GNSS- Setting Threshold for Acceptable Accuracy

See the Quectel extensions (SDK) commands, the PQEPE can output the estimated position error (meters).  Applies to Quectel L70, L80, L76, L76-L, L86 and L26 modules.
Also generally the HDOP (in the standard $GPGSA, $GPGGA and other messages) can also provide an estimate of the position error probability (accuracy).

The PQEPE is already an estimation of the precision in meters, we do not know the probability of it. It’s probably 95%. If you use PQEPE in combination with DOP, you’ll get a good impression of your positioning accuracy.

$PQEPE,5.3050,3.2000*53 //Output Horizontal and Vertical (The HACC value is .3050). HACC is in m
Horizontal:  5.3050 means 5 meter, 30 cm, 5 mm 

You could filter the PQEPE results in your host. For example, you could set the horizontal  limit to  10 m.

The DOP is a relative value, the EPE is in meters. The DOP values can be read from the NMEA GSA string. Example:
$GPGSA,<Mode>,<Fix status>,<PRN1>,<PRN2>,…<PRN12>,<PDOP>,<HDOP>,<VDOP>,<GNSS sys id>

DOP ValueRatingDescription
<1IdealHighest possible confidence level, the highest possible precision at all times.
1-2ExcellentPositional measurements are considered accurate enough to meet all but the most sensitive applications.
2-5GoodRepresents a level that marks the minimum appropriate for making business decisions. Positional measurements could be used to make reliable in-route navigation suggestions to the user.
5-10ModeratePositional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended.
10-20FairRepresents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location.
>20PoorAt this level, measurements are inaccurate by as much as 300 meters with a 6-meter accurate device (50 DOP × 6 meters) and should be discarded.

Read about DOP:

SNR – signal to noise ratio (dB)
For a good GNSS, the SNR should be above 35 dBc/Hz.
It depends on :
o The satellite view (open sky, obstructions, reflections all have an influence)
o The Antenna Quality
o The Receiver path
o The power source quality (using a Low Noise LDO close to the GNSS is best)
o Noise level from the PCB (surrounding components which radiate will raise the Noise Floor)

General Factors Affecting Performance, Position Accuracy
It depends on the receiver sensitivity, supported constellations, single of multi frequency receiver, input circuit buildup.
o Autonomous/generic Cold Start: the GNSS receiver has the capability to age the ephemerides by itself in order to reuse this information during the next receiver activation, without the need for a complete re-acquisition (e.g. 30 seconds min. for GPS)
o Offline/EASY: the GNSS receiver has the capability to store pre-calculated ephemerides, previously received thru the satellite reception, in order to use this information during each receiver activation, without the need for a complete re-acquisition.
o Online/EPO: the GNSS receiver has the capability to acquire pre-calculated ephemerides, simultaneously received thru a cellular network, in order to use this information during each receiver activation, without the need for a complete re-acquisition.

Example, for L76 series, the position accuracy is 1.5 m at its best.