SL869T3-I Product User Guide
1VV0301546 Rev. 3
Page 37 of 68
2021-03-31
9.
RF FRONT END DESIGN
9.1.
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, 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, 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, and the
signal level typically increases if a satellite has higher elevation angles.
The NavIC signal is defined by ISRO-IRNSS-ICD-SPS-1.1 dated August 2017. This
document describes the NavIC constellation and its signal structure. The SL869T3-I
receives and processes the NavIC L5 signal.
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 NavIC signal level the firmware will report a C/No of
approximately 39 dB-Hz. This is due to the rece
iver’s higher losses (NF) and a higher
signal processing noise for these 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)
