Standard refractive modulus gradient
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4-1-905
Alternative term: Standard M gradient
That uniform variation of refractive modulus with height above the earth's surface which is regarded as a standard for comparison. The gradient considered as normal has a value of 0.12 M units per metre (3.6 M units per hundred feet).
Reference: C.C.I.R.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)