Re: Using a Passive Front-End Filter
Upvote for an excellent post! You're quite right that frontend saturation is the main RF concern here.
A relatively cheap surface- or bulk- acoustic wave filter can do the filtering job but would add a fair amount of insertion loss - several dB - and that directly reduces the signal/noise ratio of a receiver. These would cost from single digit dollars to a few tens of dollars each. The systems engineering question becomes whether one is willing to accept the loss of SNR.
A cavity, ceramic resonator, or printed circuit "stripline" filter would work nicely. I did some back-of-envelope calcs that suggest a relatively low complexity design would provide >52dB of rejection over the 5G lower band (*). Cavities tend to cost around $100+ in bulk, ceramics probably $20-$50, integrated stripline varies depending on how much one would have to modify their existing PCB design to accommodate.
Another approach is to use low noise amplifiers on the frontend that have high dynamic range. Marginal cost for such an upgrade is minor, but this involves spinning new PCBs - effectively a new product.
So here is the real problem: radalt are safety critical. If I add a filter, mod a PCB, look at it funny, or even think dirty thoughts in it's vicinity... The FAA regs will require a recertification. Thats extremely expensive. Is have to draft maintenance instructions for all the radalt in the field, and these would need to be reviewed. The radalts FCC certs, CE certs, MIL-STD-461 EMI testing, MIL-STD-810 shake n bake testing, etc would have to be re-done. Somebody has to pay for the labor to installl, inspect the installation, certify it's done to spec... None of that is cheap.
What's going on here is an effort to get someone else to pay for the systems engineering to figure out if there even is an issue, and be on the hook for any mitigation if there is. Lastly, people want a paper trail so that there is someone else to chuck under the bus if an airliner ever has an issue.
(*) Butterworth, fifth order, would work nicely.