George W. Stimson introduction to Airborne Radar (Se)

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A tanker’s view of the B-2 stealth

bomber prior to refueling.

Strategic Bombing

B-2 Bomber (APQ–181)

The APQ–181 is the multimode

pulse-doppler radar for the B-2 long-

range stealth bomber. It employs a

low-RCS passive ESA antenna and

incorporates advanced LPI features.

Except for that and the fact that, like

the APQ-164, it gives the aircraft the

autonomous ability to navigate safely

around hazards and use them to mask

defensive systems, very little can yet

be said about the radar at an unclassi-

fied level.

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PART X Representative Radar Systems

556

The B-1B Bomber

One of the antenna’s

1,526 phase shifters.

B-1B RADAR (APQ–164)

The APQ–164 is an X-band mul-

timode pulse-doppler radar tailored

to the requirements of the long-range

strategic bomber. These include the

ability to (a) penetrate deep into

enemy territory at low altitudes night

or day in fair or foul weather, unde-

tected by enemy defenses; (b) detect,

accurately identify, and destroy

assigned targets; (c) immediately fol-

lowing a demanding 15-hour mission,

be fully available for another mission.

Implementation. The radar fea-

tures a 44 x 22 inch passive ESA,

employing 1,526 phase control mod-

ules. Together with its beam-steering

computer, the antenna is mounted in

a roll gimbal having detents for lock-

ing it in forward, broadside, and verti-

cal (down) positions. Besides being

able to switch beam positions virtual-

ly instantaneously (order of 200 ms),

the antenna provides extreme beam-

steering accuracy, optimizes beam

patterns for different modes, and

offers a choice of either linear or cir-

cular polarization.

Most of the radar’s units have a

high degree of commonality with

those of the F-16’s APG-68. The

transmitter employs the same dual-

mode TWT (though it’s liquid cooled

in the APQ-164), and the receiver

enables full two channel monopulse

operation. To ensure a high degree of

availability, except for the antenna,

which is inherently fault tolerant, two

independent chains of line-replace-

able radar units are provided. In

essence, two separate radars are car-

ried in the B-1B: one, in operation;

the other, in standby waiting to be

switched in.

Navigation Modes. For naviga-

tion, the primary mode is high-reso-

lution SAR mapping. Typically, it’s

employed as follows: The B-1B’s

avionics give the radar the coordi-

nates of a check point. The radar

trains its antenna on the point, makes

a patch map centered on it, and turns

off. The map is stored and frozen on

the display, giving the operator ample

time to analyze it. Having located the

check point, the operator designates it

with a cursor, thereby updating the

bomber’s position and destination

heading in the B-1B’s INS.

Supplementing the SAR mode for

navigation are real-beam mapping,

weather detection, and velocity

update modes. The weather mapping

mode is essentially the same as real-

beam ground mapping except that, if

weather penetration is necessary, the

antenna may be switched to circular

polarization.

At altitudes up to 5,000 feet

absolute, altitude is measured by a

radar altimeter. From 5,000 to 50,000

feet, altitude updates may be obtained

by moving the APQ-164 antenna to

its vertical detent position.

For rendezvousing with tankers

and other aircraft, an air-to-air beacon

mode and a short-range air-to-air

search mode are provided.

For penetration, automatic terrain

following and terrain avoidance

modes are provided. In terrain follow-

ing, the radar supplies the B-1 avion-

ics with a height versus range profile

of a corridor centered on the project-

ed flight path out to a range of 10

nmi, thereby enabling the automatic

generation of appropriate climb and

dive commands.

Through a unique azimuth and

elevation extent algorithm, the radar

differentiates between terrain and

spurious returns from rain, towers, or

electronic interference. By scanning in

azimuth, terrain avoidance detects

objects on either side of the flight

The radar’s 44 x 22-inch pas-

sive ESA, together with the

beam-steering computer, is

mounted in a detented roll

gimbal.

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Strategic Bombing

557

Each of the bomber’s three

crew members has the equiva-

lent of simultaneous indepen-

dent use of different modes

through their own displays.

Given the coordinates of a

check point, the radar makes

a patch map centered on it

and turns off.

The B-1B not only looks like a fighter, but its radar employs key hardware technology trans-

ferred from the radar for the F-16 C/D.

path that are higher than a selected

ground clearance plane, enabling the

pilot to maintain the lowest possible

overall altitude.

