Its a long time from my last contribution to the Forum. “Fortunately” Im being really really busy with a lot of work.
I would like to point a few things about NMEA.
Allowing this protocol for the ROV could open a wide range of applications, but not all NMEA sentences could allow for them.
My recomendation is not using any GP__ sentence. Albeit they are native in all GPS devices, the information formatting system is very limited.
I’d go for the $AISVDO sentences. Information is compressed into a coded data chain, that today is compatible with all the navigation software.
AISVDO includes all GP-- data, plus “ship” identification and activity information into a shorter message. At the same time, allows for traking systems to read those datas.
Today, all chartplotters and GIS systems read AIS data better than GP-- sentences.
Being AIS the actual standard, data can be very easily shared and monitorized through Internet. Increasing the RF signal range for allowing remote control, comes also easier, as AIS data can be trasfered via marine VHF radio in two channels (86 and 87) located around 162 MHz, with an effective range of 5 miles with a 5Watts output.(My company is actually building a VTS tower, hence the data Im providing come from real tests).
Control commands can be included into the AISVDI channel, which range is only limited by the land station particulars.
AIS is directly compatible with any autopilot that could be designed, as it includes all required data.
The only problem with AIS, is the data compression and coding , but I could help with that, as I wrote a few months ago, a data coding/compression data software that mix and compress GPS, IMU and ROV particulars, into an AISVDO chain, able to be sent via TCP/IP or marine VHF radio.
For AIS decoding, plotting and sharing, there is a lot of free software in the net.
About autopilots …
Best way is a NMEA compatible autopilot, becouse it could read remote sentences that could be sent from any place via internet or VHF included into the AIS sentence.
Checking the AP accuracy is performed by means of programmed standard search patterns that are executed by the AP alone.
Standard used maneuvers are: Full, Williamson and Scharnow turns that provide a lot of information about the evolution curves, “rudder” response and system errors.