WASHINGTON - As the Senate finally considers long-stalled legislation this week to speed modernization of the air traffic system, the bill won't resolve one crucial question: Who pays for the technology that planes need to make it work?

Airlines and private jet owners want taxpayers to fund the gear that would let them benefit from an upgrade from radar to satellite-based navigation. The Federal Aviation Administration says the technology will make flying safer and enable controllers to accommodate more planes in the sky.

The Senate bill mandates that aircraft owners buy the equipment, but it doesn't give them what they want - between $2 billion and $4 billion to pay for it. And while the Senate bill, which could be voted on as early as today, offers incentives for adoption, the House version of the legislation doesn't include direct funding or incentives.

U.S. Transportation Secretary Ray LaHood said this week that the administration is discussing ways to be "helpful" to airlines and would make a decision soon, but it has not agreed to pay for it. Meanwhile, airline executives are still fuming from being shut out of the $787 billion stimulus bill last year.

"If government is going to mandate it, which they would do in the Senate bill, they should fund this technology," said Karen J. Lewis, vice president of governmental affairs at Dallas-based Southwest Airlines. Fort Worth-based American Airlines shares that position.

GPS technology

The technology, known as Automatic Dependent Surveillance Broadcast, or ADS-B, uses GPS technology to broadcast a plane's speed and location, and to receive weather data. It would allow pilots and air traffic controllers to almost instantly know where planes are.