December 3-4, Green Bank GBT PTCS In Progress Review Laser Rangefinder Update 9/5/03 Experiment K. Constantikes

GBT PTCS In Progress Review – December 3-4, Experiment Designed to correlate thermal, astrometric data with LRF data ~24 hours of: –Point/focus at source near NCP –“Track” at 360°, 38° for 2 minutes –LRFs running continously, 15 sec sample interval 11 targets, some with 4 rangefinders designated (FA tip, el bearing) Zero points for each instrument Leakages for each instrument –Weather data, 10 sec sample interval LRFs had pointing optimized for all target at this GBT pose

GBT PTCS In Progress Review – December 3-4, Range data processing Zero points and leakages smoothed: 75 sec median Group refractive index smoothed: 120 sec median LRF integration time 1 sec SNR >= 10: Phase estimate noise ~<200  m –Selected low SNR to get more range data… –RMS range residual in trilateration will ID noisy range data 97F Fem model used for range prediction and resolving range uncertainty Ranges corrected for glass offset, GRI, monument and rangefinder geometry For 4 el bearing targets (2 each side) and 2 FA tip targets: –Select last 60 secs of data (up to 4 ranges, 4 rangefinders), GBT at 360°, 38°  1” –~< 650  m FA y, ~< 250  m FA x,z –~< 125  m el bearing x,y

GBT PTCS In Progress Review – December 3-4, EMS Flowgraph

GBT PTCS In Progress Review – December 3-4, Bounding actual FA position errors 2.4” ~= 0.8mm at FA tip

GBT PTCS In Progress Review – December 3-4, Bounding actual el bearing position errors 3” at el bearing ~= 0.4mm

GBT PTCS In Progress Review – December 3-4, ZAG736D: The best 1900 outage due to SW failure Some trend visible in X No data after ~2130 –LRF pointing –Dew, fog Expectation of 1  range error ~40  m –3 rangefinders –Consistent PTCS/PN/8

GBT PTCS In Progress Review – December 3-4, ZAG736D: The best - Dispersion of range error broader due to low SNR threshold  m * PDOP ~ 300  m total position error SHORT TERM (60 secs) ? - Actual 1  total position error ~ 2mm, 1  y error ~0.7mm, or ~ 6” azimuth pointing error equiv. ~ 25 minutes

GBT PTCS In Progress Review – December 3-4, ZEG41040L: Monument errors No data after ~2100 Position jumps due to redundant ranging (4 LRFs) and monument posn errors Consistent with ~1 mm monument posn error. X (mm) Number of LRFS

GBT PTCS In Progress Review – December 3-4, ZY107: Erratic pointing Very high SNR occasionally, but long outages

GBT PTCS In Progress Review – December 3-4, ZY102: Low laser power? Consistent SNR Degrade after sundown- –Dew? –Thermal pointing shift? SNR would be still be 0.1sec integration 1900 outage was software

GBT PTCS In Progress Review – December 3-4, ZY103: Inadequate laser power, poor pointing Laser power too low for 0.1 sec integration time Pointing erratic too…

GBT PTCS In Progress Review – December 3-4, Data quality statistics 6 Targets, ~ 20 hours, 4 ranges per minute: –2 retros per el bearing, 2 FA tip retros –Possible 20x60x4 = 4800 trilaterations –About half lost due to fog, dew –Others lost to pointing/SNR issues (even though SNR=10 threshold used) Total “good” trilaterations, percent excluding fog/dew: ZAG731D ………….210 or 10% ZAG731P ………….142 or 5% ZAG736D ………….611 or 25% ZAG736P ………….44 or 0% ZEG41040L ……….241 or 10% ZEG41040R ………337 or 15%

GBT PTCS In Progress Review – December 3-4, Reliability Out of six experiments, not one case where all 12 ground LRFs were operational –~ one week preparation time each case. Pointing problems most common, but also –Oscillator failures –Servo failures –Computer (ZY and ZIY) failures

GBT PTCS In Progress Review – December 3-4, Fixes LRF outgoing and incoming beams need to be broadened, pointing errors identified Some laser diodes replaced (obsolete, will require mechanical retrofits Other parts nearing obsolesence (IRIG, Oscillators…) 360 Degree retros to improve ground geometry Beam split and track to ameliorate detector group delay issues (centriod and carrier density)? How to ameliorate dew problem? More lab characterization to ensure we’re ready… Major, time consuming, expensive undertaking

GBT PTCS In Progress Review – December 3-4, Summary Not only are there fundamental issues- –Geometry and group refractive index And major system design issues- –E.g., LRFs on feed arm But also suitability, usability, cost, and schedule issues. And we know how to fix them…