SE868-V3 Product User Guide
1VV0301205 r4 – 2016-01-20
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 52 of 76
Mod. 0805 2015-02 Rev.4
The satellite transmit power is specified in each constellation’s reference documentation,
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 tree
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.
11.2.
GNSS Antenna Polarization
The GPS broadcast signal is 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.
If the multipath signal is attenuating the line of sight signal, then the RHCP antenna would
show a higher signal level than a linearly polarized antenna because the interfering signal is
rejected.
However, in the case where the multipath signal is replacing the line of sight signal, such as in
an urban canyon environment, then the number of satellites in view could drop below the
minimum needed to determine a 3D position. This is a case where a bad signal may be better
than no signal. The system designer needs to understand trade-offs in their application to
determine the better choice.
