The Rafale hidden beauties and its future

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What is the name of the program for the high capacity datalink? Is it part of TRAGEDAC? INCAS? Or anything else?

One of the main result of Tragedac step 1 would be to demonstrate a new data link with high capacity and low latency.

 

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The Indomitable Squall: Rafale’s cutting-edge radar
The Indomitable Squall: Rafale’s cutting-edge radar | Hush-Kit

 

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As you know the first Rafale with an AESA Radar was procured to French Armée de l'air on October 2, 2012.
I want to show you how this was possible with a description of the development. For that I have made a translation of the PEA (Programme d'Etude Amont) Antenne Active Radar Rafale

For a delivery in End 2012 the first PEA begin in 1999

Objectives:

- Establish, for advanced active antenna radar an efficient and independent industrial sector, based on European components constraints.

Constraints:

- Overcoming technological barriers from active antenna components until the integration of the radar system in a fighter plane.
- Ensure consistency of "active antenna" calendar with Rafale export projects
- Propose a radar detection range at least similar to the competition
- Consider the environment (thermal, mechanical, electro- magnetic ...) of a fighter

Milestones:

- PEA AMSAR "Airborne Multirole Solid-state Active-array radar"

o 1999: Design and manufacturing of subassemblies
o 2003: Checking the ability of French industrialists, British and Germany to produce an antenna with active modules
o 2006: assembly and hybrid ground testing of the first prototype European antenna
o 2008: Campaign on flight test bench, performance improvement (jamming, range ...) and validation of new high-performance radar modes​

- PEA DRAAMA "Demonstrator Active Antenna Radar and Advanced Modes"

o 2003: Continuation of work at national level, building of two demonstration radar with active antennas on the basis of a RBE2 radar
o 2009: Flight tests on the Rafale, validation of new advanced RBE2 modes​

Results:

- Validation of an independent and viable European sector of critical components (UMS) and active modules for an X-band active antenna with Gallium arsenide technology (GaAs)
- Fully removed risks for a new radar with World State of the art performance: range, anti-jamming, new detection modes ...
- Maintain of French combat aviation competitiveness: the Rafale is the first European fighter with an active antenna radar

Expected impact:

- High performance capacities of a subnets antenna for the Rafale radar of F4 standard
- Improved usability of Rafale whole weapons domain (including METEOR missile)
- Declension of expertise to other radars, communications ...

DGA Added value:

- Industrial Policy in cooperation with Germany
- Upstream investment on Gallium Arsenide sectors
- Keeping of European goals on the components and of National goals on Rafale radar
- Accelerated Validation of new radar functions using Expertise means from DGA .

 

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How we address Critical material problem

https://www.thalesgroup.com/sites/default/files/asset/document/p10-13_criticalmaterial_final.pdf

 

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Pulse compression.

This technique is old and is not specific to AESA, but then we'll see that its use with AESA allows a flexibility of use that is new.

The detection range of a radar is not a function of the power but a function of energy which is sent to the target. For this to be true it is necessary that the energy is concentrated in a "peak" of short duration and high power. Thanks to the pulse compression we can use longer pulses, the return will be compressed, which gives the same detection performance as a short pulse on transmission.

The pulse compression is not very hard to implement: in the years 80/90 the ATL2 radar had a periscopes or snorkels search mode that used significant compression, well beyond what we could implement on a Rafale. Indeed pulse compression has advantages, but also disadvantages, and from a certain level, the drawbacks may outweigh the benefits.

Casually, the search periscopes akin to looking stealth boat ...

The problem of pulse compression, is that the longer the pulse, the longer it takes to scan the same volume of space. If we compress twice, we can scan an area reduced by 21% at the same time or take a 27% increase to scan the same area. But we will have an improved range of 19%.

In TRADITIONAL radar, you had to change the mode to change the compression rate and the rate applied to the entire space. But AESA are much more versatile, the rate can be controlled by software and we can implement strategies that maximize the use of time available for optimal detection.

The update of known tracks consumes only a few time. For a given bearing, pulse lengths are in micro second, we can say one millisecond max with nice margins. If you have 100 tracks to update that will consume less than 100 milliseconds that is 0.1seconde. A basic strategy is to update the tracks already known by sending just the energy required, to increase the discretion, and use the remaining time to scan the space to be monitored.

A frequency of update of the order of 2 seconds is really great, it goes further four seconds, after that depends on the tactical conditions.

Now suppose that the normal detection range of an F-35 is 30 km and it is desired to detect 100km. You should have to compress by a factor of 124, so you would scan the area to watch in 30.7 seconds. If the two planes are in frontal head on, they may have closer to over 23.7 km in the meantime. This shows the limitations of pulse compression, but it does not mean that, sharing the work, we can not achieve good results.

The simplest tactic that has been proposed is to "interlace" in, the detection volume and the update of the tracks already known, including detections by passive means on that we generally lack the distance. For a X-band stealth plane these detections may be longer than the "normal" distance detection of the radar, so we will use a significant compression to achieve a positive result. Similarly a track that moves away to be out of the normal range, may continue to be updated thanks to an increase in energy sent to it.

