Ross 4500DR-104-07, User Manual, Page: 140 / 161

140 • GPS Option SRG-4500 User Guide (v7.0)
Signal Requirements
The SRG-4500 with GPS Receiver can achieve Cold Start acquisition with a signal level of -147dBm at its input.
This means the SRG-4500 can find the necessary satellites, download the necessary ephemeris data and compute
the location within a 5 minute period.
The SRG-4500 will display a reported C/No of 40dB-Hz for a signal level of - 130dBm into the RF input.
• Each GPS and GLONASS satellite presents its own signal to the SRG-4500, and best performance is obtained
when the signal levels are between -125dBm and -117dBm. These received signal levels are determined by:
• GPS and GLONASS satellite transmit power
• GPS and GLONASS satellite elevation and azimuth
• Free space path loss
• Extraneous path loss such as rain
• Partial or total path blockage such as foliage or building
• Multi-path caused by signal reflection
• GPS/GLONASS antenna
• Signal path after the GPS/GLONASS antenna
Even though for frequency and phase locking the SRG-4500 does not require a good 3D fix, it still requires three
satellites in view to produce a fix.
Required Time Corrections
From a GPS Receiver, the SRG-4500 Master Clock can derive UTC and also add the appropriate time offset and
provide corrected local time. Table 16.3 contains a list of the various corrections which need to be made by the
SRG-4500.
System Noise Floor
As mentioned earlier, the SRG-4500 will display a reported C/No of 40dB-Hz for an input signal level of -130dBm.
The C/No number means the carrier (or signal) is 40dB greater than the noise floor measured in a 1Hz bandwidth.
This is a standard method of measuring GPS receiver performance.
Thermal noise is -174dBm/Hz at room temperature. From this we can compute a system noise figure of 4dB for the
SRG-4500. This noise figure consists of the loss of the preselect SAW filter, the noise figure of the LNA as well as
implementation losses within the digital signal processing unit.
Troubleshooting
This section provides a flowchart ( Figure 16.10 ) to troubleshoot some common issues that may be encountered
during setup or operation of your SRG-4500. The sections after the flowchart provide additional tips and notes for
steps in the troubleshooting process.
Table 16.3 Required Time Corrections
Offset SRG-4500 Update
1
Earth Speed Drift
in one day
1ms
Leap second corrections are made approximately
every 18 months as determined by International
Earth Rotation Service (IERS).
2
Local Time User defined The user usually defines the offset from UTC at
time of installation
3
Daylight Saving 1 hour Twice per year