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Laser Rangefinder Update 9/5/03 Experiment. K. Constantikes. 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 - PowerPoint PPT Presentation
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December 3-4, Green BankGBT PTCS In Progress Review
Laser Rangefinder Update9/5/03 ExperimentK. Constantikes
2GBT PTCS In Progress Review – December 3-4, 2003
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
3GBT PTCS In Progress Review – December 3-4, 2003
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
4GBT PTCS In Progress Review – December 3-4, 2003
EMS Flowgraph
5GBT PTCS In Progress Review – December 3-4, 2003
Bounding actual FA position errors
2.4” ~= 0.8mm at FA tip
6GBT PTCS In Progress Review – December 3-4, 2003
Bounding actual el bearing position errors
3” at el bearing ~= 0.4mm
7GBT PTCS In Progress Review – December 3-4, 2003
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 ~40m
– 3 rangefinders– Consistent
PTCS/PN/8
8GBT PTCS In Progress Review – December 3-4, 2003
ZAG736D: The best
- Dispersion of range error broader due to low SNR threshold.- 40m * PDOP ~ 300m total position error SHORT TERM (60 secs) ?- Actual 1 total position error ~ 2mm, 1y error ~0.7mm, or ~ 6” azimuth pointing error equiv.
~ 25 minutes
9GBT PTCS In Progress Review – December 3-4, 2003
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
)N
umbe
r of
LRFS
10GBT PTCS In Progress Review – December 3-4, 2003
ZY107: Erratic pointing
• Very high SNR occasionally, but long outages
11GBT PTCS In Progress Review – December 3-4, 2003
ZY102: Low laser power?
• Consistent SNR• Degrade after
sundown- – Dew?– Thermal
pointing shift?• SNR would be still
be adequate @ 0.1sec integration
• 1900 outage was software
12GBT PTCS In Progress Review – December 3-4, 2003
ZY103: Inadequate laser power, poor pointing
• Laser power too low for 0.1 sec integration time
• Pointing erratic too…
13GBT PTCS In Progress Review – December 3-4, 2003
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%
14GBT PTCS In Progress Review – December 3-4, 2003
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
15GBT PTCS In Progress Review – December 3-4, 2003
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
16GBT PTCS In Progress Review – December 3-4, 2003
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…