SL876Q5-A
Product User Guide
1VV0301333 Rev. 2
Page 47 of 68
2017-06-27
10.
RF FRONT END DESIGN CONSIDERATIONS
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 ephemeris data and compute the location within a 5 minute period. In the GNSS
signal acquisition process, decoding 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, lower signal levels can be used for acquisition.
The GPS signal is defined by 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 by
IS-GPS-200, and the signal level typically increases if a satellite has higher elevation angles.
The GLONASS signal is defined by GLONASS ICD 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 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 4dB. 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 the receiver’s higher losses (NF) for GLONASS signals
and a higher signal processing noise for GLONASS signals.
Each GNSS satellite presents its own signal to the receiver, and best performance is obtained
when the signal levels are between -130 dBm and -125 dBm. These received signal levels are
determined by:
•
GNSS satellite transmit power
•
GNSS satellite elevation angle
•
Free space path loss
•
Extraneous path loss (such as rain)
•
Partial or total path blockage (such as foliage or buildings)
•
Multipath interference (caused by signal reflection)
•
GNSS antenna characteristics
•
Signal path after the GNSS antenna
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
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