TYPES OF PLATFORMS
The base, on which remote sensors are placed to acquire information about the Earth's surface, is called platform. Platforms can be stationary like atripod (for field observation) and stationary balloons or mobile like aircrafts and spacecraft's. The types of platforms depend upon the needs as well as constraints of the observation mission.
There are three main types of platforms, namely 1) Ground borne, 2) Air borne and 3) Space borne.
1.GROUND BORNE PLATFORMS:
These platforms are used on the surface of the Earth. Cherryarm configuration of Remote Sensing van and tripod are the two commonly used ground borne platforms.
Theyhavethecapabilityofviewingtheobjectfromdifferentanglesandare mainly used for collecting the ground truth or for laboratory simulation studies.
2.AIR BORNE PLATFORMS:
These platforms are placed within the atmosphere of the Earth and can be further classified into balloons and aircrafts.
a. Balloons: Balloons as platforms are not very expensive like aircrafts. They have a great variety o f s h a p e s , sizes and performance capabilities. The balloons have low acceleration, require no power and exhibit low vibrations. There are three main types of balloon systems, viz. free balloons, Tethered balloons and Powered Balloons. Free balloons can reach almost the top of the atmosphere; hence they can provide a platform at intermediate altitude between those of aircraft and space craft.
Thousands of kilograms of scientific payloads can be lifted by free balloons. Unless a mobile launching system is developed, the flights can be carried out only from a fixed launching station. The free balloons are dependent on meteorological conditions, particularly winds. The flight trajectory cannot be controlled. All the semake extremely difficult to predict whether the balloon swill fly over the specific area of interest or not.
In India, at present, Tata Institute of Fundamental Research, Mumbai, hassetupa National balloon facility at Hyderabad. Teethered balloon sare connected to the earth station by means of wire shaving hight ensional strength and high flexibility.
The teethered line can carry the antenna, power lines and gas tubes etc. when wind velocity is less than 35km. perhourat the altitude of 3000m., sphere type balloon is used. When the wind velocity is less than 30 km per hour, natural shape balloons are restricted to be placed. Tethered balloons have the capability of keeping the equipment at a fixed position for a long time and thus, useful for many remote sensing programmers.Powered balloons require some means of propulsion to maintain or achieve station over adesignated geographic location. These can be remotely controlled and guided along with apath or fly above a given area within certain limitations.
b. Aircrafts: Aircrafts are commonly used as remote-sensing for obtaining Aerial Photographs. In India, four types of aircrafts are being used for remote sensing operations.These are asfollows:
DAKOTA: The ceiling height is 5.6 to 6.2 km and minimum speed is 240 km./hr.
AVRO: Ceiling height is 7.5 km and minimum speed is 600 km./hr. CESSNA: Ceiling height is 9 km. and minimum speed is 350 km./hr. CANBERRA: Ceiling height is 40 km. and minimum speed is 560 km./hr.
The following special aircrafts are being used in abroad for remote sensing operations in high altitude photography.
U-2:Ceilingheightis21km.(for strategic photographic).Minimum speedis798 km./hr.
ROCKELL X-15 (Research Craft): Ceiling height is 108 km. and speed is 6620 km./hr.
The advantages of using aircraft sasremotesensing platform are : high resolution of data recorded, possibility of carrying large payloads, capability of imaging large area economically, accessibility of remote areas, convenience of selecting different scales, adequate controlatalltime etc. However, due to limitations of operating altitudes and range, the aircraft find s its great est application sinlocalor regional programmeratherthan measurements on global scale. Beside sallthese, aircrafts have been playing an important role in the development of space borne remote sensing Techniques. Testing of sensors and various systems and subsystems involved in space borne remote sensing programme is always undertaken in a well-equipped aircraft.
3.SPACE BORNE PLATFORMS:
Platforms in space, i.e.satellites are not affected by the earth's atmosphere. The seplatforms move freely in their orbits around the earth. The entire earth or any part of the earth can be covered at specified intervals. The coverage mainly depends on the orbit of the satellite. It is through the sespaceborne platforms, wegetenormous amount of remote sensing data and asaresult Remote Sensing has gained international popularity. According to the orbital mode, there are two types of satellites- Geostationary or Earth synchronous and sun-synchronous.