HE910 Hardware User Guide
1VV0300925 Rev. 32
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62
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2019-11-25
RF Front End Design
The HE910 Module contains an integrated LNA and pre-select SAW filter. This allows the
module to work well with a passive GPS antenna. If the antenna cannot be located near
the HE910, then an active antenna (that is, an antenna with a low noise amplifier built in)
can be used.
7.3.1.
RF Signal Requirements
The HE910 can achieve Cold Start acquisition with a signal level of -147 dBm at its input.
This means the GPS receiver can find the necessary satellites, download the necessary
ephemeris data and compute the location within a 5 minute period.
In the GPS signal acquisition process, downloading and decoding the data is the most
difficult task, which is why Cold Start acquisition requires a higher signal level than
navigation or tracking signal levels. For the purposes of this discussion, autonomous
operation is assumed, which makes the Cold Start acquisition level the important design
constraint. If assistance data in the form of time or ephemeris aiding is available, then
even lower signal levels can be used to compute a navigation solution.
Each GPS satellite presents its own signal to the HE910, and best performance is
obtained when the signal levels are between -125 dBm and -117 dBm. These received
signal levels are determined by :
•
GPS satellite transmit power
•
GPS satellite elevation and azimuth
•
Free space path loss
•
Extraneous path loss such as rain
•
Partial or total path blockage such as foliage or building
•
Multipath caused by signal reflection
•
GPS antenna
•
Signal path after the GPS antenna
The first three items in the list above are specified in IS-GPS-200E, readily available
multiple sources online. IS-GPS-200E specifies a signal level minimum of -130 dBm will
be presented to the receiver when using a linearly polarized antenna with 3 dBi gain.
The GPS signal is relatively immune to rainfall attenuation and does not really need to be
considered.
However, the GPS signal is heavily influenced by attenuation due to foliage such as tree
canopies, etc., as well as outright blockage caused by building, terrain or other items in
the line of sight to the specific GPS satellite. This variable attenuation is highly dependent
upon GPS satellite location. If enough satellites are blocked, say at a lower elevation, or
all in a general direction, the geometry of the remaining satellites will result is a lower
accuracy of position. The HE910 reports this geometry in the form of PDOP, HDOP and
VDOP.
For example, in a vehicular application, the GPS antenna may be placed embedded into
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 GPS signal by as much as 15 dB. Again, both of these factors will affect the
performance of the receiver.
Multipath is a phenomena where the signal from a particular satellite is reflected and is
received by the GPS antenna in addition to or in place of the original line of sight signal.
The multipath signal has a path length that is longer than the original line of sight path and
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