I don't think Iran should be producing Zelzals anymore I think they should just stick to producing low cost and high cost version of the Fatteh capable of achieving a CEP as low as 3 meters for high cost precision strike version up to 300 meters for a low cost cluster version
I think the best way is to use a crude land based positioning system specifically made for your missiles allowing you to do post launch Trajectory corrections + various types of terminal guidance system
I think every Iranian missile base should have it's own atomic clock and multiple receivers and transmitters within 10km radius of each base
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