ACRONYMS AND INITIALISMS

ACRONYMS AND INITIALISMS

AFB - Air Force Base
AFGL - Air Force Geophysics Laboratory (at Hanscom AFB, MA)
AFSFC - Air Force Space Forecast Center (at Colorado Springs, CO)
ALMEDS - ALaskan MEteorological Data System
AMSAT - Radio Amateur Satellite Corporation
ARRL - American Radio Relay League
ASCII - American Standard Code for Information Interchange
AUTODIN - AUTOmatic DIgital Network
AWS - Air Weather Service (USAF)
COMEDS - CONUS MEteorological Data System
CONUS - CONtinental United States
DALAS - Disk And Limb Activity Summary
DMS - data management system
DMSP - Defense Meteorological Satellite Program
DOC - Department Of Commerce
DOD - Department Of Defense
EOS - Earth Observing System
ERL - Environmental Research Laboratories
ESA - European Space Agency
GMS - Geostationary Meteorological Satellite (Japan)
GOES - Geostationary Operational Environmental Satellite (Also called SMS/GOES)
GSFC - Goddard Space Flight Center (Greenbelt, MD)
HAO - High Altitude Observatory
HEPAD - High Energy Proton and Alpha Detector (on GOES and TIROS)
HLMS - High Latitude Monitoring Station
HST - Hubble Space Telescope
IAG - International Association of Geomagnetism and Aeronomy
ICE - International Cometary Explorer (formerly ISEE-3)
IGY - International Geophysical Year
IMP - Interplanetary Monitoring Platform
IMS - International Magnetospheric Study
INTERMAGNET - An International Consortium of Magnetic Observatories
ISEE-3 - International Sun Earth Explorer-3.
ISTP - International Solar-Terrestrial Program
IUGG - International Union of Geodesy and Geophysics
IUWDS - International Ursigram and World Days Service
JPL - Jet Propulsion Laboratory
JSC - Johnson Space Center (Houston, TX)
KPNO - Kitt Peak National Observatory
MEPED - Medium Energy Proton and Electron Detector (on GOES and TIROS)
MSFC - Marshall Space Flight Center (Huntsville, AL)
NAG - Naval Astronautics Group
NASA - National Aeronautics and Space Administration
NBS - National Bureau of Standards
NCAR - National Center for Atmospheric Research
NESDIS - National Environmental Satellite, Data, and Information Service
NESS - National Environmental Satellite Service
NGDC - National Geophysical Data Center
NGSDC - National Geophysical and Solar-Terrestrial Data Center (Boulder, CO)
NIST - National Institute of Standards and Technology
NOAA - National Oceanic and Atmospheric Administration
NOAO - National Optical Astronomy Observatories
NORMEDS - NOrthern MEteorological Data System
NOSC - Naval Ocean Systems Center
NRL - Naval Research Laboratory
NSF - National Science Foundation
NSO - National Solar Observatories (combines Sacramento Peak Observatory and the Solar Section of Kitt Peak Observatory)
NSSDC - National Space Science Data Center (Greenbelt, MD)
OLDS - On-Line Data Systems
PL - Phillips Laboratory (Air Force)
RGON - Remote Geophysical Observing Network
RSTN - Radio Solar Telescope Network (USAF)
RWC - Regional Warning Center
ScI - Science Institute (Space Telescope)
SEL - Space Environment Laboratory (ERL)
SELDADS- Space Environment Laboratory Data Acquisition and Display System
SELSIS - Space Environment Laboratory Solar Imaging System
SEM - Space Environment Monitor (on GOES and TIROS)
SEON - Solar Electro-Optical Network (USAF)
SESC - Space Environment Services Center
SFC - Space Forecast Center (at Falcon AFB, Colorado)
SGAS - Solar Geophysical Activity Summary
SMM - Solar Maximum Mission
SMS - Synchronous Meteorological Satellite
SOON - Solar Observing Optical Network (USAF)
SPAN - Space Physics Analysis Network
SXI - Solar X-ray Imager
TDRS - Tracking and Data Relay Satellite (NASA)
TED - Total (particle) Energy Detector (on TIROS)
TIROS - Television and Infrared Radiation Observation Satellite
TMO - Table Mountain Observatory
URSI - Union Radio Scientifique Internationale (thus, URSIgram: message from URSI).
USAF - United States Air Force
USGS - United States Geological Survey
USSFC - United States Space Forecast Center
WDC - World Data Center
WMO - World Meteorological Organization
WWA - World Warning Agency
WWV - call letters of the standard time and frequency radio station

Solar Events

Active Region: A region of enhanced activity on the sun's surface that is associated with a complex magnetic field. An active region may be spotless (plage) or have one or more spots. Active regions are designated by a number when they appear on the visible part of the sun (the visible disk). They are also categorized by their complexity with a rating ranging from alpha (simple) to gamma-delta (multiple complexes). The more complex a region, the more activity (M- and X-flares, etc.) that region produces.

Coronal Mass Ejections (CME): Ejection of a large mass of plasma, including electrons, which are mostly caused by large solar flares. CME's directed towards the earth usually impact the planet between 36 and 96 hours after the ejection. CME's are responsible for increased A- and K-indices by increasing the solar wind velocities. These solar wind velocities may vary from 200km/h (small flares) to 900km/h (large flares).

Coronal Stream: A stream of charged particles originating from the sun's corona. Coronal streams have similar effects as CME's by increasing the A- and K-indices but usually to a lesser extent. However, a few coronal holes may cause major storm levels at the higher latitudes on earth resulting in total propagation fade-out at these latitudes.

Filament: A slow moving "cord-like" mass of plasma which moves across the sun's surface. Since most filaments are darker in color than the surrounding surface they are often visible in optical telescopes.

Prominence: A slow moving large mass of plasma on the sun's surface. Prominences are larger than filaments and are constantly changing in shape.

Proton Flares: An eruption of protons (positively charged nuclear particles) from the sun's surface. Protons usually reach the earth within an hour after the flare and they usually impact the earth at the polar regions where the magnetic field lines converge attracting these charged particles. Protons cause the ionosphere to absorb radiowaves at the polar regions.

Solar Flares: Solar flares are large eruptions of energy and charged particles from the sun's surface. They are usually accompanied by coronal mass ejections and/or proton flares. Solar flares may last from minutes to hours.

Solar Wind: The constant stream of charged particles originating from the sun. The solar wind has speeds ranging from 200km/s to 700km/s, but under certain circumstances such as fast moving CME's or coronal streams, the solar wind speed may increase to around 900km/s.

Sunspot: A small spot on the sun's visible surface where the magnetic flux lines converge. Sunspots appear darker than the surrounding surface area because they are relatively cooler in temperature.

IONOSPHERE

The Ionosphere

Ionosphere: A collection of ionized particles and electrons in the uppermost portion of the earth's atmosphere which is formed by the interaction of the solar wind with the very thin air particles that have escaped the earth's gravity. These ions are responsible for the reflection or bending of radio waves occurring between certain critical frequencies with these critical frequencies varying with the degree of ionization. As a result, radio waves having frequencies higher the lowest usable frequency (LUF) but lower than the maximum usable frequency (MUF) are propagated over large distances. Finally, predictions for the LUF and MUF at different times and regions around the world can be found by searching the world wide web for propagation forecasts.

D-Layer: The lowest part of the ionosphere, the D-layer appears at an altitude of 50-95km. This layer has a negative effect on radio waves because it only absorbs radio-energy, particularly those frequencies below 7MHz. It develops shortly after sunrise and disappears shortly after sunset. This layer reaches maximum ionization when the sun is at its highest point in the sky and this layer is also responsible the the complete absorption of sky waves from the 80m and 160m amateur bands as well as the AM broadcast band during the daytime hours.

E-layer: This part of the ionosphere is located just above the D-layer at an altitude of 90-150km. This layer can only reflect radio waves having frequencies less than 5MHz. It has a negative effect on frequencies above 5MHz due to the partial absorption of these higher frequency radio waves. The E-layer develops shortly after sunrise and it disappears a few hours after sunset. The maximum ionization of this layer is reached around midday and the ions in this layer are mainly O₂⁺.

E_s-layer: Also called the sporadic E-layer. This layer is characteristically very different from the normal E-layer. Its altitude may vary anywhere between 80km and 120km. This extraordinary part of the ionosphere is capable of reflecting radio waves well into the VHF-band (30-300 MHz) and even into the lower parts of the UHF-band (300-3000 MHz). It is still a mystery as to how this layer actually develops, but, it is clear that this layer appears mostly during the summer months and briefly at mid-winter, with the peak occurring in the early summer. Furthermore, it can appear at any time of the day, with a preference for the late morning and early evening. The sporadic E-layer may produce skip distances ranging from 400km to 2000km, with unusually high signal strengths. Even with a fraction of a Watt and a small ground plane antenna, long range contacts are very common.

F-layer: Highest part of the ionosphere. The F-layer appears a few hours after sunset, when the F1- and F2-layers merge. The F-layer is located between 250km and 500km in altitude. Even well into the night, this layer may reflect radio waves up to 20 MHZ, and occasionally even up to 25 MHZ. Ions in the lower part of the F-layer are mainly NO⁺ and are predominantly O⁺ in the upper part.

F1-layer: The F1-layer is located between 150km and 200km in altitude and it occurs during daylight hours. Just before sunrise, the sun begins to shine on the upper part of the atmosphere containing the F-layer. Due to an unclear physical mechanism, the sunlight causes this F-layer to split into two distinct layers called the F1- and F2-layers. The maximum ionization of the F1-layer is reached at midday; this layer merges with the F2-layer a few hours after sunset to reform the F-layer. Finally, this layer reflects radio waves only up to about 10MHz.

F2-layer: This important layer of the ionosphere is the upper most part of the earth's atmosphere and it is located between 250km and 450km in altitude with occasional altitudes extending beyond 600km. At the higher latitudes north or south of the equator, this layer is located at lower altitudes. Near the equator, this layer can be located at twice the altitude as compared to the higher latitudes. About an hour before sunrise, this layer starts to develop as the F-layer begins to split (see F1-layer above). The maximum ionization of the F2-layer is usually reached one hour after sunrise and it typically remains at this level until shortly after sunset. However, this layer shows great variability with peaks in the maximum ionization occurring at any time during the day, displaying its sensitivity to rapidly changing solar activity and major solar events. In contrast to all other layers of the ionosphere, the maximum ionization of the F2-layer usually peaks during the winter months. Most importantly, this layer can reflect radio waves up to 50MHz during a sunspot maximum and maximum usable frequencies (MUF) can extend beyond 70MHz on rare occasions.

Geomagnetic field (GMF): The magnetic field which originates from the rotation of the molten iron core of our planet. This magnetic field produces the well known magnetic flux lines which run between the two magnetic poles allowing us to navigate by use of a compass. The shape of the geomagnetic field, GMF, is very similar to a water drop, with the tail pointing away from the sun. This shape is formed by a constant stream of charged particles originating from the sun (i.e. solar wind) and exerting a constant "pressure" on the side facing the sun. The GMF plays a major role in the dynamics of the earth's atmosphere and without the protection of our GMF, which traps charged particles before they reach the earth's surface, our planet's surface would be undergoing a constant bombardment of these charged particles. Furthermore, without this charged particle trap, the ionosphere would cease to exist and without an ionosphere, sky wave propagation wound not exist and neither would DX contacts! Finally, the GMF is weakest near the polar regions and strongest near equatorial regions and on the night side of the earth opposite the sun, the GMF can extend millions of kilometers into space. Because of the importance of the GMF in trapping charged particles necessary for sky wave propagation, the short term variability of the GMF influences propagation; therefore, these short term variations are included in propagation forecasts. These forecasts categorize the GMF into the following categories: quiet, unsettled, active, minor storm, major storm, severe storm, very severe storm (very rare).

The Basics of Radio Wave Propagation

Edwin C. Jones, MD, PhD (AE4TM) Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996-1200

Radio Wave Propagation

Aurora: A favorite propagation. When more than the usual levels of charged particles arrive at the earth (i.e., increased solar wind), as a result of a CME or coronal stream, many of these charged particles penetrate the weakest parts of the GMF near the polar regions. This is because the GMF field lines guide these charged particles into these regions; at these polar regions, extreme ionization can result at altitudes up to 1000km. Due to this increased ionization, a dynamic curtain shaped layer develops instead of the more typical horizontal shaped F2-layer. This auroral layer may reflect radio waves from the HF-band (3-30MHz) all the way up to and including the entire UHF-band (300-3000MHz). However, due to its very irregular shape and constant movement, heavy fading (QSB) is common in the reflected radio signals. This QSB can also result from multiple reflections within these auroral layers, causing rapid phase shifting. An auroral signal is easily recognized at 30MHz as a bubbling sounding modulation or "under-water-like" modulation. Finally, because of the extreme and sudden phase shifts, narrow band modes such as CW and digital are the most reliable modes for DX contacts.

