How can I tell immediately when no satellites are visible (tunnel entrance)?

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We use a MAX-M8W (SPG 3.01). For testing, we use a satellite simulation device (LabSat). Tests in a shielded room have shown that when we switch off the satellite signals, valid satellite signals continue to be displayed via the NMEA interface. This behavior lasts between 6 seconds and 20 seconds. How can this behavior be turned off?

by Dani_99 asked May 16
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Define "valid satellite signals", does NAV-SAT/SVINFO indicate they are used in a solution?

The receiver generates local copies of the signals to correlate with the signals that are input, the tracking engine is going to try to find the continuous signal even in the absence of them.
by clive1 answered May 16
NAV-SAT and NAVSVINFO behave as follows after switching off the satellite signals:

NAV-SAT     cno    Carrier to noise Ratio      
=> goes back to a value of 8-11 in about 3 seconds => FIX  keep   3D                                                                                                  => if the value drops to 0 => NO FIX

NAV-SAT     qualityInd   Signal Quality indicator
=> falls immediately to 4 and worse => FIX  keep   3D                                                                                                  
=> only when the values fall to 1 => NO FIX

NAV-SAT health    Signal health flag
=> always stays at 1

NAV-SVINFO   cno    Carrier to noise Ratio      
=> behavior as with NAV-SAT

NAV-SVINFO   svUsed   SV is used für Navigation
=> stays until NO FIX

NAV-SVINFO     qualityInd   Signal Quality indicator
=> behavior as with NAV-SAT

Additionally, I have still observed RMX (Receiver Menager) data.

SFRBX  Subframe Data NG
=> After turning off the satellite signals, MSG stops changing.

MEASX  Measurement Data
=> Values continue to be displayed. If no values appear anymore => NO FIX

Where can you now see immediately and reliably that no more satellites are available?
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