One can also "interlace" an "early warning" mode that would have such a compresion rate 15 times higher than the normal mode. It would issue two seconds in normal mode and a second in remote mode, the normal mode tracks update would be therefore every 3 seconds, whereas the distant mode would be every 19.8 seconds, but the far mode range will be doubled.

It may also share the two modes in two different planes which would give updates every 2 seconds and every 6.6 seconds.

 

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To better calculate the slowdown caused by the compression, I'll take typical values of RBE2 EASA. To have a travel time I consider the max intended range. I take as a typical value 240 km.

Round trip travel will be in 1.6 ms.

The average power of the radar seems to be 10% of the instantaneous max power as a typical value, which means that the radar emits 10% of the time that is 177 micro seconds for a duration of 1777 microseconds (1600 +177).

This simple calculation shows that we already have a long pulse and that therefore in the "normal" mode the AESA RBE2 already practice pulse compression.

Our aim is to see what happens if more compressed. We start slowly with a rate 2 times stronger.

The range will be increased by 19% so it goes to 286 km range and 1.9 ms of travel times.

We'll have to double the length of the pulse and going from 177 to 354 microseconds.

Total time password from 1777 to 2254 an increase of 27%

If we apply a rate 15 times stronger would have a travel time of 3.2 ms, a 2.7 ms pulse and a 330% increase in scanning time.

This shows that increasing the scan time is not the main factor that prevents the use of high compression ratio.

May be we would have to turn it to the evacuation of the heat?

Indeed, going from 10% to 15.7% of instantaneous max power when we double the compresssion increasing by 57% the heat to be removed.

With 15 for the rate instantaneous max power is going from 10% to 46% which increases by 360% the heat to be removed. This seems a most important factor limiting the increase in the scanning time.

So very high compression will be possible but only for short time, that is to say not to scan the entire space.

 

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Flexibility of Installation with modular active antenna.

Just to show you the 2007 French state of the art on Tile Antenna. I have extracted two slides but there is many more to see if you follow the link.

http://cct.cnes.fr/system/files/cnes_cct/459-mce/public/03-Radars%20Modules.pdf

Tile module on conformal antenna

http://cct.cnes.fr/system/files/cnes_cct/459-mce/public/03-Radars%20Modules.pdf

 

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The ability to use long pulse signal and compress the return make it hard to know the performance in range of an AESA Radar. You can increase the detection range in the following cases:

• You decrease the area to scan
• You make a cued search for exemple you get an electromagnetic or optic bearing with SPECTRA, you cue your Radar on the bearing and you send pulse longer and longer until there is a detection. This is done automatically on Rafale when asked by the pilot.
• You can continue to track outside the normal detection volume

The two last approach are usefull to detect and track stealth aircrafts.

Finally when you get SPECTRA and an AESA, it is more interesting to be smart and to remove efficiently the heat than to augment the size of the antenna.

 

[randomradio]

Finally when you get SPECTRA and an AESA, it is more interesting to be smart and to remove efficiently the heat than to augment the size of the antenna.

Switch to GaN.

 

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Switch to GaN.

It's just a problem of cost

Compared to GaAs, which was quickly needed in the wireless telecommunications, GaN components present fewer potential market outlets in civilian business. According to Xavier Grison, it will take at least five years before we see first operational applications of GaN on airborne systems.

Source: The Rafale hidden beauties and its future | Page 3
2012+5=2017

 

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Assemble An Active Cancellation Stealth System
Modern signal-processing components make it possible to design an active stealth cancellation system to hide a target from an inquisitive radar system, even under changing conditions.
Jul 10, 2012Xu Sheng and Xu Yuanming | R F Design

Achieving stealth requires minimizing the radar cross section (RCS) of a vehicle or system as it appears to an opponent’s radar detection capabilities. To achieve this objective, an active cancellation stealth system was designed by means of a phased-array technique, digital radio-frequency memory (DRFM), and field-programmable-gate-array (FPGA) technology. The DRFM enables precise replication of stored radar waveforms, while the phased-array technology is used to generate the required waveforms to cancel reflected radar returns. The FPGA is essential for signal analysis, database search, and waveform generation and control.

The system relies on an offline calculation approach in which omnidirectional RCS, clutter, and noise databases are established in advance. The active system’s signal processing and control module analyzes a measured radar signal parameter and then finds the corresponding target echo data in the RCS database, making a real-time adjustment of the coherent echo amplitude and phase parameters. By creating a target scattering field with coherent signal cancellation in the direction of a detecting radar system, the radar receiver remains in a null synthesis pattern. A combination of software and hardware helps realize this active cancellation stealth approach.

