Why is the AWACS' rotodome located at the back and over the aircraft?

Question

Browsing this website, I found this question dealing with torque due to the huge piece in motion, and its answers also speak of drag.

The rotodome is located above the aircraft and at the aft of the aircraft. For me it would have been logical to put this huge mass as closely as possible to the CG for stability reasons. For this aircraft, the CG is located close to the main gear, a little in front of them. When I look at an AWACS, I can see that the rotodome is located well behind the main gear.

^{AWACS image courtesy of airliners.net}

Moreover, it seems logical to put the rotodome under the aircraft. Its mission is to observe airspace under the aircraft to detect hostiles flying low enough to avoid detection by ground radar. By mounting the rotodome above the fuselage, the aircraft's wings may disturb the radar mounted above them.

This position is also shared with other airborne radar such as the A-50 or the E-2C.

Answer 1

It's mounted dorsally in large part because it's simply too large to fit beneath the fuselage.

The rotodome has to be that large because inside is actually a planar array. The radar itself isn't round, nor aerodynamic, so it's wrapped in a smooth fairing that presents the same profile regardless of which direction the array is facing.

Caption: The Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome. (Credit: Scott Carson. Twitter.)

Some airborne radars do not rotate and can indeed be mounted below the fuselage, like the E-8 JSTARS' APY-7 (primarily a ground surveillance radar): (Credit: USAF. Link.)

Pave Mover, an APY-7 prototype: (Credit: Daderot. Link.)

The cancelled E-10 (planned E-3 Sentry and E-8 JSTARS successor) also had a similar under-nose longitudinal array.
(Credit: Defense Industry Daily. Link.)

However, most larger, non-rotating arrays are mounted dorsally and longitudinally, like the Boeing 737 AEW&C: (Credit: Seattle Aviator. Link.)

[Question:] Moreover, it seems logical to put the rotodome under the aircraft. Its mission is to observe airspace under the aircraft to detect hostiles flying low enough to avoid detection by ground radar.

Not exclusively. Low-altitude penetrators aren't the only targets. AWACS is also tasked with finding medium to high altitude aircraft, both watching for enemies (eg firing "AWCAS killers") and keeping track of friendlies.

[Question:] By mounting the rotodome above the fuselage, the aircraft's wings may disturb the radar mounted above them.

You don't need a lot of depression to observe things at medium to long range. At 30,000 ft, the horizon sits just a few degree below level, and even at 30 miles (which is very close) a low-altitude target is just -10 degrees. There's also a fair bit of clearance between the elevated dome and the fuselage/wings, which reduces how much of the ground is masked (roughly -20 degrees from radar to wingtip). Together, there's probably fairly little that an E-3 wouldn't be able to see. You'd have to be practically underneath it at close range. And then you'd have to deal with the fighter escorts...

Answer 2

Not all AEW&C control aircraft have a huge radome on top of the fuselage. For example, the Chilean EB-707 'Condor' has the radars mounted on the sides and front.

"EB-707 Condor, Chilean Air Force (FACh) v2" by Hippocamelus - Own work. Licensed under CC BY-SA 3.0 via Commons.

There are multiple considerations in deciding the location of the radome/rotodome in an AWACS aircraft.

• The AEW platform must be able to fly at high altitude such that the radio horizon remains beyond the range expected for a target flying at low altitude; at the high altitudes of operation, the detection of objects at the expected detection range near ground depends on the target discrimination software, rather than putting radar on the belly as the line of sight of the radar would be the same whether it is above or below fuselage. Also, it will be extremely difficult to position the huge rotodome (~30' dia. in E3) under the fuselage.

• The range (which determines the power and coverage requirements of the radar) and coverage (the radar needs to cover 360$^{\circ}$) means that there are three options to mount the aircraft:

  • on a radome on top of the fuselage (like EA-3 sentry), high enough to avoid masking effects of the wing etc. This as the best option, as the range is the same in all directions and it gives 360$^{\circ}$ with a single (rotating) radar, In the rotating Mechanically Scanned Array (MSA) radar used in the beginning, this was the only option and is used in modern AESA too as it eliminates the requirement radars at multiple locations.

Image from State-of-the-Art in Active Electronically-Steered Array Surveillance Radar: Indian Value Addition by S. Christopher

• on conformal antennas on the sides and in front of the aircraft, like EB-707. This reduces drag, but requires atleast four antennae as the side antennae provide only 120$^{\circ}$ coverage each and two more antennae are required in the nose and the tail for covering the remaining portion.

Image from State-of-the-Art in Active Electronically-Steered Array Surveillance Radar: Indian Value Addition by S. Christopher

• on dorsal antenna on top of the fuselage (Boeing 737 AEW&C)which gives 150$^{\circ}$ coverage on both sides; this again requires multiple radars. In Boeing 737 AEW&C, there is a radar on top for ensuring 360$^{\circ}$ coverage. Also, this is possible only for modern AESA radars.

Image from State-of-the-Art in Active Electronically-Steered Array Surveillance Radar: Indian Value Addition by S. Christopher

• The rotodome itself has a stabilizing influence on the aircraft due to its orientation and location.

• One of the most important factors is that mounting the radar outside the aircraft enables the use of existing aircraft with minimal modifications compared to conformal radars. As the number of AEW radar operated is pretty low, this is important as commercial aircraft can be utilized.

• The radar systems, sensors, operators and consoles weigh much more than the radar itself and can be located in the front to reduce any effect in the center of gravity.