Airdrop Behavior of Cargo Pallets Testing

Thales Navigation, a leading provider of GPS solutions for positioning, navigation and communications applications, is playing an important role in the U.S. military's efforts to assess and optimize parachute-rigging designs for the airdropping of large cargo pallets.

In a series of tests conducted by the U.S. Army Operational Test Command, Airborne and Special Operations Test Directorate in Fort Bragg (NC), Thales Navigation's G12 GPS receiver was used in concert with an inertial measurement unit (IMU) to record and analyze parachute payload behavior - including continuous positioning, velocity and attitude -- during and after exit from an aircraft. The instrumentation package, called the Airborne Position Location Instrumentation (APLS) system, was developed for the Army Operational Test Command by SRI International, an independent research organization in Menlo Park (CA).

Operational requirements for the GPS receiver included a relatively high differential update rate of at least 10 Hz. The Thales unit offered 20 Hz. In addition, the receiver needed to supply analyzable raw data, which the G12 was also able to provide.

Reacquiring satellite signals

The G12 performed its role of providing time reference information and calibration of the IMU strongly in other ways. As the pallet was released from the aircraft in testing, the receiver would temporarily lose contact with the GPS satellites for a few seconds as an external antenna on the receiver reacquired the satellite signal. During that brief period, an inertial measurement unit (IMU) collected data as a backup. The G-12 performed well in signal reacquisition tests.

The testing also called for rugged, relatively shockproof equipment. The receiver and the IMU were both housed in a 10-pound aluminum-encased package along with a small computer, CPU card and an external battery. Pallets carrying some 30,000 pounds of cargo were dropped from altitudes of 1,500 feet from aircraft flying at 120 miles per hour. Pallet loads often undergo forces of 2 to 4 g's upon extraction from the aircraft and up to 100 g's on landing - stresses that call for unusually rugged measurement equipment. The combination of good update rates and reliable performance provided meaningful data and positioning readings.

FMI: www.thalesnavigation.com