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SC872-A Product User Guide Electrical Interface
1VV0301202 Rev. 6 Page 36 of 53 2018-12-21
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 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 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:
•
Satellite transmit power
•
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 GNSS signal is relatively immune to attenuation from rainfall. However, it is heavily
influenced by attenuation due to foliage (such as tree canopies, etc.) as well as outright
blockage caused by buildings, terrain or other objects near the line of sight to each 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 general direction, the geometry of
the remaining satellites will be worse (higher DOP) and will result in a lower position accuracy.
The receiver reports this geometry effect in the form of PDOP, HDOP and VDOP numbers.
For example, in a vehicular application, the GNSS antenna may be placed on the dashboard or
rear package tray of an automobile. The metal roof of the vehicle will cause significant blockage,
plus any thermal coating applied to the vehicle glass can attenuate the GNSS signal by as much
as 15 dB. Again, both of these factors will affect the performance of the receiver.