In fact, while driving in the tunnel, I can see in Lefebure that the estimated uncertainty is increasing to meters (but I didn’t watched carefully values).
Indeed, while checking NMEA logs, I don’t know where it get accuracy. Sample at tunnel exit (GSV sentences removed for clarity):
$GNGLL,4412.0837477,N,00556.7948333,E,134507.00,A,E*7B
$GNGST,134507.00,0.0000,20,7.9,154,7.5,4.6,3.2*40
$GNTHS,328.02,E*16
$GNRMC,134508.00,A,4412.0974116,N,00556.7827365,E,58.187,326.75,020324,2.31,E,E,V*6C
$GNVTG,326.75,T,324.44,M,58.187,N,107.763,K,E*0E
$GNGGA,134508.00,4412.0974116,N,00556.7827365,E,6,00,99.99,472.411,M,47.505,M,,*7B
$GNGSA,A,2,,,,,,,,,,,,,99.99,99.99,99.99,1*30
$GNGSA,A,2,,,,,,,,,,,,,99.99,99.99,99.99,2*33
$GNGSA,A,2,,,,,,,,,,,,,99.99,99.99,99.99,3*32
$GNGSA,A,2,,,,,,,,,,,,,99.99,99.99,99.99,4*35
$GNGSA,A,2,,,,,,,,,,,,,99.99,99.99,99.99,5*34
...
$GNGLL,4412.0974116,N,00556.7827365,E,134508.00,A,E*7B
$GNGST,134508.00,0.0000,23,8.5,153,8.3,5.1,3.4*41
$GNTHS,326.75,E*18
$GNRMC,134509.00,A,4412.1123090,N,00556.7698725,E,58.700,324.84,020324,2.31,E,D,V*67
$GNVTG,324.84,T,322.53,M,58.700,N,108.712,K,D*03
$GNGGA,134509.00,4412.1123090,N,00556.7698725,E,2,12,0.75,472.204,M,47.505,M,1.0,0000*6C
$GNGSA,A,3,28,18,16,29,26,31,,,,,,,1.55,0.75,1.35,1*0C
$GNGSA,A,3,78,77,88,87,,,,,,,,,1.55,0.75,1.35,2*06
$GNGSA,A,3,34,36,02,27,30,,,,,,,,1.55,0.75,1.35,3*01
$GNGSA,A,3,13,24,08,26,25,,,,,,,,1.55,0.75,1.35,4*0F
$GNGSA,A,3,,,,,,,,,,,,,1.55,0.75,1.35,5*01
...
$GNGLL,4412.1123090,N,00556.7698725,E,134509.00,A,D*75
$GNGST,134509.00,89,8.1,5.6,162,3.2,2.4,2.1*79
$GNTHS,324.84,A*10
$GNRMC,134510.00,A,4412.1255542,N,00556.7565310,E,58.653,322.32,020324,2.31,E,F,V*6A
$GNVTG,322.32,T,320.01,M,58.653,N,108.626,K,D*0C
$GNGGA,134510.00,4412.1255542,N,00556.7565310,E,5,12,0.67,472.365,M,47.505,M,1.0,0000*6D