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, 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) jitter18 ps rms dark count rate~ C delay drift+0.38… +0.8 ps / K delay stability TDEV~ 200 fs / day detection delayabsolute ±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 SEGMENT DT1,2 optical DT3 photodiode DT4 cable ET 1 timing system response GROUND – SPACE DTr via SLR SPACE SEGMENT DT5 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 system Epoch 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 GOALdetermination of a sum of DT6 & ET2 modest accuracy & stability requirements ~ 150 ps DT5delay Optical / electrical of the ELT detector package determined by laboratory tests accuracy and stability much better than 50 ps DT6propagation delay of signal cables pair ET2characteristics of the space based Event timing system

I. Prochazka et al., Prague, March 2015 ELT space segment delay determination #2 measurement procedure HW neededELT detector BB modified for tests including its Control box MWL_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 #3 measurement scheme

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 SLR accuracy and stability better than 20 ps ELT detector only space segment will be characterized by CTU tests accuracy and stability better than 20 ps ELT related cabling and epoch timing can be characterized by simple measurements during the FM assembly “open box” phase measurement procedure was described in this document accuracy and stability of this delay (expected) better than 150 ps = > The entire ELT chain delays and stability may be determined with the overall accuracy and stability on 200 ps level.