Weapon Delivery. For weapon

delivery—both conventional and

nuclear —high-resolution SAR map-

ping is used as just described. In

addition, a ground-beacon tracking

mode and a ground-moving-target-

tracking mode enable precise target-

ing of both fixed and moving targets.

Because of the extreme flexibility

of electronic beam steering, several

radar modes can be sequentially time-

shared, giving the pilot, copilot, and

offensive systems officer (OSO) the

equivalent of simultaneous indepen-

dent use of different modes through

their own displays.

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558

The AH-64D carries up to 16 RF or semi-active laser-guided Hellfire missiles and 76 70-mm fold-

ing fin aerial rockets or a combination of both, and up to 1,200 rounds of 30-mm ammunition

AH-64D Apache Helicopter

(Longbow Radar)

Longbow is a fast-reaction, low-

exposure, high-resolution, millimeter-

wave fire-control radar designed for

the AH-D Apache attack helicopter.

Mounted atop the main rotor mast to

take advantage of terrain masking, the

radar can pop up and, in seconds

scan a 90° sector; then, drop down

out of sight.

During that brief interval, it can

detect, classify, and prioritize more

than 100 moving and stationary

ground targets, fixed wing aircraft, and

both moving and hovering heli-

copters—discriminating between

closely spaced targets of the same type

with an extremely low false-alarm rate.

It then displays the 10 highest

priority targets to the aircrew, and will

automatically cue either an RF or a

semi-active laser guided fire-and-for-

get Hellfire missile to the first target.

Immediately after its launch, the sys-

tem cues the next missile to the next

priority target, and so on.

The radar also provides obstacle

warning to alert the pilot to naviga-

tion hazards, including man-made

structures, towers, etc.

Radar data is displayed on the

pilot’s night-vision helmet-mounted

display and on two color-coded flat

general purpose displays in each

cockpit.

A derivative of the Longbow

radar will be forthcoming for the

RAH-66 Comanche helicopter. Using

the same millimeter radar and the

same Hellfire missiles as Apache, it

will include a number of advanced

features such as a smaller antenna.

Attack Helicopter

Lurking behind cover with only the radome of

its millimeter wave radar showing, Longbow

can quickly detect, classify, and prioritize

more than 100 moving or stationary targets.

Flat, fully interchangeable

color displays are provided in

both cockpits.

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559

C-130 (APN-241)

Meeting all of the requirements of

tanker/transport operations, the APN-

241 is the baseline radar for the C-

130J. It also can be employed in a

number of other aircraft, when exist-

ing modules that interface with their

avionics, controls, and displays are

included.

Implementation. A light-weight

X-band coherent pulse-doppler radar,

the APN-241 consists of just two

basic elements: an antenna and a

receiver-transmitter-processor.

The antenna is a 26 by 32-inch,

dual-channel, monopulse planar

array. Stabilized about three axes, it

provides ±135° of azimuth coverage

and +10 to

–

25° of elevation coverage.

The receiver-transmitter-processor

is all solid state. Operating at 9.3 to

9.41 GHz, it has a power output of

116 W peak; 9.5 W, average.

Modes of Operation. To meet all-

weather delivery requirements, a vari-

ety of modes are provided:

• Weather—detects weather

through weather, out to 320

nmi; turbulence out to 50 nmi

• Windshear detection—gives up

to 90 seconds of warning of a

microburst (probability of false

alert <10

-4

per flight hour)

• Ground mapping—monopulse

(2.5 to 10 times improvement

over real beam); DBS, for higher

resolution mapping off the nose

• Air-to-air detection—20 nmi

against a C-130-sized target;

indicates whether nose or tail

aspect

Overlay Modes

• Beacon—interrogates both air

and ground beacons

• Station-keeping

• Flight plan—navigation data

from self-contained navigation

system or other reference

• Traffic collision avoidance sys-

tem (TCAS)

The radar is designed for two-per-

son cockpit operation. Pilot and navi-

gator can view and control different

modes simultaneously.

Growth. A SAR mode has already

been developed and is available as a

software update. In development are

terrain following, drop-zone wind

measurement, and autonomous land-

ing guidance modes.

C-130J

Transport/Tanker Navigation

APN-241 can map weather

out to 320 nmi, turbulence

to 50 nmi, and give 90-sec-

ond windshear warning.

