Car GPS systems have to have a dead reckoning system for when you are shaded from the signals from the satellite like when you are shaded by a tall building, in a deep valley, in a tunnel or in a forest with overhanging trees in full leaf so the use the built in compass from the GPS and a pickup from the speedo head for the speed input - the above issues do not affect marine craft so they can manage without a speed input but then you get a display of course and speed made good between fixes (in GPS this is tiny but in the old Transit sat-navs the time between fixes could be hours so having the heading and speed through the water is important to give you a calculation of the set and rate of the current - especially when you are driving something like my ship with 12.5m draft or where knowing accurately where you are is important - so you can avoid being pushed onto shoals and going aground in locations where there may be no other means of position fixing (also we NEVER use GPS solely for position fixing we use the sextant or visual bearing and radar ranges as well to triangulate our position. So we also have other speed indicating devices - usually a log, often a Doppler system or an electromagnetic one and a heading input from the gyro compass system onboard. I appreciate that on the road there are not outside influences like tides and currents or windage but you still need a form of dead reckoning for when there is poor or no reception of satellite transmissions. 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

I know it's a bit but it's good to know ships masters still use traditional navigation measures as well as modern technology  Robin
2008 Range Rover Sport TDV8
Gone:
Jaguar S-Type V8 4.2 Sport
MGZTT 190 2.5

We still have to be able to use them in the event of total electrical power failure as happened in November to the ship I'm Master of at the moment and why we have been sat in Yalova in Turkey at a repair yard since 5th December. I will send you a PM about it.................  2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW