Charles Vick has worked up a drawing of what USA 193/NROL-21 probably looks like. And Charles is noted for being pretty accurate. You can see that image on the Global Security website at http://www.globalsecurity.org/space/systems/images/e-305-concept.jpg.
You can learn more about this satellite and the E-305 Radar capability on the Global Security website at http://www.globalsecurity.org/space/systems/e-305.htm. USA 193 is believed to be a new generation military radar imaging recon. This Future Imagery Architecture (FIA) Imagery Radar Satellite is probably a replacement for the operational Lacrosse/Vega/Onyx spacecraft that are believed to carrying two elongated radar imaging dishes.
And here is the latest TLE set from Ted Molczan and the SEESAT-L gang.
USA 193 5.0 2.5 0.0 4.3 v 260 X 268 km
1 29651U 06057A 08040.19031987 .00154186 00000-0 23822-3 0 09
2 29651 58.4879 79.7345 0006557 83.6022 276.5824 16.03599602 00
Arc 2008 Feb 04.28 - 09.21, WRMS residuals = 0.039 deg
Mean rate of altitude decrease over the past five days was about 850 m/d (metres per day).
Definition of 2-line elements format: http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP
_Help/tle_def.html
Using the above elements and a 10.7 cm solar flux of 71, Satevo v0.51 estimates decay from orbit on 2008 March 19, in reasonable agreement with earlier estimates. The uncertainty is at least one week.