Originally Posted by
snowfalcon2
A question to the radio engineers among you: If you would need to design a radio altimeter today taking account of the increasing scarcity of radio frequency spectrum - what would be the minimum needed bandwidth including guard bands? I know it is always a trade off between cost, performance and need for spectrum, but looking at the whopping ~600 MHz now used by the radio altimeter service (including guard bands) one would think there would exist more spectrum-efficient solutions?
Briefly,
The band allocated internationally for this purpose is 4.2-4.4 GHz, so 200MHz wide. The adjacent spectrum either side is not officially guard band but allocated by national administractions, again within international regulations, in the knowledge of the radar and other systems.
The actual spectrum requirement of a radar transmission can be calculated from the pulse repetition frequency and pulse risetimes. I suspect that different manufacturers may use their own proprietary parameters. But
prima facie the 200 MHz bandwidth seems quite adequate. The problem seems to be that the radar receivers respond to a much wider bandwidth than the 200 MHz, so they are susceptible to strong signals in adjacent frequency bands.
This has not been a problem until now, with 5G cellular being introduced into spectrum below the radar band. How much the receiver bandwidths can be reduced without limiting their performance is a matter for the manufacturers, but a simple modification to existing fitted equipment seems impractical.
There is also the issue of 5G transmissions radiating outside their own band and into the radar band. These are quite tightly controlled and are likely to be less of a problem.
The interference potential from 5G in C band is greater in the US because the allocated band goes up th 3.98 GHz, whereas in some other places including Europe it stops at 3.8 GHz. Also higher transmit powers are proposed
edited to add:
WillowRun 6-3's description of the relationships between the FAA and FCC may well explain the
disgraceful lack of prior testing to evaluate potential problems, during the ample time available to carry out the work.
Was this a standoff, with each leaving the other to do something about it? If anyone is now suffering financial loss, they should be looking to see who was responsible.