Analysis of the Battle of Dong Hoi

By Stuart Slade
Updated 20 June 1999

Editor’s Note for update of 20 June 1999:

Recently, I received some new information regarding the electronic equipment on the USS Sterett.  I forwarded this on to Stuart for his reaction.  Stuart was impressed by this new information and responded by updating this essay to include this new data plus other insights that have occurred to him since the essay was first posted.

Tony DiGiulian

Editor's Note for 26 February 1999:

I had asked Stuart Slade to comment on the Battle of Dong Hoi.  During that battle, the USS Sterett shot down a North Vietnamese Mig and engaged Russian-built MTBs.  The Sterett reported that she had shot down a Styx missile (Russian SS-N-2 or P-15/20) launched by one of those boats.  Stuart responded with an essay presenting an argument for the possibility that the Sterett crew was mistaken in their identification.  We then proceeded to thrash this out on one of our discussion boards, soliciting input from others with knowledge of the weapon and electronic systems involved.  The article below is an edited composite of Stuart's original article plus his responses to various issues raised by the participants during the open discussion over the following week.

Any subject jumping in this article is probably the result of my poor editing techniques for which I take full responsibility.  I've also tried to provide translations for the acronyms Stuart slings around so proficiently (I confess to having to go look up a couple of them)

For a full account of the Battle of Dong Hoi as seen by the crew of the USS Sterett, please see the following link:

USS Sterett DLG-31/CG-31 Page

Tony DiGiulian

Analysis of the Battle of Dong Hoi
by Stuart Slade
(Originally written 26 February 1999)

Edited quote from the Sterett History on their Website

***** Sterett's fire control radar operators had locked onto several of the surface tracks in CBT mode (Continuous Boat Track).  As they were observing one of the tracks, they detected vertical video separation.  The radar automatically maintained its lock onto the video that separated from the surface target.  At the same time Sterett's ECM (Electronic Counter Measures) crew in CIC (Combat Information Center)  received an electronic signature of an ASCM (Anti Ship Cruise Missile) and the fire control radar associated with a missile launch.  It was immediately identified as a Russian SS-N-2 Styx missile

This was the first time a Navy ship had been attacked with a guided missile in a combat situation.  The positive lock by the SPG-55 fire control radar allowed Sterett to fire a salvo of two Terrier missiles immediately following the Styx launch. If Sterett would have had to acquire the Styx normally with her air search radars, plot the course via several paints (radar sweeps) and then hand it over to the missile fire control radars, she could never have fired missiles in time to intercept the Styx missile.  Bridge lookouts reported seeing Sterett's Terriers enter a cloudbank and explode.  The Styx was never visually spotted.  Following the detonation of our Terrier, the missile target disappeared from radar and the ECM signature signal ceased*****

First thing to note is that, according to Russian sources, the North Vietnamese didn't have the P-15 or P-20 (in this context, the export version of P-15) missiles at this time.  The Russians are quite emphatic that they did not supply the Vietnamese with anti-ship missiles and the launch craft until 1974, two years after the Battle of Dong Hoi.  We don't know if the Chinese supplied any of their equivalents but there's no evidence that they did.  If they did, it must have been very early production versions - the Chinese Navy only started to receive domestically-built P-15s (as HY-2) in 1975.  Although the possession of P-15 by the North Vietnamese was considered "probable" by the US from 1967 onwards, some enquiries have revealed that by 1972, US Naval Intelligence had discounted these reports which were regarded as an over-hasty reaction to the sinking of the Eilat. The arrival of P-20 (SSN-2 Styx) missiles in North Vietnam wasn't unambiguously confirmed until 1975, which fits the Russian claim as to when they supplied the things

A key point is the geographical location of Dong Hoi.  This is a bay with high hills forming a semi-circular background.  Using the MI (Missile Illuminator) radar, which has a narrow pencil beam for surface search would have given them a lot of false echoes.  This would have been further confused by another feature called Anomalous Propagation, more commonly called “Anaprop.”  This occurs in warm, humid environments where a layer of particularly warm, moist air gets trapped to form a duct above the sea surface.  Although always referred to as “anomalous,” anaprop conditions occur something like 80 percent of the time in tropical waters The effect of anaprop is that radars pick up targets at far greater ranges, often well over the apparent radar horizon, than are usually expected.

Anaprop is also a major cause of another critical problem in radar picture compilation, the notorious "second time around problem."  Radars transit a pulse and measure range by recording the time taken for the pulse to reflect off the target and return.  However, at long ranges a second transmitted pulse may go out before the echo from the first is received and the outgoing pulses would be confused with returns from earlier pulses.  The following “track” in which capital letters indicate outgoing pulses and lower case type the received echoes shows the problem.


The plot extracted from this chain of signals can be found by the apparent pairing of the responses using brackets.

