Space shuttle Endeavour at launch pad 39A for the STS-99 mission. Photo: NASA

The six astronauts that will embark on shuttle Endeavour's upcoming flight are at Kennedy Space Center this week for a practice countdown, but NASA has yet to secure a launch date for the mission.

The U.S. space agency wants to launch the shuttle on January 31, beginning an 11-day mission to produce a 3-D map of the Earth's surface.

"We are still working towards the 31st and have made no shift from that," said agency spokesman James Hartsfield.

However, there is a conflict with the U.S. Air Force-controlled Eastern Range, which provides critical communications, tracking and safety services for all Space Coast launches and shuttle landings.

The Range currently has a down period scheduled between February 7 and 23 to upgrade its 1950s-era systems.

If Endeavour is launched on January 31 like NASA wants, the shuttle would be slated for landing at KSC on February 11 -- right in the middle of the Range blackout. The Range's services are needed to support the ship's homecoming, making January 31 an unviable launch opportunity.

NASA reportedly is lobbying the Range to delay the down period, possibly even pushing it into March.

An Air Force meeting on Tuesday afternoon concluded without resolving the issue. Another meeting is planned for next Tuesday, Air Force spokeswoman Lt. Alana Austin said.

The STS-99 astronauts pose the traditional crew portrait. Front: Mamoru Mohri of NASDA and Gerhard Thiele of ESA. Back: NASA's Janice Voss, Kevin Kregel, Dom Gorie and Janet Kavandi. Photo: NASA

While the liftoff date is uncertain, the astronauts and KSC launch team are continuing with pre-flight preparations with the traditional Terminal Countdown Demonstration Test, or TCDT.

The crew, led by veteran shuttle commander Kevin Kregel, arrived at the space center just after 4 p.m. EST (2100 GMT) Tuesday for a three-day countdown dress rehearsal and launch pad evacuation training.

On Friday, the astronauts will don their day-glow orange pressure suits and board Endeavour just like they will on the real launch morning.

Clocks will tick off the final three hours of a mock countdown leading to a simulated ignition and shutdown of the ship's three main engines.

NASA is also using the practice countdown to ensure the shuttle, ground systems and computer networks are Y2K compliant.

The a majority of the so-called Y2K end-to-end test will be completed during the countdown simulation, but the launch team will finish other checks later Friday.

"It is a precautionary test that takes advantage of TCDT," NASA spokesman Joel Wells said.

"We fully expect everything to go smoothly," he added.