Pollux: a dedicated satellite for

light pollution studies

Yvan DutilIAU Commission 50

Yvan.Dutil@sympatico.ca

Introduction

• Global analysis of the distribution of light pollution has used satellite measurement as input (Cinzano et al 2001)– Nevertheless, these data were produced by an

instrument not optimised for this application.

• The Pollux mission is based on a microsatellite platform equipped with a multispectral imager.

• Both improvement in modeling of the light pollution and the characterisation of the atmospheric aerosol is expected from this mission.

Light pollution satellite observation

Based on

Satellites DMSP

Altitude 830 km

Period 101 min

Resolution 2,7 km

Photometric measurement

only

Normal observing mode impose a continual gain adaptation

•Special observing mode required

<11% 11%-33% 33%-100% 1-3 fois 3-9 fois 9-27 fois > 27

Light pollution map of Eastern Canada

Problems with data we use now

• Poor temporal sampling

– Model output sensitive to seasonal change in atmosphere

– Data mostly taken in winter (snow mess everyting)

• Photometric measurement only

– Impossible to isolate various source of night light

• Contamination by Moon, Aurora, Nightglow

– Impossible to separate aerosol from molecular diffusion

• DMSP OLS has a poor photometric stability

• Angular sampling is inadequate

What you should know about of light

pollution!

Light emitted near the horizon

pollute 2.7 more than the diffuse

light

An increase of 1% of the light

pollution emitted above ground

lead to an increase of 25% of the

total light pollution!

V

B

Blue photons (440 nm) pollute

1.6 time more than green

photons (550 nm)

Pollux key requirements

• Ground resolution: 2.4 km at nadir

• Sensitivity: 10-9 W cm-2 str-1 µ-1 @ SNR=3

• Spectral coverage: 400 nm - 760 nm

• Spectral resolution: 10 nm

– Enable light source characterisation

• Zenithal angle coverage: 5° to zenith

– No more model guessing

Pollux system parameters• MMMSB Bus of CSA

– 75 kg

• Sunsynchronous orbit

– 22h equatorial passage

– 20 days repeat cycle

– Altitude: ~ 650-800 km

– Inclinaison: ~ 98°

• Potential problems

– Bus design for dusk to dawn orbit

– Onboard memory storage should be increase

– Downlink capability?

Pollux system parameters

Three spectrometers side by side

• Input optics

– Diameter 5 cm

– F# 1.4

– FOV 22°

• CCD3011 E2V 1024x256

– Pixel size 26 m

– Quantum efficiency >50%

– Read-out noise 18 e¯

– Binning 8×8

• Integration time 0.3 s

Dreadful data rateMultispectral imager produce a large amount of data

• Raw: 256×1024 pixel2×16 bit × 3 frames/s×3 spectros

– 37.7 Mbits/s

• Onboard Binning 8 × 8 : 600 000 kbits/s

Daily Observation time: 35000 s (40 %)

• Daily data volume: 21 Gbits!

– MOST: 20 Mbits! (1000 × less)!!!

Data handling strategy

• Targeted observation: ÷ 4

• Onboard compression: ÷ 10

• Microsat receiving station with arrayed antenna

– × 20 in downlink capacity (Wells & Zee 2003)

Others channels

Panchromatic imager

• Much higher spatial resolution

– 300 m or less

• Much high sensitivity

– TDI integration

• Allows to pinpoint the sources

– Confirm the geolocation

TIR imager

• Would allow simultaneous detection of cloud

• Possible data rate reduction by onboard data clipping

Conclusion

• Pollux would represent the next step for global light

pollution modelling and monitoring

• Moon illumination restrict the light pollution

measurement to 10 days per lunation near new moon

– Opportunity for hyperspectral imaging on other period