ELT delays absolute calibration

Ivan Procházka (1), Josef Blazej (1), Anja Schlicht (2)

Prepared for ESA Webex meeting(s) March 2015

(1) Czech Technical University in Prague, 119 19 Prague 1, Czech Republic(2) Technical University in Munich, Munich, Germany

I. Prochazka et al., Prague, March 2015

ELT detector status, early 2015 The ELT detector package EM# tests completed

Flying unit expected June 2015

Detector parameters (within specs including BIG margins) jitter 18 ps rmsdark count rate ~400 kHz @ +30 Cdelay drift +0.38… +0.8 ps / Kdelay stability TDEV ~ 200 fs / day detection delay absolute ±15 ps

Ground system delay calibration scheme designed and tested (Wettzell SLR) for SLR network

Testing and device calibration procedures for ELT detector package designed, tested and approved(including the ELT Calibration Device).

I. Prochazka et al., Prague, March 2015

ELT Laser Time Transfer delays

GOAL Ts versus Tg

GROUND SEGMENTDT1,2 optical DT3 photodiode DT4 cableET 1 timing system response

GROUND – SPACEDTr via SLR

SPACE SEGMENTDT5 ELT detector DT6 cabling ET 2 timing system response

I. Prochazka et al., Prague, March 2015

ELT Ground segment (SLR) – delay determination The ELT ground segment delays will

be calibrated individually for each participating SLR system

Calibration value computed from - epoch dif. (E2-E1)- geometry distance L

The result is a single constant describing all the delays related to a ground segment including the response of a ground timing system ET1

ELT Calibration Device consisting of a detector (twin of FM) and a timing system.

Epoch E1 is recorded using the SLR systemEpoch E2 is recorded using ELT Calibration Device Both systems use common time & frequency sources (including “1pps” cable)Procedure tested repeatedly at SLR Wettzell, Germany, results presented at EFTF 2014

I. Prochazka et al., Prague, March 2015

ELT ground – space delay determination The ground – space delay will be determined

by means of satellite Laser Ranging (SLR) technique. This techniques provides the pulse propagation delay ground – space – ground with high precision and accuracy.

The ground – space propagation delay will be evaluated as one half of SLR determined value with the application of correction related to:

Retro-reflector array internal delay Geometry : retro array <-> ELT det.

The atmospheric delays are fully removed

I. Prochazka et al., Prague, March 2015

ELT space segment delay determination #1

GOAL determination of a sum of DT6 & ET2 modest accuracy & stability requirements ~ 150 ps

DT5 delay Optical / electrical of the ELT detector packagedetermined by laboratory tests

accuracy and stability much better than 50 ps

DT6 propagation delay of signal cables pair

ET2 characteristics of the space based Event timing system

I. Prochazka et al., Prague, March 2015

ELT space segment delay determination #2measurement procedure

HW needed ELT detector BB modified for tests including its Control boxMWL_ELT timing system, cabling, test sw

digitizing oscilloscope, >= 10Gs/s, >=1GHz BW one high BW (>= 1GHz) probe for the scope frequency source for timing, 1pps signal available

Procedure - the ELT detector package will be replaced by a BB - BB will be gated by 1pps, thus generating outputs ~ 20ns after - BB output(s) will be recorded by scope exter. triggered by 1pps - alternatively the 1pps signal will be recorded - the time diff. “output/1pps” will be compared to epoch reading - calibration constant is a difference of these two numbers

I. Prochazka et al., Prague, March 2015

ELT space segment delay determination #3measurement scheme

ELTBB MWL

timing

1 ppsgate

probe position 1

probe position 2

ΔT

SCOPE

ext. trigger

I. Prochazka et al., Prague, March 2015

CONCLUSION - summary

Ground segment is characterized by “ELT Calibration Device”accuracy and stability far below 100 ps

Ground to space delay including atmosphere will be mapped by SLRaccuracy and stability better than 20 ps

ELT detector only space segment will be characterized by CTU testsaccuracy and stability better than 20 ps

ELT related cabling and epoch timing can be characterized by simple measurements during the FM assembly “open box” phasemeasurement procedure was described in this documentaccuracy and stability of this delay (expected) better than 150 ps

= > The entire ELT chain delays and stability may be determinedwith the overall accuracy and stability on 200 ps level.