Have you looked into wi-fi triangulation? Here is one example: https://www.navizon.com/product-navizon-indoor-triangulation-system https://www.navizon.com/pro

Have you looked into wi-fi triangulation? Here is one example: https://www.navizon.com/product-navizon-indoor-triangulation-system https://www.navizon.com/product-navizon-indoor-triangulation... If you went ultrasonic, this part can provide "0 to 765cm (0 to 25.1ft) with 1cm resolution" at 10 Hz https://www.sparkfun.com/products/9495 https://www.sparkfun.com/products/9495