Radar stealth technology can be divided into passive and active techniques. Although passive methods have been traditionally used, the availability of high-speed microelectronic devices, phased-array antenna techniques, and computer processing have made active methods more feasible and practical. An active stealth system can adapt to almost any object that must be protected, such as a power plant or aircraft, and the technology can be retrofit to an existing electronics platform, with lower power consumption and other advantages compared to passive approaches.1

An active cancellation stealth system involves the use of coherent signal interference. For a target to avoid detection, it must emit a cancellation wave that is time-coincident with an incoming pulse, providing the required amplitude and phase to cancel the reflected energy from an enemy radar. This can be an effective means of blanking enemy radar pulses, although the difficulty in implementing such a system lies in the need to obtain cancellation signal parameters in real time, and to achieve precise control of the amplitude and phase of the cancellation waveform.2

Active cancellation stealth depends on adaptive real-time control of electromagnetic (EM) waveforms within a three-dimensional (3D) space. When a radar target is illuminated, return signals are produced by the target’s reflected radiation.

According to EM inverse scattering theory, if the source distribution of the radiation field is known, the properties of the scatterer and the scattering field distribution can also be known. If the radar signals are considered confined within a small solid angle for the sake of EM wave cancellation, a target can made to appear “invisible” or stealth to a radar system.

An important part of developing an active cancellation stealth system is understanding a given target’s RCS, which is a comparison of the scattered power density at the radar receiver with the incident power density at the target. The formal definition of RCS is:

where:
σ^0.5 = the complex root of the RCS
scatterer;
Ei = the electric field strength of the incident wave impinging on the target;
R = the distance between the radar and the scatterer;
êr = a unit vector aligned along the electric polarization of the receiver; and
ĒS = the vector of the scattered field.

Using either active or passive means, the principle of cancellation or reducing the RCS relies on reducing the field strength incident on the target to reduce the power reflected back to the radar receiver. Reducing the target scattering intensity can also reduce the RCS. A target’s RCS can be measured for different scattering directions and, according to Eq. 1, the direction of a radar target’s scattered field can be identified as:

ES = lim [Ei ∙ √σ ∙ êr)2√πR] (2)

Active cancellation methods are based on generating an EM field equal to a target’s scattered field, but with opposite phase. The effectiveness of active radar cancellation depends on the measurement precision of the radar signal, the knowledge of its real-time characteristics, and the accuracy of the generated cancellation field, among other factors.The principle of an active cancellation stealth process: The incident radar wave frequency, phase, amplitude, waveform characteristics, polarization, and radar space position are quickly and accurately measured by a reconnaissance antenna and signal processing system on the target platform.

The target reflection characteristics that correspond to the incident radar waveform must then be extracted from the target RCS database under the control of the computer information processing system. By generating a waveform with the appropriate parameters, including frequency, phase, intensity, and polarization, the target echo can be cancelled when the radar wave returns to its receiving antenna.

If a target can be resolved into a collection of N discrete scatterers or scattering centers, then the net radar return at a given frequency is:

where:
σn = the RCS of the nth scatterer and
φn = the relative phase of the scatterer’s contribution due to its physical location in space.

For a target with a large number of scattering centers, several dominant scattering centers will exist for a specific operating radar frequency and incident signal angle. Reducing the radar returns from these dominant centers can effectively reduce the RCS of the target. If the original target RCS is defined as σ0, an active cancellation system can introduce an equivalent scattering center with effective RCS of σ1. The phases of these scattering centers are φ0 and φ1, respectively. The superposition of both for the target RCS is given by Equation :

Namely:

Control of σ1 and φ1can be used to optimize these parameters to obtain:

where parameter σ = 0 indicates having achieved stealth in the direction of the enemy radar.

Compiling a target RCS database is an essential step in designing an active cancellation stealth system. Each RCS entry represents a function, rather than simply a number, varying with different incident signal direction, frequency, and polarization. It may be necessary to establish an RCS database corresponding to different directions, frequencies, and polarizations according to real-time measurements of incident signal direction, frequency, polarization, and power from relevant data. This database must support real-time adjustment of transmitter parameters to generate an effective cancellation wave for transmission.

The capability of making real-time measurements of every radar signal incident on the target is also essential to creating an active cancellation stealth system. Also, the system must be capable of real-time tracking of such things as the relative motion between the detection radar and the desired stealth target, so that the cancellation signal has the proper parameters under dynamic conditions.

A received radar signal can be analyzed in several ways. It can be channeled into the digital-signal-processing and control unit for analysis. Alternately, it can be sent to the forwarding mode active system (used for storage and reproduction of received radar signals), where it can be compared to stored waveforms to find a corresponding signal. Dynamic corrections within the forwarding mode active system can ensure that the echo signal is consistent with the received radar signal. The output signal is then processed by means of Doppler frequency-shift modulation, with coherent superposition of noise and clutter. The power synthesis and beam-forming network and transmitting antenna are then used to form the active cancellation wave.


Assemble An Active Cancellation Stealth System | Systems & Subsystems content from Defense Electronics Magazine

 

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Continued

In an active cancellation system structure the reconnaissance receiver is used mainly for reconnaissance and reception of radar signals from enemy transmitters. The forwarding mode active system consists of five components: the DRFM, digital phase shifter, digital attenuator, adder, and detector. The DRFM stores a received radar signal and copies it with high precision. The signal passes through the digital attenuator for amplitude adjustment and the digital phase shifter for phase adjustment. It then travels to the adder to couple with the wave signal produced by the digital-signal-processing and control unit.

