Impact of the direct sequence spread spectrum signals from the global satellite navigation system GLONASS on radio astronomy: problem and proposed solution

The GLONASS satellites use direct sequence spread spectrum with a P-code chip rate of 5.11 MHz. On their L1 channels near 1600 MHz they also carry a 0.511 MHz chip rate C/A-code. The satellites are distinguished by FDMA having carrier frequencies spaced 0.5625 MHz apart. The digital code sequences are used directly to switch the phase of the carriers. This results in an emitted power spectrum of envelope which falls off only as (frequency from the carrier)^-2. This allows the signals to be seen more than ±80 MHz from the carriers. As a result GLONASS is causing unacceptable interference to radio astronomy in the band 1660-1670 MHz and devastating interference in the band 1610.6-1613.8 MHz which was allocated PRIMARY to the radio astronomy service (RAS) at WARC-92. The problem is to achieve EMC between the RAS and GLONASS. It is proposed that one measure to mitigate the impact of GLONASS should be for it to adopt a form of continuous phase modulation (CPM) which would result in its side-bands falling off as (frequency from carrier)^-6. A novel “proof-of-concept” 1/100th (in frequency) scale model modulator has been constructed in hardware which produces a simulated GLONASS CPM signal with this property. This type of modulator could find service with other DSSS systems