Interference

Typical GNSS signal levels are in the order of -130 dBm. Spurious emissions from digital logic (especially a microprocessor) or a nearby radio transmitter can easily meet the appropriate EU or FCC emission limits and still be several orders of magnitude above the level of the GNSS signals.

These emissions may desensitize the GNSS receiver and lead to performance degradation (low carrier to noise ratios, position accuracy, or loss of lock in extreme cases).

There are two bands of interest:

• Low Band (1164 MHz to 1300 MHz): contains GPS L2, GPS L5, GLONASS L2, BeiDou B2, BeiDou B3, Galileo E5a, Galileo E5b, Galileo E5 and Galileo E6.

• High Band (1525 MHz to 1610 MHz): contains GPS L1, GLONASS L1, BeiDou B1, Galileo E1, and L‑band.

OEM7 receivers are designed to mitigate RF interference on the GNSS card itself. However, if an emission (fundamental or harmonic) from equipment co-located with the GNSS receiver or antenna falls into one of these bands, there are a few things that can be done to ensure good system performance:

• Place radio transmitters and their associated antennas as far away from the GNSS antenna and OEM7 card as possible (for example, a cellular radio antenna).

• Provision for RF shielding over all high-speed digital logic circuitry (even if the design meets the required emissions regulations). A CPU operating at 400MHz will produce an emission at 1200MHz (3rd harmonic) and another at 1600MHz (4th harmonic) – the emissions may be strong enough to affect the GNSS system performance. Similar provisions should be made for any RF circuitry near the GNSS receiver or antenna.

• Run the OEM7 antenna coaxial cable away from high-speed digital logic, other radio sections and high-current nets (as in switching power supplies).