Special Signal Lights (Isophase)
Port of London General Direction for Navigation in the Port of London (2016) No. 30 requires that all Reporting Vessels underway between Cherry Garden Pier and Bell Lane Creek shall carry on board an operational electronic keying device capable of activating the special signal lights which signify that one or more Reporting Vessels are navigating or about to navigate through the arch displaying the signal.
Click on image to enlarge
The PLA has a limited number of electronic keying devices available for hire subject to certain. An Application Form along with Terms and Requirements is available to download here.