Tl;dr: Not that we know of. At least not deployed anywhere outside a lab. Some proposals have been floated, however.

This post was inspired by the recent report by the US Congress Government Accountability Office on ADS-B, which was discussed in many articles around the web, such as this one, for example: GAO: Pentagon, FAA Lag In Addressing ADS-B Risks

Let’s have a look at the source, these are quotes from the GOA’s actual document, citing some of the work from contributors here at OpenSky: [1]