Diarad_SovaP instrument performance ( IRMB C. Conscience for S. Dewitte )

Picard Consortium meeting – PMOD/WRC, Davos, 29-30 Nov 2010

Diarad_SovaP instrument performance

( IRMB C. Conscience for S. Dewitte )

Radiometric equations Ground characterizations Flight calibrationsTasks to finalize


A02, A03 Radiometric equations

Ideal radiometer :

Real radiometer :

43'

2615 cos.''1..

1.CC

A

CCCPPCSI

eff

ocl

A

PPSI ocl

23_1_ tcltcl

cl

PPP

“It becomes absolute when all corrections are characterized and the measurements are traceable to international standards.”


Radiometric equation

• Precision aperture : A Power measurements : P Cavity efficiency : α

eff

Corrections– C1 : wires heating effect – C2 : precision of the servo-system– C3 : shutter emission – C4 : mirror emission– C5 : optical effects (diffraction, scattering, backscattering ) – C6 : sensors thermal equilibrium– Δ : mirror free thermal dilatation– Δ' : mirror thermo-mechanical effects– Δ'' : mirror initial deformation due to the fixture torque – cos θ : depointing

43'

2615 cos.''1..

1.CC

A

CCCPPCSI

eff

ocl


Ground characterizations

Electrical characterisations :

– Current measurement Resistor, R100 , in the lab.

– Reference voltages, V1-->V6, V'1-->V'6 during Th.Vac.

– Heating Resistor, R120 , during Th.Vac.

– the sensitivity is measured in air & vacuum

– Determination of parasitic Resistors, rc & rd , based on sensor sensitivity.

– Adjusting power equilibrium of the cavities– Adjusting sensors equilibrium– Calibration of thermometer sensor

Optical characterisations :– Quasi cylindrical tube profile– Cavity bottom uniformity – Cavity reflectance

Mechanical characterisations :– Precision aperture, A, measured by NIST & NPL


Electrical characterization : Calibration voltages

2210 .. resresn TaTaaV


Electrical characterization : Heater resistor

33

2210120 ... basebasebase TaTaTaaR

- A third order model of the heater resistor was established during the Th.Vac, and is valid for [-30°C 45 °C], with a traceable AGSE. This model serves also for quality control of the acquisition chains.

Rd(Tbase) en Rad05

120.26

120.265120.27

120.275

120.28120.285

120.29

120.295

120.3120.305

120.31

120.315120.32

120.325

120.33120.335

120.34

-30

-25

-20

-15

-10

-5 0 5 10

15

20

25

30

35

40

45

°C

Ohm

Rd


Electrical characterization : Heater parasitic resistor

Current flowing through the wires and the soldered joints produce a small power that is detected by the sensor and thus must be taken into account in the radiometric equation.

Charaterization done with traceable DVM, and based on the sensivity of the detector

[mOhm] u [mOhm]

rc,left 3.23 0.15

rd,left 3.70 0.16

rc,right 3.86 0.29

rd,right 2.70 0.21

dcparasitic rrR


Optical characterization :

C5: Correction factor for optical effectsDiffraction on precision aperture (δ’)Scattering around the intrance aperture (Σ)

Backscattered radiation from volume between shutter

and precision area (Σ')

''

15 C

Σ 7.5 10-6

from Solcon, to be updated

Σ’ 45 10-6

from Solcon, to be updated

δ’ 500 10-6

to be updated by NIST calculation


Mechanical characterization : Precision Apertures

Area measured by NIST at 20°C

– based on optical method

NPL has measured 4 diameters at 20°C

– based on a mechanical method

– with a measure of the departure from roundness

Aperture Sun side

Aperture Sensor side


Mechanical Correction : free dilatation Δ

k 8.6 ppm/°C

Titanium

T0 20.0 °C NPL, NIST

T 22°C 32 °C

Diarad in

flight

Δ = 0,00021 @ T = 32 °C

20

20 ).().(.2 TTkTTk

negligible


Flight calibration : corrections C3&C4

This correction has been calculated from the first stellar pointing.

– ~ - 1,15 W/m² ( loss of energy when the shutter is opened & T mirror decreases )

Solar pointing

Stellar pointing

Tright shutter (closed)

Tleft shutter (opened/closed)

ΔP = L_(shutter + mirror)


Flight calibration : corrections C3&C4

ΔTmg = -1.5 °C

ΔTog = -13 °C


Flight calibration : acquisition system

During the 3’ that lasts a radiometric cycle, 6 calibration voltages are fed into the acquisition system to calibrate it. A linear equation is establishedbetween each counter and the 6 voltages.

MUX

Amplifier V/F Counter

V1

V6

Sensor


Flight calibration : acquisition system

V_channel = A . counts + B

First switch on

switch off/on

Coefficients A, and B since the beginning of the mission : they track variations in the acquisition system. The effect of the decreasing of the Temperature of the V/F, due to “eclipses”, is visible in the green circles.


Uncertainty of the SI measurements in A02, A03

Uncertainty in [PPM]

u Left cavity

Right cavity

Traceable

Aperture A 107 117 yes

Thermal effects Δ 30 30 literature

Thermo-mechanical

effects

Δ’ 250 250

Fixation effects Δ’’ 250 250

Cavity absorption factor

αeff

190 190 not yet

Optical effects C5 110 110

Solar pointing Cos

TBAnalysed

TBAnalysed

Shutter correction

C3 39 39

Diaphragm correction

C4 19 19

Static error of the servo

C2 1.2 1.2 yes

Sensor thermal equilibrium

C6 TBAnalysed

TBAnalysed

Wire heating effect

C1 0.8 1.5 yes

Electrical chains

294 518 yes

Combined uncertainty

Uc 523 677

Expanded uncertainty (k =2)

U 10461354


Tasks to be finalized

WP1 : Optical corrections (Σ, Σ’, σ) need to be updated for SovaP

WP2 : Depointing correction – Analyse CNES report for depointing between SovaP and reference

surface.

WP3 : shutter & mirror corrections- Analyse in details stellar pointings to improve the correction.

WP4 : efficiency of the cavity Extra calibration of tiles has been asked

WP5 : corrections Δ’ , Δ’’ need to be characterized Description of new setup and test is requested

WP6 : estimate the global Uncertainty of the SI measurement – Case of A08 ( other radiometric equation for this mode ).

Diarad_SovaP instrument performance ( IRMB C. Conscience for S. Dewitte )

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