A carrier lock detector for a QPSK or 4-QAM system implements, a lock detection algorithm that maps detected signals onto one of first and second areas associated with nominal states defined by (I2⊕I3)·(Q2⊕Q3) and {overscore (Q1⊕Q2)}·(I1{overscore (I2I3)}+{overscore (I1)}I2I3)+({overscore (I1⊕I2)})·(Q1{overscore (Q2Q3)}+{overscore (Q1)}Q2Q3), or alternatively, by ({overscore (I1⊕I2)}·{overscore (Q2⊕Q3)})+({overscore (Q1⊕Q2)}·{overscore (I2⊕I3)}), respectively. When detected signals map onto one of the first areas, a first signal is generated. When detected signals map onto one of the second areas, a second signal is generated. When a difference between the first and second signals exceeds a threshold, a carrier lock detection signal is generated to enable a decoder. The carrier lock detector is able to detect carrier lock at a raw BER of 1e-2 or greater at a very low signal-to-noise ratio.