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Hughes/Raytheon AN/APQ-181


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TITLE : AN/APQ-181 radar for the B-2
Airborne Ground Mapping and Terrain-Following Radar (GMR/TFR).
Airborne Fire Control Radar (FCR).

The AN/APQ-181, designed specifically for the Northrop Grumman B-2 Spirit
stealth bomber, operates in the J-band (12.5 to 18 GHz) using 21 separate
modes for terrain-following and terrain-avoidance; navigation system
updates; target search, location, identification and acquisition; and
weapon delivery.
The radar is a completely redundant modular system which employs two
electronically scanned antennas, sophisticated software modes and advanced
low probability of intercept techniques that match the aircraft’s overall
stealth qualities.
To meet reliability specifications and provide for operational
redundancy, the AN/APQ-181 provides two separate radar sets, each
consisting of five Line-Replaceable Units (LRUs): antenna, transmitter,
Radar Signal Processor (RSP), Radar Data Processor (RDP) and
receiver/exciter, with all but the antennas able to function for either or
both radar units. These LRUs weigh 955 kg and have a volume of 1.485 m{3}.
The LRUs are installed in the aircraft in three zones. Each 260 kg
antenna is mounted behind a large radome some 8 ft outboard of the
aircraft centreline, just below the flying wing’s leading edges. Antenna
locations are marked by large, slightly darker, rectangular patches
visible on the underside of the aircraft. Each antenna has a large
unobstructed field of view forward and to the side of the aircraft
fuselage reference line. Six units (two each of the transmitter, receiver
and RSP units) are located symmetrically in openings in each sidewall of
the nosewheel well; the two RDPs are located one above the other in an
opening in the aft wall of the nosewheel well.
The antenna is electronically steered in two dimensions and features a
monopulse feed design to enable fractional beamwidth angular resolution.
It includes a beam steering computer that determines and commands the
phase settings of the beam steering phase shifters in response to a
pointing direction command from the RDP. The antenna is fitted with a
Smiths Industries’ motion sensor subsystem, which is a modified strapdown
inertial platform used to measure antenna motion to enable compensation
during SAR mode operation. The antenna is equipped with its own power
supply and is liquid cooled. It is designed to have carefully controlled
and very low scattering performance (low radar cross-section) with respect
to both in- and out-of-band Radio Frequency (RF) illumination.
The radar transmitter is a single unit, including dedicated high-voltage
power supplies and utilising a liquid-cooled, high-power gridded
travelling-wave tube RF amplifier, similar to other Hughes radars.
The receiver/exciter LRU performs several functions usually requiring
more than one LRU in contemporary radars. These include generating RF
waveforms for amplification by the transmitter (exciter) and
amplification, detection and frequency down-conversion to baseband
(receiver) of signals received from the antenna. The receiver/exciter also
digitises the received signal stream and performs pulse compression to
enhance range resolution.
The radar signal processor extracts target images and measurement
information from the digitised signal stream and converts this information
into a format usable by interfacing avionics or displays. In addition to a
digital data output bus, it also has a video output bus that enables
direct drive of cockpit displays. The RSP is fully programmable.
The RSP is a dual-central processor unit, general purpose-type computer.
It is the command controller for all radar units and serves as the radar
terminal on the B-2 avionics databus.
All radar system components communicate over a dual-redundant
MIL-STD-1553 databus, and are hardened to withstand transient radiation
and electromagnetic pulse effects. The radar was designed for very
stringent environmental requirements exceeding those of other radars, due
to the extreme Noise, Vibration and Harshness (NVH) environment expected
during low-altitude flight in the rigid B-2 airframe.


Frequency band: 12.5 - 18 GHz
Modes: 21 modes including terrain following and terrain avoidance
Line Replaceable Units (LRUs): 10
Weight: 955 kg
Volume: 1.485 m{3}
Power: 22 kVA AC; 500 W DC (peak)

Operational status
The AN/APQ-181 is in service in US Air Force B-2 Spirit strategic bombers.
The USAF is embarking on a five-year spending plan from Fiscal Year 2004
(FY04) to address a potential frequency-interference issue, which concerns
the possibility that the radar system will interfere with commercial
satellite communications after 2007. When the USAF acquired the B-2 in the
early 1980s, the issue of frequency interference was not widely foreseen.
However, in recent years, more and more of the frequency spectrum has been
made available to commercial users. At the end of 2007, the frequency in
which the radar operates will be turned over to commercial users.
Unmodified, the B-2 radar will disrupt their transmissions and could
damage commercial communication satellites, for which, it is likely, the
USAF would be liable, according to industry sources.
To address this, during October 2002, Northrop Grumman Corporation’s
Integrated Systems sector was awarded a US$34.2 million contract by the US
Air Force for the first phase of the B-2 radar Pathfinder programme, a
multiyear effort to design and integrate a new radar antenna on the
Stealth Bomber. The total programme value for Northrop Grumman, prime
contractor for the B-2 programme, is estimated at more than US$900 million
to the company through to 2011.
Installation of the new radar antenna on the B-2 fleet is scheduled to
be completed by the end of this decade. The modification consists of an
Active Electronically Scanned Array (AESA) radar system that will resolve
the aforementioned conflicts in radio frequency usage between the B-2 and
commercial systems and, importantly, facilitate future upgrades to improve
radar performance.
Initial phase effort, completed during 2003, consisted of system
engineering leading to the establishment of performance requirements. The
Raytheon Company’s Space and Airborne Systems in El Segundo, California,
which provided the original B-2 radar, is the principal subcontractor to
Northrop Grumman on the radar programme.
During the second quarter of 2003, Northrop Grumman and Raytheon were
awarded further contracts for radar upgrade work. Northrop was awarded a
US$85.9 million contract for the establishment of specifications, the
design of new and modified components and risk-reducing technical
demonstrations, with work scheduled for completion by the end of August
2004. Raytheon was awarded a US$63 million contract for work with phase 2
of the APQ-181 upgrade’s Component Advanced Development (CAD) effort, to
include design and prototyping activities with regard to the proposed AESA
antenna. This work was also due to be complete by the end of August 2004.
During September 2004, Northrop Grumman was awarded a US$388 million
contract by the US Air Force for the next phase of the programme. During
this System Development and Demonstration (SDD) phase, Northrop Grumman
will develop and test the antenna and integrate six new radar systems on
B-2 aircraft for initial demonstration and operational training. The
subsequent production and deployment phase will include Low-Rate Initial
Production (LRIP) and Full-Rate Production (FRP) to field the upgraded
radar. Installation of the new antenna into the B-2 fleet will take
several years to complete. In addition to replacing the antenna, Northrop
Grumman will modify the B-2 defensive management system and the radar
transponder to support the change in operating frequency. In addition to
Raytheon, subcontractors include Lockheed Martin Corporation in Owego, New
York, for defensive management system modifications and BAE Systems
Information and Electronic Systems Integration in Greenlawn, New York, for
radar transponder modifications. The radar modernisation and other efforts
to upgrade the B-2 will improve its ability to communicate and exchange
data with joint force commanders and share updated target information
during a mission.
Under another programme, Northrop Grumman recently delivered the first
B-2 with a specially formulated coating developed to improve the
aircraft’s combat readiness. Another significant improvement in the
aircraft’s capability is a bomb rack assembly that enables the B-2 to
deliver up to 80 GPS-guided weapons on a single pass, five times as many
as its current capacity.


2004 Oct 13


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