Before we start making baseless assumptions on the radar cross section (RCS) of the latest designs some basic understanding of radar detection is necessary...
In radar detection, there are vital 'target resolutions':
- Aspect angle
In order to calculate those resolutions, a constant stream of EM energy across the distance will not help. All of those resolutions are in the time domain, as do most of everything we do: Measure in and by time indexes or (slices).
So instead of a constant stream of EM energy that give us no information in the time domain, we must create individual time slices or 'pulses'...
Each pulse has:
- Leading edge
- Trailing edge
All three give us a characteristic call 'finite pulse length'. The corollary to 'finite pulse length' is a finite amount of energy per pulse. Therefore, a pulse has two time indexes: When the pulse begins life and when it ends. The radar system remembers this crucial information along with duration. Other pulse characteristics listed in the above illustration are for a different discussion. The above is applicable to all wavelengths, from the meters length HF/VHF/UHF bands to the ghz centimetric and millmetric bands.
For any wavelength, the shortest pulse we can create is that equal to one cycle. However, such a short pulse will deny us the ability to create other pulse characteristics such as the variable PRI and manipulate them for other purposes such as ECM and that is for another and more complex discussion.
It is then obvious that the greater the amount of time slices we have inside a duration and across a physical distance, the higher those vital target resolutions. The analogy here is high speed photography where the shutter speed creates time slices of visible wavelengths to give us those sports 'slo-mo' action. Or in nightclubs where strobe lights create 'jerky' and 'disjointed' movements. We want pulses that are greater than one cycle but not too long that it will give us too coarse information. This is a delicate balancing act for any system engineering team when designing a radar for a specific purpose.
What this mean is that each frequency band, from the meters length HF/VHF/UHF to the ghz centimetric and millimetric bands, has their advantages and disadvantages because of the characteristic 'finite pulse length' which equals to useful packets of energy. The centimetric and millimetric bands will allow us to create pulses that are much closer to each other to give high target resolutions the same way the camera's high shutter speed can give us so fine motion resolutions that we can 'slo-mo' the athlete's run.
Air defense radars will have different antennas transmitting different wavelengths because of these advantages and disadvantages:
- Long wavelengths give us long pulses that contains the highest amount of energy but the poorest target resolutions. This is useful for long distance search where coarse target information will suffice. For an airport, traffic at 200 km distance is not as important as traffic at 50 km.
- Low end centimetric and millimetric give us shorter pulses and finer grain target resolutions but because there is less amount of energy per pulse, we are restricted to use these bands at closer distances. For the military, it is for threat assessment and assignment, for the civilian airport it is for landing permission and priority.
- High end centimetric and millimetric give us even shorter pulses and higher target resolutions but the least amount of energy per pulse. Atmospheric attenuation (loss) can sap the pulse of its energy before it reach the target. This highest target resolution capability but against this disadvantage confined these bands to missile guidance, either from the ground or contained in a missile. At this point there no longer are potential threats. All targets are assigned as threats and missiles launched.
The answer to the highlighted question is based upon some unofficial official 'laws' of RCS control measures for the military and under the fact that active cancellation is not yet available. The first and most important law is:
1- Target the threat frequency. In other words, the design should be shaped to act against the X-band.
2- Use angled facetings technique to deny the seeking radar large expanse of surface areas as much as possible.
3- Use absorber (lossy) material whenever possible to control surface wave behaviors.
4- Enforce tolerances across surfaces.
5- Treat trailing edges to control edge diffraction signals. This includes plan forming all flight control elements.
6- Avoid corner reflectors of any degree whenever possible. If not possible, then avoid the 90 deg kind.
7- Avoid straight line cavities such as inlet tunnels whenever possible. If not possible, diffuse entrant signals prior to them entering said cavities. Inlet tunnels can create 'resonance' or 'ringing' in the EM spectrum that will exit both ends of the tunnels.
8- Shield one's own high-gain radar antenna from non-threat frequencies. In other words, use law 2 to deny other frequencies from exposing one's aircraft via direct reflection from the antenna.
9- Avoid surface discontinuities whenever possible. If not possible, see law 5.
Some examples of how these laws are applicable: During competition, the Boeing design had a single vertical stabilator, severely violating law 6 and forced the company out of competition. Law 6 is why all 'stealth' designs will have twin canted vertical stabs or in the case of the B-2 -- none at all, and law 6 is why all weapons will be carried internally. Laws 2, 5, and 9 are most evident on the F-117. Law 5 is applicable to all. Law 3 is minimally used on the F-22 and F-35.
Some critics of the F-35 called out the alleged fact that it is less 'stealthy' than the F-22. The critics missed the point completely that mission requirements can trump certain laws, in other words, mission requirements compelled the design team to focus more on some laws than others.
