On the following High IFR chart from skyvector.com I notice along the meridian at 30' intervals there is a green symbol the same shape as an RNAV waypoint. Next to them are 5-digit identifiers, but it is alphanumeric rather than the usual 5-letter fix name. What are these symbols and what are they used for? Can they be entered into the FMS?
That's part of the High Altitude Redesign (HAR) program.
As far as the available information, it's now available for flights above FL390, and only in certain ARTCC's in the U.S.
The waypoint naming convention is as follows:
There are plans to have this system implemented worldwide.
The pilot will no longer be limited by airways based on ground navaids. I.e. they'll make their own airways based on those waypoints. It's called Non-Restrictive Routing (NRR).
This way they can plan to fly around the weather, yet remain close to the great circle route. The waypoints are also easy to relay and eliminates confusion when re-routing, as the identifiers are based on their location.
Comparison to the ground-based navaids is shown below: