The GGA and GSA strings transmitted over NMEA give a different number of satellites used to calculate the position. Why?

Question

satellite navigation

$GNGGA,155000,4700,N,03201.46451,E,1, 12 ,0.78,9.5,M,27.7,M,,*4E

- 12 satellites

$GNGSA,A,3, 21,18,20,16,10,15,27,08,32 ,,,,1.27,0.78,1.01*11

But the GSA lists only 9 satellite numbers - Why?

PS Total observed satellites 27 pieces

UPD: "Full" output:

$GNGGA,155000,4700,N,03201.46451,E,1, 12 ,0.78,9.5,M,27.7,M,,*4E

$GNGSA,A,3, 21,18,20,16,10,15,27 .08.32 .
_ _ _

_ ,1,16, 08 ,46,304,11, 10 ,
83,114,10 , 11,18,295 ,, 14,05,169, * 70 .19.088.14 , 20.50.060.09 * 79 $
GPGSV.4.3.16 , 21.37.090.25 , 26.03.198.13 _
$GPGSV,4,4,16, 36,36,181,, 40,32,150,, 41,17,121,, 49,30,215,*7E

$GLGSV,3,1,11, 68 ,20,031,10, 69 ,66,074,08 , 70,38,186,, 75,10,274,*6D
$GLGSV,3,2,11, 76 ,15,322,14, 77,02,009,, 83 ,06,117,08, 84,59,090,*67
$GLGSV,3,3 ,11, 85 ,50,323,13, 86,00,305, ,,,,24*63

Answer 1

The NMEA version 4.10 protocol does not provide for more than 12 satellites for GxGGA - this was done for backward compatibility with protocols of older versions (when no one could have thought that it would be possible to catch so many), so even if there are more than 12 active satellites, the GGA will still show 12 PCS.

Answer 2

The GxSA message indicates the number of satellites in a particular constellation. Those. there will be one message that GPS is 9, and another GLONASS is 3. Although, judging by the documentation, not all versions indicate which data.

NMEA defined GNSS System ID
NMEA v4.1 and above only