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12 Common evaluation pitfalls
A parameter may have the same name but a different definition. GNSS receivers may have a
similar size, price and power consumption, but can still have different functionalities (e.g. no
support for passive antennas, different temperature ranges). Also, the definitions of hot, warm,
and cold start times may differ between suppliers.
Verify design-critical parameters; do not base a decision on unconfirmed numbers from data
sheets.
Try to use identical or at least similar settings when comparing the GNSS performance of different
receivers.
Data which has not been recorded at the same time and the same place should not be compared.
The satellite constellation, the number of visible satellites and the sky view might have been
different.
Do not compare momentary measurements. GNSS is a non-deterministic system. The satellite
constellation changes constantly. Atmospheric effects (i.e. dawn and dusk) have an impact on
signal travel time. The position of the GNSS receiver is typically not the same between two tests.
Comparative tests should therefore be conducted in parallel by using one antenna and a signal
splitter; statistical tests shall be run for 24 hours.
Monitor the Carrier-To-Noise-Ratio. The average C/No ratio of the high elevation satellites should
be between 40 dBHz and about 50 dBHz. A low C/No ratio will result in a prolonged TTFF and more
position drift.
When comparing receivers side by side, make sure that all receivers have the same signal levels.
The best way to achieve this is by using a signal splitter. Comparing results measured with
different antenna types (with different sensitivities) may lead to incorrect conclusions.
Try to feed the same signal to all receivers in parallel (i.e. through a splitter); the receivers will not
otherwise have the same sky view. Even small differences can have an impact on accuracy. One
additional satellite can to a lower DOP and less position drift.
When doing reacquisition tests, cover the antenna in order to block the sky view. Do not unplug
the antenna since the u-blox M8 positioning technology continuously performs noise calibration
on idle channels.
UBX-20028627 - R01
C1-Public
Common evaluation pitfalls
EVK-M8L User Guide
Page 31 of 33
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