GLAST Large Area Telescope: NCR 529 Systems Engineering R. Bright

1. Gamma-ray Large Area Space Telescope GLAST Large Area Telescope: NCR 529 Systems Engineering R. Bright

2. Outline • Problem Statement & Current Results • History/Background • Future Options • Potential Testing

3. Problem Statement & Current Results • TKR FM3 errors started after connection to TEM GLAT1835. • Revised integration sequence pairs TKR to Flight TEM for first time in Grid. • Errors are frequency but intermittent. • Errors appear to be sensitive to clock frequency. • Unlikely that MCM has degraded. • Most likely that TEM to MCM interaction induces problem. • Other 5 towers were tested over frequency and do not exhibit the failure.

4. History & Background • Tracker Module FM3 testing did not exhibit MCM errors. • Trackers are tested at lower voltage than Flight TEMs provide. • Lower voltage was stress case for other failures. • MCM 11377 tests were error free. • TEM GLAT1835 test results were nominal. • Review of TEM EICIT data for CCA board indicates in family performance. • TEM Clock signal quality is not measured.

5. History & Background (continued) • Error signature is identical to Tracker MCM level error, NCR 104 which was for failures on other MCMs. • Exact cause of error at MCM level was not determined. • Higher voltage increases occurrence of error. • MCM GCRC resistor value changed from 75 ohms to 100 ohms to mitigate error. • Paper work from Teledyne for MCM rework cannot be trusted. • Random inspection revealed multiple cases of reworked MCMs with no documentation. • No MCMs were found that had not been reworked to new resistor value. • Most likely that MCM has 100 ohm resistor. • No other known failure modes could induce this error signature.

6. Future Options • Repair Tracker FM3 MCM • Risk to hardware perceived to be high. • Risk to schedule perceived to be high. • R&R TEM GLAT1835 • Risk to hardware perceived to be medium. • Risk to schedule perceived to be medium. • May not be successful in removing failure. • Use-As-Is (UAI) • 1 of 6 towers installed in Grid exhibit this error. • Other towers yet to be integrated may exhibit this error. • Environmental testing may provoke error. • Flight S/W impact is expected to be none based on discussions with M. DeKlotz. • ISOC impact is unknown. • SAS impact is unknown. • User community impact is unknown. • NASA opinion has not been solicited.

7. Potential Testing • Possible that power off resistance measurements at GLAT1835 GT-6 connector will be nominal. • Repair of Tracker FM3 MCM is unlikely. • Future errors in Integrated Flight Towers may be mitigated by return to single flight tower integration before installing in Grid.