3D RADAR

A radar which measures the elevation angle along with the azimuth angle and range is called a 3D radar.

One which measures the range and one angle coordinate (usually azimuth) is called a 2D radar.

3D RADAR

The range -azimuth coordinates of a target can be obtained with a vertical fan-beam antenna, the elevation coordinate can be obtained with a horizontal fan beam (The three coordinates can thus be obtained with two 2D-radars).

Such a height finder would be directed by the 2D air-surveillance radar to the azimuth of the target it then scans its horizontal fan beam in elevation to make an elevation angle measurement of the target found at the range designated by the air-surveillance radar.

NODDING-BEAM HEIGHT FINDER:-

The nodding-beam height finder scans its beam in elevation by mechanically locking the entire antenna. It is possible to mechanically slew a nodding-beam height finder a full 180deg in a relatively short time.

The nodding-beam height finder is one of the oldest techniques for measuring the elevation angle of aircraft targets.It is also one of the best.Its accuracy is probably as good or better then any other technique. Even though a separate radar is employed to measure target height, the combined cost of the 2D air-surveillance radar and the nodding beam finder can be less then cost of a comparable single 3D radar.

Another advantage of the separate nodding height finder in military application is that it generally operates at a higher frequency than does the 2D radar .This increases the ECCM capability of the system since a jammer must rad

iate in both radar bands simultaneously to deny the location of the air craft target.

Instead of mechanically rocking the entire antenna structure , the horizontal fan beam of a height finder can be scanned in elevation means, such as with the Robinson scanner, organ pipe scanner or eagle scanner. It is also possible to scan the beam with electronic phase shifter.

This radar generates two fan beams one vertical and other slanted at some angle to vertical. The time separation between the echoes received in the vertical and slant beams is measure of target height. A short time separation signifies low altitude, while longer separations occurs with high altitude target. Two separate reflector may be used to generate the two fan beams, the reflector might be back to back, a single reflector with two feeds can be used, or the two beams can be generated with a single phased array antenna.

V-BEAM RADAR:-

The V-beam radar is satisfactory method for obtaining three coordinate target information if the no. of targets is not excessive. Larger the no.of targets ,more difficult is the problem of correlating the echoes from each of the two beams. Closer the beams, easier it is to correlate the echoes, but the less the accuracy. Another useful modification is to separate the beams at zero elevation so that data from the vertical and slant beam do not appear close in time.

An elevation angle measurement can be made similar to that of monopulse radar. Two fan beams are displaced in elevation angle and the sum and difference patterns are obtained .The measurement of the angle is similar to that of an amplitude comparison monopulse tracking radar except that it is made open loop i.e. the output voltage from the angle error detector is calibrated to read elevation angle.Two displaced horns with combining circuitry,or single multi mode horn, is the principle modification required of the antenna.

MONOPULSE:-

The use of contiguous beams stacked in elevation has been employed for 3D radar. It is sometimes called as stacked beam radar. It is good technique from a fundamental point of view since it uses simultaneous pencil beam radiation pattern from a single aperture to cover the elevation angles of interest .Each beam can be considered as a separate radar. It is, however, costly and complex. The transmitter radiates the fan beam from the summation of all overlapping pencil beams to give the desired elevation coverage. A separate receiver is provided to each pencil beam and some means of interpolation between the beams is used to refine the angle measurement.

MULTIPLE ELEVATION BEAMS:-

3D information can be obtained on each rotation of the radar by electronically scanning a single pencil beam in elevation while mechanically rotating the antenna in azimuth.The beam is rapidly scanned through the entire elevation coverage in the time the antenna rotates one azimuth beam width. The 3D information is obtained by the sequential scanning of a single beam as compared with the simultaneous multiple beams.The relatively simple form of electronics scan in this application is frequency scan in which a change in frequency result in change of elevation angle , the beam can also be scanned with phase shifter.

SCANNING PENCIL BEAM:-

One the limitation of a long range 3D radar with single scanning beam is that the angular coverage consists of large no. of individual beam positions so that the dwell time in any one resolution cell is small. The minimum dwell time in each position is determine by the time it takes for the radar energy to travel to maximum range and back .The no. of beams position multiplied by the dwell time of each equals the scan time. Long dwell time and short scan time are not compatible.

