Why should they have a litter tray if they're allowed out? We never had litter trays for our cats once we had a cat flap.
MY MIL hates cats coming in her garden and has one of the contraptions that makes a sound outside of our hearing but well within the cats'. Scares the daylights out of them apparantly and they scoot through her garden at a great rate of knots without so much as stopping for a sniff.
BR