Reading some more on AM broadcast transmitter frequency stability: 
Modern transmitters >25kW seem to come with +/-2ppm over temperature 
range and can be 'improved by connecting to a GPS'. That would be at 
least 10 times better than an unadjusted watch crystal even without GPS 
and likely much better than 5 seconds/month.

This is very tempting to try out imho. Just take a cheap AM radio, PLL 
it with its own IF and connect to a PIC ? Of course some work is 
required.

I suppose that in more advanced countries the powers that be monitor the 
big transmitters and send them love letters if they wander off their 
allocated frequencies so they should be spot on, if for no other reason 
then to be able to talk themselves out of remote (dx) interference 
complaints.

Peter

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