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Self-Calibrating Satellite Imagers, how far can we go?
Steve Mackin
Aim
“The aim is to develop the means to have truly selfcalibrating satellite imagers with no human intervention and no onboard calibration equipment.”
Why do we (DMCii) need it ?
DMCii is a commercial company that currently provides calibration and data quality services for three satellites.
Always we seem to have 36 satellites to manage, but we expect the number to increase.
We also expect each satellites capability to increase (more bands, higher resolution, more images per orbit) so more data.
Sadly, there is no corresponding increase in staff numbers. We have approximately 0.08 of a year for one staff member to do this task.
Choices
Given the increasing data quantities with no increase in support we have no choice but to automate as completely as possible all of our procedures.
Our current satellites
3 to 5 bands in the VNIR2.5m to 22m GSDNo onboard calibration
Our future satellites ?
We would like to use 50kg satellites (B, G, R, NIR, SWIR), 12 bit data, two star cameras.
Its feasible to launch five together to get 24 hour repeat at 5m resolution, always on over land.
Data volumes would be massive for our small group
Current Calibration / data quality checks
Calibration Bias Dark images (Pacific at night)
Relative Calibration White images (Antarctic / Greenland)
Absolute Calibration (CrossCalibration) – Dune field (Libya 4)
Problems
Labour intensive process to extract information over the identified test sites.
Requires specific targets which may be cloudy (DomeC, Libya 4) or not suitable (Greenland and DomeC with low sun angles).
Uses satellite capacity which could be used on commercial contracts, as out systems are not currently “always on”.
Tends to be a less frequent activity than we would like given the other demands on our time.
Problems
If infrequent…
Vertical striping residuals (very common)
Changes in relative gain curve (has happened)
Absolute calibration drift (always)
Changes in SNR characteristics of sensor (does happen)
Also we can have calibration errors
Not yawing properly (relative calibration over DomeC)
Surface not homogeneous enough for relative calibration
Dome- C Issues
Surface VariabilityIssues in homogeneity across the swath of the instrument. We need to be careful when yawing the spacecraft to reduce illumination slopes in the profiles
What would be ideal ?
Use all images (heterogeneous)Crops, water, soil, urban
No specific calibration acquisitionsNever calibrate manually again…!!
Key parameters checked once per week
The System updates its own coefficients
The System flags changes to operators
The updates take place before a user can actually detect the data quality issue
Typical Images
Ordinary images collected over any target type
Is it feasible?
Individual detector drift (vertical striping)
Relative calibration curve changes (banding)
Signal to Noise Ratio changes
Individual Detector Striping
No matter how good the calibration of linear CCD’s there tends to be drift of individual detector response with time, producing striping effects.
Corrected in the past using white (snow) scenes which we consider uniform, but are only approximate flatfields.
Dome- C Issues
Surface VariabilityIssues in homogeneity across the swath of the instrument. We need to be careful when yawing the spacecraft to reduce illumination slopes in the profiles
Individual Detector Striping - Experiment
0.2% change >1% change
Individual Detector Striping - Experiment
• Coefficient number 222 was reduced by 0.2% for 10 NigeriaSat2 scenes
• This is about 40,000 lines of data
• As we increased the number of scenes we reduced the background clutter to reveal underlying features
Relative Calibration Changes
One can monitor changes in groups of detectors.Temporal changes, CCD boundaries, external influences
Relative Calibration Changes
One can monitor changes in groups of detectors.Prelaunch calibration errors
Series 1 is from heterogeneous images, Series 2 is pre-launch
Full SNR Profiles can be determined
The upper edge defines the SNR
Signal to Noise Ratio (SNR) estimation
Uses heterogeneous images (approximately 30 to 50% accepted)Improved SNR as we have changed data processing (8 to 10 bit, dynamic gain allocation)Could be implemented per detector…!
Absolute Calibration (automatically)
PseudoInvariant Calibration Sites
2001
Absolute Calibration using all images?
Examining the limitations currently using simulations to try and understand the fundamental accuracy we can achieve.
First tests using all data, show it to be less sensitive than using PseudoInvariant Calibration Sites.
Considering a new method based on modelling and measurement of the saturation radiance.