Where on Earth are you?

The most common means of answering the question: where am I? is GPS. You will need at least one GPS. It is possible to set up a backup GPS unit.

All chartplotters accept GPS input, but some can supplement it with other automatic aids such as LORAN (P-Sea WindPlot II. MaxSea (all))

The chartplotter gets its primary information from GPS. Marine GPS use an interconnection interface called NMEA 0183. A GPS is a "talker" under this standard, while a chartplotter is usually a "listener".  Some charters can be "talkers" to autopilots as well as listeners to GPS, compasses, LORAN, etc. Be sure that you have enough GPS "talker" outputs for all your vessel's needs. This doesn't necessarily mean buying more GPS if, for example, you have two GPS connected to two VHF radios.  There are several hardware and software solutions to distributing GPS output to several devices, which may be cheaper and more flexible than buying more GPS units. Let's discuss your needs.

Mostly for education, but sometimes for voyage planning, several plotters offer dead reckoning, celestial navigation, and manual plotting. The latter might use radar bearings.