How Satellite SAR Radar sees large (thousand foot) ships at 25 meter resolution.
Posted 5/4/2006
US Nuclear powered aircraft carrier
Posted 5/4/2006
Large bulk carrier merchant ship, about the same
length as US carrier.
April 15, 2004 China plans to put it's first high resolution radar satellites
in orbit within two years. Oceanic reconnaissance with radar satellites was
pioneered by the Soviet Union, which began launching such satellites in the
mid-1970s. But their EORSAT program proved too expensive, and no long needed
when the Cold War ended, and was gone by the late 1990s, when the last EORSAT
bird wore out.
The United States has some 200 military satellites in
orbit. This includes several equipped with radar for ocean surveillance,
continuing a program that began in 1978. Most of the American military satellite
are used for SIGINT (signals intelligence, picking up wireless messages) and
communications. At the end of the Cold War, Russia had about a hundred military
satellites in orbit, but that number fell to less than a dozen by the late
1990s. The Russian satellite numbers have since increased to the point where are
thought to have a few dozen military satellites in orbit, with more on the way.
The Russians are still secretive about their space satellite activities, and
often use the same satellites for commercial and military purposes (but under
military control).
Russia also still possess considerable technical
knowledge on how to build and operate radar satellites and it is thought that
the Chinese have provided the money to keep the Russian radar satellite
engineers employed. Thus the Chinese radar satellites will probably be based on
proven Russian technology.
But radar satellites do not automatically
give you a picture of what ships are out there. To find and identify warships at
sea, you have to focus the radar satellite's attention, otherwise you can cover
a wide ocean area and only see dots, which only indicate large ships. You can
get much better detail if you are willing to settle for a "soda straw" view.
With a 25 meter focus, you can make out the unique outline of an aircraft
carrier (which is obviously different from a large container ship or tanker of
the same length). If you go to 200 meters resolution you can see a chunk of the
Pacific, but the ships only show up as dots. In effect, unless you know within
about 50 kilometers where a ship is, more or less, you aren't going to find
specific ships via broad area surveillance. When you do spot dots on the ocean,
you can then refocus the satellite radar on the individual dots. Of course,
since satellites only have a limited "pass" time, and do not necessarily return
to the same area on each pass, you may lose the target before you can properly
identify it. Even having a handful of satellites will not necessarily be very
helpful. The radar images transmitted back to earth are rough, but in the case
of an American aircraft carrier, pretty obvious. Smaller ships are of less
distinctive shape, but it is possible to tell a large amphibious ship from a
destroyer.
For the Soviets during the 1970s and 80s, (and the Chinese
today) the most important thing their satellite radars were looking for were the
American carriers. Next came large merchant ships, that, like the carriers,
could be attacked by submarines or anti-ship missiles. For the Chinese, this
means having radar satellites (using modern synthetic aperture radars) that can
regularly scan the western Pacific, to spot approaching American carriers, and
provide targeting information needed by Chinese submarines and missile carrying
ships. The Russians never got a chance to use their large, "carrier busting"
anti-ship missiles before the Cold War ended. But now these missiles are being
sold to China, where they may get a second chance to do what they were built to
do.
