The Active Marker deck is a Bitcraze product primarily meant for Qualisys motion capture systems.

It contains four infrared LEDs of wavelength 850nm - the same used by Vicon Vantage cameras. (source: deck datasheet, Vicon manual)

Its onboard microcontroller does something nontrivial when used with a Qualisys system, but it also has a PWM control mode. The PC sets the pulse width for each LED via the parameter system (source: bottom of driver code)

We could use this deck to resolve the mapping between radio addresses and initial positions for libobjecttracker initialization. The user would only need to provide the list of (radio address, channel) pairs. We initialize by enabling only one Crazyflie's LED at a time.