Is it common for ships to transmit mode S signals?

FlightAware ADS-B Flight Tracking
1

I’m currently watching what appears to be a ship with a MLAT track.

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The track corresponds to the channel through Port Phillip Heads in Victoria, Australia and ship trackers identify the vessel as the Australian Navy’s HMAS Stalwart:

https://www.navy.gov.au/hmas-stalwart-iii

I noticed a MLAT track when Stalwart entered Por Phillip a few days ago and assumed that someone had left the transponder switched on in the ship’s embarked helicopter. However, the barometric altitude is a constant -1000 ft and the callsign VKCU is the ship’s International Callsign, making me think this a signal from the ship itself.

We don’t get a lot of naval vessels passing, so I wondered if someone could tell me if this is typical of naval vessels and how the signal is used?

2

Certainly not typical as seen here on the south coast of the UK. Plenty of naval vessels appear on AIS but I have never seen a ship appear on ADSB, not even aircraft carriers, only ever their aircraft.

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3

Could it be a support helo pinging?

4

Could certainly be, the constant -1000 ft are really strange though.

5

That is strange. I wonder if a test bench was left on and forgotten?

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