The elevation angle accuracy of single scanning beam 3D radar is usually less then that of other systems. This due to relatively few pulses per beam position, as well as the error introduced by the target echo amplitude fluctuations when sequential rather then simultaneous angle measurement are made. Changes in echo amplitude cause errors since the sequential measurements are necessarily made at different instants of time. This is similar to the effect of amplitude fluctuations on tracking radar. If frequency scan is used, there is even more likelihood of echo fluctuations because of the change in target echo with frequency.

In the frequency scan 3D radar a single pencil beam is step scanned in elevation . Each pulse is at a constant frequency but the frequency is changed every pulse or every few pulses to position the beam at different elevation angles. Another method of utilizing a frequency scan antenna is to sweep the frequency over the entire frequency range on each pulse so that energy is radiated throughout the entire elevation coverage for the duration of single pulse.

WITHIN PULSE SCANNING:-

The frequency of the received echo signal will depend upon elevation angle of the target. A bank of contiguous receivers, each tuned to a different frequency, provides coverage of the elevation sector with the equivalent of parallelreceiving beams . The elevation angle of the target is determined by which receiver is excited. This technique is called within pulse technique.

The height resolution depends on the antenna beam width, but the range resolution is determined by frequency spectral components, received from a given elevation angle, that corresponds to one beam width.

An interferometer consists of two individual antenna spaced so as to obtain a narrow beam width for accurate angle measurement .The phase difference between the signals of two antenna elements of the interferometer provides the elevation angle.This type of angle measurement is similar to phase comparison mono pulse radar except that the size of the individual antenna is small compare to spacing between them.Grating lobes results from the wide spacing between the antennas. These grating lobes can cause ambiguities in the measurement.The ambiguities can be resolved by use of more than two antennas with unequal spacing.

INTERFEROMETER:-

This technique employs a single reflector with a rather unique type of feed. The effect of multiple beams is generated with this feed but only two receivers plus a phase detectors are needed to determine angular location.

PHASE IN SPACE:-

High range resolution, as might be obtained with a short pulse, is important for many radar application.There can be limitation, however, to the use of short pulse. Since the spectral bandwidth of a pulse is proportional to its width, the bandwidth of a short pulse is large. Large bandwidth can increase system complexity, make greater demands on the signal processing, and increase the likelihood of interference to and from other users of the electromagnetic spectrum.

PULSE COMPRESSION RADAR

A limitation to achieve long ranges with short duration pulses is that a high peak power is required for a large pulse energy the transmission line of high peak power radar can subject to voltage breakdown at higher frequencies where wave guide dimensions are small. A short pulse have wide spectral band width. A long pulse can same bandwidth as a short pulse if long pulse is modulated in frequency or phase. .

The modulated long pulse with its increased bandwidth B is compressed by the matched filter of the receiver to width equal to 1/B. This process is called as pulse compression. It is use of long pulse of width T to obtain the resolution of short pulse by modulating the long pulse to achieve pulse B>>1/T,and processing modulated long pulse in a matched filter to obtain a pulse width (t)=1/B. Pulse compression allows radar to simultaneously achieve the energy of long pulse and resolution of short pulse without the high peak power required of a high energy short duration pulse.

PULSE COMPRESSION RADAR

The block diagram of LFM is similar to convention pulse radar except the transmitter is frequency modulated and receiver contains a pulse compression filter. The transmitted wave from consist of rectangular pulses of constant amplitude A and duration T.the frequency increases linearly from f1 to f2 over he duration of pulse.This is known as up chirp.The frequency could as well decrease with time called as downchirp.

LFM PULSE COMPRESSION

On reception the frequency modulated signal is passed through pulse compression filter, which is a delay line whose velocity of propagation is proportional to frequency. Its speeds up the higher frequencies at trailing edge of pulse relative to lower frequency at leading edge so as to compress the signal to a width 1/B,where B= f2-f1. The pulse compression filter is a matched filter and its output envelope is auto correlation function of input. The peak power of the pulse is increased by pulse compression ratio BT=T/(t) after passages through the filter.

LFM PULSE COMPRESSION

BLOCK DIAGRAM OF PULSE COMPRESSION RADAR

mixer VA

LO

IFA O/p

AeTR Freq.

modulatorTx

Tx IFA PCF VADET

LO

Pulse compression wave forms

t1 t2

f1f2

B

2/BT

T

T

TA

A

A