Most airliners and business aircraft, on the other hand, already have traffic, through TCAS (traffic and collision avoidance system) and TCAS II, and weather, through onboard radar. Moreover, because of the decades-old TCAS mandate, most already have Mode-S transponders. While Mode S will give many light GA operators a relatively cheap upgrade path to ADS-B, for the airlines, that might not be the case. The problem is that not many airliners have GPS - they typically rely on other forms of navigation, including inertial navigation, that are not easily compatible with ADS-B. Many older airliners, believe it or not, still get by with VOR/DME. For the airlines, ADS-B could be an extremely expensive proposition. Indeed, the FAA and the airline industry agree that the cost will be in the billions, between around $2 billion and $6 billion, in fact.