BACK TO BASICS: SENSORS



Early radar mounted on a Me Bf110
Welcome to "Back to Basics"; an ongoing series in which we will attempt to "get back to basics".  Each week (or so) we will examine one crucial aspect of a fighter and how the fighters vying for Canada's FFCP compare. 

When comparing modern jet fighters to one another, one tends to focus on the more tangible aspects:  How fast they are, how many weapons they can mount, and how maneuverable they are.  Yet none of these factors really matter until a target is found, identified, and tracked.  

For years, a fighter aircraft's sensor suite was limited to pilot's vision.  Not only was 20/20 vision (or better) a prerequisite, but the design of the aircraft itself was an important factor in giving pilots a clear view of the sky around them.  Everything from cockpit position to bubble canopies have been implemented to help improve performance of the "Mark I Eyeball".  

The later days of World War II would bring a quantum leap in detection capability.  The use of electromagnetic radiation to "see" out to great distances was becoming practical after years of experimentation.  This new system, known as "RADAR" (radio detection and ranging) would result in a radical shift for both civil and military systems.  No longer constrained by the limits of the human eyeball, targets could be detected much further away, day or night, in any weather condition.  As crude as it was compared to modern systems, it proved invaluable for allied forces.  

Like other aspects of fighter technology, radar systems have seen constant improvement over the years.  Modern radar systems are both more powerful while simultaneously being more sensitive than every before.  The most modern systems, known as active electronically scanned array (AESA) radars do away with the traditional "dish" antenna in favor of multiple antenna elements that transmit multiple and receive multiple "beams" which are then combined into one image.  

(This is REALLY simplifying things.  For a more detailed explanation, check here.)




Despite its usefulness, radar does have its limitations.  Being an active form of detection, it depends on radar waves being transmitted, reflected off an object, then received.  Like a game of flashlight tag, the use of a radar away both presence and location of the one using it.  It is also susceptible to being jammed or "spoofed".  Some modern fighters and bombers have developed stealth designs to minimize their radar visibility as well.  

The solution to this dilemma exists on another part of the electromagnetic spectrum; infrared.  

Unlike radar, systems that work on the infrared spectrum are passive; there is no need to transmit a signal.  There are also less options when it comes to reducing the infrared signature.  Infrared also works incredibly well for targets that produce a lot of heat, like a jet engine exhaust.  

For jet fighters, infrared detection usually comes in two different forms, IRST (infrared search and track) and FLIR (forward looking infrared).  While both of these systems operate on similar principals, they tend to focus on or different objectives. 

IRST systems help improve a fighter's situational awareness by searching the surrounding area for heat signatures.  This works similar to the sensor used in heat-seeking missiles like the AIM-9 Sidewinder.  This sensor is typically mounted near the front of the aircraft, either at the base of the front canopy or under the "chin" of the aircraft to give it the widest field of view possible.

First used in "Century Series" interceptors like the F-101 Voodoo and F-102 Delta Dagger, early IRSTs were rather crude.  They soon fell out of favor in American-built fighters, being omitted in favor of an onboard cannon in later F-4 variants.  The F-14 would be the last American fighter to incorporate an IRST for quite some time.  Fighters like the F-15, F-16, F/A-18 and even the F-22 would skip the IRST entirely...  Until recently.  The former Soviet Union must have felt different, as they have incorporated IRSTs on their main fighters since the MiG-23, continually improving them for fighters like the MiG-27 and Su-27.  This has led western forces to revisit the idea, mounting IRST on newer fighters like the Eurofighter Typhoon and retrofitting older fighters with pod-mounted IRSTs.  

While IRSTs search a large area, FLIR focuses in a single focal point.  This allows for a much more detailed view, at the expense of field of view.  An analogy would be like comparing the picture from a ultra-wide lens to that of a super-zoom.  In the context of fighter aircraft, FLIR is useful for identifying and targeting ground targets.  For this reason, it is usually combined with laser rangefinders, and other electro-optical sensors into a single "targeting pod" that can be mounted on a weapon hardpoint.  

Sniper XR targeting pod mounted on a CF-18

Comparing the various radars, IRSTs and other sensors is difficult task.  While some information exists in the public domain, sensor capabilities are among the most tightly guarded secrets in the military.  There is also a near infinite amount of variables to account for as sensor ranges can be affected by target size, shape, materials,  heat levels, ambient temperature, etc.

For our purposes here, we will concentrate on "the basics".  What sort of sensors are available, how modern those sensors are, and what sort of "extras" can be brought to bear.

F/A-18E/F's AN/APG-79 AESA radar.
The Super Hornet's IRST located forward of the belly tank.


The F/A-18E/F Super Hornet is an excellent case study in the evolution of modern fighter jet sensors.

When it was first introduced, the Super Hornet mounted the same conventional AN/APG-73 radar found on Canada's current CF-18s.  This has since been updated to the AN/APG-79, which an AESA version of the previous pulse-doppler radar.  This unit has been met lukewarm response, with reports stating that it "has experienced multiple radar shutdowns that necessitated airborne radar restarts".  This system has seen more than ten years of use since then, so it is safe to assume most of the issues have been sorted out.  

Developed during a time when airborne electro-optical sensors were out of favor with the US military, the Super Hornet lacks built-in IRST or FLIR.  Instead, the Super Hornet depends on externally mounted pods added after-the-fact.

