SL869T3-I Product User Guide
1VV0301546 Rev. 3
Page 38 of 68
2021-03-31
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Multipath interference (caused by signal reflection)
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GNSS antenna characteristics
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Signal path after the GNSS antenna
The satellite transmit power is specified in reference documentation for each
constellation, which is readily available online.
The GNSS signal is relatively immune to attenuation from rainfall.
However, the GNSS signal is heavily influenced by attenuation due to foliage (such as
forest canopies, etc.) as well as outright blockage caused by buildings, terrain, or other
items near the line of sight to the specific GNSS satellite. This variable attenuation is
highly dependent upon satellite location. If enough satellites are blocked, say at a lower
elevation, or all in one general direction, the geometry of the remaining satellites will
result is a lower position accuracy. The receiver reports this geometry effect in the form
of PDOP, HDOP and VDOP numbers.
For example, in a vehicular application, the GNSS antenna may be placed on the
dashboard or rear package tray of an automobile. The metal roof of the vehicle will cause
significant blockage, plus any thermal coating applied to the vehicle glass can attenuate
the GNSS signal by as much as 15 dB. Again, both of these factors will affect the
performance of the receiver.
Multipath interference is a phenomenon where the signal from a particular satellite is
reflected and is received by the GNSS antenna in addition to or in place of the line of sight
signal. The reflected signal has a path length that is longer than the line of sight path and
can either attenuate the original signal, or, if received in place of the original signal, can
add error in determining a solution because the distance to the particular satellite is
actually shorter than measured. It is this phenomenon that makes GNSS navigation in
urban canyons (narrow roads surrounded by high rise buildings) so challenging. In
general, the reflection of a GNSS signal causes the polarization to reverse. The
implications of this are covered in the next section.
9.2.
GNSS Antenna Polarization
The GNSS broadcast signals are Right Hand Circularly Polarized (RHCP).
An RHCP antenna will have 3 dB gain compared to a linearly polarized antenna (assuming
the same antenna gain specified in dBic and dBi respectively).
An RHCP antenna is better at rejecting multipath interference than a linearly polarized
antenna because the reflected signal changes polarization to LHCP. This signal would be
rejected by the RHCP antenna, typically by 20 dB or greater.
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