The result of the compromises between mission requirements and RCS control laws will give us the three generally accepted RCS shapes above. The process goes: Modeling, Estimation, and Measurement. With today's sophisticated software, the first two items can change positions but nothing is known until the measurement step, and the data from measurement will be hidden from the public.
Not to sound rude, are these laws that are generally accepted by engineers and physicists in theory like Newton's laws or are these your interpretation of stealth?
Anyway a noobie question , how well do you feel both PAKFA and J-20 have incorporated these laws?
Aren't composites 24% by weight on the F-22. The Sukhoi director was known to say the PAKFA will also have 24% composites by weight and 70% by surface area. So wouldn't it suffice to say the F-22 would be similar if not the same.
A question that a lot of people may be looking for. In the chance that the PAKFA or J-20 reach -30dB or greater reduction, similar to a F-22(maybe), then how will the USAF balance force levels with only small orders for F-22? The F-35 may not be enough to beat a F-22 equivalent aircraft theoretically. Are there more orders in the pipeline for the F-22 or is this it? I am of the opinion more may come with time, based on changing geo-political situation and also the American and world economic health.
The cancellation of the F22 program may not be accidental at all or due to the worsened economic climate of the US. Remember that the Americans will not give an inch for anything for their security. I'd like to believe that there is a rational strategic thinking into the cancelation. The war planners in Pentagon seem to be shifting their focus from the traditional fighters to a new platforms. But whatever it is we will find clarity to the strategy in about 5-10 years from now.
In the meantime, the Russians and Chinese are racing to catch up with the F22.
Thank you gambit. I had picked up a fair bit about RCS from different forums but this is really good explanation.
My questions are -
1. For F-22, the frontal RCS is claimed to be 0.001 m^2 and for F-35 it is claimed to be 0.01 m^2. How do these calculate against your Bowtie DB figures and how accurate are these? Also, what will be the lateral RCS for the F-22 and the F-35?
2. For PAK-FA, what will be the comparable figures, in your opinion?
As for P2Prada - the USAF together with Boeing, LM and General Dynamics are looking to build a 6th generation fighter. A supersonic stealthy fighter drone - about the size of the F-22, but remotely operated, with 25G+ performance and using hypersonic A2A missiles. Something like that will be even deadlier and can have a 5:1 kill ratio to a 5th gen fighter. The Chinese bloggers claim that the US 6th Gen aircraft might have directed energy weapons (laser/ plasma) etc.
Generally accepted in light that active cancellation is not yet feasible, at least for others anyway. Physicists reveals how things behave. Engineers exploits those behaviors. An old saying attributed to Einstein goes: 'Scientists investigate that which already is; Engineers create that which has never been.' These laws are not so much laws as they are guidelines IF there are X, Y, and Z goals.
Quite well, in my opinion.
Composites do not necessarily mean absorbers. If we go by weight alone, we could argue that composites are used for weight saving and not for RCS control. Look at it this way: Concrete and plywood are composites.
In the event that a potential adversary field an F-22 equivalent or even better, the solution then is to be more creative in tactics in both the technical and non-technical arenas. I understand that the cancellation of the F-22 may be out of financial necessities, but that does not mean the line cannot be resurrected out of military necessities.
There are no credible information regarding the F-117's RCS value, let alone the F-22 or F-35. Keep in mind that a 10dB difference mean a %50 difference in detected distance, meaning if A and B are detected at 200 km and A did <something> to drop -10dB in output, A will be detected at 100 km. Give or take a few km. The ONLY way we will know within the accepted %3 range of statistical certainty is to put each aircraft inside a controlled anechoic chamber, like Benefield at Edwards AFB in California, and take our time measuring them.
We can derive physical dimensions and surface features from photos and insert each aircraft into software written specifically for RCS modeling and estimation, and have no doubt plenty of people have done that, but just as 'Garbage In. Garbage Out.' any data from these calculations should not be construed as the true values because of the differences in physical dimensions between actual and estimated. Saying 'garbage' may be extreme because the commercial software are very well written and even Lockheed uses them so at least we have enough to make these discussions interesting.
Post 1516 was about the X-band as the targeted band and explained it in principle. This supplement will explain the why in the engineering.
It is known that there are several characteristics of a radar transmission that directly affect the quality of target resolutions, the main ones are:
- Antenna shape
- Antenna dimension
The last item 'Power' affect distance. The other items have complex relationships to each other.
Definition: radar resolution cell The above illustration is about radar resolution cell. In most cases, a narrow beamwidth in both the horizontal and vertical planes are desirable, as in the left situation. When volume search is the mission, the wider the beam the greater the volume that can be scanned per sweep, however, it will be at the expense of target discrimination in a multiple targets environment.
RADAR BEAM CHARACTERISTICS What this mean -- so far -- is that for a 'fighter' class aircraft where internal volume is already limited, the X-band proved to be the most useful in terms of beamwidth for superior target discrimination in a multiple targets environment.
