Did PLAN retrofit the VDS to the first block? That sounds like a perfectly reasonable thing to do for me.
p.s. Can you explain why the Sino-SURTASS towed sonar isn't being used on the frontline vessels? I'm pretty curious about that one.
I don't think the VDS was retrofitted to the first block, or at least not yet. What I mean by first batch should be the first 16 ships, first four ships being the first block, no. 5 to no. 16 being the second block.
First four ships are identified by unbroken bezel from the point of the bow to the root of the superstructure with a flared bow. Has Type 730 CIWS and used SMT Pielstik engines.
No. 5 to No. 16 now has the bezel and the bow restored to the original 054 style and used domestic engines.
These ships have the Type 206 towed array sonar.
Starting from the 17th ship, the Type 311 VDS or Variable Depth Sonar is added to the ship. Unlike other TAS/VDS systems where the towed array and variable depth sonar array are placed on the same single line, the Type 206 and Type 311 has separate lines and winches on their own. The Type 206 TAS uses a small rectangular port, while the Type 311 VDS has a much larger port door that opens. As the 17th ship and above also has the Type 1130 CIWS, so a 054A that you can spot with a Type 1130 CIWS should have the VDS also.
The SURTASS sonar, which I think is called the Type 208 has a much larger reel and winch which can't be incorporated on an existing warship but on a dedicated ship on its own. The array might be bigger, too big that it requires a dedicated ship. Large array sizes and much longer line lengths, which allows the sonar to be deployed much farther from the ship, less noise from the ship, are there for long range, low frequency detection, while the smaller VDS, like you see on the Type 054A, is for medium frequency, medium range. The range of detection should be way greater on the SURTASS vessel. Both Type 206 TAS and 311 VDS are standard fit on the 052D and 056A, while the 052C has the 206 TAS.
The better analogy would be to compare the radar of a fighter vs. the radar for an AEW aircraft.
The bigger the buoy like thing the VDS drops, the lower the frequency and the longer the range.
The ships have a third line which is a towed sonar decoy for antitorpedo. This also takes the form of a small rectangular port. A ship that has two small rectangular ports on the transom would have both TD and TAS, but if there is also a door opening, that would be the VDS. So it goes like:
052B - Towed Decoy only. Lets see what the refit will do.
052C - TD, TAS
052D - TD, TAS, VDS
054 - TD only. Let's see what the refit will add.
054A (1 to 16) - TD, TAS,
054A (after the 17th ship) - TD, TAS, VDS
056 - only bow sonar
056A - TAS, VDS, but no TD.
It appears the 053H2G has the TAS or TLAS added. The 053H3 refit may have added both TAS and VDS.
The major refit of the two Type 052 destroyers saw them having both the TAS and VDS retrofitted. This can mean while the current 1 to 16 Type 054A doesn't have the VDS, it remains an option in the future during a major midlife refit.
It is possible the recent 054 refit may have added both TAS and VDS but I want to see a good view of the rear or transom. Need a good quality open source photo of the two ships.
This sums up the spotter guide.