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Weekly UpdatesAccording to Thales, the RBE2 AA will offer 50% greater range than the current RBE2 and a huge increase in reliability--major overhaul every 7-10 years, compared with a current TWT service life of around 100 hours. It will also be possible to generate SAR images in air-to-ground mode with 1 m resolution or better, and to detect at long range low-reflection airborne targets, including stealthy UAVs and UCAVs.
No state funding has yet been made available to fund the active-array transition for Typhoon and Gripen. Euroradar (Selex SAS/EADS/Galileo Avionica/Indra) launched its own Caesar demonstrator programme for Typhoon in 2003. The demonstrator made its first flight on a BAC 1-11 testbed in February 2006. Caesar combines the back end from the existing Captor with an antenna partially featuring active GaAs modules from UMS (Germany) and Filtronic (UK). Captor air-to-air modes have been partially adapted to the new antenna. Caesar was flight tested on a Typhoon development aircraft (DA5) in May.
Industry is hopeful of an order as part of the Tranche 3 batch of Eurofighters, currently due to be ordered in 2009 for delivery starting in 2012. According to industry officials, the operating cost gains due to improved reliability would compensate for the extra cost due to development of the new antenna.
In Sweden, Saab Microwave Systems (the former Ericsson) is following a similar path, also without government funding. Saab, which hopes to start flight demonstrations this summer, aims to have an active antenna radar on Gripen by 2015, slightly later than the offer European programmes but with more ambitious technology goals. The antenna for its so-called Not Only Radar (NORA) concept would be mounted on a vertical axis allowing the scan angle (120[degrees] in pure electronic mode) to be extended to 200[degrees]. Nora would also offer jamming and data link functions, similar to what the Americans are testing today on the F-22's APG-77.
RELATED ARTICLE: Multifunction radars.
Much of the potential offered by GaN can be seen today with GaAs. It is already possible to produce very-high-bandwidth medium-power amplifiers for self-protection jammers covering the entire upper portion of the the electronic warfare spectrum (618-GHz). Thanks to a major technology investment, industry can now produce more powerful amplifiers, delivering around 10W at the high efficiency levels (around 50%) required for airborne X-band (8-12GHz) radar applications. In doing so, however, bandwidth has dropped to around 10% of the operating frequency. The experts predict that, within a relatively short timeframe, it will be possible to produce still more powerful GaN amplifiers ... with at least twice the bandwidth. The US Defense Advanced Research Projects Agency (Darpa) is targeting a six-fold increase in power compared with existing GaAs modules.
In Europe, the Korrigan project (see main article) aims to develop X-band power amplifiers in excess of 20W (i.e. suitable for radar and long-distance telecom applications) and with a bandwidth of around 2GHz, sufficient to jam other transmitters in the same frequency band.
In theory, modules with twice as much power could be used to produce radars with twice as much power, i.e. twice the range. However, engineers could well select another avenue, initially at least, since the useful range of a radar is related to the range of the weapons that it is being used to control. On the other hand, if the power of GaN is used to trim the number of modules, this means that the size of the antenna--and the nose section of the aircraft--can be significantly reduced, with an obvious payoff in terms of aerodynamics and stealth. The aircraft's stealth characteristics would be further improved by the fact that, by sharing the same antenna for radar, jamming and communications functions, there would be a reduction in the number of reflectors for enemy radars.
Another consequence of the smaller antenna is on increase in beam width. The tradeoff is a slight loss of resolution, but this is not a major problem in air-to-air situations where missiles have their own active seekers that can compensate for shortcomings in target designation. In air-to-ground modes, however, a wider beam enables a given area to be covered more quickly to establish cartography. Also, in jamming mode, the aiming accuracy in relation to a hostile transmitter would be less demanding.
The combination of all these modes (radar, communications jamming) on a GaAs radar is also possible. In the US, trials have been performed using the Northrop Grumman APG-77 radar on the F-22 and the Raytheon APG-79 on the F/A-18E/F Block 2. However, local media reports have highlighted the limits of what can be achieved. First-generation APG-77s reached their temperature limits already in radar mode. This problem seems to have been resolved on more recent versions, but in jamming mode the APG-77 cannot transmit for more than one second without damaging the radar. Also, experts have commented that jamming is effective over a frequency band that is too narrow to effectively counter all airborne threats.
This helps to explain why the US, despite their lead in GaAs technology, is currently accelerating research into a future alternative.
RELATED ARTICLE: Power plus bandwith.
The intrinsic properties of gallium nitride (GaN) make it the designated successor to gallium arsenide (GaAs) for radar applications. The three major properties are: substrate thermal conductivity and breakdown electric field 10 times greater than GaAs, and a very high output impedance, allowing GaN transistors to operate across very large bandwidths.
The higher breakdown electric field means that components will be able to operate at higher voltages (typically 20 and 40V, compared with 10V for X-band GaAs components) and will possess greater tolerance to impedance mismatch, rendering them less sensitive to hyperfrequency aggressions.
Operating at higher voltages, GaN amplifiers should reduce heat losses--which the good thermal conductivity of the substrate will help to evacuate more effectively. Hence the possibility of either deriving more power from components, or reducing component size for the same power.
GaN can be used to produce amplifiers up to several hundred watts which could be used to replace travelling wave tubes on telecommunications satellites. A major advantage in this case would be the elimination of very-high-voltage power supplies and the risk that these represent for the onboard environment. Transmit/receive modules for radar antenna (which today measure 6-7cm in length, with a 15mm section) could be packaged in 13mm cubes ... small enough to insert into conformal antenna and open the way to "smart skin".
Finally, the high breakdown voltage of the semiconductors means that the low-noise amplifiers in the reception stages of the radar will be less sensitive, i.e. more resistant to external aggression, such as offensive jamming and leakage from the transmit circuit at the antenna stage. Today, GaAs receive module stages require protection in the form of bulky and expensive ultra-rapid ferrite circulators. These circulators could be replaced by simple switches, also using GaN technology. In this way, all the high-frequency components of the radar antenna modules could be built using the same process, thus further reducing production costs.
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