The results from the adder are sent to the detector and a DC voltage is sent to the digital-signal-processing and control unit through an analog-to-digital-converter (ADC) interface. When the detector has measured a minimum output value, the system has achieved a zero balance. At this point, the digital-signal-processing and control unit will send a command to the forwarding mode active system, so that the radar signal is transmitted to the Doppler frequency-shift modulation module.

The system’s memory is mainly used for storing the databases, including the target echo database, the noise database, and the clutter database. The system operates with the assumption that a radar echo consists of three parts: target echo, noise, and clutter. As a result, a radar echo signal can be idealized as follows:

x(t) = s(t) +n(t) + c(t) (7)

where:

s(t) = the target echo signal;
n(t) = the noise signal; and
c(t) = the clutter signal.

Because a great deal of processing and calculation power is needed to determine the radar cancellation wave, it is difficult to achieve real-time calculations without pipeline delays. For this reason, an offline calculation approach is used to establish a target RCS database. The main RCS prediction method is based on the approximation for obtaining a complex target RCS; the error between the predicted value and the actual RCS value can be minimized within a few decibels.6 Approximate solutions to a target’s RCS can be found in a number of ways, including by geometric optics, physical optics, geometrical theory of diffraction, equivalent currents, and areal projection/physical optics.

The database for clutter and noise usually employs a Gaussian distribution of white noise, which can be generated by the Monte Carlo method. Clutter can include ground, sea, and weather variants, among others. Methods for modeling and calculating clutter and noise are detailed in refs. 7-13.

Clutter data is related to airborne altitude, aircraft speed, radar carrier frequency, radar point, radar pulse repetition frequency (PRF), and distance to target. To reduce this large amount of data when clutter data are calculated, aircraft altitude, speed, and radar frequency are fixed, and only radar point and PRF are changed.

The digital-signal-processing and control unit is an active cancellation system core module that is used mainly for radar signal analysis and processing, database searches, and control of other system modules. It is comprised of field-programmable-gate-array (FPGA) chips, using the design diagram. FPGA1 is used for analysis of a received signal; it sends instructions about a signal to memory via a Peripheral Component Interconnect (PCI) interface control chip. Retrieved data are available to FPGA2 and FPGA3 through PCI connection. Based on the signal information obtained, the target echo and clutter generation module produces a variety of target echo and clutter signals. Because of the range of signals, the computing time may vary, so the first device to complete a calculation will send a data cache to the Double Data Rate 2 (DDR2 ) memory. An output is synthesized once the calculation is completed.

The Doppler frequency-shift modulation unit superimposes the Doppler frequency on the forwarding mode active system output signal to produce an effective Doppler shift in the radar wave. The Doppler unit can be used to reflect the relative position of the target to the incident radar. The power synthesis and beam-forming unit, following instructions from the digital-signal-processing and control unit, can form the desired digital beam. It can also switch off the receiving channel and turn on the transmitting channel, allowing transmission of the modified beam via the transmit antenna.

For the purpose of evaluation, the active cancellation stealth system was simulated using commercial software and the following conditions:

1. The radar transmit signal is a coherent pulse train with modulation rate of 1 MHz.
2. The simulation signal has a pulse width of 4 μs and PRF of 1 kHz.
3. The target moves with uniform speed (in a straight line), with initial distance from the radar transmitter of 100 km, initial radial velocity of 300 m/s; elevation and azimuth angles of both 0 deg.; and target RCS of 2 m2 based on the Swelling II model.
4. The reconnaissance reference pattern function is described by Eq. 8:

where:
κ0 = (cosΘ0)0.5 = the factor for control of the phased-array antenna beam gain with scanning angle variation;
θ0 = the beam scanning angle;
θ1 = the unbiased-beam mainlobe beamwdth of 3 dB;
θ2 = the unbiased-beam first sidelobe 3-dB beamwidth;
A = the unbiased-beam mainlobe gain value;
B = the unbiased-beam first sidelobe gain value;
a = 2.783;
a1 = πθ1/a = the unbiased-beam first zero (in rads); and
a1.5 = π(θ1 + θ2/a = the unbiased-beam first sidelobe peak point of view (in rads).

The three-dimensional EM pattern can be simplified into an azimuth and elevation pattern multiplication result. Namely:

F(θ, φ) = Fθ(θ)Fφ(φ) (9)

where:
Fθ(θ) = the azimuth pattern and
Fφ(φ) = the elevation pattern.

Assuming that the radar antenna vertical mainlobe beamwidth is 2 deg., the mainlobe gain is 40 dB, the first sidelobe width is 1 deg., and the gain is 9 dB, the 3D antenna pattern will be produced. The system can also be used for an electronic-countermeasures (ECM) function using an M x N rectangular array antenna with a reference pattern function described as:

(θ, φ) = G(θ,φ)|E(θ, φ)||e(θ, φ)| (10)

where:
g(θ, φ) = the antenna pattern;
G(θ, φ) = the directivity factor, which only affects antenna gain variations;
E(θ, φ) = the array factor, which determines the beam shape;
e(θ, φ) = the array element factor, with e(θ, φ) ≈ 1;
θ = the azimuth angle on the array of spherical coordinates;
φ = the elevation angle on the array of spherical coordinates;
φ ∈ [0, π/2]; and
Θ ∈ [0, 2π].

