73 yards

You may be looking for the episode of the same name or its novelisation.

The TARDIS's perception filter had a normal range of 66.7 metres, or 73 yards. (TV: Empire of Death ^[+]Loading...["Empire of Death (TV story)"])

In a timeline created by the Fifteenth Doctor and Ruby Sunday interfering with a fairy circle, Ruby found herself followed by a woman who always remained 73 yards away from her. The woman was shielded by a perception filter that prevented her from being noticed by most; when Ruby positioned her in front of a police car it simply drove around her. Kate Lethbridge-Stewart theorised that it had a connection to the TARDIS being parked by the fairy circle. (TV: 73 Yards ^[+]Loading...["73 Yards (TV story)"])