ashtech DG16 Reference Manual page 373

Y-Code
The designation for the end result of P-
Code during Anti-Spoofing (AS)
activation by DoD.
Y-code tracking, civilian
Signal squaring (now obsolete)
multiplies the signal by itself, thus
deleting the carrier's code information
and making distance measurement
(ranging) impossible. Carrier phase
measurements can still be
accomplished, although doubling the
carrier frequency halves the
wavelength.
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