HDS speed and time settings??

[clarkend]

On my old Lowrance I could get my speed to show in hours:minutes:seconds. On my HDS7 I can not get seconds....is there anyway I can change this?

On my old Lowrance it showed my speed to the hundredth of a MPH (EX 1.45mph). On my HDS7 I can only show tenths (1.4).

Any way to change this??

Thanks!
Clarkie

[jhebert]

If you are using the GPS receiver to provide speed over ground (SOG) the accuracy of the GPS receiver speed measurement is only about plus-or-minus 0.5-MPH. It makes no sense to display a speed reading to a resolution of one-onehundrethd of a mile per hour when the real accuracy is only about half a mile per hour.

A typical marine GPS receiver cannot measure speed at all. It just measures the time interval between two position fixes and deduces the speed of travel between them by assuming a straight course line was followed. Since the position fix is not precise, the speed deduced by this track point interval measurement is correspondingly imprecise.

To overcome this inherent limitation in speed measurement using the track point interval measurement, there are a few specialized GPS receivers made which can measure speed by observing the Doppler shift of the L1 carrier of the GPS signal, but, unfortunately, these receivers are rare and are not found in the general run of the mill marine chart plotter.

On the other hand, time is known to great precision by a GPS receiver. Unfortunately the specification for the NMEA data sent by the GPS receiver tends to limit the time to one-second resolution. The HDS seems to further limit display of the time to one minute resolution, although I believe that there are some screens, perhaps in the STEER display, that show values of time, such as TIME TO GO (to destination), with a resolution of one second. That does not make much sense for a device that otherwise limits itself to one minute of resolution in time. I don't know of any handle to adjust the time display resolution in a HDS unit.