SE868Kx-Ax Family Product User Guide
1VV0301201 Rev. 5
Page 54 of 83
2021-07-15
Not Subject to NDA
10.
RF FRONT-END DESIGN
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, 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 and/or ephemeris
aiding is available, acquisition can be accomplished at lower signal levels.
The GPS signal is defined by the 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 a 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, assuming 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 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:
1.
Satellite transmit power
2.
Satellite elevation angle
3.
Free space path loss
