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Ortho ready (L1C)

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Product description

Ortho Ready, also known as L1C, is a 4-bands imagery product designed for users who want to perform their own orthorectification process with some DEM (Digital Elevation Model) of their choice. The 16 bits, 4-bands (visual and near infra red) rasters are shipped alongside RPCs (Rational Polynomial Coefficients). These two can be combined with a DEM to orthorectify the images and obtain geometrically corrected rasters with a uniform scale.

The main components of this product are the raster and the RPCs. The former comes in GeoTIFF format and the latter comes duplicated both as GeoTIFF tags inside the raster file and as a separate text file with the same base name as the image (for example image.tif and image_rpc.txt). These two RPC formats, inner tags and external text file, are well understood by main GIS software 1. The reason for the duplication is that some software supports one version but not the other one.

The RPCs are obtained after computing Ground Control Points over the images against a reference 2 and performing a bundle block adjustment to minimize the errors and recompute the satellite position and pose. The physical models obtained are transformed and provided as polynomials.

Each raster has a fixed size that depends on the satellite generation, currently available for MarkIV and MarkV. The nominal resolution is also dependant on the satellite generation, the orbit altitude and the off-nadir angle of each capture. For one given scene, several L1C images are provided, where each one of them has some overlap (around 15%) with the following one.


An example MarkIV L1C raster.

Product formation

Ortho ready product level is composed by a collection of anchor frames, these frames are 4-bands stacked images that contain continuous spectral information. With the available spectral and geometrical information the anchor frames or composites are constructed.


L1C product coverage.

Technical characteristics

Parameter Mark IV Mark V
Spectral bands Blue : 450 - 510 nm
Green: 510 - 580 nm
Red : 590 - 690 nm
NIR : 750 - 900 nm
Blue : 450 - 517 nm
Green: 517 - 583 nm
Red : 597 - 690 nm
NIR : 759 - 890 nm
GSD 0.7m - 1.35m 3 0.73m - 0.81 3
Swath @ nadir 4.8 - 5.5 Km 3 6.8Km - 8.5Km 3
Projection Geographic coordinates at WGS84
Max Off-Nadir angle ± 25 deg
Image size 5120 x 5120 pixels 9344 x 7000 pixels
Image bit depth Unsigned integer 16-bit
Bands 0: Red
1: Green
2: Blue
3: NIR
Band to band alignment <= 1px ce90
Anchor frames overlap 12% - 22% 3
Image format GeoTIFF, LZW lossless compression
Ancillary files RPCs (TXT) and preview (PNG)

Product components

Each L1C product contains multiple assets as well as metadata. All assets follow the same basename template.

{DATE}_{TIME}_{SECOND_DECIMALS}_{PAYLOAD}_{SATELLITE}_{PRODUCT}_{VERSION}_{SUFFIX}.{EXTENSION}

  • DATE: Year(4 digits)month(2 digits)day(2 digits)
  • TIME: Hour(2 digits)minute(2 digits)second(2 digits)
  • SECOND_DECIMALS: All the decimal part from the timestamp seconds
  • PAYLOAD: MS for multispectral (4 bands)
  • SATELLITE: Satellite name
  • PRODUCT: L1C
  • VERSION: Check Changelog
  • SUFFIX: depends on the asset, might be _RPC, or nothing for example.
  • EXTENSION: depends on the asset type.

All time values refer to the capture time in UTC. Hence the string 20220619_153346_938947represent actually 2022-06-19 15:33:46.938947.

Raster

drawing The main raster, also called anchor frame, contains the ready to orthorectify data. It is a fixed size (check Technical specs) 4 bands (RGB and NIR), 16 unsigned bits raster. Its nodata value is 0. Its CRS defined by its RPCs is geographic WGS84. The RPCs are written inside the GeoTIFF as tags 1 and also as a different file in another asset. The raster format is GeoTIFF with LZW compression.

Example filename: 20220619_153346_938947_MS_NS13_L1C_0_0_0.tif

RPC

drawing The RPC 4 is a text file with a common base name with the raster plus _rpc.txt as suffix. This is a common format to store this information that is understood by most software with orthorectification capabilities.



