Understanding the importance of Head Up Display and North Up Display is important and so is the difference between the both. So let us first understand these both terms first.
Head Up Display
In Head-Up display, the 12 o'clock position on the PPI represents own ship's course and hence the heading shows zero on the graduated scale around the PPI. All bearings taken on this display will be relative to the heading.
The right side of the heading marker on PPI is the Starboard side of the ship and the left side of the heading marker is the Port side of the ship. If Radar is used in Headup mode while altering the course the Heading marker remains at the same position and the targets start moving on the screen, due to this the picture gets smudged.
North Up Display
In North-up display, the zero of the fixed scale of the PPI represents true North and the heading marker represents the true course of the own ship. All bearings of targets are true.
As the own ship alters course, the heading marker swings around to the new course on the PPI, but all targets remain on their true bearings.
Difference between Head-Up Display and North Up Display
|Head Up Display||North Up Display|
|Picture smudges in azimuth during alterations of course.||The picture does not smudge during alterations of course.|
|Because of smudging, accurate bearings cannot be taken during large course alterations||Bearings can be taken accurately, even during large alterations of course.|
|Plotting becomes highly inaccurate during yaw in bad weather, as ship's head keeps changing several degrees per second, resulting in severe smudging.||Plotting accuracy unaffected by yaw.|
|All bearings are relative.||All bearings are true|
|Time wasted in the conversion of relative bearings into true bearings.||No such time lost as true bearings are directly obtained.|
|Chances of the clerical error in conversion from relative to true bearings.||No such chances of clerical error as true bearings are directly obtained.|
|Radar picture is head-up whereas the chart is North Up||Both being North Up, easier comparison with the chart.|
|After large alterations, of course, observer tends to get disoriented with plotting as all targets have shifted to their new relative bearings on the radar screen.||No such disorientation as all targets remain on their true bearings on the radar screen, despite large alterations of course.|
|Automatic radar plotting aid (ARPA) cannot be used.||Automatic radar plotting aid (ARPA) can be used.|
|No indication, on PPI, of gyro course steered. Temporary alterations or wandering off course can remain forgotten/undetected unless the compass repeater or course recorder is inspected.||Heading marker indicates, at all times, the gyro course steered. Every time the observer inspects the radar screen, the course steered gets checked by him.|