APQ-181 has 21 distinct operating modes, including two versions of built-in-test for fault detection
and isolation to a Line-Replaceable Unit (LRU), or in the case of the antenna a line-replaceable module.
Each mode has its own software programme, which is an assembly of functional code modules that
provides detailed radar instructions. In fact each mode has two sets of software; one for the Radar Data
Processor (RDP), which is a militarised general purpose computer and one for the Radar Signal
Processor (RSP), which is a high-speed special purpose computer. The RSP is programmed in
high-efficiency machine language to maximise signal processing throughput.
In addition to the signal and data processors, there are three additional units in a `radar string':
antenna, transmitter and receiver/exciter. There are two of each unit arranged in a string in a B-2 aircraft
set. The strings are functionally connected so that any antenna may be connected to either of the four
units in a string. This approach to redundancy maximises the radar mission probability of success.
The radar equipment is installed in the aircraft in three zones. Each antenna is mounted in a cavity
behind a large radome, approximately 2.4 m outboard of the aircraft centre line and below the leading
edge of the wing/body. 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 side wall of the nose wheel well; the two RDPs are located
one above the other in an opening in the aft wall of the nose wheel 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 motion sensor subsystem, which is a modified
strap-down 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 and includes its own high voltage power supplies within its
chassis. It employs a gridded travelling-wave tube RF amplifier, similar to other Hughes radars.
Because it is such a high-power density package it is liquid cooled.
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 has also a video output bus that enables direct drive of cockpit
displays. The RSP is fully programmable.
The radar signal processor 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. It
has a throughput of 2.5 Mips and a one million, 16-bit word (16 Mbits) bulk memory.

Specifications
Number of LRUs in one aircraft set: 10
Total weight of an aircraft set: 955 kg
Total volume of an aircraft set: 1.485 m3
Operating prime power demand: 22 kVA AC; 500 W DC

Type
J-band (10 to 20 GHz) multimode radar for the B-2 strategic bomber.

Development
Hughes (now part of Raytheon Electronic Systems) is reported to have begun work with Northrop (now
Northrop Grumman) in the early 1980s to develop radar concepts for what was then known as the
Advanced Technology Bomber. The prototype radar first flew in 1987 and the first AN/APQ-181
production contract was proposed in 1986, authorised in 1987 and fully negotiated in 1988. Ten
pre-production radars are understood to have been delivered during the early 1990s and the system is
thought to have been in full scale production by 1995.