Receiver-Transmitter-Proces-

sor, includes interfaces for

aircraft avionics, controls,

and displays

The 26 x 32-inch dual-

channel antenna pro-

vides ±135° azimuth

coverage and is space

stabilized about all

three axes.

DBS ground map (far left) enables precise location

of drop zones or target areas.

Monopulse ground map (left) enables blind radar

approaches to small or unimproved landing sites.

1. Can be displayed auto-

nomously or overlaid

on any radar map.

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560

The RDR-4B is currently oper-

ating in nearly every type of

commercial transport aircraft.

RDR-4B Civil Weather Radar

The RDR-4B is a pulse-doppler

forward-looking weather radar, oper-

ating in nearly every type of commer-

cial transport aircraft. Besides the

ability to penetrate weather systems

and accurately map rainfall and tur-

bulence, it meets all FAA require-

ments for stand-alone windshear

detection.

At altitude, the radar provides a

detailed ground-clutter-free color

weather display of a ±40° forward

sector out to a selectable maximum

range of up to 320 nmi.

Whenever the absolute altitude is

less than 2,300 feet, the windshear

detection mode is automatically acti-

vated on alternate antenna scans.

During clockwise scans, the radar

continues to operate in the operator

selected mode. But during counter-

clockwise scans, it operates in winds-

hear mode, with the antenna tilt tem-

porarily changed to the optimum

angle for measuring horizontal winds

caused by the outflow from a

microburst core. The only apparent

difference in the radar’s operation,

though, is that the display updates

only on the clockwise scans.

If a windshear is detected at any

range out to 5 nmi, an advisory icon

appears on the display, and a chime

or oral caution, “monitor radar dis-

play” sounds.

Below 1500 feet, if a windshear is

detected within 3 nmi and ± 25° of

the aircraft’s heading, a windshear

alert, sounds, and a warning appears

on the radar display.

At 1.5 nmi, a windshear icon

indicating the exact location of the

microburst is superimposed over the

normal display, and an oral warning,

“windshear ahead, windshear ahead”

sounds,

giving the pilot roughly 15

seconds to avoid the potential hazard.

Civil Applications

Antenna, radar electronics,

controls, and display unit.

Pulse width:

radar modes, 6

and 18 µsecs (interlaced);

weather, 2 µsecs.

PRF:

weath-

er and maps, 380 Hz; turbu-

lence mode, 1.6 kHz; wind-

shear mode, 6kHz.

Peak

transmitter power:

125 W.

Frequency agility for interfer-

ence reduction.

When windshear is encoun-

tered, red and yellow icon is

superimposed over the weath-

er display, indicating the

microburst’s exact position.

Standard Weather Display

■ Level 2 rain ■ Level 4 & above

■ Level 3 rain ■ Turbulence

In windshear mode, antenna tilt is temporarily

changed to the optimum for measuring the hor-

izontal winds due to the out flow from a

microburst.

1. If the radar isn’t already on,

it automatically turns itself

on and operates continu-

ously in windshear mode.

2. During landing, this would

be changed to “go around,

windshear ahead.”

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Civil Applications

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HISAR

This is an advanced high-resolu-

tion, real-time radar mapping system

developed for a wide range of primarily

civil uses. It can monitor environmental

conditions such as flood damage, oil

spills, and sea ice over vast regions in

inclement weather. It can uncover ille-

gal activities and detect illegal border

crossings and threatening buildups

without provoking a response.

Implementation. Suitable for use

in small executive-class aircraft, HISAR

consists of an X-band, multimode

radar which looks out through a

radome in the bottom of the aircraft,

plus a computer workstation which

controls the mission and displays the

radar’s outputs to the operator.

The radar has two planar array

monopulse antennas mounted back-

to-back. When aligned with the flight

path, they enable surveillance to be

switched quickly from one side of the

aircraft to the other.

The workstation has two displays:

one for mission planning and control;

the other, for displaying SAR images

and target data. A second workstation

may be installed in a ground facility,

enabling preflight mission planning and

post-flight analysis to be performed

there rather than in the aircraft.

Mission Control. For radar and

mission control, the aircraft’s GPS-

aided INS provides HISAR with the air-

craft’s longitude, latitude, velocity, alti-

tude, and attitude. To guide the aircraft

on its mission, HISAR’s workstation

gives the autopilot flight commands. To

collect the desired data at appropriate

points in the preplanned mission, it

cues the radar’s selection of modes and

controls their operation. At any point

in the mission, of course, the radar can

be redirected by the operator.

