Serving mankind as a "lighthouse in the sky," the 50th Global Positioning System satellite launches Saturday to sustain the space-based navigation constellation.

A Delta 2 rocket will launch GPS 2R-11. Photo: NASA

Riding atop Boeing's workhorse Delta 2 rocket, the GPS 2R-11 spacecraft is scheduled for liftoff at 12:39 p.m. EST (1739 GMT) from Cape Canaveral, Florida.

The $45 million satellite, built by Lockheed Martin, is being launched to replace an ailing spacecraft that has surpassed its design life.

The orbiting GPS fleet is critical to U.S. military forces around the globe, helping guide aircraft, ships, troops and precision bombs.

GPS 2R-11, also known as Space Vehicle No. 59, is expected to complete post-launch testing and enter service by mid-April. It is destined to fill the Plane C, Slot 3 position in the GPS network.

The spacecraft being replaced -- GPS 2A-19 or Space Vehicle No. 31 -- was launched 11 years ago this month.

"SVN-31 has a suspect Navigation Data Unit. In the previous 2.5 months, the NDU has experienced upsets which has required the vehicle to be set unhealthy," the Air Force GPS program office told Spaceflight Now.

As the 50th satellite goes up Saturday, it carries a lasting tribute to one of the fathers of GPS.

Words from the late Dr. Ivan A. Getting, "Lighthouses in the Sky, Serving All Mankind," are inscribed on the satellite.

Getting, founding president of The Aerospace Corporation and credited as the visionary behind GPS, died in October 2003 at the age of 91.

GPS satellites send continuous navigation signals that allow users around the world to find their position in latitude, longitude and altitude and determine time. The signals are so accurate that time can be figured to less than a millionth of a second, velocity to within a fraction of a mile per hour and location to within a few feet.

The first experimental GPS craft was launched in February 1978 aboard an Atlas booster from Vandenberg Air Force Base, California. Ten additional launches followed through 1985 as the U.S. military tested the space-based navigation concept using the so-called Block 1 GPS satellite design.

Today's operational GPS system began launching in February 1989 on Delta 2 rockets from the Cape. Nine Block 2, 19 Block 2A and ten Block 2R spacecraft have lifted off.

An artist's concept of a GPS Block 2R satellite orbiting Earth. Credit: Lockheed Martin

Saturday's available window in which to launch extends 15 minutes, closing at 12:54 p.m. EST (1754 GMT). The weather forecast is favorable.

The 126-foot tall booster will dart off Cape Canaveral Air Force Station's pad 17B on the thrust generated by the liquid-fueled main engine and six solid-fueled motors. A minute into flight, the solids will burn out and separate as three more fire to life.

Heading on an east-southeast trajectory from the Cape -- an initial flight azimuth of 110 degrees -- the rocket will perform a so-called "dog-leg maneuver" two-and-a-half minutes into the flight. This 20-second maneuver will bend the flight path slightly more southerly as range safety permits.

Four-and-a-half minutes after liftoff, the exhausted first stage will be jettisoned. The liquid-fueled second stage engine then ignites for the first of its two firings during ascent.

A second "dog-leg maneuver" begins at T+plus 4 minutes, 43 seconds -- shortly before the rocket's nose cone is ejected.

Within 11 minutes, the rocket will settle into a temporary orbit with a high point of 108 nautical miles, low point of 94 nautical miles and inclination of 36.85 degrees to the equator.

The rocket soars across the Atlantic on its southeasterly trek, skirting the southern tip of Africa before making a northeasterly pass over the Indian Ocean during a 51-minute coast.

The mission patch for this 50th GPS satellite launch. Photo: Boeing/Air Force

Once in view of the Air Force tracking station in Guam, the second stage engine is restarted. This 40-second firing raises the orbit's high point to 622 nautical miles, the low point to 101 nautical miles and increases the inclination to 37.2 degrees.

Moments after the second stage finishes the burn, tiny thrusters are fired to put the third stage motor and attached GPS satellite into a rapid spin.

Spinning like a child's toy top, the duo is released from the second stage. The solid-propellant third stage is then lit, boosting its cargo into a highly elliptical orbit stretching from 101 nautical miles at perigee to 10,998 nautical miles at apogee. The inclination is expanded to 39.0 degrees on either side of the equator.

Sixty-eight minutes after the launch began, the $45 million satellite is deployed from the rocket to complete the 111th flight of Delta 2 and first of 2004.

In the following days, the satellite will circularize its orbit at 11,000 miles and raise the orbital inclination to 55 degrees to join the GPS constellation.

The space network features 24 primary satellites split into six orbital planes with four craft in each. Some planes also have additional satellites to enhance coverage and serve as backups.

"The health of the constellation is excellent," the program office says. "There are currently 27 satellites in operational use with a mean age of 7.9 years and median age of 9.9 years."