frequent observation of the illuminated region of the earth possible from an orbital system;
ii) the possibility of scanning the same point on theearth repetitively, generating a series of spatially coregistered images;
iii) the largest area coverage possible from an orbital system;
iv) the effective use of telecommunications.
The main disadvantages of a geosynchronous satellite are:
i) the economical and technological difficulties of placing the system in such a high orbit and of obtaining an adequate performance from the sensors carried onboard (e.g., an adequate spatial resolution);
ii) the poor polar coverage.
This is a much lower orbit, (about 900 km) than the geosynchronous orbit. The inclination of the orbit, relative to the equator, is near to 90° (polar or near-polar) and the satellite (e.g., LANDSAT, NOAA, SPOT, etc.) crosses the equator at the same sun time each day. This means that a particular point on the earth is seen regularly (depending on the period of the satellite) at the same hour, which is useful for comparative analysis of multi-temporal data. By selecting a particular orbit it is possible not only to obtain a repeated coverage of the whole surface of the earth, but also to select the interval between observations at a particular location. This is achieved with low level satellites in a polar or near polar sun- synchronous orbit. For example, LANDSAT-4 has an inclination angle of 98.3° and an altitude of 687 km. It crosses the equator every 98.5 minutes and, during that interval of time, the earth has rotated “s”:
R = radius of the earth = 6400 km
T = period of the earth =24 hours = 1440 minutes
The number of rotations per day is given by the ratio:
The satellite overflies a given location every 233 revolutions (i.e. 16 days).
The main advantages of a sun-synchronous satellite are:
i) the economical and technological ease of placing the system in a low orbit and of obtaining an adequate performance from the sensors carried onboard (e.g., high spatial resolution in the order of tens of metres);
ii) the possibility of servicing the orbital system with manned space missions.
The main disadvantages of a sun-synchronous satellite are:
i) the low repetitiveness in coverage (e.g., in the order of weeks) this problem, however, can be overcome with the proper combination of orbital parameters and sensor imaging characteristics. The same location can thus be imaged as frequently as every 1 to 3 days, depending on latitude even though corrections are then required to compensate for the variable incidence angle;
ii) the lost imaging opportunities due to cloud cover, small area coverage and low repetitiveness.
