IFF signals may be captured using Cambridge Pixel's HPx series of radar interface cards, and then decoded using the SPx software to extract target ID and altitude. The latest SPx Server and SPx Development releases offer enhanced capabilities.
A core part of these enhancements is in the handling of "garbled" video, where transponder responses from multiple aircraft overlap. The new software now detects and overcomes more types of garbling than before and offers enhanced robustness in the case where the transponders don't reply to certain interrogations (e.g. military), resulting in gaps across the target response. It is also more robust where transient IFF code errors occur on individual azimuths, now ignoring unconfirmed single code changes.
In parallel with these developments we have upgraded our simulation capabilities, enabling SPx Radar Simulator to output IFF video with embedded interrogation mode information.
A white paper discussing our IFF support is available here