Ed Rasimus wrote:

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.

As noted below, this was the WWIII fit, and they were just starting procurement.
The question was whether they'd confirm
procurement of the APR-26 or go with the HRB-Singer set. Almost no sets of either
type had yet been fitted to trials a/c, and only a few of the APR-25/-26/IR-133 to
the F-100F WWs.

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.

H'mm that seems a bit off. Normally, fire control sets search at a lower PRF, then
track at a higher one. FWIW, the first available site I could find credits Fan
Song C/E with the following PRFs:

PRF 828-1440 Search. 1656-2880 Trk.

Fan Song B and F would show similar differences in PRF, although the specific
numbers would probably be different. You need the lower PRF for search/acquisition
to eliminate second time around range ambiguity, which also allows you to use
longer (hence more powerful) pulses. But you lose range resolution, so once
detected the radar will normally switch to a higher PRF for tracking (same with the
F-4, btw).

So, Low PRF would indicate general search mode, High PRF would indicate tracking
_somebody_ (at shorter range). That at least would be the case with the APR-25.
While you'd undoubtedly BE in both beams while the radar was tracking you or
someone close to the same LoS (as Marshall mentions in his LB II book, tracking
usually had to done manually after pods arrived), the PRF lights wouldn't be
indicating position in the beam per se, but just the radar PRF, a far simpler
procedure. Location in the beam sweep was a later addition -- That was what the
ALR-31 (and the same or similar circuit in the APR-36, attached to the A/S light)
would do. From the Air & Space article Bob referenced:

"Klimec set out to improve on the existing RHAW system, which only told you that a
SAM was looking, or launching, and gave only a general bearing to the radar source.
At this early stage in anti-radar development, before specially designed missiles
that home in on radar signals were available, the target still had to be visually
acquired and attacked with conventional weapons like rockets, guns, or bombs.

"The Fan Song was one of the first electronic scanning radars--it directed its
energy without having to move its antenna. "The way the Soviets built the Fan Song
was to have [one] radar that tracks both the aircraft and the missile," Klimec
says. "It would scan across 20 degrees and then go off the air, because you had to
shut the radar down in order to preclude any kind of problems with the energy
coming back inside and blowing out equipment--and then it would fly back, come back
on again, and scan 20 degrees, and go off the air."

"The radar cycled several times per second and was directed so that a targeted
aircraft was located at the center of the scan sector, which enabled the missile to
be maneuvered freely inside, while the target was simultaneously tracked by the
radar.

"So it dawned on me that if we could detect when the radar came on, and we could
determine when the aircraft was illuminated on the radar in the main beam, and we
could detect when the radar shut down to fly back, we could calculate the position
of the plane relative to the scan sector," Klimec says. It was known that the Fan
Song took about 100 milliseconds to complete a scan, so if an aircraft was
"painted" by the radar 50 milliseconds after the radar turned on, the aircraft was
in the mid-point of the scan sector. "And the aircraft ordinarily did not get to
the center of the sector unless somebody put him there--and since the tracking scan
system could only track one aircraft to make an intercept on one aircraft, if you
found yourself in the center of the scan sector and you found you stayed there,
then you knew somebody had selected you as a target," he says.

"After design engineers devised equipment to verify Klimec's theory, he began
monitoring the Eglin Fan Song simulator's emissions from the top of a hangar. "I
talked on the phone to the radar site and got them to move it a little bit, and we
verified that we could detect when the radar came on to start the scan, we could
detect when it went off the air, and we could detect when we got the large spike of
energy as the main beam came by," Klimec says. Klimec's innovation eventually
allowed fighter crews to know whether or not they were targets and to take action
only if they were."

So, prior to the ALR-31/APR-36, you might be in one or both beams and getting High
PRF, but not be in the center of the Fan Song sweep because it was actually
targeting some other a/c close to the same angle between the radar and you, leading
to unnecessary maneuvers and high pulse rates. You'd also pick up the L-band
guidance signal, again without necessarily being the target.

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.

Right, the guidance used the C-band dish on the left of this picture (the E/F or
G-band tracking antennas are the horizontal and vertical troughs):

http://fas.org/nuke/guide/russia/airdef/fan_song2.jpg



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.

I know, you're misinterpreting what I wrote. The A/S and the above mod are two
separate things. Here's what I'm talking about (again from the Air & Space article
Bob pointed out):

"As tactics were developed in the air, field modifications to the Wild Weasel
systems continued on the ground. A key weakness of the equipment was that if
several SAM sites were displayed on the scope and the light that signaled a launch
was illuminated, there was no way to know which site had fired and from which
direction the SAM was coming. "I heard the crews complaining about that," says
Weldon Bauman, who in 1967 was a junior enlisted technician at Takhli. "And I
thought Well, if I knew more about the signal, then maybe we could do something
about it." Bauman became a Wild Weasel legend for devising a system similar to Bob
Klemic's but that sidestepped cumbersome and lengthy procurement procedures and
could be hot-wired into the aircraft in the field immediately. But to do it, he
first needed access to sensitive data about the nature of SAM site radar emissions,
and after convincing an EWO to escort him into the intelligence section, he got the
information he needed. "I sat down and got the real-time data--the same day then
was real time," Bauman says. "I found out what they were seeing and then went back
and designed a circuit and it worked." When activated, Bauman's modification
cleared the scope of all information except for a blip that indicated the launching
site. Tom Wilson, a former F-105 EWO, marveled at Bauman's ingenuity and his
modesty. "This kid had two stripes, and he was so damn smart it was unreal," Wilson
says. "When I asked him how he came up with the mod, he said, "It was real easy.
Just three little parts wired into the line for the scope, and a switch, and it was
done.' "

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.

Right, see above.


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.

Thanks. BTW, in the case of say a two-ringer growing to a three, did the strobe
extend in from the periphery, or out from the center? I've always assumed it was
the latter (looking similar to a PPI display), but I've never seen a source I trust
which says which it was.

Thanks,

Guy