SATELLITE AND SENSORS

 Satellite Characteristics Satellite Power sources Satellite Orientation Orbits and Swaths  Sensors Active Sensors Passive Sensors  Platform Ground-based Platform Airborne Platform Spaceborne Platform

SATELLITE Man-made object launched into space to orbit the Earth, moon, sun or other celestial body. Satellite communication : Ground segment Space segment Moon is a natural satellite Platforms launched for remote sensing, communication, and telemetry (location and navigation) purposes.

Satellite : Power sources Solar cells Nickel Cadmium battery Technology has not progressed sufficiently for nuclear power sources to be used as a power source.

Satellite : satellite orientation Satellite orientation in space is important for continuous solar cell and antenna orientation. The satellite antenna must also be pointed at the appropriate earth terminals. Spin stabilization operates on the principle that direction of the spin axis of a rotating body tends to remain fixed in space.

An example of spin stabilization is the effect of the rotation of the earth in keeping its axis fixed in space. Satellite that has a spin axis parallel to the axis of the earth will maintain this position since both axes are fixed in space. Figure illustrates the use of this principle.

Once the system is in motion, spin stabilization requires virtually no additional energy. A spin-stabilized satellite is usually constructed like a flywheel. After reaching its orbit, the radial jets are pulsed to start the satellite spinning. The satellite spin axis is orientated to the axis of the earth by means of small axial jets. Velocity jets are used to place the satellite in orbit position and provide velocity correction.

Figure 4-9 is an example of spin stabilization

Satellite : Orbits & Swaths The path followed by a satellite is referred to as its orbit. Satellite orbits are matched to the capability and objective of the sensor(s) they carry. Orbit selection can vary in terms of altitude (their height above the Earth's surface) and their orientation and rotation relative to the Earth. Satellites at very high altitudes (altitudes of approximately 36,000 kilometers ) revolve at speeds that match the rotation of the Earth. Weather and communications satellites commonly have these types of orbits.

Many remote sensing platforms are designed to follow an orbit (basically north-south). Allows them to cover most of the Earth's surface over a certain period of time. Many of these satellite orbits are also sun-synchronous such that they cover each area of the world at a constant local time of day called local sun time. At any given latitude, the position of the sun in the sky as the satellite passes overhead will be the same within the same season. This ensures consistent illumination conditions when acquiring images in a specific season over successive years, or over a particular area over a series of days.

Most of the remote sensing satellite platforms today are in near-polar orbits. The satellite travels northwards on one side of the Earth and then toward the southern pole on the second half of its orbit. If the orbit is also sun-synchronous, the ascending pass is most likely on the shadowed side of the Earth while the descending pass is on the sunlit side. Sensors recording reflected solar energy only image the surface on a descending pass, when solar illumination is available. Active sensors that provide their own illumination or passive sensors that record emitted (e.g. thermal) radiation can also image the surface on ascending passes.

The area imaged on the surface, is referred to as the swath. Imaging swaths for space borne sensors generally vary between tens and hundreds of kilometers wide. As the satellite orbits the Earth from pole to pole, its east-west position wouldn't change if the Earth didn't rotate. The satellite's orbit and the rotation of the Earth work together to allow complete coverage of the Earth's surface, after it has completed one complete cycle of orbits.

SENSOR “Sensor” is preferred because it refers to a broader way of getting information than a camera(only be seen by the eye). Is a device. Used to acquire data i.e. to measure the radiation arriving to the satellite instrument.

TYPES OF SENSOR  Have 2 types of sensor, they are Passive Sensor and Active Sensor  Passive sensor - In remote sensing, many different sensor are used that varying sensitivities to radiations at different wavelengths in the electromagnetic spectrum.

Passive Sensor:  Passive sensors detect electromagnetic radiation emitted from an object.  Record incoming radiation that has been scattered, absorbed and transmitted from the Earth in transit from its original source, the Sun.

The electromagnetic spectrum

 Some are sensor are designed to receive all ‘green’ wavelengths, other that more targeted toward infrared wavelengths.  In infrared viewer, is specially made to ‘see’ objects emitting infrared radiation (even in the dark).  In general terms, sensor that use external energy sources to “observe” an object are called “Passive Sensor”.  Sun is main sources.

