SL871 Family Product User Guide
1VV0301170 Rev. 9
Page 54 of 89
2021-07-15
Not Subject to NDA
10.
RF FRONT-END DESIGN
The SL871 and SL871-S modules contain a preselect SAW filter in front of the RF input.
The SL871L and SL871L-S modules add an LNA in front of the (post-select) SAW filter
which allows the modules to work well with passive GNSS antennas. For improved
performance, or if the antenna cannot be located near the receiver, an active antenna
(that is, an antenna with a built-in low noise amplifier) should be used.
RF Signal Requirements
The receiver can achieve Cold Start acquisition with a signal level above the specified
minimum at its input. This means that it can acquire and track visible satellites, download
the necessary navigation data (e.g. time and ephemeris) and compute its position within
a period of 5 minutes. In the GNSS signal acquisition process, demodulating the
navigation message data is the most difficult task, which is why Cold Start acquisition
requires a higher signal level than navigation or tracking. For the purposes of this
discussion, autonomous operation is assumed, which makes the Cold Start acquisition
level the dominant design constraint. If assistance data in the form of time or ephemeris
aiding is available, acquisition can be accomplished at lower signal levels.
The GPS signal is defined by Interface Specification IS-GPS-200. This document states
that the signal level received by a linearly polarized antenna having 3 dBi gain will be a
minimum of -130 dBm when the antenna is in the worst-case orientation and the satellite
is 5 degrees or more above the horizon.
In actual practice, the GPS satellites transmit slightly more power than specified, and the
signal level typically increases if a satellite has higher elevation angles.
The GLONASS signal is defined by GLONASS ICD (currently 2008 Version 5.1). This
document states that the power level of the received RF signal from GLONASS satellite
at the output of a 3dBi linearly polarized antenna is not less than -131dBm for L1 sub-
band provided that the satellite is observed at an angle of 5 degrees or more above the
horizon.
The receiver will display a reported C/No of 40 dB-Hz for a GPS signal level of -130 dBm
at the RF input. This assumes a SEN (system equivalent noise) of the receiver of 4 dB.
System Equivalent Noise includes the Noise Figure of the receiver plus signal processing
or digital noise. For an equivalent GLONASS signal level, the GLONASS signal will report
a C/No of approximately 39 dB-Hz. This is due to th
e receiver’s higher losses (NF) for
GLONASS signals and a higher signal processing noise for GLONASS signals.
