Doc. No: Unex-QSG-21-003
14/30
A printed version of this document is an uncontrolled copy
© 2023 Unex Technology Corporation – Company Confidential
8.1.2. GNSS
For stable fix acquisition, at least 4 satellites with enough signal strength (C/N
0
value
above 30 dB-Hz) are required. If the signal strength goes below 30 dB-Hz, the fix will
become unstable.
8.1.2.1. GNSS Antenna Detection
The antenna detection mechanism is a 2-step process: First check the signal strength.
If the signal strength drops to zero, then check the antenna status flag.
When all the C/N
0
values becomes null, it means that the GNSS antenna or its cable
may either become detached or damaged. The GNSS antenna port has a built-in antenna
detection function. The detection mechanism is based on the current consumption of an
active antenna (I
ANT
). If the active antenna consumes less current than 12 mA, then it is
considered as the OPEN status; if the antenna consumes more than 58 mA, then it is
considered as the SHORT status. Anything between 12-58 mA is considered as the
NORMAL status.
The GNSS 3.3V antenna bias will be continuously supplied in OPEN/NORMAL status.
Once the SHORT status is triggered, the SHORT flag will persist, and the 3.3V bias will be
turned off until the reset of the GNSS module. After the reset of the GNSS module, the
detection process will start all over again.
It is possible for a good active antenna to be reported OPEN because it consumes less
current than 12 mA, or a good passive antenna to be considered SHORT because it is DC
shorted (e.g., a slot antenna). However, as long as there are more than 4 satellites with
enough signal strength (C/N
0
value above 30 dB-Hz), the GNSS is in good state.
The antenna status flag is reported in a proprietary NMEA message:
$PSTMANTENNASTATUS,<status>*<checksum><cr><lf>
where <status> can be:
• 0 - The antenna current is in the normal range (NORMAL)
• 1 - The antenna current is below the normal range (OPEN)
• 2 - The antenna current is above the normal range (SHORT)
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