A jammer denies radio, data or GPS, or some of each. Inside a field: the voice net degrades, TAC//MSG sends go late or are refused, and the GPS loses its lock ("Jamming: GPS NONET - receiver has no lock"). You always still receive messages.

The JAM tile shows RADIO / DATA / GPS as OK, DEG or NONET and how jammed the spectrum is; the page keeps the recent alerts. The scanner needle points at the emitter.