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@Mean
Citat:To je tacno za ne-AESA radare koji imaju jedan izvor radio talasa.
Ja se izvinjavam za malo duzi tekst, ali tema je izuzetno interesantna!
Citat:Ie you have 1500 modules, you can dedicate a 4th to A2A, another 4th to datalinking/comms, another 4th to A2G, another 4th to EW- this if achieved will represent a pinnacle of current ability. Right now, per indications they arent talking of interleaving modes, but actually implementing comms modes & EW modes on the JSF/F-22 etc- though some modules may be specifically tasked for this- which would allow simultaneous modes to run & hence interleave.
But of course, if one splits the available Tx/Rx modules up- you take a hit in total available power for the given function, which has an effect on range. (Basically average power & peak power) Another thing is that most ranges given for the high end AESA radars, are normally under non LPI conditions
Citat:In the first gen AESA, you just add active circuitry (amplifiers) into the individual modules. They are still fed with a master signal, because they should be synchronous to each other PRECISELY. This master signal is phase shifted and fed to a power amplifier. the received signal is fed through the phase shifter and combined at the back.
This scheme also can not do multiple roles for the modules, so it do not offer any functional advantage over PESA. However, it improves reliability and power efficiency
Citat:Next improvement is using multi-channel TX/RX modules. Each module can be fed with multiple master signals. One channel can be selected at a time for operation. So, with a four channel system (eg. APG-79), you can form four different beams at the same time. Note that one module can do only one function at a time, so you got to divide up the modules. The received signals also travel back the same way.
Citat:The second gen AESA (eg Vixen) uses Direct Digital Synthesis (DDS). Here the outgoing signal is NOT from a master. It is generated using DSP techniques. You still nees a master clock, so all the modules are synchronized in frequency and phase. Each module will have DSP and processors and the master RC will talk to them setting up various waveforms and phases.
This scheme can generate any combination of waves, limited only by software. But this has a major problem. That is received signal.
In previous models, you do DSP processing at the combined signal, which has more amplitude. Here we have to process a lower amplitude signal, so the signal to noise ratio is much lower. So, the effective sensitivity of the receiver is much lower here. That in fact offset a lot of advantages.
Citat:
The notion that each of the modules scan a small area is totally incorrect. If you turn on a single module, it will create a low power wide angle beam almost reaching the entire FOV of the array antenna. The beam is created by interference of the multiple emitters at the correct phase relationship, and the larger the number of modules participating in the beam, the sharper the definition. Needless to say, all emitters participating in a single beam MUST have the EXACTLY same frequency. Otherwise you can not form the beam properly. This is achieved by either a digitally synthesized master signal or using a low power phase shifter approach.
Now this requirement also debunks the notion that being able to use multiple frequencies, AESA is more immune to jamming. The only advantage in this regard would be that it is possible to create multiple beams each with a different frequency.
It is not a simple task to vary the frequency emitted by a number of modules while keeping the phase relationships appropriate. It is next to impossible to form a beam out of spread spectrum signals using phased arrays because each component needs a different phase relationship. The only viable method is to dedicate one module for one frequency. It is also possible to use multiple channels in one module, but that is essentially multiple modules in one package.
Also, the signals of all the modules participating in forming a beam MUST be synchronized to each other.
Bottom line, spread spectrum as you know from communication systems doesn't exist in Radars. The best you can do is a bunch of individual frequencies. Now, those returns can not be combined in time domain. You can only combine them on raster. That will not improve detection, but will only aid in resolution.
Poenta je da u LPI modu nije moguce efikasno eksploatisati pojedinacno funkcionisanje TR modula, zbog povratnog signala koji bi bio previse slab ili ciju bi frekfenciju bilo nemoguce razaznati od okolne sredine.
Razlika AESA od PESA je taj sto svaki pojedinacni modul kod AESA moze da radi u sirem frekfentnom opsegu i sto je moguce podeliti module na nekoliko grupa koje istovremeno obavljaju razlicite funkcije.
PESA moze da obavlja isto vise funkcija odjednom, ali na taj nacin sto ce u delicu sekunde preusmerivati glavi signal sa recimo V-V moda na V-Z mod i obrnuto, sto je drugacije i sporije u odnosu na AESA radar, koji to bukvalno radi istovremeno.
Druga stvar je da gubis snagu svakog pojedinacnog snopa kod njegovog deljenja recimo na 4 dela kao kod APG-79, jer imaju isti izvor napajanja.
To znaci da je svaki signal 3 cetvrtine slabiji od snage koju bi imao da su oni povezani u jedan veliki snop.
Sledeca stvar je da ipak moras smanjiti snagu signala u LPI modu, jer iako RWR nece moci da obradi vrstu signala zbog velike brzine i promene frekfencije, moci ce da prepozna anomalju i samim tim pretpostavi da je signal vestacke prirode zbog njegove vece snage u odnosu na okolinu.
Citat:Da ali samo ako bi RWR bio dovoljno brz kao AESA radar. Vec rekoh da se kod AESA radara brzine mere u milisekundama. Dakle snop se zadrzi na meti par milisekundi. Po netu sam vidjao podatke da RWR treba oko 1sekunde da bi efikasan.
Naravno, to se podrazumeva!
Ali izgleda da je to ostvarivo.
Dao sam jedan primer u proslom postu, a evo i sta tvrde rusi:
Citat:U.S. radar designers insist that the latest AESA radar models are all frequency-hopping and therefore they do not emit a signal long enough at a specific point on the bandwidth for such a seeker to lock on to them. But Russian specialists reply that the threat model used by the U.S. to develop these radars relies on older versions of the 9B-1032 seeker. The new version for export, they claim, employs a number of new-generation components that are more sensitive and process signals faster.
Cinjenica je da rusi razvijaju nove V-V rakete sa pasivnom glavom za samonavodjenje na izvor radarskog zracenja drugog aviona.
Ako ne postoji nacin da uhvatis i identifikujes signal koji odasilje AESA radar onda nema smisla ni praviti taj tip rakete.
Ocigledno da tehnologija za to postoji.
Izvinjavam se ako sam bio previse opsiran.
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