The backbone of NATO advanced early warning, the E-3 Sentry AWACS plane, is getting old. It first flew in 1975, and although it has been regularly upgraded over the years, it will eventually be impossible to maintain. NATO plans for a replacement in 2035, and although some NATO official are unimaginatively calling for an off-the-shelf purchase of some E-737 Wedgetail to replace it, the Wedgetail itself will be thirty years old at that point.

So there is an need and an opportunity to think twice about what is expected of a future AWACS, and design or purchase one accordingly.

Currently, AWACS fulfill two roles at the same time:

• They act as long-range radars, seeing beyond the horizon of the ground-based radars and beyond the range of the smaller radars on the fighters. They also carry electronic intelligence receivers.
• They are an airborne command post, coordinating the air operations around them, by vectoring fighter to intercept threats for instance.

These roles could be split between two platforms, or even between a plane and a command post on the ground. However the latter would require satellites links to work on long distances, and satellites are vulnerable. So the radar and the command post will need to stay in line of sight if they are not on the same platform.

For the airborne radar roles, a few more precise requirements can be drafted

• Future AWACS need to detect future threats at useful ranges. That sounds obvious, but in the context of air-to-air missiles having an increased range, the AWACS needs to stay further from the frontline. The Chinese are fielding a long-range PL-15 and are thought to be planning even longer-range missiles, either using quasi-ballistic missiles or ramjet propulsion. The Russian already have anti-AWACS long-range missile, the R-37, and might develop a new one.

• The threats are getting stealthier: all newly developed fighters incorporate some degree of stealth, and the same is true for cruise missiles.
• Faster threats have emerged, namely hypersonic cruise missiles and hypersonic glide vehicles. While ballistic early warning is traditionally the purview of large ground-based radars, the hypersonic projectiles fly within the atmosphere, so much lower than ballistic missiles. Consequently, the radar horizon of ground radars against them is shortened, and an airborne radar might prove useful to have a higher warning time.

Some sample use scenarios are a good way to evaluate the relevance of candidate designs, as just laying down some quantitative requirements can miss some of the complexity of modern warfare. Here is one, from a French Air Force perspective:

• Raid escort is the most demanding mission for the Air Force, especially “first entry” raids on the first day of a conflict. The AWACS has to provide situational awareness of the enemy forces ahead of the raid, and manage the fight if necessary

The nuclear raids are a special kind the Air Force trains a lot for in the Marathon and Poker exercises, and they typically include an AWACS, one or several tankers and Rafale fighters. They can last for up to 11 hours, for a 9000km/5000nm range. The objective is to deliver a ~500km-range nuclear missile on a defended target, which means defeating enemy combat air patrols that might be positioned in the way.

Radar choice and integration

There are a few different possible architectures for the radar integration onto the airframe. Long, flat phased-array antennas like on the Wedgetail below are aerodynamic and can be very large and tall, providing good angular resolution. They are very popular in recent Western AWACS design:

However they cannot look directly forward and backward, as the beam can only be steered up to +-60° of the perpendicular to the antenna without incurring important losses, so another, much smaller radar has to be used for the front and back directions. This makes them ill-suited for raid escort, when the AWACS wants to see what is ahead. It could zig-zag +-30° to get the large antenna pointed in ahead of the raid, but that would slow it down by 15% and result in an intermittent coverage.

Rotating antennas, like on the E-3 and E-2, do not have that problem. They can be combined with electronic scanning to focus on a particular sector, even stopping the rotation. The AN-APY-9 on the E-2D has the following modes:

In ETS mode, the rotation of the antenna is stopped and the radar stares in a particular direction. Thus, more energy is put into that direction and the detection range increases. This can almost double the range, as seen on this datasheet for the ground-based Lockheed TPY-X search radar:

Even though having a larger range than the distance to the radar horizon can seem useless, it also means that targets will produce stronger returns, making them more detectable. So it will increase the range against targets with small radar cross section (RCS), like stealth aircraft.

There are other possible architectures, like having a fixed radome on top of the aircraft instead of a rotating ones, and putting three phased arrays inside. This gives slightly smaller arrays than with the rotating option, put is simpler mechanically and has a higher refresh rate. This is used on the Chinese KJ-2000:

Finally, another option to put a forward-facing antenna is to put it in the leading edges of the wings, like the L-band array on the Su-57:

Putting an array in each wing gives a very wide coverage of at least the frontal hemisphere: with a 30° wing sweep like on a commercial airliner, the +60° electronic steering gives a +-90° scannable volume.

There is a downside though to using the wings: they are very thin, so the array cannot be tall, which means it cannot get information on the altitude of the objects it detects. On the Su-57, the wings are the only place where a wide L-band counterstealth array could be placed, so the designers accepted that tradeoff.

Speaking of stealth, many countries are producing or developing stealth aircraft. As the Su-57 shows, they are no longer reserved to the US Air Force, so a future AWACS will have to be optimized to detect them. This means operating at low frequencies, where stealth shaping and radar-absorbent materials are less effective. Search radars usually operate in the low L band, between 1 and 2GHz, as it penetrates clouds and rain better. Interestingly, the AN/APS-145 on the E-2 operates in the low UHF band at 400MHz. The Russian ground-based Nebo RLM radars, which are optimized to detect stealth aircraft, operate in the VHF band at around 100MHz.

The problem operating at lower frequency is that the angular resolution of the radar degrades, requiring a larger antenna to get back the resolution. This is a problem for 360° arrays carried above the aircraft: they cannot be too large as it would cause too much drag, reducing the range of the AWACS or even making it unstable.

Interestingly, French radar manufacturer Thales is working on UHF radar with a focus on counterstealth applications, so it is a likely choice for a future French-designed AWACS, if such a thing were to happen:

The UHF antenna on the vehicle above is interesting in that it is not filled: there is a wide space between the elements because they are half a wavelength apart, and the wavelength is large. So it might be possible to build an airborne forward-facing UHF array with little drag.

With a UHF radar at 500MHz, the angular resolution will be four times poorer compared to a L-band radar at 2GHz. However, adding a high-frequency radar, for instance in X band at 10GHz, would allow to get back a good resolution: the X-band array would only operate in tracking mode and focus on the tracks detected by the lower frequency array instead of searching the whole sky. Thus, it can concentrate its energy on them. A X-band array using the full height of the radome would have 20 times the vertical resolution of the 500MHz array. That could be enough to get weapon-quality tracks from very far away, even against relatively stealthy targets, reducing the need for the fighters supported by the AWACS to use their own radars. As a rule of thumb, an X-band array with twice the diameter compared to a fighter’s radar would have twice the range, for around four times the cost.

Using higher frequencies also has a benefit for identification of contacts: the higher the frequency, the better the range resolution. At 10GHz, a radar with 10% of bandwidth has a range resolution of 15cm. It becomes 15m for a 100MHz VHF radar with 10% of bandwidth. Range resolution is interesting because it gives an estimate of the size of a contact. For instance, if contact has measures 50m in the range direction, it is not a fighter. However, with only range information, it is not possible to make a precise contact classification.

To do so, a synthetic aperture radar (SAR) mode is needed. SAR is commonly used to image the ground, or to classify surface contacts with the more advanced ISAR mode:

In SAR modes, a 2D picture of the target is built. The higher the radar frequency, the better the resolution of the picture, so it is a good reason to operate in X-band. Now, there are no radar manufacturers that I am aware of that advertize air-to-air SAR mode, but that does not mean it is not implemented. Indeed, there are scientific articles on air contact ISAR imaging, demonstrating the feasibility. The challenge is that the target can move more quickly and in more direction that surface contacts.

With ISAR, the length, wingspan and number of engines of contacts can be determined, so it becomes possible to classify targets at long ranges, even in bad weather.

Adding an X-band radar would also open interesting possibilities of bistatic radar operations: since the fighters’ radars operate in the same band, they can act as receiver for the signal coming from the AWACS, without emitting themselves. That way, they stay stealthy, and since they are closer to the target, they get a stronger return from it than the AWACS does.

Platform

To minimize cost, the platform will likely be derived from a commercial aircraft. Business jets are a possibility for a side-looking architecture. To get back to an hypothetical French-designed solution, the Dassault 8X has already been selected by the French Air Force for its next-gen ELINT planes, and is slightly smaller than the Gulfstream G550 used for the Israeli AWACS.

Another option is to use an Airbus plane. A Neo version of the A320, such as the A321XLR, could be an option. At 101t maximum take-off weight, it would be slightly heavier than the baseline Boeing 737-700 used for the Wedgetail. With its operational radius of 2300nm, it is a good fit for the 11-hours, 5000 nm missions which the Air Force is fond of. It would only need one partial refuelling.

To avoid taxing tankers too much, the heavier A330 airframe could be used. The A330 is the basis for the MRTT tanker which is in service in the French Air Force, so using it would bring some commonality to the two platforms, hopefully reducing cost. The MRTT is based on the A330-200, which has been reengined into the A330-800. The latter has a 4000nm operational radius. At a list price of 250M$, it is twice the price of an A321 though, and the range far exceeds what is need for long-range raids. It would have a very long on-station time but would probably need two crews to make full use of it. So it is oversized and the A321 is a better fit.