Adjacent to the array element spacing of d = λ/2 in the x and y directions, E{Θ, φ) can be expressed as:

where:
k = 2π/λ = the wave number;
Imn = the weighting coefficient; and

where:
θ0, φ0 = a beam pointing vector.
If M = 51, N = 21, θ0 = 30 deg., φ0 = 20 deg., the 51 x 21 array antenna pattern will result.
The cancellation signal can be defined as:15

where:
ΔĒ = the cancellation residual field and
ĒS = the target scattering field.

When S = 0, complete stealth is realized. It can be seen that ΔĒmax = 6 x 10−2 dB and the corresponding cancellation signal, S, is 0.51 dB, so that the maximum radar detection range has been reduced to about 25% of the original value.

In conclusion, these simulation results show that an active cancellation system can greatly reduce the chance that a target will be detected. The approach can be applied to a number of different radio echo scenarios. The active cancellation stealth system presented here has a modular design for simplicity of maintenance.

Assemble An Active Cancellation Stealth System | Systems & Subsystems content from Defense Electronics Magazine

Stealth Technology: How Not To Be Seen
Sep 11, 2015
Bill Sweetman | Aviation Week & Space Technology

Photo: DGA
Measuring Stealth

Predicting the radar cross-section (RCS)—a measure of a target’s apparent size on radar—of a complex shape over a wide range of aspect (viewing) angles or frequencies is so difficult that it was impractical before the development of high-powered computers. In 1975, Lockheed Skunk Works engineer Denys Overholser developed a software program called Echo that could accurately model a shape composed entirely of flat surfaces and straight edges. (Russian mathematician Pyotr Ufimtsev had earlier developed formulae to compute diffraction from edges.) Within a few years, Lockheed had developed the system to deal with simple curves, and Northrop was working with complex spiral curves. More powerful computers have made it possible to model small surface details and more complex shapes and account for materials and cavities.
The first outdoor RCS test ranges—the stealth equivalent of a wind tunnel—were built in the late 1950s, including one at Groom Lake in Nevada that supported the design of the Lockheed A-12 Oxcart. A full- or sub-scale model of the vehicle, or even a complete aircraft, is mounted on a pylon and illuminated by a radar that is far enough away to approximate a far-field condition. The space between the radar and target has to be flat and free from obstacles, so outdoor ranges often resemble runways when seen from the air. Indoor or “compact” ranges—large anechoic-walled chambers with theater-screen-size parabolic reflectors to create far-field conditions—became popular in the 1980s: the world’s largest is operated by France’s DGA defense materiel agency at Bruz, in Brittany (above), and can test a complete Dassault Rafale aircraft. With better modeling, the demand for RCS ranges has declined, and at least three U.S. ranges have shut down: the former McDonnell Douglas range at Gray Butte and Northrop Grumman’s Tejon Ranch facility, both in California, and Boeing’s range at Boardman, Oregon.

Photo: DGA

Future Stealth

Like any technology, stealth evolves. Although a good deal of effort over the decades has focused on reducing the cost of stealth—acquisition, operations and aircraft performance—there have been performance improvements as well. For example, Northrop Grumman and the U.S. Air Force say that the B-2 is both less maintenance-intensive and stealthier than it was when it entered service. It has been suggested that clean-sheet designs can now be classified as extreme low observables (ELO) as they are stealthier than the B-2 in all respects.
One enhancement to stealth is active cancelation (AC): onboard electronics detect a radar signal, locate the emitter and transmit a signal that exactly matches the echo received by the radar—but exactly half a wavelength out of phase, so that the radar sees nothing. AC has existed in theory for decades, and may have been considered for the B-2. In 1997, a French engineer said in an interview that the Rafale’s Spectra electronic warfare system (above) includes “stealth-jamming modes that make the aircraft invisible” and it was disclosed later that MBDA had tested an “active-stealth” system on a C-22 target UAV in 1999.
Other improvements to stealth survivability may be system-wide. There have been references, for instance, to “stand-in jamming”—if the target has a low RCS, less power is needed to jam a radar, and an effective jammer can be carried on a small, semi-expendable platform.

Photo Gallery: Stealth Technology: How Not To Be Seen | Aviation Week

 

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Stealth technologies

Aircraft has to employ tactics and technologies that improve its survivability, if not it will be vulnerable to counter attack. Stealth aircraft are able to accomplish and survive missions where others cannot. Their operational flexibility provides users the ability to penetrate even the well defended zones with less risks. Stealth technologies provide more flexibility in tactics and mission planning.

