The operation of a GPS receiver can severely be limited or completely disrupted in the presence of in-band interference signals. The threat of in-band interference signals increases daily as new communication systems are put in place and the radio frequency spectrum becomes more populated. The threat is not only from the interfering signals themselves but also from their harmonics that fall inside the GPS band.
We have devised an efficient and totally different interference suppression system (patent pending) integrated into our GPS chip. This all-digital compensation type system estimates the parameters of the interfering signal (e.g. bandwidth, frequency, amplitude and phase) and suppresses it. With six separate such suppressing systems, we can suppress up to six simultaneous in-band narrow-band, pulsed and CW interference signals by up to 60 dB. This interference suppression system protects our integrated direct spread spectrum communication channels for RTK data transmission as well as GPS and GLONASS L1 and L2 bands.
A good demonstration of the suppression system efficiency is the following experiment. An narrow-band interference (in C/A GPS band) from the signal-oscillator output was admixed to GPS L1-signal arriving at antenna's low noise amplifier. With zero-power interference and with disconnected suppression system, the receiver tracked 8-10 satellites. Then, after smooth increasing interference power, the number of tracked satellites decreased. After that, interference power increased up to the value when the receiver lost lock on all satellites. Finally, being switched on the suppression system resulted in receiver's lock on 7-9 satellites.
The following picture shows how the suppression system reduces computation errors: