SARNEG stands for Search and Rescue Numerical Encryption Grid. It is a prepared correspondence between numbers and letters for communications between isolated personnel and recovery forces. It allows numerical information, such as a person's position, to be reported as a sequence of letters over an unsecured communications channel. US Army terminology also uses the expansion Search and Rescue Numeric Encryption Grid.
How the number-to-letter substitution works
The basis is a designated word containing ten different letters. Each letter corresponds to exactly one digit from 0 to 9. The sender substitutes the appropriate letters for the digits in the report; the recipient uses the same mapping to turn them back into numbers. Having no repeated letters in the word makes the reverse substitution unambiguous.
Geographic coordinates or a Military Grid Reference System reference describe the location; SARNEG changes how their numerical content is represented for transmission. The coordinate format and the elements of the position report to be sent are prepared jointly. The agreed format gives the recovered sequence of digits its geographical meaning.
Preparing both ends of the report
Isolated personnel and receiving forces need the same valid mapping and compatible reporting procedures. In the US Army PR system, the Personnel Recovery Coordination Cell distributes this information through PR coordinating messages. It includes SARNEG procedures, the coordinate format and other communications arrangements. Changes must reach both sides so that they continue to interpret a numerical report in the same way.
A different mapping can produce different digits at the receiving end even when the letters have been heard correctly. A wrong coordinate format can, in turn, place a correctly decoded string of digits at the wrong location.
SARNEG, SARDOT and authentication
A SARDOT is a predesignated geographical reference point. Direction and distance describe the isolated person's location relative to that point. SARNEG serves a different function: it assigns letters to digits. With SARDOT, both sides need the same point and the same convention for direction and distance; with SARNEG, they need the same numerical mapping.
Recovery forces also verify the identity of the person they have contacted. Separate authentication information and procedures are prepared for that purpose, including information held in an ISOPREP. Decoding a SARNEG report supplies the transmitted numbers; authentication establishes who is behind the contact.
What concealing the numbers depends on
Concealment depends on the shared mapping being protected from outsiders. Anyone who knows that mapping can convert the letters directly back into numbers. While the mapping remains unchanged, the same digit is always represented by the same letter, so repetitions remain visible in the transmitted pattern.