RADAR AND ITS MILITARY USES

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-a.

2.     Continuous scanning in space Typically with 5 -> 8 minutes between scans of the same volume.

3.     Reasonable resolution. For a typical 2 msec 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 Z_e 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 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

MILITARY RADAR CASE:-

Recommendation ITU-R M.1638 [7] and CEPT Report 006 [8] provide characteristics of a wide range of military radars. Annex 2 of this report provides some examples of characteristics of military radars.

             

Coexistence between radar and RLAN in the 5 GHz range and work on the efficiency of DFS have been studied in several working groups. In France practical testing campaigns have been performed with military radars in 2004. The situation can be summarized as below:

-           Studies within CEPT (e.g. JPT 5G, SE38, JPT BWA, SE41…): see ERC Report 072 [9], ECC Report 068 [10] and ECC Report 110 [11],

-           EN 301 893 standard v 1.2.3 (for RLAN in the 5150-5350 and 5470-5725MHz bands) has been published in 2003,

-           Tests have shown that DFS characteristics in compliance with EN 301893 v1.2.3 were not sufficient to protect all military radars,

-           EN 301 893 has been improved (but frequency hopping signals are not taken into account) in versions 1.3.1 and 1.4.1,

-           EN 301 893  has been further improved in version v1.5.1 and future version v1.6.1 but frequency hopping signals are still not taken into account.

Tests done with off-the-shelf RLAN equipment in the period 2004-2008 with a variety of radars, including a high performance air defense system and a mobile, theatre air defense system, have shown that the detection and avoidance of such radars by DFS can be very effective. It is important to note that the protection of frequency hopping radars depends on the specifics (e.g. hopping rate, rotation speed, PRF, beam width, etc…) of the operation of the radar.

Also in Sweden practical testing campaigns regarding coexistence between military radar and RLAN (equipped with DFS mechanism) have been performed during the same period. The main results indicated shortcomings w.r.t. the detection of radars using staggered PRFs [12].

Regarding the impact on radars, some uncertainties are related to the mobile nature of RLAN on-board aircraft whereas all previous studies and analysis were performed so far on DFS applied to fixed or nomadic scenarios.

This difference creates additional difficulties in the coexistence with radars, especially for those which have a function of air surveillance.

A first analysis is based on a mutual link budget calculation. This is based on the assumption of a symmetrical propagation path between the RLAN and a typical mobile military radar.

“RLAN à  radar”  link budget			Unit
RLAN IFE eirp	200mW	23	dBm
Aircraft Attenuation	17dB	-17	dB
Radar Antenna gain	34 to 50 dBi	35	dBi
10log(BWLAN/BRADAR)	10log (20/4)	-7	dB
Radar Sensitivity	-105 dBm	-(-105)	dBm
Radar protection criteria (I/N)	-6 dB	- 6
	Necessary Attenuation loss	145	dB
	Distance (free space)	73	km

Table 7: “RLAN à  radar”  Link budget

“Radar à  RLAN”  link budget			Unit
Radar eirp		105	dBm
Aircraft attenuation		-17	dB
Antenna gain		0	dB
DFS threshold		-62	dBm
	Necessary attenuation loss	154	dB
	Distance (free space)	164	km

Table 8: “Radar à  RLAN”  Link budget 

This analysis shows that, in theory, with the DFS detection threshold contained in EN 301893, the DFS mechanism detects this radar before the radar ‘sees’ RLAN interference.

The following sections highlight some practical scenarios that can lead to coexistence difficulties between RLAN on-board aircraft and military radars.

COEXISTANCE SINARIO & OPERATIONAL IMPACT:-

The following scenarios are considered.

	Scenario 1: radar near an airport
	Usually air traffic surveillance is performed with radar in L or S band; but sometimes, a 5 GHz band radar can replace the fixed radar in case of failure As well, protection of an area near an airport can be performed by a 5 GHz band radar.
1.1	Aircraft traffic detection:  X aircrafts The radar is able to detect many aircraft in its coverage area, which, per the data given in Table 7, has a radius of e.g.  73 km.
1.2	Hypothetical worst case: RLANs without DFS (or with DFS not detecting frequency hopping  radar signals) may cause that the radar detects RLAN signals that may appear like  false detections or “radar spoofing” by a foreign aircraft. This could lead the radar to create false tracks or to identify a potential threat. Other possible consequences include: - loss of range in the direction of each aircraft with on-board RLANs - time computer reduced for real threat Notes: Saturation of the radar screen, even of a small part, is highly unlikely given the low power level of RLANs and the distances involved.  -   The potential loss of process time caused by the interference depends on the number of airplanes in view and the intensity of the interference.

Figure 4: Scenarios 1 and 2 radar near an airport

Without DFS or with an inefficient DFS mechanism, radar functioning is not realistic (scenario 1.2). Each aircraft fitted with RLANs will be equivalent to a low power jammer.

