Youtuber Suchomimus* has many brief but informative videos based on commercial satellite imagery of Russian defensive lines being built in recent weeks:

https://www.youtube.com/watch?v=B3cceeYu8wA https://www.youtube.com/watch?v=KB-n79HKcU8 https://www.youtube.com/watch?v=CtolRDhxy_Y https://www.youtube.com/watch?v=y0kLBTMk7RE