Recent GPS Launch Vehicle Tracked with GPS Rather Than Radar

May 18, 2014 – Signaling a new chapter in the long-sought modernization of the U.S. Air Force’s launch ranges, Friday night’s flight of a Delta 4 rocket was tracked via satellite instead of by radar in a move officials say is a money-saving upgrade to the military’s aging range infrastructure.

A special avionics system mounted on the 20-story launcher transmitted its location to controllers on the ground using Global Positioning System navigation data. The Delta 4 rocket Friday happened to be boosting a fresh GPS spacecraft into orbit to replenish the Air Force-run satellite fleet circling Earth more than 12,000 miles up.

Walter Lauderdale, the mission director for Friday’s launch, said United Launch Alliance’s Atlas and Delta rockets are transitioning to GPS metric tracking for range safety functions, which protect the public and property should a launch vehicle veer off course.

All U.S. rockets carry a flight termination system to destroy the booster if a serious problem occurs. The pyrotechnic destruct charge is commanded from the ground by an Air Force official who monitors the rocket’s flight path.

"For GPS metric tracking, there is an avionics box that’s on the vehicle that sends down position information about where the rocket is back to the range for them to track for public safety," Lauderdale told reporters before Friday’s launch.

Most rockets launching from Cape Canaveral, Fla., and Vandenberg Air Force Base, Calif., have for decades been tracked by a C-band radar. Each rocket is fitted with a C-band transponder which helps the radar lock on to the launcher on its flight into space.

"It provides the same position information as if you used a C-band tracking radar, where you’re actually following and interrogating the rocket … The GPS signal goes down to the rocket, in this case for metric track, [and] it tells the range safety folks where the rocket is, so they can track that the rocket is following its appropriate flight profile," Lauderdale said.

The GPS avionics unit completed a series of demonstration flights on previous rocket launches before being permitted to take over as a primary tracking method.

According to Lauderdale, test launches of unarmed Minuteman missiles have used GPS metric tracking before, but space launchers are just getting into it.

With the Delta 4 now GPS-capable, two more Atlas 5s will carry C-band tracking beacons before it switches over to GPS metric tracking, Lauderdale said.

An Atlas 5 launch "at the end of July will be the last Atlas 5 to use the C-band radar; it flies out the last of the C-band tracking beacons. For Atlas 5 launches after the one at the end of July, we’ll also be using only GPS metric tracking and will not require the C-band tracking radar as a mandatory [range asset]," Lauderdale said.

Officials say GPS metric tracking adds to the accuracy of the position and velocity data received by range safety officer during a launch. It is also cheaper because it means the C-band tracking radars at Cape Canaveral and Vandenberg Air Force Base are no longer needed.

GPS metric tracking is a cornerstone of the Air Force’s effort to create a more "responsive" launch range. Finicky ground systems, many of which are based on 1950s or 1960s technology, occasionally cause launch delays even when weather, the rocket and the payload are "go" for liftoff.

Budget cuts have also stretched the Air Force launch ranges, forcing managers to decommission backup systems.

The perils of relying on antiquated technology in an era of lean funding was exhibited in late March, when a fire damaged an Air Force-owned tracking radar at the Kennedy Space Center.
The fire kept two rocket launches, one carrying a U.S. national security satellite and another ferrying supplies to the International Space Station, on the ground for more than two weeks as the Air Force rushed a backup radar into service.

SpaceX officials have said the company is working on a GPS metric tracking capability for the Falcon 9 rocket, but a company spokesperson did not respond to an inquiry on the status of its development.

According to a technical document posted on ULA’s website, the company’s GPS metric tracking system is one of three independent range tracking sources to help the Air Force range safety officer follow the flight path of a rocket. Alongside the GPS data, the range safety officer uses telemetered inertial guidance data and tracking information from a launch head skin track radar.

The GPS metric tracking system on the Delta 4 and Atlas 5 rockets receives satellite signals from two L-band antennas mounted 180 degrees apart on the second stage, processes the data to formulate its position and velocity, then transmits the information to the ground through an S-band radio link for comparison to the launch vehicle’s predicted location.

Before GPS metric tracking, engineers installed transmitters on rockets to allow them to send telemetry to control centers through NASA’s constellation of Tracking and Data Relay Satellites, reducing reliance on a network of ground station scattered around the globe.

The next improvement under study by the Air Force is the introduction of autonomous on-board flight safety systems, which would replace today’s man-in-the-loop range safety paradigm. Instead of requiring an engineer to send a manual destruct command to an errant rocket from the ground, a computer on-board the launcher would do the job by itself.

An autonomous flight termination system flew on an Air Force Minotaur rocket launch from Virginia in November in the first of what officials then said would be several test flights needed to certify the technology.

Ultimately, simplified space-based ranges could eliminate the need for ground infrastructure, allowing launch ranges to exist virtually anywhere in the world, officials said.

"The idea behind space-based range is you literally take all that range infrastructure, which is time-consuming and costly, try to streamline it and put it on the rocket," a former Air Force space official said.

Source: Spaceflight Now