Reducing aircraft radar detection range and its infrared, visual and acoustic signatures enhances air superiority and the freedom to attack surface targets. It is not possible to become completely stealthy, but low observability delays detection and decreases the effectiveness of tracking systems. Stealth aircraft increases the operational success rate of precision bombs by allowing their use closer to the target optimizing the size of force and the amont of ammunitions.

Stealth and ECM can cooperate to defeat Radar capabilities and improve survivability in air operations. A low observable aircraft may not need to use electronic counter measures intensively, compared to its counterparts, its low signature capabilities increase the operational benefits of electronic warfare suites. Friendly jammer aircraft can support low observables from greater distances which increase their own survivability.

Additionally, stealth aircraft maintain the sudden attack advantage in the battlefield, and they increase the ability to escape from defensive fire before, during and after a mission.

The first disadvantage of stealth aircraft is that they are designed according to requirements for RCS reduction, and in general this results in handling aerodynamic difficulties. Modern aircraft are made unstable at one axis for greater manoeuvrability but stealth aircraft are usually unstable in all axes.

The second disadvantage of stealth aircraft is the requirement to either restrict electromagnetic emissions completely or emit them in a very careful manner, such as via LPI radars. LPI technology is more necessary to low observables than any other aircraft. LPI can be used to support systems, such as altimeters, tactical airborne targeting, surveillance and navigation, but such sophisticated LPI systems, which require continuous development to counter new receiver designs, result in very high costs and deployment of complex electronically instrumentation and software.

The third disadvantage is the high maintenance costs associated with stealth.

The fourth disadvantage is that stealth aircraft are limited by the amount of ordnance they can carry. This is because in full stealth mode, aircraft are required to carry all of their ordnance internally.

The fifth disadvantage of stealth aircraft is their visual signatures. Stealth aircraft are still visible with optical means.

The sixth and the most important disadvantage of stealth technology is the cost for acquisition and operational expenses.

Stealth aircraft are usually unstable in all axes. To manage this unstability designer have to build a kind of artificial stability using highly redundant, fly-by-wire systems for flight safety, which increase the cost and add extra weight to the airframe.

To enhance this artificial stability and solve some critical situation F22 get also TVC. For that purpose a 2D TVC is convenient, so the F22 doesn't have a 3D TVC.

During training and experimental flights, there were many failures of these flight control systems, some of which resulted in crashes. One B-2 crash, one of seven F-117 crashes, and two F-22 crashes. All these crashes were related to flight control unit malfunctions.

A basic technique to improve the detection capability of radar is transmitting more energy and using a more sensitive receiver. This increase the size and weight, decrease mobility and increase cost. Additional sensitivity of the receiver means extra clutter and a greater number of false targets.

Extra clutter and false targets overload the radar and slow down the computational performance. These effects can be sorted out only with highly sophisticated processors that also increase cost. Despite these disadvantage increasing the transmitted power and receiver sensitivity enable the radar to obtain stronger reflections from the target.

Normally, traditional radars cancel out signals which are under a threshold value and display only strongest targets. As a result of their small RCS values, the energy returned from low observables is typically below threshold and therefore, the target is undetected. However, electronically scanned radars, with fast scanning speeds, allow the evaluation of suspected signals over a greater time period, observing targets of interest with the main transmitted beam and several received beams.

A similar operating mode, without a selected threshold, uses the “track-before-detect” method. This technique uses “computing-intensive algorithms” to discriminate the real target from the clutter and other undesired data. This discrimination occurs while tracking all received signals, over a certain time period, then determining and cancelling false targets from their "unreasonable and unrealistic behaviors.

http://users.isy.liu.se/rt/fredrik/reports/05RSN.pdf

In general, radars using VHF and UHF, as well as high frequency (HF) and lower bands, are called low frequency radars. These low frequency radars operate at C (0.5-1 GHz.), B (250-500 GHz.), and A (up to 250 MHz.) bands. Low frequency radars have long-range performance utilizing surface waves and are less affected by atmospheric interference and absorption. However, they have some operational disadvantages, including the requirement for physically big antennas and poor resolution.

Target tracking systems, which are tasked to direct missiles to their intended targets, are operated at higher frequencies, such as X (8-12 GHz.) or Ku band (12-18 GHz.).

When low observable assets are designed, their RCS reduction features like RAM, edge alignment, faceting and other shaping methods, are focused primarily on defeating new military early-warning/surveillance and target acquisition radars. In general, when the wavelength of radar frequency is much shorter than the dimensions of the low observable airframe then reducing total RCS is relevant by shaping methods which can be used to change the directions of reflections and provide low observability.

However, these methods are less effective in the Raleigh and Resonance regimes of scattering. The length of radar wave according to the physical dimensions of the target determines the physics of radar scattering. In the Raleigh scattering regime, the physical size of the target is close or smaller than the wavelength and the reflection increases when the physical size is larger. The resonance phenomenon is produced when an airframe’s dimensions (or their exterior body surfaces) are the same size as a half wavelength, creating in-phase reflections from the ends of the targets.

