Identification of radio equipment and its purpose in Vietnam

The following photograph is from the first years of U.S. involvement in Vietnam. These soldiers are wearing baseball caps and their patches are in color. Later, the patches were olive drab. Obviously, it's some sort of listening equipment, but what kind? And what for?

They are wearing the 25th Infantry Division patch. Their equipment is from the 27th Infantry Regiment ("Wolfhounds"). Thoughts on possible location would be helpful.

Other images from same source: photo 1, photo 2, photo 3.

2 Answers
2

I think what you have there may be the AN/TPS-21 battlefield radar:

Further details about the unit are available in the Department of the Army Technical Manual, TM 11-487C-1: Military Standardization Handbook: United States Radar Equipment.

The functional description of the AN/TPS-21 radar is given as follows:

Radar Set AN/TPS-21 is lightweight, portable battlefield surveillance equipment that will search for and detect moving ground targets. The range of the radar set is 100 to 20,000 yds. A characteristic audio signal with a frequency variation dependent upon the speed and direction of the moving target is the 'ontarget' indication. It is capable of an automatic search or 'sector scan' function in which a terrain area, with a depth of 880 yds and a width variable from 30 to 140 degrees or a continuous 360 degrees, is scanned for evidence of moving targets. In addition, the equipment may be manually operated to follow the target and indicate its range and azimuth.

(My emphasis)

If you are interested, the technical specifications of the AN/TPS-21 radar were as follows:

Electrical Input and Output Data

Antenna

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It looks like equipment for artillery sound ranging - which uses the time difference of arrival for artillery fire to calculate either (a) the location of enemy guns; or (b) how close to a target one's own artillery has hit.

Note the semi-parabolic reflectors behind the mikes. These are parabolic only in one plane (slightly off vertical) in all photographs rather than in two planes. Thus the accurate direction of the sound being ranged is unnecessary of course, as that is to be calculated), yet collection occurs in the one plane. I expect that the angle is set to point the mikes just above a rise on the other side of the valley.

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