No Public Key

[Greg John Goodsir <greg.gd@woosh.co.nz>]

I have been trying to add a repository from this site to Synaptic. I am
using Ubuntu 7.10. When I add "deb http://ftp.wa.au.debian.org/debian
sid main" or any other repository in the mirror list, after Synaptic has
finished downloading the list of packages, it says "W: GPG error:
http://ftp.wa.au.debian.org sid Release: The following signatures
couldn't be verified because the public key is not available: NO_PUBKEY
A70DAF536070D3A1"
There is no update available after FlightGear0.9.10 the version that I
am using now from Ubuntu repository.
How do I get a public key?
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