Backscatter: A useful form of propagation which mostly occurs when the maximum usable frequency (MUF) rises above 30MHz. During these conditions, when radio waves reach the ionosphere (usually the F2-layer), they are reflected towards the earth's surface at a larger detectable continuum of angles than usual. In other words, a detectable fraction of a radio signal is now reflected at a very sharp angle back into region just surrounding the transmitting station but usually beyond the range of ground wave communications (i.e., blind zone). Therefore, backscatter signals are heard within a radius of 2000km from the transmitting station. Backscatter signals are generally weaker than the normal reflected radio waves and during periods of low solar flux, only radio stations using directional antennas can produce readable signals. However, during periods of very high solar flux, even small stations using 10 Watts and vertical ground plane antennas may produce readable signals. Backscatter signals are generally very stable and rarely influenced by QSB. Finally, backscatter signals are easily recognized as a "hollow" or "barrel-like" sound originating from the expected blind zones of a radio station.

Blind Zone: The blind zone is the area around a radio station which cannot normally be worked by either ground waves or normal ionospheric sky waves. Usually stations in the blind zone can only be worked via intermittent backscatter propagation. This zone is also called the "skip zone" by the US Military.

E_s: A mode of propagation producing well known short skip radio contacts off the E-layer of the ionosphere. This propagation occurs most frequently during the summer months with a major node occurring during the summer, a minor node occurring during the winter, and "valleys" occurring around both equinoxes. During the summer, this mode is popular due to its high signal levels. Finally, the skip distances are generally around 1000 statute miles.

F2: The most common mode of propagation is sky waves reflected off the F2-layer of the ionosphere; these reflections are responsible for most DX contacts.

Gray-line: The area occurring along the sunset and sunrise zones (i.e. also called the terminator in astronomy) is known as the gray line and it has special significance to radio communications. Signals which travel along this gray line region often experience significant improvements in received signal strengths as compared to the direct shortest distance communications. This is because the radio wave absorbing D-layer disappears faster than the higher altitude radio wave propagating F2-layer around the time of sunset (and vise versa for sunrise). Because the F2-layer of the ionosphere remains strongly ionized along this gray line, HF signals often have less attenuation when they travel along the gray line as compared to the more direct shorter route.

LUF: Lowest usable frequency.

Meteor scatter: A remarkable type of propagation caused by the ionization by meteors (also known as "shooting stars") entering the earth's atmosphere. Meteors are small rocks orbiting in space and every year on certain dates, the earth passes through streams of these meteors. When the earth crosses an orbit of meteors, meteors hit the earth's atmosphere at a speeds of over 10.000km/h causing them they burn up at extremely high temperatures. The resulting high temperatures leave traces of ionized air behind them at 80-150km in altitude. Fortunately for radio operators, this trace of ionized air can reflect radio waves up to 500MHz and sometimes beyond. It can also reflect HF signals in the range of 30MHz. Each meteor entry results in a radio wave scatter that can be categorized into either a "ping" or "burst". Pings are short openings lasting a few seconds and bursts are openings lasting for minutes. During meteor storms (i.e., when meteors occur at high rates), both pings and bursts can occur so regularly that long QSO's are possible. The most famous meteor shower is called the Perseids and it occurs when the earth crosses the Perseid meteor orbit around August 12th of each year. This particular shower is known to have up to 120 meteors per hour. For instance, in 1994 the Perseids supported radio conversations having strong signal strengths for several hours and the skip distances ranged from 200 to 1800km. However, meteor scatter contacts are usually more brief; and a result, APRS and VHF packet radio is considered to be a good means of communication during meteor showers due to the mode's short packets of data containing useful information such as the transmitting station's callsign as well as location in each packet sent.

MUF: Maximum Usable Frequency.

TA: Trans-Atlantic. A mysterious and rare type of propagation named after the mysterious openings that occur between Europe and North America during the summer months, at a sunspot minimum, and well after sunset. In theory, openings such as these are unlikely, but there have been many occasions in 1995, 1996, and 1997 when such openings like these have occurred which allowed DX contacts across the Atlantic when DX seemed impossible. Even more mysterious is the fact that TV-amateurs received signals across the Atlantic well into the VHF-band during these openings. The mechanism of propagation is still unclear, but one proposed theory suggests that a gigantic E_s-cloud forms above the entire Atlantic resulting in sky wave propagation.