Radar Modes. Imaging is per-

formed in three of HISAR’s five radar

modes:

• Wide-area search (DBS)—radar

scans a 60° sector extending from

37 km to 110 km with resolution

of 25 meters in range and 0.4

milliradian in azimuth

• SAR strip map—strip 37 km

wide can be positioned in range

anywhere between 20 and 110

km; resolution of 6 meters in

both range and azimuth

• SAR spotlight—patch 3.5 km

square, with 1.8 meters resolution

in azimuth and elevation, can be

placed anywhere in range be-

tween 37 and 110 km and in

azimuth within ±45° off broadside

Supplementing these modes are

(a) ground-moving target detection

which can be interleaved with wide-

area-search and strip-map modes, and

(b) air-to-air search. The latter is a

coherent low-PRF mode providing

±150° coverage in two elevation bars.

It is capable of detecting helicopters

and low-level, medium-speed aircraft

out to a range of 70 km, enabling the

operator to monitor the “air picture”

over an immense area.

Multisensor Integration. HISAR is

integrated with and exchanges cueing

and data with a forward-looking

infrared sensor (FLIR) and communi-

cation and electronic-intelligence sen-

sors.

HISAR antenna looks out

through radome in the bottom

of this executive-class aircraft.

Radar has two planar array

antennas mounted back-to-

back on an azimuth gimbal.

Long-wavelength FLIR images cued by HISAR: truck convoy (above, left), and tanker

trucks (above, right). Note white image of wheels produced by heat of the tankers’ tires.

The operator’s workstation

has two large displays. One

(top) for mission planning and

control; the other for SAR

images and target data.

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RADAR SYSTEMS COVERED

RADAR DEVELOPER

APS-145 Lockheed Martin

APY-1/2 Northrop Grumman

ESSD*

(Westinghouse)

JointSTARS Northrop Grumman

Norden Systems

APG-77 Joint venture:

Northrop Grumman

Raytheon TI Systems

APG-68 Northrop Grumman

ESSD*

(Westinghouse)

APG-73 Raytheon

(Hughes)

APG-76 Northrop Grumman

Norden Systems

APQ-181 Ratheon (

Hughes)

APQ-164 Northrop Grumman

ESSD*

(Westinghouse)

Longbow Joint venture:

Lockheed Martin

Northrop Grumman

APN-241 Northrop Grumman

ESSD*

(Westinghouse)

RDR-4B Allied Signal

HISAR Ratheon

(Hughes)

*ESSD – Electronic Sensors & Systems Division

547

E-2C Hawkeye

The APS-145 is the latest version

of the Airborne Early Warning radar

for the US Navy’s carrier based E-2C

Hawkeye. Early versions of the aircraft

went into service in 1963. Since then,

it and the radar have undergone

numerous upgrades.

Designed for operation over both

land or sea, the APS-145 can provide

surveillance over 3,000,000 cubic

miles of air space and can simultane-

ously monitor and track up to 2,000

targets. Looking beyond the horizon,

it has a maximum detection range of

350 nmi.

Implementation. Operating at

UHF frequencies (0.3 to 1.0 GHz) to

minimize sea clutter, the radar employs

a linear array of yaggi antennas, having

monopulse sum and difference out-

puts. This array is housed in a 24-

foot-diameter rotating radome, called a

rotodome, which rotates at 5 rpm.

The transmitter is a high-power

coherent master-oscillator power-

amplifier (MOPA). It is switched

through three different PRFs to elimi-

nate doppler blind zones and employs

linear frequency-modulation (chirp)

pulse compression.

Adaptive Signal Processing. By

means of DPCA and a double-delay

AMTI clutter canceller, mainlobe clut-

ter is eliminated, thereby avoiding the

problem of low-closing (or opening)

rate targets being obscured by spread-

ing of the clutter spectrum due to the

aircraft’s advance when looking in

broadside directions (normal to the

aircraft velocity). Clutter cancellation

is followed by coherent signal integra-

tion with the FFT. To minimize false

alarms, the detection threshold is

adaptively adjusted—resolution-cell

by resolution-cell—in accordance

with the clutter level and the density

of targets.

