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It is known that GPS satellites transmit information in the frequency 1575.42MHz MHz while GLONASS satellites transmit information at the some other 1602.0 MHz.

How come most GNSS module makers say their modules receiving frequency is 1575.42MHz in the specification sheet?

In that case, even though their module supports GLONASS how could it view/read from those satellites when the antenna cannot pick up info sent by GLONASS Satellites?

For examples check these leading manufacturers

http://www.linxtechnologies.com/en/products/gps-modules/tm-gnss-receiver This module claims to support GLONASS but its suported frequency is 1575.42MHz. So in that case how do we know even by inserting a GLONASS supported antenna, this module will fully make use of it?

Dont we have to use antennas that capture both GLONASS and GPS frequencies with such modules?

Im sorry if the answer is too obvious but these vendors are really confusing us, beginners.

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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
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The GLOSSNASS frequency is mentioned in the detailed PDF datasheet, the answer to this question is like RTFM.

There is only a small difference between the GPS and GLOSSNASS frequencies, so most GPS antenna designs will pick up both just fine.

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