TEP: Trans-Equatorial Propagation. This is another form of mysterious radio wave propagation which occurs during the spring and fall months during the sunspot minimum. This form of propagation allows two stations at nearly identical middle latitudes on opposite sides of the geomagnetic equator to communicate at frequencies up to 150 MHz. For example, communications can occur between Italy and South-Africa or between the West Indies and South America. Like Trans-Atlantic propagation, there is no widely accepted scientific explanation for this type of propagation.

Tropospheric scatter: The only form of propagation that is directly influenced by the surface weather of the earth. Our troposphere (0-10km altitude) is composed of layers of air having different temperatures and moisture contents. When a sharp transition, called an inversion, appears between a cold dry layer and a warm moist layer of air, this transition causes refraction of radio waves. This is analogous to the refraction caused by the transition between water and air. For instance, when you put a stick into the water, it looks like it is bent. This same type of refraction occurs when a radio wave travels through a climate inversion; if the inversion is strong enough, radio waves can be refracted back to the surface of the earth after traveling significant distances (up to several hundred kilometers on the 6m band). Finally, this propagation effect is seen most often in the VHF and UHF bands, especially the 6m band.

Ducting: On rare occasions, two or more inversions may appear at different altitudes. Sometimes certain radio waves can be transported between these two inversions. Therefore, this type of propagation is called "ducting" (or "tunnelling"). Records of over 2500km have been set due to such ducting on VHF and UHF. Unfortunately, the effect is usually confined to 2m, but it can occur as high as 1.2 GHz (usually along frontal systems), and it almost never occurs below frequencies of 50MHz. When ducting does occur on these frequencies, communication distances are typically in the range of ~400km. Inversions usually develop under the influence of high pressure weather systems when there is very little air movement. Also, low pressure systems may produce an inversion when a cold air mass collides with a warmer air mass (called a frontal system in meteorology). Inversions that occur along frontal systems support propagation along a line parallel to the weather front, and radio amateurs using frontal inversion often point their antennas parallel to the frontal system to take advantage of this form of propagation.

