*(for posts related to the newest flight of the DDG-51 class, starting with info on it's most significant design difference from the previous flight: the SPY-6 AMDR, data recovered from old forum post)***Just how good will the new SPY-6 (AMDR) on the DDG-51 Flight III be?***(TLDR version)*AMDR

free-space range (vs. 1 sqm rcs target) is on the order of ~1,750 km, by comparison spy-1d(v) free-space range is on the order of ~740 km...

put in perspective, the 14-foot diameter shipborne AMDR will give around half the performance of the original, land-based, building sized, 90-foot ballistic missile defense PAVE PAWS phased arrays...

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*(full version)*the stated design parameters for AMDR (air and missile defense radar) to be deployed on the flight-3 arleigh burke destroyers (currently intended for the 3rd ship procured in fy16, ie. ddg-127*), and given the designation spy-6, are for an approx 14-foot diameter active array w/ a sensitivity improvement over the current spy-1d(v) of +15 dB... this means a sensitivity improvement of approx ~31.6x better than spy-1d(v)... in range performance terms this corresponds to an approx ~2.4x greater range (due to the 1/4th-power relationship of sensitivity to range)...

and just what would this ~2.4x range improvement work out to? well in the mar-2004 issue of the "US Field Artillery" magazine, an article on CRAM written by CWO3 J.Robinson titled "Employing the Spy-1D Radar", had this to say on the spy-1d(v) performance: "it also can track golf ball sized targets at ranges in excess of 165 km"... so let's scale this up to it's performance vs a 1 sq-m rcs target...

using the method of computation utilized by mostlymissiledefense.com (a site which specializes in ballistic missile defense topics), because a golf ball has a diameter of around ~0.0427 m, which in comparison to the s-band spy-1d(v)'s wavelength on the order of 0.1 m (ie. assuming a freq of around 3 GHz, or around center s-band), this results in a ratio of object diameter to wavelength of around ~0.427, which means the RF interaction with the object is in the Mie or resonance region...

since this is in the Mie/resonance region, the rcs is not exactly the same as the rcs in the optical region (which in the physical optics approximation would just be on the order of pi*radius^2)... so instead (as based again on the computations used in mostlymissiledefense.com) we use the NASA size estimation model as found in the "Haystack and HAX Radar Measurements of Orbital Debris Environment" (NASA 2003) paper, which results in an rcs of approx ~0.0025 sqm...

so taking this back to the spy-1d(v), we now have a range perf of ~165 km vs. a ~0.0025 sqm rcs target... scaling this up to 1 sqm and using the 1/4th power relationship of rcs to range, we thus get a free-space range perf of ~737.9 km vs. 1 sqm rcs...

since the AMDR is going to be ~31.6x more sensitive than the spy-1d, which corresponds to a ~2.4x better range performance, then the free-space range of the AMDR is approx ~1749.8 km vs. 1 sqm rcs...

to put that into perspective, the original fps-115 PAVE PAWS ballistic missile defense phased arrays are 90-102 foot diameter arrays, housed in buildings, with a claimed range of ~5550 km vs. ballistic missile targets of 10 sqm rcs... scaling this down to 1 sqm rcs targets results in a range performance of ~3121 km...

by comparison, the AMDR is a 14-foot array, housed in the superstructure of a ship, and yet will get a little over half the performance of those building-sized PAVE PAWS arrays...

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**the first 2 ships procured in fy16 are the flight-2a ddg-123 and -124, so it would seem the third ship (ie. the first flight-3 variant) should be -125, but due to the multi-ship block buy executed by the USN, -125 and -126 are already entered into the USN procurement system as fy17 ships... thus when the US congress funded a third ship for fy16 this would then get the next available hull number, hence ddg-127, (ref: may-2016 presentation by USN PM for ddg-51 program, Capt. Vandroff: https://www.youtube.com/watch?v=iJ3iZHLFtWU)*