Embodiments of the present invention disclose systems and methods for providing an avionics overlay data link. Through embodiments of the present invention, existing ATC (or other) modulated signals using existing frequencies (or other frequencies) may be utilized to transmit (e.g., from an aircraft transponder) additional information in a manner that does not render the transmitted signal unrecognizable by legacy ATC equipment. In various embodiments, legacy equipment may demodulate and decode information that was encoded in the transmitted signal in accordance with preexisting standard modulation formats, and updated equipment can also extract the additional information that was overlaid on transmitted signals.