Radar Cross Section The X-band is centimetric (cm) which will have any target in the 'Resonance' or 'Optical' region. An aircraft is meters in length and wingspan. In order to place an aircraft into the 'Raleigh' region, the wavelength would have to be in the mhz meters length HF/VHF/UHF bands. The F-15E antenna is 0.9 m in diameter. If this antenna transmit in the mhz bands, the beamwidth would be so large -- double digits of degrees -- that it would be worthless, whereas the more desirable beamwidth is between 1-5 deg.
Diameter of a Raindrop The higher the freq the more vulnerable to atmospheric attenuation (loss) and the example above is the reason why. The X-band is centimetric and when a pulse encounter a raindrop the raindrop will be in the 'Raleigh' region. However, if a millimetric freq is used, when this millimetric pulse encounter a raindrop the raindrop will be in either the 'Resonance' or 'Optical' region, resulting in a clutter display of weather phenonmena instead of other intended targets such as armed fighters and bombers.
Add all these factors together and there are those complex relationships that system engineers must take into consideration when designing a system for a specific mission. This is why the X-band proved to be the most useful for the 'fighter' class aircrafts and because of this usefulness, RCS shaping must target this particular threat freq.
These factors and their complex relationships are also the reason why AWACS antennas are relatively 'flat' disks that they are: Antenna shape. A 'vertical' fan is from an antenna shape that is 'long'. A 'horizontal' fan is from an antenna shape that is 'tall'. Basically, the fan is always the opposite of the antenna's orientation and each orientation has its purpose. A 'horizontal' fan sweeps in an up-down motion and is useful as a height finding radar. A 'vertical' fan sweeps either side-side or 360 deg is useful in finding targets' locations in respect to one's own position. The wider the fan the more volume is can cover per sweep cycle so a height finding radar can also find target positions but generally if target position and volume search are the goals then a 'vertical' fan is preferred because of the 360 deg ability with the benefit of altitude information per revolution.
Can you please tell me at what range is the F22's rcs 0.0001m^2. Because I know that range is one of the major factor as dissipation of radar waves takes place and it loses power at a certain distance and dosent get reflected back
If you scroll right to the bottom, you will find a reference to Anechoic chambers purchased and installed by our defence industry and air force. HAL will have 2 chambers meant to fit fighter sized aircraft along with a slightly smaller third one. No idea if we have any radar range planned.
As for Russia they have had Anechoic chambers since a long time. The Bars radar was tested in the 5th Central Research Institute nearly 2 decades ago.
Larger radars like the Greenpine, Phalcon, M3R, Wedgetail etc use L band. Erieye, Sampson,EL/M 2248(MF-STAR) use the S band. The radars linked to Patriot program, AN/MPQ-53 and 65 work in the C band. Is there any particular reason to why other bands are preferred over X band for such large radar systems?
Here's my 2 cents -
The L-band operates at a longer wavelength (15-30 cm) at 1-2 GHz. They have extended range and can pick up bogies at a long distance (300+ km) even from air-turbulence from an approaching aircraft. they might not be well suited to identify the type of aircraft or the exact number, but as an early warning system they can provide the direction of approach, speed, possible size and number for the aircraft.
The C-band operates at "medium" 4-8 cm wavelength 4-8 GHz. Because of the wavelength and freq of this band, this radar maybe able to detect, identify and analyze the flight paths of smaller / leaner flying objects like a ballistic missile at a relatively long distance (200-300 km) Hence the patriot missile systems use this type of radar. However, the C-band is not ideal for small ballistic missiles, which is why the patriots had 10% success rate against the Scuds.
The X-band is much more sensitive with a small wavelength and require less power/ smaller antenna (which is why it is used in aircraft), however, the large freq means it has high attenuation in the lower atmosphere.
Not possible. Not because I know and refuse to tell because of OPSEC considerations but simply because no one knows. At least no one in the general populace that is not connected to the F-22's program. Based upon my posts 1516 and 1522, we know that a 'fighter' class radar antenna will limit its effective detection range to 150-200 km. A clean F-16 is about 1 meter square in that range and from my experience that was very tough. Once the F-16 dropped below 1000 ft altitude, sea surface clutter hid him completely. So if we are reasonably certain the vast majority of the F-22's opponents are already limited to that 150-200 km detection capable range, we can be reasonably assured that the F-22 is shaped to target that distance.
Yes. That antenna dimension and beamwidth relationship as you accidentally answered your own question. For any freq, to get a desired beamwidth, you must either change the freq or change antenna dimensions or work out a compromise between the two. Those lower freq longer wavelengths have more energy to get a longer reach, but to have reasonably accurate target discrimination in a dynamic and multiple target environment, ground radar antennas are usually much larger than its airborne brothers to have the same beamwidth. However, once a missile is launched and if the missile has its own radar guidance it will have one of the X-band freqs. The missile can work in concert with ground radar data, but whether if ground data can override its own data is a different issue.