Definitions of RADAR on the Web:
An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. RADAR = RAdio Detection And Ranging.
Radio Detection And Range See also: MTI, ATC, VTS, VTMSAcronym for RAdio Detection And Ranging. Radio waves are bounced off an object, and the time at which the echo is received indicates its distance.a method of estimating the distance or travel speed of an object by bouncing high frequency signals off the object and measuring the reflected signal.A system of detecting and locating targets which are capable of reflecting high frequency radio waves (microwaves), generally in the wavelength range from a fraction of a centimetre to some tens of centimetres. Radar is used in meteorology to detect and measure cloud and precipitaion elements. Radar images are regularly used in television weather broadcasts to show the movement of rain bearing cloud formations across the country.
An instrument useful for remote sensing of meteorological phenomena. It operates by sending radio waves and monitoring those returned by such reflecting objects as raindrops within clouds.
Short for "radio detection and ranging," radar sends out short pulses of microwave energy and records the returned signal's strength and time of arrival.an electronic radio detection and ranging system that determined the location, speed, and number of water vessels and/or the azimuth, location, height, speed, and number of aircraft. RA in RAdio, D in Detection, A in And, and R in Ranging.
an electronic instrument that broadcasts and receives microwave signals back from precipitation areas, and determines their location, height, movement, and intensity. rainbow: an arc displaying all colors in the visible light spectrum. Formed when light from the sun is reflected and refracted through water droplets. Always appears on the side of the sky opposite of the sun. ...
A technology used in earth observation that uses microwaves instead of light energy. Whereas optical sensors are passive (rely on the sun's energy to shine on an object in order to detect it), radar detectors are usually active in that they first emit a pulse of microwaves. This microwave energy travels to the surface of the earth and the reflected energy is detected by the sensor (a bit like flash photography). ...
(Radio Detecting and Ranging) - an electronic means of determining distance (not an acoustic means).
radio detecting and ranging; a device used to detect and determine the range to distant objects (eg, hydrometeors) or atmospheric discontinuities by measuring the time for the echo of a radio wave to return from it and the direction from which it returns.
ray-dar i) A system for detecting the direction, range, or prescence of aircraft, ships, and other (usu. moving) objects, by sending out pulses of high frequency electromagnetic waves, ii) the apparatus used for this.Beamed radio waves for detecting and locating objects. The objects are "seen" on the radar screen or scope.A technology used to locate objects by bouncing radio waves at them and observing the returning reflected patterA system using pulsed radio waves to detect the position of objects by measuring the time it takes a single pulse to reach the object and be reflected back.
is of two types, X band and S band. The ship's radar scans the sea only for about 1 and 1/2 vertical degrees in all directions. Modern radar can plot the the course and speed of any ship within range simply by a couple clicks of the mouse (actually track ball). This makes life a heck of a lot easier for the bridge officer
An electronic instrument used to detect objects (such as falling precipitation) by their ability to reflect and scattered microwaves back to a receiver. In Southern Colorado, the National Weather Service operates a radar system (called Nexrad, Doppler, or the WSR-88D) in extreme Northeastern Pueblo County. It is the most powerful and accurate radar in the region.
Measures audience estimates, total radio usage, network radio commercials and network commercials within radio programs. Estimates are based upon daily telephone interviews over a week. RADAR measures listenership for person twelve years of age or older and is published twice a year.
a device for radio detection and ranging. Radar measures the time interval between transmitted and received radio pulses and provides information on the range, azimuth, and/or elevation of objects in the path of the transmitted pulse. A primary radar system uses reflected radio signals. A secondary radar system is a system wherein a radio signal that is transmitted from a radar station initiates the transmission of a radio signal from another station
Radio detection and ranging. An electronic instrument that uses radio waves to find the distance and location of other objects. Used to avoid collisions, particularly in times of poor visibility.(acronym for Radio Detection And Ranging) an electronic instrument that broadcasts and receives microwave signals back from targets to determine location.Stands for "radio detection and ranging." An instrument that detects and ranges distant objects by measuring the scattering and reflection of radio beams. Radiation- The transferring of energy through electromagnetic waves. Rain- Liquid precipitation with drops larger than .02 inches in diameter. Rainbow- An arc or circle of colored light caused by the refraction of light by water droplets in the air. ...
Acronym for Radio Detection and Ranging. A method, a system, or a technique for using beam, reflected, and timed electromagnetic radiation to detect, locate, and track objects, to measure distance (altitude), and to acquire terrain imagery. The term 'radar' in remote sensing terminology refers to active microwave systems (from about 1 GHz - 100 GHz; the majority of current instruments operate below 10 GHz).RAdio Direction and RAnging, has a long history in the development of radio communication and was applied most successfully by the British during the Second World War as part to their coastal defence system. Click here to learn more.
measuring instrument in which the echo of a pulse of microwave radiation is used to detect and locate distant objects
RADAR is an acronym for RAdio Detection And Ranging or Radio Angle Detection And Ranging. It is a system used to detect, range (determine the distance of), and map objects such as aircraft and rain. Strong radio waves are transmitted, and a receiver listens for any echoes. By analysing the reflected signal, the reflector can be located, and sometimes identified. Although the amount of signal returned is tiny, radio signals can easily be detected and amplified.

