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…