diff --git a/cubesatsim/cubesatsim.h b/cubesatsim/cubesatsim.h
index 87acffdd..0fed2585 100644
--- a/cubesatsim/cubesatsim.h
+++ b/cubesatsim/cubesatsim.h
@@ -379,6 +379,7 @@ RPI_PICO_Timer ITimer1(1);
 
 bool timer0_on = false;
 volatile int timer_counter;
+bool filter_present = false;  // BPF installation state for transmitter
 
 char callsign[20];
 int morse_timing = 60;  // ms for a dit