GPS & GLONASS systems Antenna Selection
The system defined frequencies for GPS and GLONASS are the center frequencies. The data and signals aretransmitted with different bandwiths. Since the GPS and GLONASS frequencies are close together, you can desing the antenna to cover both frequencies. It is possible that a single GPS antenna can 'see' GLONASS frequencies and push them forward as observations to the receiver-unit (processor). BUT (!) Although the antenna seems to 'see' the GLONASS satellites, the problem is that the data has to be prepared in a appropriate way! This is carried out by the LNA (low noise amplifier) of the antenna. 

Why?

This has to do with the bandwith of the antennas LNA and how sharp the edges of the filter inside the antenna front end are designed. As long as the LNA is not optimized for GLONASS reception, you will never be sure to use the correct data for your receiver. 

Solution

Check the quality of the incomming data (signal strength, how stable is the code or carrier solution) and the repeatability of a single GLONASS solution compared to a GPS-only solution. But the best way will be to use an antenna with multi-constellation support (GPS+GLONASS+derivatives) - also named as Global Navigation Satellite Systems (GNSS). Therefore you need to collect some information of the antennas LNA.

further reading:


Ivan G. Petrowski and Toshiaki Tsujii (2012): Digital Satellite Navigation and Geophysics, Cambridge University Press, Cambridge
Philip Mattos and Fabio Pisoni (2014): Quad Cosntellation Receiver, GPS World 25(1):34-63, online