Types of Passive Sensor:  Have 5 types of Passive Sensor, they are:- 1) Gamma-ray spectrometer –Passive sensor that detects gamma rays. –The sources for the radiation is are generally upper-soil layers as well as rock layers. –Caused by radioactive decay. – Used to explore mineral deposits.

Passive Sensor: 2) Aerial cameras –Used in aerial photography. –Aircraft serve as a platform as well as many low- earth orbiting satellites deploy many aerial cameras. –Used for topographic mapping.

Passive Sensor: 3) Thermal infrared video cameras –Equipped to detect radiation in the near-infrared range. –Sometimes combined with active sensors, such as radar, to provide additional information. –Aircraft as well as satellites can serve as platforms.

Passive Sensor: 4) Multispectral scanner –Records information in the visible and infrared spectrum. –Scans the Earth's surface for various wavelength bands. –Satellites act as platforms for such passive sensors. –Used for geological purposes.

Passive Sensor: 5) Imaging Spectrometer –Similar to the multispectral scanner. –Scans very narrow wavelength bands of the spectrum. –Satellites are used as platforms. –Used for determining the mineral composition of the Earth's surface and concentrations of suspended matter in surface water.

Passive sensor Disadvantage – if the sky is covered with clouds, they cannot be used to observe the Earth surface (or oceans)

Active sensor:  Sensor that able to direct energy at an object in the form of electromagnetic radiation (EMR).  Object is scanned and the sensors detect any radiation reflected back from the object.  Types of active remote sensing: –Active Optical Remote Sensing –Active Thermal Remote Sensing –Active Microwave Remote Sensing

Active Optical Remote Sensing  Active optical remote sensing involves using a laser beam upon a remote target to illuminate it, analyzing the reflected or backscattered radiation in order to acquire certain properties about the target.  The velocity, location, temperature and material composition of a distant target can be determined using this method.  Example: –LIDAR( Light Detection and Ranging)

Active Optical Remote Sensing LIDAR( Light Detection and Ranging) –The instrument works by using a transmitter and a receiver. –The laser generates pulses which excite the specified target, causing it to absorb radiation at certain wavelengths. –The target then reflects radiation in the form of photons which are detected by the LIDAR's sensors and converted to an electrical signal.

Active Thermal Remote Sensing  Thermal remote sensing deals with information acquired primarily in the thermal infrared range.  The majority of the thermal remote sensing is done using passive sensors.

Active Microwave Remote Sensing  Active microwave remote sensing uses sensors that operate in the microwave region of the electromagnetic spectrum.  Example: –RADAR (Radio detection and ranging)

Active Microwave Remote Sensing RADAR –The sensor transmits a microwave (radio) signal upon a specified target. –The reflected or backscattered radiation from the target is then detected by the active sensors which measure the round trip time delay to targets allowing the system to calculate the distance of the target from the sensors.

Passive sensing relies on reflected sunlight and emission from hot objects (top). Active sensing illuminates the object with its own light source; a laser in this example (bottom).

Platform:  A satellite platform is the service module section of a satellite.  Or the vehicles or carriers for remote sensors 3 types of platform: Ground Based Platforms Airborne Platforms Spaceborne Platforms

Ground Based Platform:  Is the remote sensing platform that position the sensor at the Earth's surface  Used for close-range, high-accuracy applications, such as architectural restoration, crime and accident scene analysis, landslide and erosion mapping,...  It is either static (tripod or mast) or dynamic (moving vehicle).

 These systems are fixed to the Earth  And the ground-based sensors are often used to record detailed information about the surface  or measure environmental conditions such as air temperature, wind characteristics, water salinity, earthquake intensity and such.  Example: -DOE ARM (Atmospheric radiation Program) -NASA AERONET (Aerosol Robotic NETwork).

Airborne platforms: Are primarily stable wing aircraft, although helicopters are occasionally used. Used to collect very detailed images and facilitate the collection of data. Up to 50 km from earth. Examples: NCAR, NOAA, and NASA research aircrafts.

Spaceborne platforms: Platforms that located about 100 km to km from earth. Examples: -rockets, satellites, shuttle Types of spaceborne platforms: -Space shuttle: km -Space station: km -Low-level satellites: km -High-level satellites: about km