We had the same problem as our cat got older. It seems she didn't want to turn around to squat correctly in the littler box. Not sure if your cat is getting to that place where her joints are hurting her. If so, then you might try turning her around while she is in the process while lovingly reassuring her that this is what you want.
One other option is we found a litter box tall enough where she had to step over to get in and then when she peed the wall was higher than the pee so it stayed in the box.
Lastly, we began to place cloths at the place where the urine was hitting the carpet to absorb it and then throw it away. Because she reached a point where she wouldn't even go all the way into the box.
Luckily, we sell a product that really neutralizes the urine smell so after she departed we could erase those smells from the home. Here is a link if you don't have a source for a good neutralizer. http://www.healthypetcorner.com/24-pet-force-urine-and-stain-remover
Hope that helps.