Radars which use UHF, and especially VHF bands with wavelengths on the order of a meter, satisfy the physics of resonance or Raleigh scattering, thus stealth aircraft are detected by them at greater distances compared to more common higher frequency radars. However, there are problems gaining meaningful detections. Angular accuracy of these radars has been deficient until recently.They also have poor low altitude detection performance. Another problem is that these low frequency bands are usually used by other communication and broadcast networks, thus these bands carry a great deal of noise.

Advanced Radar

LPI Radars use advanced radar and signal processing techniques:

- Very high sensitivity,
- High processing gain,
- Low sidelobe antennas,
- Irregular antenna scan patterns,
- High duty cycle/wide band transmission,
- Accurate power management,
- Coherent detection,
- And monostatic/bistatic configurations.

Several LPI radar techniques can be used to Reduce the radar’s peak effective radiated power (ERP). Using pulse compression technique is the most common LPI radar technique. The objective is to spread the radar’s signal over a wide bandwidth and a period of time. This is typically done with frequency modulation (FM), which is the most common technique, phase shift keying (PSK), and frequency shift keying (FSK) techniques.

F/A-22 Raptor tactical fighter’s AN/APG-77 (Northrop Grumman with Raytheon) multimode radar incorporates a low-observable, Active Electronically Scanned Array (AESA - incorporating approximately 2,000 transceiver modules) and is described as offering long-range, multi target, all-weather, stealth vehicle detection, electronic intelligence gathering and multiple missile engagement capabilities.

The active array provides frequency agility, low radar cross section, agile beam steering, and a wide bandwidth capability typical of LPI radar. APG-77 has a typical operating range of 193 km and is specified to achieve an 86 per cent probability of intercept against a 1 m² target at its maximum detection range using a single radar paint. It seems it is able to change of frequency 1000 times by second.

LPI radars have low power, and phase or frequency coding. The coding and frequency are unknown and are complex, hence coherent detection is not possible and non-coherent detection must be performed first. To achieve the maximum sensitivity, by detecting the total signal energy, the RF and video bandwidth must be matched to the signal modulation.

Detection of LPI radar signals requires large processing gain due to the wideband nature of the LPI radar. Detection is possible if the signal is integrated over a long observation time. It also requires sophisticated receivers that use time frequency signal processing, correlation techniques and algorithms to overcome the processing gain of the LPI radar.

Fourier analysis techniques have been used as the basic tool. From this basic tool, more complex signal processing techniques have evolved, such as the short-tme Fourier transform (STFT). More sophisticated time-frequency and bi-frequency distribution techniques have been developed to identify the different modulation schemes used by LPI radars. These techniques include the Wigner Ville Distribution (WVD), Quadrature Mirror Filter Bank (QMFB), and Cyclostationary Processing (CP).

These signal processing algorithms require large amounts of computing speed and memory. The trend in ES receivers for LPI radar detection is toward digital receivers and incorporates the concept of digital antennas in which the analog-to-digital converter (ADC) is at the antenna.

Monostatic/Bistatic Configuration

Monostatic and bistatic configurations may both be used in LPI radar designs. For monostatic radar, the leakage of the CW signal from the transmitter must be isolated from the receiver. For bistatic radars, the transmitting antenna and receiving antenna(s) are separated by distance.

Bistatic radar designs face technological challenges preventing widespread operational use, such as the synchronization of time and direction, etc. From all considerations, the bistatic spread spectral CW radar is the most ideal form of LPI radar. In addition, bistatic radar can minimize the attack of ARMs and increase the detection of stealth targets (GuoSui Liu et al. 2001, 120; 120-124; 124).

Bistatic synchronization of time and direction

One way to adress synchronisation of time and direction in an bistatic LPI configuration could be to use some techniques used in GPS and Galileo. To compute your position, the GNSS receiver has to acquire 4 satellite signals minimum, then determine time and distances from your position to the 4 satellites. The signal is extremly weak and the satellites are very far. It is done because the receiver know what will be the signal and use correlator to find it. The detection become a problem of computer power and time of computing (and not signal sensibility (mainly)). In fact the only objective of the detection is to know at what time precisely the signal was detected by the receiver. The time of emission is known because there is a time stamp added at the emission by the satellite.

For a bistatic radar the emitter could process as the satellite and the receiver as the GNSS receiver. Problem of synchronisation in time is solve because it's a native and mandatory characteristic of the system : in satellite the precise time is given by atomic clocks, in the bistatic radar the precise time is given by GNSS satellites. For the direction it could be added to the signal (as the time stamp) by the emitter. Multiple frequencies, compression rate and so on has to be defined and sent to the receiver in advance. It needs a military grade cryptography but this exist already for GNSS satellite.

The receiver exploit the direct signal until it is synchronised and then it exploit the indirect (reflexion) to detect. For that it has to mask the direct signal.

Counter stealth radar systems

- Radars with high Power emitters and sensitive receivers can be considered for all types of radar systems to improve their capabilities.

- AESA has a Low probability of interception (High electronic counter measure resistance) and may be one of the supporting technologies for detecting low observables if used within networks formed by a number of radars and augmented by additional computing power.

