Grounded military weather satellite finally repaired

Posted: June 21, 2001

  DMSP in pad clean room
File image of DMSP weather satellite. Photo: Lockheed Martin
You could say fate was with the U.S. Air Force back in January when, after two scrubbed countdowns, a catastrophic problem that would have likely doomed the launch of a military weather satellite was detected while the craft was still on the pad.

Now officials say they are sure technicians have found and fixed the trouble with the Defense Meteorological Satellite Program F16 spacecraft's ailing guidance system.

"We're very confident that we solved the anomalous condition and plan to fly a real good healthy satellite," Col. Randy Odle, the DMSP program office director at Los Angeles Air Force Base, said in an interview this week.

Originally scheduled for blastoff on January 20 atop a refurbished Titan 2 missile from Vandenberg Air Force Base in California, the Lockheed Martin-built DMSP satellite failed to make it off the planet that day when the countdown was halted at T-minus 3 minutes due to a ground support equipment glitch.

Liftoff was rescheduled for the following morning.

The second countdown reached T-minus 28 seconds when computers detected one of the rocket's fuel valves had not opened as planned. In the end, the valve worked just fine but a sluggish indicator switch didn't register the opening fast enough for the computers' liking.

Faced with just a 10-minute window to launch the Titan 2, officials tried to reset liftoff for the end of the period but simply ran out of time.

That afternoon, as workers were preparing to roll the protective mobile service tower back around the rocket, faint traces of hydrazine fuel were detected in the air at the Space Launch Complex 4-West launch pad.

Titan 2 rocket awaits liftoff from SLC-4W at Vandenberg. Photo: Spaceflight Now
After several hours of work to clean up the minor leak, workers were allowed back into the pad. However, so much time had been lost officials decided another launch attempt the next morning wouldn't be made.

Then, later that night ground controllers watched as the DMSP's Inertial Measurement Unit -- the craft's navigation brain -- began acting erratically.

First, the gyroscopes within the IMU suddenly dropped out of flight mode. That incident was followed 20 minutes later by the AC and DC power supplies switching from primary to backup systems on their own.

"If those anomalies occurred during the ascent phase of the launch, the likelihood is we would have lost the satellite and the mission," Odle said.

The DMSP F16 mission is valued at $430 million -- $80 million for the launch and $350 million for the satellite hardware and development, according to Odle.

Without its IMU working properly, the spacecraft would have been unable to navigate itself during the climb to orbit. Most troublesome would have been the portion of launch when a kick motor attached the satellite fires to propel the craft the rest of the way to space after separating from the Titan 2 booster while on a sub-orbital trajectory.

Without the IMU functioning up to par, the satellite wouldn't have been able to point itself correctly, likely resulting in the craft failing to reach orbit and instead reentering the atmosphere.

"Looking back perhaps fate was with us more than we thought. It is always nicer to fix problems on the ground than in space," Odle said.

"Whether we would have launched on the first attempt, perhaps the anomaly wouldn't have even manifested itself. But once we saw it, clearly we could not launch with that catastrophic potential."

Once safely in space, the IMU isn't critical to success of the satellite's mission to monitor weather conditions around the global for U.S. military forecasters. A backup system would be able to replace the IMU chores, said Lt. Col. Grant Takahashi, DMSP program engineer.

Given the severity of the problem, the 3,300-pound DMSP satellite was pulled off the Titan 2 rocket at the launch pad and returned to a payload processing facility at Vandenberg on February 27.

It was in that building that engineers discovered a cable on the satellite had broken due to of poor workmanship during assembly. The investigation revealed the cable hadn't been manufactured with the proper "strain relief" as designed, which led to the failure.

Engineers determined the IMU glitches were caused by an interruption of the clock signal from the spacecraft to the IMU as a result of the broken cable.

The remedy was replacing the damaged cable, and to be sure there weren't any other potential causes left unfixed, the satellite's Controls Interface Unit (CIU) and another cable connecting the CIU to the IMU were switched out.

Subsequent re-testing has gone well and officials believe the problem has been solved. The Air Force puts the cost of finding and fixing the problem at $7.5 million.

There was a similar IMU problem back in December in which a suspect cable had to be fixed. It is possible the earlier cable failure could have "masked" the more serious problem that occurred during the countdown in January.

"Whether that is true or not, we don't know. But it certainly could have been the case," Odle said.

Space Launch Complex 4. Titan 2 pad on left, Titan 4 pad on right. Photo: Air Force
Even though the satellite is now fixed and could be ready for liftoff as soon as August 4, it will have to sit on the ground until sometime in November because the launch of a secret national security payload has taken priority at Vandenberg.

The Titan 2 rocket shares Space Launch Complex 4 with the massive Titan 4 vehicle. The complex's two pads -- SLC-4W for Titan 2 and SLC-4E for Titan 4 -- are extremely close.

As a result, officials don't want satellite cargoes on both pads at one time because the vibration during launch can damage the spacecraft on the neighboring tower, not to mention the chance of destroying an expensive craft if a rocket were to explode.

Earlier this week the Air Force opted to juggle its launch schedule and delay DMSP to November, after the planned September 25 launch of the Titan 4 rocket on its classified mission for the National Reconnaissance Office. The DMSP satellite will be moved to the launch pad soon after the Titan 4 departs.

Previous plans had called for the Titan 2 to fly first, with the NRO payload moving to the launch pad for mating with the Titan 4 in support of a November 14 liftoff.

"From our perspective the DMSP spacecraft and the Titan 2 booster were on a game plan to meet the August 4 launch date, but because of other priorities for other missions the country flies we were directed to push out to the November time frame," said Odle.

Officials are still working out details of the new DMSP schedule and a specific target launch date will be forthcoming.

An artist's concept of a DMSP weather satellite in space. Photo: Lockheed Martin
The extra postponement will require technicians to remove, re-calibrate and re-install the Special Sensor Microwave Imager Sounder (SSMIS) and Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instruments on the DMSP because their certification will expire after August.

Plus the IMU must be re-calibrated, a "life extension" analysis will have to be performed for the satellite's pyrotechnic devices to ensure they can be used for the new launch date and a full spacecraft electrical retest must be conducted and mechanical deployments demonstrated again.

That means the satellite's launch campaign will start over from day one.

Once the satellite finally gets into orbit it will replace the DMSP F-13 satellite that has surpassed its expected lifetime. It was launched in 1995.

"I wish my car did as well as the F-13 satellite is doing. It is hanging in there extremely well," Odle said.

"We are all talking statistics here. Tomorrow something could happen, but our technical assessment is that F-13 will remain operational and doing its mission though the end of this year and perhaps into the early part of next year."

The U.S. military relies on two primary DMSP satellites to orbit around Earth's poles to gather the data meteorologists need to generate forecasts for strategic and tactical planning.

The satellites circle 458 nautical miles above the planet to track clouds, hurricanes and storm systems around the world for weather forecasting, plus monitor ice and snow coverage, pollution and fires.

Flight data file
Vehicle: Titan 2 (G-9)
Payload: DMSP 5D-3-F16
Launch date: November 2001
Launch window: 1353-1408 GMT (9:53-10:08 a.m. EST)
Launch site: SLC-4W, Vandenberg AFB, Calif.