I participated in a DERA trial last year of a HUD system used on the ground that utilised Mode S transmissions to ensure aircraft seperation in LVP's/Cross Runway Ops etc. It also had features that helped rapid runway exit and taxiway nav/gat finding. It too would require transponders left on until on stand etc. Worked like a dream.
The only issue I see is how to persuade beancounters as to the benefits of paying for the extra functionality.
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