Going to War - GPS Declassified: From Smart Bombs to Smartphones - page 105

altitude-sensing radar to compare the terrain beneath the flying missile to
topographic maps of the intended flight path stored in its computer. 7 Program-
ming each flight is time consuming, and each missile launch must occur very
close to its programmed start point, so the system can lock in on the terrain.
This works best where the terrain is bumpy and unique. Iraq's flat desert forced
mission planners to send weapons using tercom zigzagging among whatever
landmarks were available.
gps, by contrast, provides easier targeting and more launch flexibility. If the
missile has precise target coordinates and the receiver can acquire signals, it
can determine its location and the route to its destination in the same way a
dashboard navigation system will respond by “recalculating” when the driver
veers off the expected path. That sounds commonplace today, but in the mid-
1980s there was no precedent for having a single-channel gps receiver in a
missile acquire multiple satellite signals on the fly, much less for having it feed
that information seamlessly into an inertial guidance system. 8 Boeing engi-
neers overcame those and other technical challenges within twelve months. 9
The resulting weapon was the agm- 86c conventional air-launched cruise mis-
sile, or calcm. The missile is nearly twenty-one feet long, two feet in diam-
eter, and has a wingspan of twelve feet. Weighing more than three thousand
pounds and powered by a turbofan engine, it can cruise up to 690 miles (its
unclassified range) at speeds of 500 mph. 10
Anyone knowledgeable about alcms who saw the b-52s en route to the Per-
sian Gulf might have reasonably suspected the imminent use of nuclear weap-
ons, since the secret conventional missile looked just like the nuclear version.
Extreme secrecy was necessary not only regarding the mission, which gave
the Air Force a potent surprise against the adversary, but also regarding the
development of the calcm missiles. Had news of the existence of gps- guided
cruise missiles leaked out, the Iraqi military could have predicted possible
attack times. With the gps constellation incomplete and only sixteen satellites
in orbit, there were gaps in coverage, leaving specific windows of opportunity
when the required minimum four satellites would be overhead for several hours.
Mission planners drew the flight timetable carefully to avoid any possibility
that Libyan radar operators might spot the b-52s over the Mediterranean Sea
and warn Iraq before the first f-117 stealth fighters attacked. 11 Disclosure of
the calcm's existence could also have complicated negotiations in the Strat-
egy Arms Reduction Treaty (start I), signed July 31, 1991, by President George
H. W. Bush and USSR president Mikhail Gorbachev.