Topic: 798 si internal antenna going bonkers

[larry e frazier]

Have a buddy with a 798 si . It has started doing some crazy stuff. He has it mounted on the front on the trolling motor. When he is running on plane his gps works fine . But when he stops to fish  and hes coming on to a waypoint or a map point the waypoint will revert a 180 dgr when the boat position has not changed. Could this be interference if so why does it not affect sonar? Thanks for reading if you have any ideas please help him out. Thanks again Larry

[lolar3288]

I'm not quit sure I understand the question but in general, if a GPS unit is on a very slow moving object sometimes it has trouble determining direction.  GPS has no direct direction sensing.  It compares current position with the previous reading.  If the two readings are within the error (estimated positional error or EPS) usually around 2 - 5 metres then the reading is undetermined and will read as not moving in most units.

Someone who better knows the software Humminbird uses may be able to give a more accurate answer!

[Roddy]

Lolar, You got it right on the head. Estimated position error can large and can read 180* out at times at low speed when at or passing a waypoint. Roddy

[lolar3288]

Has a suggestion to Humminbird, this could be easily addressed by relating the update theorem to the physical speed hence slowing down the positional updates at lower speeds.

[ITGEEK]

You need to first keep the trolling motor off and look at the waypoint.
If it acts screwy with the trolling motor on, then it's interference.

You said the sonar isn't affected, but I'd try the same
thing, especially looking at the depth reading.
If the depth is stable with trolling motor off and jumps around
with it on, then it's interference.