Other peoples' cats are a very serious nuisance for a city gardener. No fencing can keep them out and repellent aromas are totally useless. They are most active at night when you can't be ready with the water pistol. And there are just so many of them, that its an Herculean task to try to scare each one individually anyway. It's not only the disgusting fouling, but also the massive damage they cause and the ghastly noise they make which are intolerable. That the owners of these horrible pests aren't subject to any kind of legal duties or restraints (as are dog owners,) is an insufferable outrage. Step in electronic scarers. In practical terms, these machines offer the only hope of keeping the cats out. But don't be fooled into thinking all cat scarers are as good as each other. That's very far from the case. The Defenders repeller is worth the money. Others of the kind certainly aren't. It is a large machine with a very sturdy and adaptable ground stake. This detaches from the main body of the unit and can be driven into firm ground with a mallet safely. It's important to get it exactly upright and also get the elevation right as the unit's coverage depends on it. The sensor should be at the same height as the cat's body above ground. This machine runs perfectly well on rechargeable batteries. It takes two 9v PP3s. Other machines, which only take one and insist on a Duracell type, do not work as well, and I suspect that the extra battery helps make the ultrasound signal louder, pushing it above the cats' tolerance level. The animals I've seen in range of my Defenders units have been absolutely terrified- a very welcome sight. It's a bit of a struggle to get the batteries inside the compartment- you have to lay them in flat, and positioning the bottom one is fiddlesome. But this aside, installation is a quick and simple process. I like, too, the test button on the front panel which allows a very quick and easy check on the unit's operating status. The only other control is the unit's on- off switch, hidden away next to the battery compartment on the machine's base. Quite simple to find with one's fingers and easy to operate, so you can switch it off to work in the garden without wasting power, and on again to initiate the unit afterwards. It is, of course, vitally important to be aware that the operating range of the Defenders scarer is limited to an arc of only 94 degrees; 47 either side of the unit's axis. So you must be very careful to site and position the unit properly, and for odd shaped and/ or large spaces, more than one unit will probably be needed for full coverage. Around bay windows or wheelie bins, for example, the little pests will certainly find any unprotected areas and make their smells and messes. But that said, the Defenders is less than half the price of older products which didn't work so well, so buying multiple units (and the spare rechargeable batteries to power them,) really isn't such a great hardship.