User's Manual

HE910 Hardware User Guide
1vv0300925 Rev.28 – 2015-06-24
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 52 of 52
7.2 GPS Signals Pinout
The Pads related to this function are the following:
PAD Signal I/O Function Type
R9
ANT_GPS I GPS Antenna (50 ohm) RF
R7
GPS_LNA_EN O Output enable for External LNA supply CMOS 1.8V
7.3 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.