The Doppler Effect and Sonic BoomsDaniel A. Russell, Kettering University
The sudden change in pitch of a car horn as a car passes by (source motion) or in the pitch of a boom box on the sidewalk as you drive by in your car (observer motion) was first explained in 1842 by Christian Doppler. His Doppler Effect is the shift in frequency and wavelength of waves which results from a source moving with respect to the medium, a receiver moving with respect to the medium, or even a moving medium.
The perceived frequency (f ´) is related to the actual frequency (f0) and the relative speeds of the source (vs), observer (vo), and the speed (v) of waves in the medium by
The choice of using the plus (+) or minus (-) sign is made according to the convention that if the source and observer are moving towards each other the perceived frequency (f ´) is higher than the actual frequency (f0). Likewise, if the source and observer are moving away from each other the perceived frequency (f ´) is lower than the actual frequency (f0).
Although first discovered for sound waves, the Doppler effect holds true for all types of waves including light (and other electromagnetic waves). The Doppler effect for light waves is usually described in terms of colors rather than frequency. A red shift occurs when the source and observer are moving away from eachother, and a blue shift occurs when the source and observer are moving towards eachother. The red shift of light from remote galaxies is proof that the universe is expanding.
The animations below will illustrate this phenomena for a moving source and stationary observer.
Stationary Sound Source
The movie at left shows a stationary sound source. Sound waves are produced at a constant frequency f0, and the wavefronts propagate symmetrically away from the source at a constant speed v, which is the speed of sound in the medium. The distance between wavefronts is the wavelength. All observers will hear the same frequency, which will be equal to the actual frequency of the source.
For a movie showing how circular waves can be created (in terms of particle motion and wave motion) go here.
Source moving with vsource < vsound ( Mach 0.7 )
In the movie at left the same sound source is radiating sound waves at a constant frequency in the same medium. However, now the sound source is moving to the right with a speed vs = 0.7 v (Mach 0.7). The wavefronts are produced with the same frequency as before. However, since the source is moving, the center of each new wavefront is now slightly displaced to the right. As a result, the wavefronts begin to bunch up on the right side (in front of) and spread further apart on the left side (behind) of the source. An observer in front of the source will hear a higher frequency f ´ > f0, and an observer behind the source will hear a lower frequency f ´ < f0.
Source moving with vsource = vsound ( Mach 1 - breaking the sound barrier )
Now the source is moving at the speed of sound in the medium (vs = v, or Mach 1). The speed of sound in air at sea level is about 340 m/s or about 750 mph. The wavefronts in front of the source are now all bunched up at the same point. As a result, an observer in front of the source will detect nothing until the source arrives. The pressure front will be quite intense (a shock wave), due to all the wavefronts adding together, and will not be percieved as a pitch but as a "thump" of sound as the pressure wall passes by. The figure at right shows a bullet travelling at Mach 1.01. You can see the shock wave front just ahead of the bullet.
Jet pilots flying at Mach 1 report that there is a noticeable "wall" or "barrier" which must be penetrated before achieving supersonic speeds. This "wall" is due to the intense pressure front, and flying within this pressure front produces a very turbulent and bouncy ride. Chuck Yeager was the first person to break the sound barrier when he flew faster than the speed of sound in the X-1 rocket-powered aircraft on October 14, 1947. Check out the movie The Right Stuff for more about this significant milestone, and the beginnings of the US space project. The figure at right shows a n F-18 at the exact instant it goes supersonic. Click on the figure to see more information and a MPEG movie of this event.
Source moving with vsource > vsound (Mach 1.4 - supersonic)
The sound source has now broken through the sound speed barrier, and is traveling at 1.4 times the speed of sound (Mach 1.4). Since the source is moving faster than the sound waves it creates, it actually leads the advancing wavefront. The sound source will pass by a stationary observer before the observer actually hears the sound it creates.
As you watch the animation, notice the clear formation of the Mach cone, the angle of which depends on the ratio of source speed to sound speed. It is this intense pressure front on the Mach cone that causes the shock wave known as a sonic boom as a supersonic aircraft passes overhead. The shock wave advances at the speed of sound v, and since it is built up from all of the combined wave fronts, the sound heard by an observer will be quite intense. A supersonic aircraft usually produces two sonic booms, one from the aircraft's nose and the other from its tail, resulting in a double thump. The figure at right shows a bullet travelling at Mach 2.45. The mach cone and shock wavefronts are very noticeable.

The picture at the left shows the shock wave front generated by a T-38 Talon, a twin-engine, high-altitude, supersonic jet trainer (below).
This picture shows a sonic boom created by the THRUST SSC team car as it broke the land speed record (and also broke the sound barrier on land). Click on the image to download a larger version of the image.
Other important applications of the Doppler Effect:
Doppler Radar uses the doppler effect for electromagetic waves to predict the weather.
The Doppler shift for light is used to help astronomers discover new planets and binary stars.
Echocardiography - a medical test using ultrasound and Doppler techniques to visualize the structure of the heart.
Radio Direction Finding Systems
There is also an instrumental rock group called The Doppler Effect

The Doppler Effect
A Familiar ExampleHeard an ambulance go by recently? Remember how the siren's pitch changed as the vehicle raced towards, then away from you? First the pitch became higher, then lower. Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves, as illustrated in the following picture.

