re: My new chartplotter manual says it needs a Heading Sensor in order to overlay the Radar on the chart.
You may not need to install a dedicated heading sensor; but may be able to take Heading data from your autopilot, if it is available as an NMEA output to be placed on your NMEA-183 or -2000 bus.
Further info: our original 400-series Raymarine autopilot was upgraded to the 'G' specification by the addition of a rate gyro input, specifically a Raymarine "Gyroplus2 SHS" (Smart Heading Sensor). As I understand it this was necessary because the original fluxgate compass (the black puck) did not by itself provide the quality of heading data needed by the new chartplotter+radar combo, for display in the "heading up" mode. The heading hold and performance of the autopilot is also improved by the additional of the rate gyro input.
The A/P needs to be powered up for the "Heading Up" mode to be available; when the A/P is off, then only "North Up" display is available.
Hope this is helpful.
SV Perigee, SM#396
On anchor, Sint Maarten