LECTURE 12
RADAR AND COMMUNICATION SATELLITE SYSTEMS
· The word Radar is an acronym for Radio Detection and Ranging.
· As it was originally conceived, radio waves were used:
1. To detect the presence of a target
2. To determine the distance or range of target.
· The primary purpose of radar systems is to determine the range, azimuth, elevation, or velocity of a target.
· In order for a radar system to operate, it must transmit and receive electromagnetic radiation known as radio frequency (RF).
· RF transmissions have specific characteristics that determine the capabilities and limitations of a radar system to provide these target discriminants, based on an analysis of the characteristics of the target return.
· The frequency of transmitted RF energy affects:
1. The ability of a radar system to analyze target return, based on time, to determine target range.
2. The ability of the transmitting antenna to focus RF energy into a narrow beam to provide azimuth and elevation information.
· The wavelength and frequency of the transmitted RF energy impact the propagation of the radar signal through the atmosphere.
· The ability of a radar system to use the Doppler effect in analyzing the radar return impacts the velocity discrimination capability of the radar.
· Each of radar produces a RF signal with specific characteristics that differentiate it from all other signals and define its capabilities and limitations.
· Radar signal characteristics determined by the radar transmitter are:
1. Pulse width (pulse duration).
2. Pulse recurrence time (pulse recurrence interval).
3. Pulse recurrence frequency.
4. Power.
· Radar receiver characteristics are:
1. Listening time.
2. Rest time.
3. Recovery time.
· There are critical relationships between RF, pulse recurrence frequency (PRF), pulse width (PW), and beamwidth in targeting detection and making resolution.
· The method radar antennas employ to sample the environment is called the radar scan
· Radar scan is actually a critical design feature of a radar system.
· Four examples of radar systems widely used are basic pulse radar, continuous wave (CW) radar, pulse Doppler radar, and monopulse radar.
· Satellite is an object that circulates around the Earth via its orbital plane or path.
· Satellite communication began after the 2nd world war (WWII).
· The 1st satellite known as Sputnik (“Fellow Traveler”) was firstly launched in October 1957 by the Soviet Union.
· Sputnik was basically a small sender (22-inches diameter) used for transmitting a periodic beep sound.
· In 1965, the 1st commercial geostationary communication Intelsat 1 (“Early Bird”) went into operation.
· Intelsat is an acronym derived from International Telecommunications Satellite.
· It was in service for 1.5 years, weighing 68 kg and offering 240 duplex telephone channels or alternatively a single TV channel.
· Traditionally, satellites have been used in the following areas:
1. Weather forecasting.
§ Several satellites deliver pictures of the earth using either infrared or visible light.
2. Radio and TV broadcast satellites.
§ Hundreds of radio and TV programs are available via satellite.
§ This technology competes with cable in many places, for it is cheaper to install and in most cases no extra fees have to be paid for this service.
§ Today’s satellite dishes have diameters of 30~40 cm in central Europe, whereas the diameters in northern countries are slightly larger.
3. Military satellites.
§ Many communication links are managed via satellites for they are much safer from attack by enemies.
4. Satellites for navigation.
§ The Global Positioning System (GPS) is nowadays well known and available for everyone even though it was initially only used for military purposes.
§ The GPS allows for precise localization worldwide where the precision is in the range of some meters with the application of some additional techniques.
§ Almost all ships and aircraft rely on GPS as an addition to traditional navigation system.
§ Many trucks and cars come with installed GPS receiver mainly used for fleet management of trucks or for vehicle localization in case of theft.
5. Satellites for earth-resources remote-sensing.
§ The applications are shipping and fisheries, ocean feature mapping, oil pollution monitoring, sea ice mapping, iceberg detection, crop monitoring, forest management, geological mapping, topographic mapping and land use mapping.
· In the context of mobile communication:
1. Global telephone backbones.
§ One of the 1st applications of satellites for communication was the establishment of international telephone backbones.
§ However, satellites functioning as backbones for telecommunication networks are increasingly being replaced by fiber optics crossing the oceans.
§ The main reasons for this are the tremendous capacity of fiber optical links (some 10 Gbps using wavelength division multiplexing (WDM)) and the much lower delay compared to satellites.
2. Connections for remote places or developing areas.
§ Many places all over the world do not have direct-wired connection to the telephone network or the Internet due to geographical location (e.g. researchers on Antarctica) or to the current state of the infrastructure of a country.
§ Satellites now offer a simple and quick connection to global networks.
3. Global mobile communication.
§ The latest trend for satellites is the support of global mobile data communication, e.g. the Iridium service.
§ In this case, satellites using lower orbits are needed.
§ The basic purpose of satellites for mobile communication is not the replacement of existing mobile phone networks, but their extension into areas without coverage.
· Allocating frequencies to satellite services is a complicated process requires international coordination and planning.
· The ITU facilitates frequency planning by dividing the world into 3 regions:
1. Region 1: Europe, Africa, formerly the Soviet Union and Mongolia.
2. Region 2: North and South America and Greenland.
3. Region 3: Asia (excluding region 1 areas), Australia and the Southwest Pacific.
· Satellite frequency band designations:
Freq. Range (GHz) / Band Designation0.1 ~ 0.3 / VHF
0.3 ~ 1.0 / UHF
1.0 ~ 2.0 / L
2.0 ~ 4.0 / S
4.0 ~ 8.0 / C
8.0 ~ 12.0 / X
12.0 ~ 18.0 / Ku
18.0 ~ 24.0 / K
24.0 ~ 40.0 / Ka
40.0 ~ 100.0 / mm
· Four different types of orbits used for communication satellites:
1. Geostationary or Geosynchronous Earth Orbit (GEO).
§ GEO operation distance: Almost 36 000 km to the earth.
§ The lifetime of GEO is expected about 15 years.
§ Used for TV and radio broadcasting, weather forecasting and telephone network backbone.
2. Medium Earth Orbit (MEO).
§ Operation distance: about 5 000 ~ 12 000 km.
3. Low Earth Orbit (LEO).
§ Operation distance: 500 ~ 1 500 km.
4. Highly Elliptical Orbit (HEO)
§ This class comprises all satellites with non-circular orbits.
§ These systems have their perigee (the point of closest approach to earth) over large cities to improve communication quality.
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