You can use your smartphone as a heading sensor by connecting your smartphone and fish finder with a cable.
You can get a heading sensor for 2000 yen, about 1500 yen for the cable and 500 yen for the app.
Instructions
https://mobileheadingline.hatenablog.com/
! ! important point! !
It will not work if the android device does not have a magnetic sensor. Please install a compass app as a trial and check.
Terminals that do not have a USB host function will not work. Most of the recent terminals have it, but please check it.
The equipment you use will not work unless the FTDI chip is installed. The linked product has been confirmed to work. This is an Amazon Associate link. https://amzn.to/3LaQeI6