Sony STR-DG820 Operating Instructions Manual page 76

Message Explanation
appears
XM – – The preset number entered is an
invalid number.
SIG:GOOD The signal strength is good.
SIG:MARG The signal strength is marginal.
SIG:WEAK The signal strength is weak.
SIG:NONE There is no signal.
76
US
Remedies
Enter a number from 1 to 30.
Adjust the antenna so that the signal is as strong as
possible. For details, see "Aiming the XM antenna
(aerial)" (page 69).