code: public <T extends ModeSReply> Integer getGeoMinusBaro(T reply) {
  		if (reply == null) return null;
  		DecoderData dd = this.decoderData.get(reply.getTransponderAddress());
  		return dd == null ? null : dd.geoMinusBaro;
  	}
docs: Get the difference between geometric and barometric altitude as tracked by the decoder. The value is derived
  from ADS-B {@link AirspeedHeadingMsg} and {@link VelocityOverGroundMsg}. The method returns the most recent
  value.
  @param reply a Mode S message
  @param <T> {@link ModeSReply} or one of its sub classes
  @return the difference between geometric and barometric altitude in feet