The expected SNR for a GNSS SV varies based elevation angle. I have created a simple plot for this data, which can be accessed via TrimbleTools.com, http://de.trimbletools.com/Average_SNR_Plot.html
This is a quick and simple way of checking if you have obstructions, jamming or antenna issues.
The data is from a more than 5 million measurements with a Zephyr 2 Geodetic. There is some difference between all the Zephyr models, but for troubleshooting this table can be used for all of them, as normally you are looking for significant (6dB of more) of difference.