Tobyhanna Army Depot, Pa. -- Technicians quickly responded to an urgent request to repair critical Identification Friend or Foe systems in support of Operation Iraqi Freedom.

Tobyhanna received the request in January from the U.S. Army Missile Command to repair and test four TS-1843B/APX In-Flight Transponder Test Sets.

"The 1843 is installed in aircraft or ships as part of the IFF transponder systems; it's connected between the IFF antenna and the RT-859A/ APX-72 Transponder," explained John Ross, Transponder Division chief, Avionics-Intelligence Electronics Warfare Systems Directorate. "It allows the crew to monitor the performance of the transponder when it is challenged by signals from external interrogators."

The test set can also interrogate the AN/APX-72, evaluating functions such as receiver sensitivity and tuned frequency, and reply frequency.

The AN/APX-72 provides automatic radar identification of friendly units and is used by all military branches. It receives interrogations, decodes them and transmits replies.

Due to back orders of the test sets caused by high usage rates, four aircraft were grounded. Division personnel expedited the repairs and shipped them within days of the request.

"We got the call on Jan. 21 and hand-carried the test sets to DDTP [Defense Distribution Depot Tobyhanna] for shipping on Jan. 23," Ross said.

The work involved mechanical repairs and repair of circuit cards and A5 modules, which ensure radio frequency and pulse accuracy, said Bob Harvey, an electronics mechanic.

"We also align the system; however, alignments require an AN/ UPM-362 Transponder Test Set," he added. "We don't have that test set so we worked with Production Engineering to develop a solution."

Alignments ensure that the test sets, especially the A5 especially the A5 module, work properly with components such as the different-length cables used in various aircraft and ships.

"We developed an external tester as a troubleshooting aid for the A5 that nobody else had before," said Martin Simko, electronics mechanic. "It has more capability, giving us a better idea of the condition of the A5 module."