Defensive Electronic Countermeasures (DECM) against Surface-to-Air Missiles (SAMs) was a given for large Navy attack airplanes but not initially for the A-4 Skyhawk because of the size and weight of the electronics required. Losses over North Vietnam dictated its addition, in part as Project Shoehorn.
The exterior change was the addition of five antennas, three forward and two aft. One forward or one aft detected the radar pulse of four different kinds of fire-control radar. The ALQ-51 electronics then automatically transmitted a pulse from the other two forward antennas or the other one aft that misled the radar operator or confused him as to the location of the airplane, preventing guidance of the SAM to it. For an instructive video on the process, click HERE.
The antenna resembled an ice cream cone:
The forward transmitting antennas were located on either side of the nose landing gear actuator well. Two were probably required because there was no place to mount one aft of that or if there was, the airframe or stores might block or diminish the signal from it in some directions.
The antennas were exposed and therefore sometimes broken off. In that event or if they were not required, they would be replaced by a half-round black rubber ball as in the mount labeled 9 here:
A subsequent addition to the Skyhawk's ECM suite provided the pilot with a warning that a fire-control radar was tracking him to improve his chances of spotting a SAM fired at him and evading it in case it wasn't being spoofed. This was the APR-25. It utilized four antennas about the size and shape of small drink coasters, two facing forward and two aft. They were angled outward so the APR-25 electronics could analyze the strength of the pulse being received at each antenna, determine the direction it was coming from, and display that to the pilot on a scope in the cockpit.
The two aft antennas were mounted one on each side of the sugar scoop.
The two forward APR-25 antennas were colocated with the forward facing ALQ-51 receiving antenna in a larger fairing.
Thanks to Dave Dollarhide, Bill Egen, Carlton Floyd, and Jim Winchester for their help with this topic; any errors in the above were made by me. Comments, corrections, and additional information are welcome.
A very nice article - and very helpful for a modeller, too!
One thing more I'd like to read about - ECM system on an early Skyhawk. The book "A-4 Skyhawk units of the Vietnam War" shows a color profile of an A-4B with a note that one gun has been removed and replaced with an ECM system - do you have any details on such rig?
Thanks a lot for a great article, looking forward for more, have a nice day!
Other than the gun port being faired over, there wasn’t any external difference except for the added antennas that I described here. In essence, the ECM electronics were installed in the ammo compartment.Delete
Another wonderful article, I'm a big fan of your blog and have always been interested in EW subjects. Any info on when the A-4 (and Navy F-4's for that matter) was fitted with Chaff / Flare dispensers?ReplyDelete
I was the project shoehorn test project officer. I was a LT NFO ECM person and assigned to Weapon Systems Test ECM group. The project was to first install the ALQ-51 in an A-4 Skyhawk. As the 51 was rather large and there was no room in the A-4, the 20mm amo container was removed a d the 51 installed in its place. That's where the name shoehorn came from, the 51 was shoehorned into the aircraft A team of engineers from Douglas Aircraft and Sander Associates, the jammer manufacturer along with the WST metal shop designed the attachment parts and had the jammer and antennas and flexible waveguide and control box installed in about a week. The test team consisted of a pilot, ground crewman, civil engineer and me. We flew to a test facility that had a simulated SA-2 Fansong radar and in less than 3 weeks I was satisfied that the jammer installation worked. As the test project officer I had to send the message to NavAir that the installation worked. And within 3 months A-4 aircraft were flying from carriers to the Repair facility in the Philliteans then back to carriers and war Soon after the testing was finished, an F-4 and an F-8 had 51s instaiied.and flight testing startred.. I was used as a consultant and worked on testing passive ecm receiversReplyDelete