A [Ba] [Cb] [Dc] E [Fd] [Ge] [Hf] I [Jg] [Kh] [Ki] M [Nj]

The radar plot extractor will ignore the pulses that (apparently) do not have a response (A, E, I and M).  The effect of second time around is that distant, stationary targets returning long-delayed echoes appear on the radar set as a close-range fast-moving, target.  Under some circumstances, second time around can not only gravely distort the apparent range and speed of a track but can also cause the radar plot extractor to interpret the data as the track splitting into two contacts.

The combination of anaprop, second time around problems and large numbers of false reflectors means that the radar picture therefore is by no means as simple as it appears from the account In fact, the best description of the radar plot is that its extremely confused.  It is quite possible that all the radar contacts quoted here were ghosts - or even that the Sterett tracking herself.  Significantly, it seems as if her surface and air search radars (deliberately designed to cope with such conditions) did not pick up anything.  This stresses the importance of the EW (electronic warfare) contribution to the situation.

Looking at the EW side of the action, we lack two important bits of data - the range to the surface contacts presumed to be launch platforms and what precisely that ESM contact was.  The first is probably inevitable due to the confused radar environment The second, however, is more awkward In the context of the time, there are two items of equipment on the Sterett that were applicable; the ships WLR-1 ESM (Electronic Surveillance Measures) system, and the ASMD (Anti Ship Missile Defense) system.  It is important to remember how primitive these items of equipment were by today’s standards; computer analysis of signals and searching of threat libraries were still 15 years in the future.  For that reason, I checked out how the WLR-1 of early 1970s vintage worked (we have to be careful because WLR-1H of the early 1980s onwards is a very different animal).

The display is Cartesian, that is, each sweep of the sensor head (a spinner in a dome) records as a single line on the display.  Each line also displays intercepted frequency as a dot along that line.  Thus, after repeated sweeps, an intercepted radar shows up as a bar on the display.  All this gives is frequency.  The operator can now manually select a specific frequency, at which point the spinner stops spinning and trains on that specific source.  The screen is then switched to analysis mode and this displays such things as exact frequency, pulse repetition factor, jitter factor, variance and all the other goodies the EW boys take such delight in.   There was no computer threat library; comparison had to be done manually.  This operating mode of WLR-1 means that only a single radar can be investigated in detail at any one time although the high gain of the system once it is investigating that signal means that a lot of data can be gathered.  That reflects the background of WLR-1 as an ELINT tool .  In this old version of WLR-1, the analysis procedure took time and I don't think the EW crew had it.

This is a pretty typical set-up for its generation, although, personally, I prefer to work with a polar display.  This is a circular display, which gives the threat bearing as one ordinate and the severity of the threat as the other with the most severe threats closest to the center of the screen.  The last time I did this I had two screens set up, one showing a polar display and the other the analysis data complete with a computer ID from the library.  Once that worked and we were able to go from a prototype to a production system, we used windowing to put both displays on the same screen.

ASMD was one of the earliest US Navy reactions to the sinking of the Israeli destroyer Eilat.  ASMD was the outcome of an extensive program called SAMID (Ship Anti-Missile Integrated Defense) that was aimed at improving passive intercept capabilities, integrating those with the output from the ship’s SPG-55B radars and using the integrated output to initiate countermeasures.  In effect, ASMD was a warning system that provided an alert of a threat from one of three Soviet missiles, initiated countermeasures and distributed that warning via the NTDS network to other ships.  The three threats were T-1 (the SSN-2 Styx), T-2 (the SSC-1 Samlet) and T-3 (the SSN-3 Shaddock).  Contrary to my normal practice, I’m using NATO designations here rather than the correct Soviet ones because ASMD didn’t respond to the characteristics of Soviet missiles but to what NATO thought those characteristics were - and we now know that information was badly wrong.  A long-term development of ASMD would have included computerized threat libraries and a dedicated information distribution network.  This was cancelled as part of the post-Vietnam cut-backs and ASMD itself was never fully developed or debugged.

The key phrase in the account is "At the same time Sterett's ECM crew in CIC received an electronic signature of an ASCM (Anti Ship Cruise Missile) and the fire control radar associated with a missile launch.  It was immediately identified as a Russian SS-N-2 Styx missile." Eyewitness data received recently makes it clear that this is a reference to a T-1 indication from ASMD It does not, however, actually say the seeker head was detected.  It could simply be the Rangout Radar tracking the Sterett.  I'm also not sure how much was known about how P-15 functioned in those days; it may be that the fact that the missile flew most of its run unguided wasn't known and tracking by a Rangout was assumed to be the launch signature of the missile

Now lets look at the other side of the equation.  I checked my manual on the care and feeding of P-15 missiles.  The "fire control radar" on the launch platform was the MR-331 Rangout (NATO codename Square Tie).  Although often identified as the fire control radar for P-15 missiles, this was actually quite widely used and was installed on both Project 183 torpedo boats and to control 130 mm coastal defense guns as well as on the Project 183R Komar class missile boats.  Rangout took a series (usually three but sometimes as many as eight) range and bearing readings on the target, which the system operator plotted to give a target course and speed.  This was fed into the fire control system which calculated the range and bearing for the missile to execute the desired intercept.  The operator then set a manual range gate around the target and added that to the fire control solution.  The FCS then fed that data to the missile.