DPCA. The phase center of a side-

looking planar array can be displaced

forward or aft in the plane of the array

by adding or subtracting a fraction of

the monopulse difference signal, in

quadrature, from/to the sum signal.

In the E-2C, following transmission

of the first pulse of each successive pair

of pulses, the phase center is displaced

forward by the distance the aircraft will

advance during the interpulse period.

Following transmission of the second

pulse, the phase center is displaced aft

by the same amount. As a result, both

pulses will travel the same round-trip

distance to any point on the ground,

Reconnaissance & Surveillance

Carrier-based E-2C Hawkeye

can monitor 3,000,000 cubic

miles of airspace and simulta-

neously track 2,000 targets.

Hawkeye’s 24-foot diameter rotodome houses a monopulse connected linear array of UHF

yaggi antennas, and rotates at 5 rpm.

making possible complete cancella-

tion of the ground return by the clut-

ter canceller.

This process, of course, requires

precise synchronization with the

PRF, the velocity of the aircraft, and

the rotation and look angle of the

antenna.

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PART X Representative Radar Systems

548

E-3 AWACS Radar

The APY-2 is the radar for the

U.S. Air Force E-3 Airborne Early

Warning and Control System

(AWACS). From an operational alti-

tude of 30,000 ft, the radar can detect

low altitude and sea-surface targets

out to 215 nmi, coaltitude targets out

to 430 nmi, and targets beyond the

horizon at still greater ranges.

Implementation. Operating at S-

band frequencies (nominally 3 GHz),

the radar employs a 24-ft by 5-ft pla-

nar-array antenna, steered electroni-

cally in elevation, and housed in a

rotodome which rotates at 6 rpm.

Besides phase shifters for eleva-

tion beam steering, phase shifters are

also provided for offsetting the beam

for reception during elevation scan-

ning to compensate for the time delay

between transmission of a pulse and

reception of returns from long-range

targets. The antenna has an extremely

narrow azimuth beamwidth and is

amplitude weighted for sidelobe

reduction.

The transmitter chain consists of

a solid-state predriver—whose output

power is increased as a function of

antenna elevation angle—a TWT

intermediate power amplified, and a

high-power pulse-modulated dual-

klystron amplifier. For reliability, dual

redundancy is employed throughout.

Following an extremely low-noise

(HEMT) receiver preamplifier, two

separate receive channels are provid-

ed: one for range-gated pulse-doppler

operation; the other, for simple

pulsed-radar operation.

Digital processing is performed by

a signal processor, employing 534

pipeline gate arrays operating at 20

MHz; and a data processor, employ-

ing four RISC CPUs.

Modes of Operation. The radar

has four primary modes of operation:

• High-PRF pulse-doppler range-

while-search, for detecting targets

in ground clutter;

• High-PRF pulse-doppler range-

while-search, plus elevation scan-

ning for additional elevation

coverage and measurement of

target elevation angles

• Low-PRF pulsed radar search with

pulse compression, for detecting

targets at long ranges beyond-

the-horizon, where clutter is not

a problem

• Low-PRF pulsed radar search for

detecting surface ships, featuring

extreme pulse compression and

adaptive processing that adjusts

for variations in sea clutter and

blanks land returns on the basis

of stored maps.

These modes can be interleaved

to provide either all-altitude long-

range aircraft detection or both air-

craft and ship detection. A passive

mode for detecting ECM sources is

also provided.

Each 360° azimuth scan can be

divided into up to 32 different sec-

tors, in each of which a different

operating mode and different condi-

tions can be assigned or changed from

scan to scan.

215 nmi

(400 Km)

215 nmi

(400 Km)

LIMIT OF AWACS

SURFACE TARGET

COVERAGE

LIMIT OF AWACS

COALTITUDE

TARGET COVERAGE

AWACS

30,000 ft

GROUND RADAR COVERAGE

AWACS RADAR SURVEILLANCE VOLUME

SEEN ONLY BY AWACS

AWACS installed in an E-3.

The AWACS antenna consists

of a stacked array of 28 slot-

ted waveguides, plus 28 reci-

procal ferrite elevation-beam-

steering phase shifters and 28

low-power nonreciprocal

beam offset phase shifters.

From an altitude of 30,000 ft., AWACS can detect sea and low-altitude targets out to 215 nmi

and coaltitude targets out to 430 nmi.

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