With no built-in IRST, the Super Hornet relies on a pod that combines an IRST with a ventral-mounted fuel tank.  While this setup does enhance the Super Hornet's abilities, it is a rather odd design decision that forces several compromises.  For one, it incurs much more weight and drag than an indwelling IRST would.  While the extra fuel is certainly useful, the Block III F/A-18E/F with conformal tanks is not exactly wanting in that department.  The positioning of the IRST sensor that far back on the ventral surface of the fighter also results in a sizable blind spot, with much of the fighter itself blocking the IRST's potential field of view.  It is akin to a snowboarder mounting a GoPro on a belt buckle instead of a helmet; it will work, but it is not the best option.  

When precision ground targeting is required, the Super Hornet utilizes the USN's ATFLIR (advanced targeting forward looking infrared) pod.  Similar to the Sniper XR currently used by the RCAF's CF-18s.  


The Gripen E's Raven ES-05 radar and Skyward G IRST (and a LITENING pod under the intake)

The JAS 39E Gripen is similar to the Super Hornet in that it adds AESA radar and IRST to a design that started out with neither.  It does this in a different way, however.  Unlike the Super Hornet, which developed sensors around the constraints of an existing platform, the Gripen E modifies the platform itself to better fit new sensors.  

Unlike the Super Hornet's AN/APG-79 in that it mounts an AESA dish to a pre-existing "back end"(although such an option will be available for older Gripen models), the Gripen E utilizes an entirely new system.

What sets the ES-05 apart from most AESA radars is that it incorporates a repositioner that greatly increases the field of view.  By spinning the dish on the roll axis, the Gripen can add an addition 40 degrees to any direction.  This gives the ES-05 a full 100 degree field of view as opposed to a fixed AESA radar's 60 degrees.    This not only increases detection capability, but allows the fighter to maneuver more aggressively while maintaining a radar fix on a target.  It is a simple and elegant enhancement.  

Compared to the Super Hornet, the Gripen incorporates a much better integrated IRST.  Instead of installing a pod under the fuselage, Saab engineers took advantage of the E model's redesign to incorporate a more integrated IRST.  Located at the base of the canopy, the "Skyward G" has the same unobstructed view as the pilot. 

For missions that require a keen eye towards the ground, the Gripen has several options available.  For precision ground targeting, it can utilize the LITENING pod.  This targeting pod has much in common with both the ATFLIR and Sniper XR.  Saab lists the Sniper XR as an option for the Gripen.   

For those unaware, the "S" in JAS 39 stands for Spaning (reconnaissance).  For these missions, the Gripen can mount an external RECCE pod called the Modular Reconnaissance Pod System (MRPS). This capability enable the Gripen to fulfill a recon role over Libya in 2011. 


(top to bottom) DAS, AN/APG-81, and EOTS!

Unlike the Gripen and the Super Hornet, the F-35 Lightning II was designed from the outset to incorporate all of the latest sensor equipment.  Nothing is added after the fact.  No external pods, no pointy-bits sticking out.  

Front and foremost is the JSF's AN/APG-81 AESA radar.  Developed from the F-22's AN/APG-77, the F-35's radar easily qualifies as one of the most advanced airborne radars in the world today.  It lacks the repositioner of the Gripen's ES-05, but it is more than capable of holding its own.  There have been some minor issues with the radar, but it is likely more capable than the Super Hornet's and likely as good if not better than the Gripen's.  

Being a stealth aircraft, the F-35 does not alway have the luxury of being able to use its radar, as doing so risks giving the aircraft's position away.  For this reason, the JSF incorporates several passive sensors located all over the aircraft.  

Instead of a typical single IRST sensor, the F-35 utilizes the AN/AAQ-37 Distributed Aperture System (DAS).  DAS uses multiple sensors located at various locations around the aircraft.  Onboard computers then stitch data from each together to form complete 360 degree coverage around the aircraft without any blindspots.  Much has been made of the F-35 pilot's ability to "see" through the aircraft.  This is what they mean.  

Located under the JSF's chin is the Electro-Optical Targeting System (EOTS).  Heavily based on the SNIPER XR, the EOTS gives all the benefits of a targeting pod without the extra drag or taking up a weapon hard point.  This system is not without its negatives, however.  Being a built-in system, there is no option to remove the EOTS to save weight when it is not needed.  It is also much more difficult to upgrade.  This has led F-35s flying with a system that is outclassed by older fighters flying with upgraded Sniper pods.  


F-35 cockpit with simulated helmet mounted display in green.

Much has been said about the F-35's "sensor fusion" in which it combines all of its separate sensor data into a a single cohesive image of the battlefield.  This information can then be shared with allied units using data links, making each individual piece a part of a much larger whole.  This ability is not exclusive to the JSF, however.  Both the Super Hornet and the Gripen incorporate similar capabilities.  

Unfortunately for the purposes of this blog, a true comparison of each fighter's sensor suite is practically impossible using public-domain materials.  There are plenty of glowing PR materials available, each one praising the superiority of their respective systems...  But true capabilities are intentionally left vague, leaving us to merely estimate.  Sensor performance is one of the most closely guarded secrets in the military.  One does not allow potential adversaries to know how close they can get without being detected.  

In any case, each of the fighters competing in Canada's FFCP offer a substantial upgrade to the CF-18's Cold War era radar.   They also offer IRST capability not found in our legacy Hornets.  None offer a substantial upgrade to the CF-18's current targeting pod, but that is a compliment to the CF-18's relatively new Sniper XR.  

One cannot stress the importance of a modern fighter's sensor suite.  Those sensors are the deciding factor in the first part of the "OODA Loop" so vital to military strategy and tactics.  Whatever fighter ends up replacing the CF-18, we can be assured that sensor performance will play a vital role in the decision.  

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