1.3

Hypothetical best case: This scenario assumes that RLAN will stop their transmissions immediately on detection of the radar signal. RLANs on board incoming aircraft detect the radar beyond the radar’s coverage area and avoid the use of its channel RLANs on board aircraft departing from nearby airport will detect the radar before or during take-off, before the issues noted in 1.2 appear. In case of a frequency hopping radar, the RLANs on board aircraft may find all channels occupied and have to shut down

Scenarios 2 and 3 Scenarios 4 and 5 present cases of RLANs on board civilian aircraft in the airspace of an air defense system involved in either a peace keeping operation or a maritime situation. The main goal of the radar is to detect a target with ECCM (Electronic Counter Counter Measure) capabilities.  In both cases, the radar may have some difficulties to detect the required target with the presence of an aircraft with 5 GHz on-board  RLAN in the vicinity. .

Scenario 4: Example of peace keeping scenario

Possible evolution of the scenario in a ‘worst case’ situation (in a short time, < a few seconds)  : Detection of aircrafts RLAN signal may be considered as a jamming signal co-located with one aircraft and will cause false alarm and jamming detection.

Scenario 5: airspace and maritime space check

Example of intentional and no-intentional jamming on aircraft

4 aircrafts detected Detection of potential jamming signal collocated with two aircrafts (one is real ECCM jamming, the other is RLAN signal) : the ability to discriminate the hostile aircraft is reduced.

Figure 7: Airspace and maritime space check

These situations can be more or less critical, according to the DFS efficiency. The EN 301893 standard (RLAN) was designed for terrestrial RLAN systems in a stationary environment or with a limited speed compared to the speed of an aircraft.

Coexistence between military radars and RLAN on-board aircraft systems (WLAN on-board aircraft) in the bands 5250-5350 MHz and 5470-5725 MHz can be summarized as follows:

- Without DFS, coexistence is impossible.

- With DFS:

-      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.

Applications

Ground Penetrating Radar -. Landmines, buried some centimetres below the surface, are very dangerous weapons not only in wartime but also for decades afterwards. The landscapes of countries like Angola, Cambodia, the former Yugoslavia and even the deserts of Libya are still contaminated with leftovers from wars that are long finished. Ground-penetrating radar can be used to find and subsequently destroy these mines because radar signals are not completely absorbed or reflected at the boundary between air and ground, but can penetrate into soil, provided that the moisture content is not too high.

Area Surveillance or Ground Surveillance -. Most of these radars are small devices (about the size of a business suitcase) mounted on tripods. They serve as a kind of sentry to keep an 'eye' on an area and issue an alert as soon as something is going on. This function is also performed by JSTARS, an airborne MTI (moving target indicator) radar that puts a whole battlefield under surveillance and monitors movements like march columns on the ground. A prototype was successfully used during Desert Storm in 1991.

Air Surveillance -. Monitoring the airspace is essential for detecting hostile aircraft and directing defensive measures against them. The first such application was the British Chain Home of World War II. In general, radars cannot look around corners - therefore, these radars are usually located on elevated places in order to achieve maximum coverage area. Better coverage, especially against ground-hugging aircraft, can be obtained if the radar is mounted on an airborne platform such as AWACS (Airborne Warning and Control System). Some radars can look around corners: Over The Horizon (OTH) radars exploit certain features of Earth's atmosphere and can detect low-flying objects out to distances of thousands of kilometres.

Target Tracking -

Tracking radars intend to keep a pair of virtual crosshairs centred on a target. They are built around Conical Scan or Monopulse antennae which yield very precise angle measurements. The readings are taken as input to direct gunfire or to control missile weapons.

The ultimate systems in terms of hardware and software complexity are tracking radars built with monopulse phased array antennae. This is accomplished by either splitting an antenna array into four sub-arrays or illuminating the array with a monopulse feed. By switching the antenna's beam position and beam shape, such radars are capable of tracking multiple targets at the same time, doing target searches in between, and transmitting guidance commands to missiles, or illuminating a target so that a missile-seeker head can find it.

Applications

Shell-tracking. -Radar can detect all kinds of airborne objects, and artillery shells are among them. Shell-tracking radars are used for improving the accuracy of an aircraft's own fire and for measuring the flight trajectory of hostile projectiles, in order to calculate their point of origin.

Ballistic Missile Early Warning (BMEWS) and Ballistic Missile Defence.- This is where the big ones are. The requirement for high angular resolution at long ranges leads to really huge antennae. The antennae for ABM (anti ballistic missile) phased array radars such as Cobra Dane or Pave Paws can easily take on the dimensions of multi-storey buildings.

The latest project in this area is the GBR (Ground Based Radar), which is a part of the American NMD (Nuclear Missile Defence) incentive. Also called X-band Radar because of the frequency band employed, it is used to find and track incoming long range missiles or individual warheads, preferably before they reach the summit of their trajectory.

                               REFERENCE 

·         http://2dix.com

·         http://Scribed.com

·         http://Seminarprojects.com

·        http://ebook.com   

·         http://www.britannica.com/EBchecked/topic/488278/radar

·         http://calteches.library.caltech.edu

·         http://www.bbc.co.uk