[TG] DSM 300 reads false bottom in 2500 feet

[jrm]

I get a false bottom of various depths usually about 30 feet in deep water. My unit works fine while tracking the bottom up to say around 1500 feet. Once it no longer can track the bottom I get this false reading.

Is there some setting that needs adjusting?

The unit runs off a MFD C140W. V2.35

[Tom - Raymarine - Moderator]

Hello jrm,

That's not unusual for a DSM300 under those conditions. What's happening is that when there's no detectable bottom echo in range, the DSM increases its gain in order to reacquire the bottom lock. It ends up increasing the gain sufficiently that the background noise in the receiver and on the cable starts to be picked up. The false depth you're seeing comes from the fact that the DSM's TVG (time variable gain) curve, which is there to selectively suppress excess near-surface clutter. ramps up at around that 6-8m range (approx. 30ft) to the point where the noise clutter becomes detectable.

In other words, it's quite normal for a DSM to pick up a false bottom at that sort of range when in Auto gain and when there is no real bottom echo received. If you want to suppress this, any one of the following (in order of preference/effectiveness) should do so:
- manual range
- manual gain
- a higher manual TVG

Regards,
Tom