How did an obsolete Surface-to-air missile like the Soviet SA-3 manage to shoot down the most advanced US stealth aircraft like the F117 nighthawk?

I would however like to provide a more ‘technical’ answer focusing on the equipment fielded by Col Dani i.e., the S-125 ‘Neva’ Surface to Air Missile System and modifications to the same, like the Soviet SA-3

The missile like the Soviet SA-3 system utilises three primary radar systems

1. The P-15 M2 ‘Squat Eye’ C-Band Target Acquisition Radar.

2. The SNR-125 ‘Low Blow’ I/D Band Tracking,Guidance and Fire Control Radar.

3. The PRV-11 ‘SideNet’ E – Band Height Finding Radar.

The F-117 was the first operational stealth fighter and the Soviet engineers quickly turned their attention towards Very Low Observability or VLO aircraft, developing radar systems operating in the lower bands such as VHF – UHF and upto L Band.

Read more about: Why can’t a pilot simply change course or turn sharply to evade an incoming missile?

Most stealth design features are intended to scatter incoming radar waves aka illumination in a controlled fashion, evidenced by the use of edge alignment, faceting and other geometrical shaping features, supplemented by the use of absorbent materials as demonstrated in the design of the F-117 Nighthawk.

All of these techniques are intended to defeat radars operating in the geometrical optics and less frequently, resonance zones of scattering.

Read more about: India to build BrahMos 3 for Russia

The precondition for this to work is that the wavelength be much shorter than the cardinal dimensions of the shaping feature of interest i.e., the specially designed fuselage or wings.

The Soviet approach has been to invest in the further development of low band radars, especially operating in the VHF band. The physics of radar scattering depend to a large extent on the size of the radar wavelength vs the physical size of the target.

In the Raleigh scattering regime, the wavelength is similar or greater in magnitude to the physical size of the target, and the magnitude of the reflection is essentially proportional to the physical size of the target.

As the wavelength is reduced, the resonant region is entered, where the wavelength is comparable in size to key shaping features on the target, and the magnitude of the reflection fluctuates strongly with wavelength and aspect. Finally, in the optical scattering regime, target shaping can be used to precisely control the magnitude and direction of reflections.

The high effectiveness of stealth designs against decimetric and centimetric band radars reflects the reality that for most aircraft sizes, these wavelengths are a tenth to a hundredth of the size of key shaping features.

This explains the Soviet affinity towards low band radars to detect VLO aircraft.

Low band radars are not however the silver bullets to defeat VLO aircraft as they are made out to be.

Their angular accuracy is notoriously poor, and the required antenna size results in ungainly systems which are usually slow to deploy and stow, even if designed from the outset for mobility.

The size and high power emissions of these radars, coupled with limited mobility, makes them much easier to detect and destroy than typical mobile systems operating in the decimetric and centimetric bands, which can relocate rapidly after a missile shot.

Despite these drawbacks, the older Russian low band radars still provide a valuable early warning capability, and enable cueing of other sensors and platforms, which is of most importance here.

Read more about: How is it possible that Pakistan authorities didn’t know of the surgical strike until India made an official statement? part – II

Which is exactly what happened that night. With the NATO communications decrypted successfully and missile batteries placed at strategic points the Yugoslavs were already a step ahead.

The Squat Eye low band tracking radars were modified to large extent by use of solid state electronics effectively digitising their operation.

Moreover speculation has it that modern Digital Moving Target Indicator (DMTI) or digital pulse Doppler techniques were employed, possibly using Space Time Adaptive Processing (STAP), exploiting the high performance of COTS computing technology, readily available in the open market and easy to ruggedised for a semi-mobile application of this kind.

That night when the F-117 opened its bomb bay doors for munitions release, the slight change/increase in the RCS was picked up by the Squat Eye tracking radar at a distance of 50–60 Kms.

Now as discussed earlier the angular accuracy wasn’t that great for these low band tracking radars, but it was enough to cue and activate the accompanying fire control/engagement radar.

Enter the SNR-125 LowBlow Engagement Radar

SNR-125 uses a pair of fixed scanned trough antennas to generate flapping fan shaped beams, but the design is inherently Scan ON Receive Only or SORO with a separate transmit antenna mounted between the characteristic chevron arrangement of trough antennas. Optical adjunct tracking using the 9Sh33A Karat 2 television telescope has been installed on later variants probably used by missile battery in question here. The antenna at the top of the turret is used for the low power missile FMCW uplink channels.

The Low Blow is designed to acquire targets using only bearing and range inputs from an external 2D acquisition radar in this case the Squat Eye.

When acquiring a target, the Low Blow radar head is rotated to the target bearing and the UV-10 antenna scanning feed engaged to produce a 1° wide pencil beam swept in elevation.

Once the target is acquired the Low Blow is switched into tracking mode, using the UV-10 antenna to transmit, the UV-10 to receive for ranging, and the scanning UV-11 chevron receive antennas for angle tracking.

The radar head is mechanically steered in azimuth and elevation to maintain track.

The Low Blow provides manual tracking, automatic tracking and television angle tracking modes. The system provides five missile guidance control laws, TT (CLOS), PS, MV (LoAlt), K (surface target attack) and DKM (ballistic). Three missile uplink signals are employed, K1 and K2 for pitch/yaw steering, and K3 for fuse control.

The Russian doctrine in the presence of heavy jamming and anti radiation missiles was often to cease emitting and use the scanning receiver to effect angle tracking of the jammer, acquire the target with the TV telescope, and perform a range unknown missile shot against the target/jammer in CLOS mode.

Which is what happened here as soon as the target was tracked by the Squat Eye the Low Blow swung into action and several range unknown missile shots were fired.

The missile fuses were calibrated prior to account for the lack of accuracy and un known range. All of this supposedly happened in 17 secs to prevent NATO counter radiation action.

The rest as they say is history as per Wikipedia:

The F-117, callsign “Vega-31“, was being flown by Lt. Col. Dale Zelko, an Operation Desert Storm veteran. He observed the two missiles punch through the low cloud cover and head straight for his aircraft.

The first passed over him, close enough to cause buffeting, but did not detonate. The second missile detonated, causing significant damage to the aircraft and causing it to tumble, out of control.The explosion was large enough to be seen from a KC-135 Stratotanker, flying over Bosnia.