Went to a shooting range I had never been to before that had targets set up from 100 to 1000 yards in various increments. Went to use my ballistic app on my I phone and found I had no signal. Fortunately, I had seen a review that Frank did on AC&TS's Field Density Altitude Compensator and decided to order one. Sure glad I did, as it let me calculate my firing solutions and was as accurate as my i Phone app, and just as fast. Modern technology is great, but it sure don't hurt to have a backup....