As the ambulance approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch. As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the ambulance's speed.
By analogy, the electromagnetic radiation emitted by a moving object also exhibits the Doppler effect. The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blueshifted. In contrast, the radiation emitted by an object moving away is stretched or redshifted. As in the ambulance analogy, blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer.
The Doppler Effect In AstronomyIn astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Radiation is redshifted when its wavelength increases, and is blueshifted when its wavelength decreases.
Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth. For example the spectral lines emitted by hydrogen gas in distant galaxies is often observed to be considerably redshifted. The spectral line emission, normally found at a wavelength of 21 centimeters on Earth, might be observed at 21.1 centimeters instead. This 0.1 centimeter redshift would indicate that the gas is moving away from Earth at over 1,400 kilometers per second (over 880 miles per second).
Shifts in frequency result not only from relative motion. Two other phenomena can substantially the frequency of electromagnetic radiation, as observed. One is associated with very strong gravitational fields and is therefore known as Gravitational Redship The other, called the Cosmological Redshift, results not from motion through space, but rather from the expansion of space following the Big Bang, the fireball of creation in which most scientists believe the universe was born.

BASICS

BASIC RADAR CONCEPTS The term radar is an acronym made up of the words radio, detection, and ranging. It refers to elec- tronic equipment that detects the presence, direction, height, and distance of objects by using reflected electromagnetic energy. The frequency of electromag- netic energy used for radar is unaffected by darkness and weather. This permits radar systems to determine the position of ships, planes, and landmasses that are invisible to the naked eye because of distance, dark- ness, or weather. Radar systems provide only a limited field of view and require reference coordinate systems to define the positions of detected objects. Radar surface angular measurements are normally made in a clockwise direction from true north, as shown in figure 1-1, or from the heading line of a ship or aircraft. The actual radar location is the center of this coordinate system. Table 1-1 defines the basic terms in figure 1-1 that you need to know to understand the coordinate sys- tem. 1-1

RADAR TYPES AND APPLICATIONS

Radar has many different types and applications:
"Search radars" scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. The waves are usually less than a meter long. Ships and planes are metal, and reflect radio waves. The radar measures the distance to the reflector by measuring the time from emission of a pulse to reception, and dividing by the speed of light. To be accepted, the received pulse has to lie within a period of time called the range gate. The radar determines the direction because the short radio waves behave like a search light when emitted from the reflector of the radar set's antenna."Targeting radars" use the same principle but scan a much smaller area far more often, usually several times a second or more, where a search radar might scan a few times per minute. Some targeting radars have a range gate that can track a target, to eliminate clutter and electronic counter-measures."Radar proximity fuzes" are attached to anti-aircraft artillery shells or other explosive devices, and detonate the device when it approaches a large object. They use a small rapidly pulsing omnidirectional radar, usually with a powerful battery that has a long storage life, and a very short operational life. The fuzes used in anti-aircraft artillery have to be mechanically designed to accept fifty thousandg, yet still be cheap enough to throw away."Weather radars" can resemble search radars. These radar use radio waves with horizontal, dual (horizontal and vertical), or circular polarization. The frequency selection of weather radar is a performance compromise between precipitation reflectivity and attenuation due to atmospheric water vapor. Some weather radars uses doppler to measure wind speeds."Marine radars" are used by ships for collision avoidance and navigation purposes. The frequency band of radar used on most ships is x-band (9 GHz/3 cm), but s-band (3 GHz/10 cm) radar is also installed on most ocean going ships to provide better detection of ships in rough sea and heavy rain condition. Vessel Traffic Centre also use marine radars (x or s band) for tracking ARPA and provides collision avoidance or traffic regulation of ships in the survallence area."Navigational radars" resemble search radar, but use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft."General purpose radars" are increasingly being substituted for pure navigational radars. These generally use navigational radar frequencies, but modulate the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar and map data from GPS position."Radar altimeters" measure an aircraft's true height above ground. Air traffic control uses Primary and Secondary Radars Primary radar is a "classical" radar which reflects all kind of echoes, including aircraft and clouds.Secondary radar emits pulses and listens for special answer of digital data emitted by an Aircraft Transponder as an answer. Transponders emit different kind of data like a 4 octal ID (mode A), the onboard calculated altitude (mode C) or the Callsign (not the flight number) (mode S). Military use transponders to establish the nationality and intention of an aircraft, so that air defenses can identify possibly hostile radar returns. This military system is called IFF (Identification Friend or Foe).Mapping radars" are used to scan a large region for remote sensing and geography applications. They generally use synthetic aperture radar, which limits them to relatively static targets, normally terrain.Wearable radar and miniature radar systems are used as electric seeing aids for the visually impaired, as well as early warning collision detection and situational awareness.