- VHF and UHF Radars use Raleigh scattering regions, thus are not affected by RAM and shaping precautions. Stealth aircraft can be detected by them at longer distances compared to common higher frequency radars. They get poor angular accuracy, poor resolution, poor low altitude detection performance and lots of noise in operated bands. Coud be used in conjunction with AESA radars.

- HF OTH Radars have the longest detection range (1000-4000 km) and satisfies better than UHF and VHF, the physics of resonance or Raleigh scattering regions thus not affected by RAM and shaping precautions. They depend on ionosphere behavior which varies with time of day. They don't provide accurate enough target position for tracking capability. Could be used for detecting low observables at early warning stage.

- Bistatic and multistatic Radars need less radiated power for detection compared to monostatic ones. It's hard for low observable target to apply countermeasures. Multistatic networks, may be required to detect stealth aircrafts. Future computing power advancements may increase the capabilities of these systems.

- Passive Radars and passive emitter location systems have Short range detection as a result of relatively low powered transmitters. They have similar capabilities and limitations with multistatic radars to counter stealth targets.

Cooperation of AESA airborne warning and control aircraft with new VHF or HF radars which are equipped with greater computing power will improve effectiveness against low observables. Low frequency surveillance radars are promising early detection systems against low observables and can indicate the broad or rough areas of the stealth threat. However, these early warning systems are limited in accuracy and are not suitable for tracking and point location.

This limitation can be overcome by obtaining general sector information of a target by means of the HF or VHF radar surveillance capabilities and then using an AESA powerful (using compress pulse for example) search modes in the right sector.

 

[Picdelamirand-oil]

Lockheed communication tries to make us forget that the stealth F-35 is primarily used to increase its autonomy, allowing it to fly higher and to make less zigzag between the radars detected threats, allowing to whack its target shots with high altitude dropped SDB .

Well, so, as it must be stealthy, it must take its in-house fuel and armaments, which automatically increases its curb weight and drag, which increases its consumption, which decreases its operational range .

Ideally, a Rafale with SPECTRA, external tanks and tactical flight capacity Very Low Altitude, which can therefore afford to avoidance, shooting the missile behind the shoulder (AASM is a missile, with an area of use and approach to much wider target than a SDB) while carrying a full load of air-to-air missiles ... Bah finally, it is not so bad!

A weapon system denotes design philosophy and an effective work culture. That's the whole point of self build their own equipment, rather than having to find the least worst (or best) in all proposed solutions on shelves.

 

[Picdelamirand-oil]

A modular hardware architecture

The basic principle of modularity, its very definition, is to move or add components as needed. It is therefore desirable to design a system that can evolve over time by adding "modules" of additional power when it will prove necessary. The Rafale has a central computer called MDPU (Modular Data Processing Unit). Each MDPU is physically composed of a backplane with 18 slots that can each accommodate a module.

Each module consisting of a motherboard, a processor (CPU) and RAM.

When switching to the fourth tranche of production, the Rafale weapon system has received substantial improvements. The new Rafale had to be supplied with more powerful modules that support the additional workload that resulted. These modules have also had to deal with larger volumes of data generated in the fusion system powered by an AESA radar capable of a greater range.

In 2003, for the production of the Rafale F2 standard, each module MDPU Rafale was composed of a PowerPC 740 running at 200MHz, and accompanied by 64MB of memory. In 2006, the modules have an embedded PowerPC 750 running at 733Mhz with 256MB of memory.

Regarding the current production standard, at best do we know that the processors, always based on the Power architecture, exceeded the Giga hertz and became multicore.

The computing capacity depending on the hardware architecture embedded in the Rafale are scalable, and virtually limitless. If this is the bottleneck that could become the width of the Bus MDPU, a problem that is not without solutions.

The MPDU modules are replaceable by more powerful modules, and there is a second location for a second rack MDPU.

Systems Virtualization.

For the modernization of a new standard, there was classically integration of new hardware, and new software should always be created in order to use them.

To oversimplify, each software depends on the hardware on which it was created to function. How to ensure that the Rafale software can continue to evolve in the coming decades, without having to rewrite everything should the equipment change? Simple answer: virtualization!

Virtualization allows a hardware abstraction. The software works in an environment that will remain the same throughout Rafale life, and only the software part (hypervisor) for the interface between the hardware and this environment will be adapted.

When installing the first MDPU models based on a new processor generation in 2006, engineers have started the system, and everything worked ... the first time!

If further tests had to be performed to ensure the non-regression of the aircraft systems, no rewriting software code on which depends the Rafale weapon system was necessary.

Rafale Update

The software development is not stopped for a combat aircraft. With the permanent addition of new weapons, equipment, or to meet the new threats, the weapon system is still evolving.

The Rafale update is performed within hours via the connection of a computer on the plane. Follows a verification of the systems and the aircraft is operational again!

When the F3R standard should be operational in 2018, all airplanes of the Air Force and Navy will then get the latest standard in the same way, and all will be able to play with the new Talios pod, or fire new long range Meteor missile.