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Topics - r3mu511

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(for posts related to the DDG-51 Flight IIA restart ships, ie. those still equipped with the SPY-1D(V) radar)

Presentation by CAPT. Mark Vandroff, Program Manager DDG-51 Shipbuilding Program:

https://www.youtube.com/watch?v=iJ3iZHLFtWU

Flight IIA restart ship procurement initiated FY2010, 14 ships currently under contract starting with DDG-113 "John Finn"

Video of DDG-113 "John Finn" undergoing shipbuilders trials last Sept:

https://www.youtube.com/watch?v=oYQkQWUPG_A


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Stealth / Stealth vs. Low Frequency Radars
« on: September 27, 2016, 11:14:27 AM »
(post based on data from the old forum discussing the Russian "Sunflower" HF radar and the F-35)

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most folks appear to already accept that low frequency (ie. long wavelength) systems are capable of detecting aircraft w/c are optimized for LO (ie. low observability) characteristics in shorter wavelength bands (and this is the major assumption made of the f35, ie. that it is optimized for LO at higher freqs = shorter wavelengths)... as some like to say about long wavelength detection of high freq LO craft, "that's old news" (hence all that old talk all over the internet about the VHF freq russian NEBO sets vs. the f35, and the downing of a f117 via a system equipped w/ the p-18 spoon rest family of sets)...

the electromagnetics involved give an indication why high freq LO techniques would be mitigated by long wavelengths... if one considers the case of achieving LO via rf energy redirection (ie. reflection of the energy away from the transmitting source) this requires that the EM interaction takes place in the optical region, ie. the region where specular returns from the irradiated object dominate the contributions to the object's rcs... this optical/specular region means that the primary dimensions of the object are on the order of 5-10x larger than the wavelength used to irradiate the object...

consider the following images of this russian "sunflower" system:

http://www.fotos-hochladen.net/uploads/podsolnuhecoanxeg6z48qt.jpg
http://www.globalsecurity.org/wmd/world/russia/images/podsolnukh-image01.jpg

if the diagrams are correct then this set has a receiver array w/c is on the order of 32 elements wide inside a width under 640m... let's say the array only occupies 480m width (since in the diagram the 640m width appears to be the longer side of a trapezoid), then this means the elements are spaced ~15m apart... and assuming the typical half-wavelength spacing used for phased arrays, this means this system has a wavelength on the order of ~30m or so (ie. a frequency of 10 MHz or so, well w/in the 3-30 MHz range for HF OTH sets)...

considering that the f35 at a head-on aspect is like 10-11m wide (ie. wingspan), this means the primary dimension to wavelength ratio is on the order of ~0.37, w/c is well below the optical region of EM interaction (ie. the Rayleigh region, above which is the Mie or resonance region, above which lies the optical region)... in this lower regions of EM interaction, creeping wave (for Mie region) and dipole moment (for Rayleigh region) returns become significant and combine either constructively or destructively w/ any specular returns w/c result in an EM interaction w/c does not follow the purely optical region mode of rf energy redirection (ie. reflection)... the result being that LO techniques thru rf redirection are mitigated by the use of these long wavelengths...

if one now considers LO techniques via energy attenuation or absorption (eg. techniques such as ram/radar-absorbing-materials, etc. which absorb part of the RF frequency radiation and re-emit the energy as heat), then an important parameter to consider is the penetration of the incident rf wave into the structure, meaning the "skin depth"... and b/c skin depth is inversely proportional to the square root of the irradiating frequency, this means if one designed the attenuating material to work for higher freqs of let's say x-band at 10 GHz, then irradiation by a much lower 10 MHz rf signal would result in an rf penetration or skin depth w/c is ~32x more than that at the higher frequency... this would mean the attenuating material would need to be ~32x thicker for it to have a possibility of attenuating the lower frequency... so if for example you had given your aircraft a certain thickness of material to attenuate 10 GHz signals, then when irradiated w/ the lower 10 MHz freq the rf energy would penetrate much deeper into the aircraft's structure where the energy can "rattle around" and bounce back out, effectively mitigating the high freq LO attenuation technique...

so for both energy redirection and attenuation techniques, use of a much longer wavelength than what the craft was optimized for leads to a mitigation of the LO characteristics... again this is all under the assumption that the LO was optimized for the higher wavelength only...

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but even if a long wavelength system can detect a high-freq LO craft, the resolution afforded does not lend itself easily for fire control purposes... if we take the images linked above for example, w/ a 32 element width and an assumed typical phased array spacing of half-wavelength separation b/w elements, this would mean the horizontal beamwidth is on the order of ~3.18 degrees... hence at a target detection range of 100 km, the azimuth or cross-range resolution would be an arc w/c is ~5.56 km wide... at 200 km detection range the arc would be ~11.13 km wide... such wide resolution arcs mean it's not well suited for use as a fire control quality track, and perhaps is better used to vector an interceptor w/c then has to "search" inside the ~11.13 km box for the actual LO target...

(there are techniques to improve the cross-range resolution such as using dbs/doppler-beam-sharpening, tho this would be useful only at larger offset angles from the array's broadside... or one could use two such systems and the intersection of their arc "boxes" would give a smaller resolution arc, but still pretty wide by fire control standards)...

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so the end result is that, though long wavelength systems can mitigate the high frequency LO techniques used and thus allow for possible detection, the tradeoff is poorer resolution which does not lend itself well for fire-control purposes...

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AFP Modernization & Defense Acquisitions / PAF Long-Range Air Defense Radars
« on: September 27, 2016, 10:44:36 AM »
(posting for reference based on data from the old forum)

PhilStar article dated 02/04/16:

Quote
MANILA, Philippines The Philippine government has signed a deal with an Israeli company for the purchase of three air surveillance radars worth P2.68 billion...

The Department of National Defense (DND) and Elta System Ltd. signed the contract for the project last Dec. 21, documents seen by The STAR showed.

The letter of credit, a document that assures the supplier that the Philippine government has money in the bank to pay for the radars, was opened in the third week of January.

Under the contract, Elta is supposed to deliver the first of three radars within 22 months after the opening of the letter of credit.

http://www.philstar.com/headlines/2016/02/04/1549522/government-signs-p2-b-air-surveillance-radar-deal

Janes article dated 02/08/16:

Quote
The Philippines Department of National Defense (DND) has contracted Elta Systems, a subsidiary of Israel Aerospace Industries (IAI), to supply its ELM-2288 air defence and air traffic control radar.

DND spokesman Peter Paul G Galvez confirmed to IHS Jane's on 5 February that the contract is valued at PHP2.68 billion (USD56 million) and features three ELM-2288 radar systems...

http://www.janes.com/article/57802/philippines-to-acquire-elta-systems-radars

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