Harmful Interference caused by GLONASS transmissions in the band 1610.6 - 1613.8 MHz degraded radio astronomical observations dramatically in the years between the launch of the first GLONASS satellites. With the full GLONASS constellation the band 1610.6 - 1613.8 MHz would be closed for any radio astronomical research. In the fall of 1992 the Radio Astronomy Service made dedicated observations in full collaboration of the GLONASS administration in a world-wide campaign to test this interference. Negotiations between IUCAF and the GLONASS administration led in November 1993 to an agreed step-by-step plan to clear the band 1610.6-1613.8 MHz of interference by GLONASS. This agreement became model for agreements between the administration of the Russian Federation and those of other countries. The plan, which is already underway, involves moving the satellite frequencies to an adjacent band, and filtering of their unwanted emissions according to the following timeframe and frequency scheme:
Stage 1: Before 1998.
No nominal GLONASS frequencies will be used in the 1610.6-1613.8 MHz band. Channels n=0 to 12 and 22, 23 and 24 will be used for normal Glonass operations, however channels 13,14 and 21 may be used under exceptional circumstances. If possible, advance notice will be given of such uses through the designated U.S. point of contact (for information: contact CRAF secretary). Satellites launched may use the -7 to +12 channels (but are not required to do so) during this period. Satellites launched may be equipped with filters to filter emissions in the 1660-1670 MHz band, down to ITU-R RA.769 limits.
Stage 2: 1998 to 2005
GLONASS transmissions will be limited to channels n=0 to 12. Satellites launched during this period may use channels -7 to +12. Satellites launched during this period will incorporate filters to protect the 1660-1670 MHz band to the ITU-R RA.769 limit (-238 dB(Wm-2Hz-1))
Stage 3: Beyond 2005
Satellites launched after 2005 will use channels n=-7 to +6, with channels +5 and +6 used only for orbital insertion, or during periods of exceptional circumstances. Satellites will incorporate filters, protecting both the 1660-1670 MHz and the 1610.6-1613.8 MHz band from out of band emissions to the required ITU-R RA769.1 limit (as above, -238 dB(Wm-2Hz-1)).
The modernized system will use the same modulation as the current one.
The GLONASS channel scheme used above is f=1602+0.5625*n MHz