Wikipedia
GLITS is an acronym for Graham's Line Identification Tone System, a test signal for stereo systems devised by BBC TV Sound Supervisor and Fellow of the IBS Graham Haines in the mid 1980s. It comprises a 1 kHz tone at 0 dBu (- 18 dBFS) on both channels, with interruptions which identify the channels.
The left channel is interrupted once for 250 ms every 4 seconds. 250 ms later the right channel has two interruptions of 250 ms spaced by 250 ms.
This arrangement has an advantage over the EBU stereo ident tone in that each channel is explicitly identified as belonging to a stereo pair. The EBU Technical Document Multichannel Audio Line-up Tone (Tech 3304) defines stereo lineup tone as having an interruption in the left channel only, lasting 250 ms every 3 s.