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GlobalSecurity.org In the News

SATELLITE WEEK October 15, 2001


NASA's Feb. 2000 Shuttle Radar Topography Mission (SRTM) made possible use of Global Positioning Satellite (GPS)- guided missiles in U.S. military effort against Afghanistan, said spokesman for Globalsecurity.org, defense research organization. "This is a wonderful technology... Without the elevation data, the [GPS-guided] bomb would be useless."

GPS system requires 3 sets of coordinates to function properly: latitude, longitude, elevation. Before SRTM, weapons utilizing GPS for guidance relied on topography information from Lacrosse National Reconnaissance Office radar imaging satellite for elevation coordinates. NASA, during SRTM, used C-band and X-band interferometric synthetic aperture radars (IFSARs) to acquire topographic data over 80% of Earth's land mass between 60 degrees N and 56 degrees S, amassing enough data during its 10 days of operation to obtain most complete near-global high-resolution database of Earth's topography. Although NASA lists many applications that will benefit from new data, Global Security spokesman said GPS guided weapons were "primary" beneficiary. Total mission cost was $142 million, including launch delay costs. NASA said other mission uses included: (1) Scientific applications in fields of geology, geophysics, earthquake research, volcano monitoring, hydrologic modeling, co- registration of remotely acquired image data. (2) Civilian applications including Enhanced Ground Proximity Warning Systems for aircraft civil engineering, land use planning, line-of-sight determination for communications (e.g., cell phones). (3) Other military applications, including battlefield management, flight simulators and logistical planning.

Joint Direct Attack Munition (JDAM) missile is used extensively in current U.S. war effort, technology that wasn't available in Gulf War. JDAM is attached to missile's tail and converts it from ordinary "dumb" gravity bomb to GPS-guided smart bomb. JDAM kits cost $20,000 each, half of price of laser guided kits and tenth of cost of Joint Standoff Weapons (JSOWs), Global Security spokesman said. Organization said it took only 30 min. to convert "dumb" bomb into smart bomb. JDAM can be released up to 15 miles from target. Its Inertial Guidance System (INS) with GPS updates allow control fins to correct trajectory right up until impact. Accuracy is nearly as good as laser-guided bombs, but isn't degraded by bad weather. JDAM was upgrade on existing inventory of Mk-83 1,000- and Mk-84 2,000-pound general purpose unitary bombs and 2,000-pound hard target penetrator bomb by integrating guidance kit consisting of Inertial Navigation System (INS) and Global Positioning System guidance kit. Version for 1,000-pound JDAM is designated GBU-31, and 2,000-pound version is GBU-32. JDAM variants for Mk-80 250-pound and Mk-81 500-pound bombs are designated GBU-29 and GBU-30, respectively.

Guidance is accomplished via tight coupling of accurate GPS with 3 axis INS. Guidance Control Unit (GCU) provides accurate guidance in both GPS-aided INS modes of operation and INS-only modes, capability that will counter threat from near-term technological advances in GPS jamming. In event JDAM is unable to receive GPS signals after launch, INS provides rate and acceleration measurements that weapon software will develop into navigation solution.

Using JDAM doesn't require intensive cockpit work for pilot such as is necessary when firing laser-guided missile, Global Security spokesman said. Pilots using laser-guided missiles must line up cross-hairs and lock successfully onto target, illuminate intended target almost until impact of bomb, and release weapon during proper window of opportunity, "all while flying the plane and avoiding enemy aircraft... It's a lot of work for a pilot operating a single-seat aircraft like the F-16. It can be very distracting. Imagine all that and then getting illuminated with surface-to-air missile threats." Aircraft equipped with JDAM weapons already have target loaded by mission planners, Global security spokesman said: "You just fire and forget... That's why they are so wonderful." Mission plans are loaded to host aircraft before takeoff and include release envelope, target coordinates and weapon terminal parameters. Weapon automatically begins initialization process during captive carry when power is applied by aircraft. Weapon aligns its INS with host aircraft's system. Targeting data automatically are downloaded to weapon from host aircraft. When host aircraft reaches release point within Launch Acceptable Region (LAR), weapon is released. Weapon maneuverability and range are enhanced by fixed aerodynamic surfaces (midbody strakes) attached to bomb body. One downside to using GPS guided weapons is possibility of incorrect data entry, which could lead errant strike, spokesman said.

JSOW and some Tomahawk cruise missiles also are GPS- guided weapons. JSOW is highly lethal glide weapon with standoff capabilities from 15 nautical miles in low altitude launch to 40 nautical miles in high altitude launch. JSOW is used against variety of land and sea targets and operates from ranges outside enemy defenses. JSOW is launch-and-leave weapon that, like JDAM, employs tightly coupled GPS with INS. Since Gulf War, Navy has improved its Tomahawk missile's operational responsiveness, target penetration, range, accuracy. It has added GPS guidance and Tomahawk TLAM Block III system upgrade with jam-resistant GPS receivers. Tactical Tomahawk adds capability to reprogram missile while in flight to strike any of 15 preprogrammed alternate targets or redirect missile to any GPS target coordinates. Tomahawk land attack cruise missile has been used against variety of fixed targets, including air defense and communications sites, often in high-threat environments.

Copyright 2001 Warren Publishing, Inc.