Vietnam era F-4s Q

Guy Alcala wrote:

Buzzer wrote:

On Fri, 08 Aug 2003 08:35:18 GMT, Guy Alcala
wrote:

The AS light on the APR-36/37 was the simple version of the ALR-31 on
the weasel which I think had been around for a couple years.

You're right and I was misremembering. It seems that APR-37 was essentially
the APR-26 with that modification, but that APR-26 itself wasn't improved.
Here's what I've got, from Jenkins' book on the Thud:

"The Air Force also conducted a quick look evaluation of a potential APR-26
replacement in April [1966].

Not to disparage Jenkins, who's done great research on the F-105, but
"potential replacement in April (1966)" doesn't track well with my
experience. I arrived at Korat in May of '66 and at that time the
APR-25/26 was just being initially installed in the operational jets.
We had maybe a dozen airplanes out of 40 or so with the "vector" gear.
Installation of the entire fleet wasn't completed until mid-June. I
hadn't even seen or been briefed on the RHAW gear while in training at
Nellis through April of '66. To be seeking replacement before initial
installation doesn't make any sense.


An HRB-Singer 934-1B missile warning receiver
was installed in 62-4416 and test flown at the Sanders facility, which had a
Fan Song missile guidance simulator not available at Eglin. The 934-1B
differed from the APR-26 in that it analyzed the modulation characteristics of
the C-band [i.e. radar L-band] guidance signal to differentiate between SA-2
missile activity and missile launch modes, while the APR-26 simply looked for
an abrupt amplitude increase. The HRB-Singer set performed well, but the Air
Force was already committed to a large APR-26 procurement and saw no
compelling reason to buy another system to perform the same function. Only
after the Wild Weasel III F-105s were in combat was it learned that the
APR-26's design was based on possibly faulty intelligence regarding the
amplitude increase. This led to numerous incidents of flase lower
threat-level 'activity' indications when 'missile launch' should have been
displayed. The APR-26 was later modified to analyze the guidance signal and
the improved sets redesignated APR-37."

I'm not a "squeaks and beeps" EW, but here's what I was taught about
the sequence for the SA-2. The initial TDU (Threat Display Unit) light
were for "Lo" indicating a low PRF (pulse recurrence frequency), as
you got lit up with both beams of the Fan Song (Az & El), you got a
"Hi" for high PRF. When missile data upload was taking place, another
frequency was employed (that's where an EW could tell you more) you
got an "Activity" light and when command guidance signals were
received, indicating control signals to the missile airborne, you got
the "Launch" light.

This was separate from the QRC-317 SEE-SAMS/QRC-317A ALR-31, which was
eventually incorporated into the APR-25 ('SPOT SAM') and turned it into the
APR-36 (the 'centered in both beams' A/S light). Jenkins, further on his his
section on the Weasels, also seems to mention the same mod you call the
'Bowman', although not by name. The description certainly fits:

"A separate modification provided the capability to correlate a C-band missile
guidance signal received by the APR-26 to a specific E-F band signal displayed
on the APR-25 azimuth indicator."

While the "correlate a C-band missile guidance signal to a specific
E-F band signal" tracks with what I said above regarding "launch"
lites, it doesn't equate with what the definition of the AS light was.
The AS (azimuth sector, but colloquially the "aw ****" light) meant
you were illuminated by both the horizontal and elevation beams of the
Fan Song at high PRF. It literally meant that you were the designated
target for that particular missile system. It did NOT relate to a
missile actually being launched.

BTW, how was this displayed by the strobe? I've seen references elsewhere to
dashed versus solid lines or something similar, but nothing authoritative.

Yes, different frequency bands displayed different strobes. A Fire Can
was a solid strobe, a Fan Song a three dash line, and something else
(CRS strikes here) for an AI (air intercept) radar.

In the high threat arena, the 25/26 was notorious for degenerating
into a big "spider" in the center of the scope.


Ed Rasimus
Fighter Pilot (ret)
***"When Thunder Rolled:
*** An F-105 Pilot Over N. Vietnam"
*** from Smithsonian Books
ISBN: 1588341038