30-04-2011, 04:16 PM
PRESENTED BY
UJJWAL KUMAR JHA
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Introduction
Radar is an acronym for Radio Detection And Ranging. A radar is an electro-magnetic device capable of transmitting a electro-magnetic wave near 1 Ghz, receiver back a reflection from a target and based on the characteristics of the returned signal determine things about the target. Radars have become indispensable in several major fields of research and in commerce. The Federal Aviation Agency (FAA) makes extensive use of radars not only to track aircraft, but to make sure landings and take-offs are uneventful. Meteorologist use radars to track severe weather and to estimate the amount of rainfall. Radar meteorology means many things to many people. Depending on what your research interests is your definition may be very different from mine. As a working definition I will use the following:
Definition: Radar meteorology is the study of the atmosphere using radar as a tool.
Radar Meteorology is not a true branch of meteorology because it is use by several true branches of meteorology, such as cloud physics and severe storms, as a tool for that particular branch. Radar meteorology is also not a branch of radio meteorology; Radio meteorology is the study of how electro-magnetic waves travel through the atmosphere. As such radio meteorology deal with refraction, reflection and propagation of electro-magnetic waves. Although these concepts are very important they are not the core of radar meteorology.
Basic Characteristics
Radar is a remote sensing tool in that it is not in contact with the object it is sensing Radar measures the characteristics of the atmosphere from a distance. Further, radar is an active sensor in that it modifies the atmosphere and then measures the atmospheres response. Radar is not a prognosticator, i.e. it does not make a forecast rather it samples the atmosphere from a close distance and there appears to make a very accurate forecast. Radar is a means of detecting locating identifying, measuring and then displaying the atmosphere and what is in it. Radar is useful because of the following characteristics:
1. Radar scans a three-dimensional volume and can be pointed any where in space. The scale of the smallest volume is meso-.
2. Continuous scanning in space Typically with 5 -> 8 minutes between scans of the same volume.
3. Reasonable resolution. For a typical 2 sec pulse at 100 nm the volume is about 5 km x 5 km x 600m
4. Total variability of the atmosphere can be measured, i.e. Radar can measure all the components of the total derivative.
5. Radar can make in-storm measurements
6. Radar can measure the actual severity of the storm, since Ze is a measure of the number of hydrometers per cubic unit.
7. Radar, if coherent, can measure the three components of the wind.
Thus from the meteorologists point of view a radar provides a large number of advantages over any other tool designed to look at the structure of severe storms and clouds. Much of what we know about the inner workings of thunderstorms and other precipitating cloud systems come from radar.
Radar uses an antenna producing a narrow beam of energy to scan a volume of space until a reflection is obtained. The direction the antenna is pointing and the time interval between the transmission and reception determine the location of the reflection in space. Further the strength and polarization of the reflection determine the characteristics of the target.
Types of Radars:-
Radars come in many forms depending on the use that the radar. As with anything, the type of radar can be broken down into a number of different types. The first major sub-category is based on the kind of antenna used. If the transmitters antenna and the receivers antenna are the same antenna the radar is mono-static. This is the most common type of radar in use by meteorologists. A bi-static radar is one where the antenna for the transmitter and receiver are different. The normal case is where I have a
single transmit antenna and many different receivers. This is the case with NCAR's SPOL radar. This radar can be setup as a mono-static system or it can be setup as a bi-static array, where the series of receive only facilities are distributed around the primary SPOL.
The bi-static Radar Network (BINET) consists of several where the series of receive only facilities are distributed around the primary SPOL.
The bi-static Radar Network (BINET) consists of several link. In addition, its small size and low power consumption allow for operation from a simple and small-integrated generator unit. A wireless data page link eliminates difficult installation and hookup even in the most remote locations in the world.
A third possibility is that the radar is mounted ona ship or aircraft. The aircraft radar presents an interesting case for the application of radar. Normally to recover the horizontal and vertical winds, you need at least two Doppler placed such that the angle between the two radars beams is 90. This often very difficult with ground based radars. With an aircraft housed radar the aircraft can fly close to the region of interest. By mounting two antennas, one pointing fore and one pointing aft, as the radar flies by the target first the fore radar will sense the target and then the aft radar. By combing the fore and aft signals I can recover the three-dimensional wind field s described in chapter 8. NCAR’s Electra aircraft has the ELDORA aircraft radar mounted in the tail (see Fig. 1.3) and has been extensively used in several projects most notably VORTEX.
Executive summary:-
This ECC Report addresses the issue of compatibility between RLAN on-board aircraft and radars (military and meteorological) in the bands 5250-5350 MHz and 5470-5725 MHz. It investigates whether the approach taken for the compatibility between ground-based RLAN and radars (i.e. DFS with the essential requirements as defined in EN 301 893 v1.5.1) is applicable in the case of the operation of RLAN on-board aircraft.
With regard to military radars in the bands 5250-5350 MHz and 5470-5725 MHz, the Report shows that:
- RLAN on-board aircraft compatibility with military radars, in these bands is theoretically feasible but should be carefully considered, in the light of the mobile nature of the aircraft. Detection of some specific military radar signals by DFS can not be ensured. In addition, in some specific scenarios, this may lead to a reduction of the ability of a military radar to identify the required target.
- Although EN 301 893 has not been specifically developed to address radars using Frequency Hopping modulation, detection of Frequency Hopping radar signals is ensured if these signals are covered by one of the existing radar test signals included in EN 301 893. In the case of RLAN on-board aircraft flying over areas where frequency hopping radars are in use, frequent DFS triggers may cause numerous channels to be temporarily unavailable for the RLAN on-board aircraft operation.
First uses of radar in military –
During the 1930s, efforts to use radio echoes for aircraft detection were initiated independently and almost simultaneously in eight countries that were concerned with the prevailing military situation and that already had practical experience with radio technology. The United States, Great Britain, Germany, France, the Soviet Union, Italy, the Netherlands, and Japan all began experimenting with radar within about two years of one another and embarked, with varying degrees of motivation and success, on its development for military purposes. Several of these countries had some form of operational radar equipment in military service at the start of World War II.
The first observation of the radar effect at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., was made in 1922. NRL researchers positioned a radio transmitter on one shore of the Potomac River and a receiver on the other. A ship sailing on the river unexpectedly caused fluctuations in the intensity of the received signals when it passed between the transmitter and receiver. (Today such a configuration would be called bistatic radar.) In spite of the promising results of this experiment, U.S. Navy officials were unwilling to sponsor further work.
The principle of radar was “rediscovered” at NRL in 1930 when L.A. Hyland observed that an aircraft flying through the beam of a transmitting antenna caused a fluctuation in the received signal. Although Hyland and his associates at NRL were enthusiastic about the prospect of detecting targets by radio means and were eager to pursue its development in earnest, little interest was shown by higher authorities in the navy. Not until it was learned how to use a single antenna for both transmitting and receiving (now termed monostatic radar) was the value of radar for detecting and tracking aircraft and ships fully recognized. Such a system was demonstrated at sea on the battleship USS New York in early 1939.
The first radars developed by the U.S. Army were the SCR-268 (at a frequency of 205 MHz) for controlling antiaircraft gunfire and the SCR-270 (at a frequency of 100 MHz) for detecting aircraft. Both of these radars were available at the start of World War II, as was the navy’s CXAM shipboard surveillance radar (at a frequency of 200 MHz). It was an SCR-270, one of six available in Hawaii at the time, that detected the approach of Japanese warplanes toward Pearl Harbor, near Honolulu, on December 7, 1941; however, the significance of the radar observations was not appreciated until bombs began to fal