While this process was in hand, the P-15 missile was being prepared for firing by running up its gyros.  This meant that the Project 183R had to hold a steady course during this time.  Once the gyros had stabilized, the missile was then fired and set off down the required bearing.  At this point it was flying on autopilot without its guidance system 6 nm from the estimated target range, the radar guidance system switched on and the missile searched for the largest radar image within its range gate.  There was no velocity filter fitted and P-15 could not differentiate between stationary and moving targets.  Also, its radar seeker used conical scan, which made it very easy to jam.

Now, the tech manual for the P-15 is quite definite; the radar seeker does not come on until six nautical miles short of the estimated position of the target.  Thus, to have picked up the seeker head, the missile must have already been that close.  Yet the report clearly says that the missile was never sighted, the Terriers vanished into the overcast and exploded there.  This suggests the inbound "missile" was high, which is not compatible with the P-15's profile.  The maximum altitude reached by the P-15 was 300 meters and it normally cruised in at 150.  The version we're looking at had a barometric altimeter and six pre-set cruise altitudes (100, 150, 175, 200, 250, 300 meters), the idea being that the longer the range, the higher the cruise altitude would be set.  In contrast, the big P-35 Shaddock missile could climb to over 1,000 meters before setting down to cruise altitude.

The EW evidence on the Sterett is quite positive; there was a radar out there and it wasn’t friendly.  The question is, what was it? WLR-1 is perfectly capable of determining that the fire control radar in question is a Rangout radar coming on and painting the Sterett.  The problem is that radar could have been on a Project 183 torpedo boat, a Project 183R missile boat or controlling a shore based battery of 130 mm guns.  Also, remember Anaprop; that radar could have been a long way away.

So what did the EW team actually see? They probably saw a single spike come up on the screen, which was the Rangout radar (it doesn't matter what the threat was at this time; all the candidates had the same radar).  Typically Russian radars switch from a low-PRF (Pulse Repetition Factor) mode when searching (low-PRF increases unambiguous range) to high PRF (and a higher frequency) when tracking.  So the Rangout picked up the Sterett, the operator switched from "search" to "track" to get range, course and speed data.  On the WLR-1, that would be seen as a sudden shift in the position of the spike of the Cartesian display.  I bet that shift was "the launch signature of the ASCM."  ASMD would have thought so as well and thrown up the T-1 warning The CIC crew were expecting a Styx launch, saw the radar plot track divergence, the frequency jump on the WLR-1 and the ASMD warning and (quite correctly) gave the alert immediately.  Apparently, they had three separate bits of data to go on; in reality they were seeing the same information presented three different ways.

So what really was going on.  Certainly WLR-1 (which was a nice bit of equipment for its day) should have been able to pick up the difference between the missile seeker and the target tracker.  The problem is that, given how P-15 worked, I don't think that there was a seeker there to pick up.  One possible explanation is that there were two or more North Vietnamese Project 183 MTBs (Motor Torpedo Boat) out there.  These had the same radars and ESM as the Project 183R (Komar) class.  These were heading out to the US ships when their ESM detected the missile illuminator emissions.  They had no means of knowing that the MIs were being used for surface search so when the illuminators locked onto them, they would immediately assume they were under attack and that an anti-ship Terrier would be following shortly.  Terrier had a known and well-publicized anti-ship mode, which would be very dangerous to a surface target.

They therefore went to surface search on their radars in order to determine the threat axis.  My bet is, if we could read the NVN reports, they claimed they broke off the action when they came under missile attack from the Sterett.  The Project 183s would also have fired off chaff rockets and take evasive action.  The watching MI radar sees a separation (note how it locks onto the presumed missile launch - the chaff rocket working as advertised) Sterett then launches her missiles.  The Project 183s decide that this is not fun and get out of Dodge.  They turn off their radars (hence the disappearance of the ESM signature) and retreat as the chaff rockets explode.  The Terriers are looking for an airborne target, don't find one and explode as per instructions.  These are warshot Terriers; they explode when they go ballistic and cannot reacquire a target.

I don't believe there ever was a missile launch at Sterett, although the CIC crew had every reason to believe that there had been one and they acted entirely correctly.  I think we had a combination of a very confused radar environment made worse by the use of an unsuitable radar for surface search and a misinterpretation of the available data.  I don't rule out the possibility of an inbound P-15 completely, but the more I look at the situation, the less likely it seems.  I think the USN had good reason to disallow the claimed shoot-down.  This does not change the fact that the CIC crew did a fantastic job in a very confused environment.  They had an air action to fight, a crippled ship to protect and also a potentially lethal surface threat developing.  Even if they didn't kill a missile, they still deserve a salute for jobs well and skillfully done.


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