I’m using Blue Robotics Fathom Tether cables for Ethernet communication with my underwater vehicle. I originally used a 30 m tether, and everything worked perfectly. Later, I switched to a 90 m Fathom tether, which also worked reliably at first.
Recently, I’ve started seeing intermittent packet loss on both cables:
Sometimes all packets go through
Sometimes only half
Sometimes the packet loss increases suddenly (e.g., from 4 packets → 2 packets → 3 packets → 2 packets)
The link never fully disconnects, but packets are being dropped inconsistently.
Has anyone experienced intermittent Ethernet packet loss over long lengths of Fathom Tether cable (30–90 m)? Could this be due to cable attenuation, aging, or connector degradation, even if the cable initially worked well? What troubleshooting steps or solutions would you recommend?
Given it’s a notable change from previous behaviour, I’d expect one of:
You have changed what you’re sending through it, and it is now above its capacity, so any given packet has a chance of being dropped in favour of other ones
There are one or more breaks in the tether wires, which are being held in place by their insulation+sheathing, so they cause intermittent dropouts depending on how the tether is bent/twisted/pulled/moved
There is corrosion on one or more of your connector pins, and/or on one of your wires (e.g. if the cable sheath has been cut/nicked and allowed water ingress to the cable)
Assuming you’re using Fathom-X* boards on either end, and BlueOS in your vehicle, you could try installing the Tether Diagnostics Extension and seeing what kind of connectivity you have?
*Note that the HomePlug signal protocol used by Fathom-X boards has a side-effect of travelling surprisingly well along nearby conductors via parasitic capacitance, so with a 4-pair tether even if your primary pair is completely severed in the middle, or you’ve plugged each end into a different pair, the signal may still through (albeit generally with reduced quality).
Other than that your main options are network speed tests, ping checks, and resistance measurements of the conductors.
Thank you for the detailed explanation — this helps a lot.
To add more context, yes, we have increased the amount of data being sent recently, so it’s possible that the higher data load is now pushing the cable closer to its limits.
Also, just to clarify, we are not using a Fathom-X board. The tether is connected directly from the Raspberry Pi to a networking switch, and from the switch to the GCS (laptop). So the path is:
Hi @Harshada -
I’m surprised you get a connection at all over 90m of Fathom tether via Ethernet! Because it uses stranded wire for flexibility, it does not meet the specifications for standard CAT5 cable- I’ve never seen a connection longer than about 60-70m function.
Shortening your tether, using a different type that uses solid wires, , or installing a tether adapter like the fathom-x should resolve your communications issue!