Preview and thumbnail

drawing The preview and thumbnail are a reduced version of the raster in 8 bits and png, for visualization and navigation purposes.




Metadata

Metadata is available per anchor frame. Common metadata can be found across composites of the same capture. The following table shows the description of each field and an example corresponding to the image used in previous section.

Field Description Example
gsd The approximate ground sampling distance of the image, in meters. 0.98
datetime Capture date and time of the item. In UTC 2023-11-28T11:28:25.279649+00:00
platform Satellite name that acquired the imagery newsat13
eo:cloud_cover The percentage of cloud found on the image, from 0 to 100. 41.7
view:off_nadir The off-nadir angle for the capture measured in degrees. 25.8465
view:sun_elevation The elevation angle of the sun at the target, at the moment of the capture, in degrees. 50.4441
view:sun_azimuth The azimuth angle of the sun at the target with respect to the north 156.912
satl:outcome_id A unique identifier of the associated capture with this item. d5627157-d1aa-472d-9daa-a55086fd32c2
satl:valid_pixel The percentage of valid pixels on the tile. If less than 100 it means that part of the image contains NO_DATA 99.457
satl:product_name Product type L1C
satl:product_version The version of the product. Uses semantic versioning. 1.1.0
satl:ba_[B1]_[B2]_ce90 Circular error at 90th percentile of alignment between bands [B1] and [B2], in pixels 0.432
satl:ba_[B1]_[B2]_rmse Radial Root mean square error of alignment between bands [B1] and [B2], in pixels 0.254
satl:uint16_to_reflectance_[BAND] The factor to apply to the DN value of every pixel to obtain reflectance units between 0 and 1 . [BAND] can be Red, Green, Blue, NIR. It has a fixed value. 0.0001
satl:uint16_to_radiance_[BAND] The factor to apply to the DN value of every pixel to obtain radiance units of W/(m² sr nm) . [BAND] can be Red, Green, Blue, NIR. 0.0005934402

Radiometric corrections

L1C products are built on top of the L1A, which includes the following radiometric corrections.

  • Dark signal
  • Non-uniformity in pixels response
  • Stray light
  • Hot and dead pixels
  • PSF deconvolution
  • Vicarious calibration

Product quality

Band alignment

Given 4 bands (R, G, B, Nir), there are 6 band to band combinations. For each band to band combination, a process based on FFT Phase Matching is run to obtain up to around 2000 matches. There are band combinations that are more difficult to match, like blue and nir due to their spectral difference. With all the matches two different statistics are calculated:

  • ce90: The circular error at the 90th percentile. In other words, expect that 90% of the contents of the image have an error below this threshold.
  • rmse radial: The Root Mean Square Error gets computed per X and Y dimensions. This value is the norm of that vector.


ce90 as a circle over the matches residuals

These two metrics are provided as metadata with each item.

Known Issues of the current version

This section lists product anomalies which sometimes elude the automatic quality assurance criteria, distinguishing those correctable by repeating the image processing steps with different configurations, and those uncorrectable or needing further analysis / pipeline improvements. The most common and best known features are listed in this section. This list may not be exhaustive, but it will be updated in future editions of this document.

Blunders

Anomalies are discerned through a distinctive contrast inversion observed in the NIR band relative to others, with a notable tendency for this irregularity to impact road segments in closer proximity to agricultural fields.


Miss matching effect.

Failure in composition

Eventually the algorithm of composition may fail when creating a composite image. This problem should be isolated respect the surrounding composites, usually due to the need of improving the algorithm with specific image contents. It is expected that this issue will be less likely to be found with upcoming product versions, where the algorithm will progressively get more robust.


Isolated failure in image composition

Changelog

Version Release date Change log
0.10.0 15/01/2024
  • Geolocation improvement by fine-tuning gcp extraction
  • Allow MarkV processing
0.9.0 15/12/2023
  • Initial version

  1. More info: gdal documentation 

  2. Reference image source: esri. Heights source: mapzen 

  3. Varies with altitude of the satellites and off nadir angle. Some Mark IV satellites will decay naturally in the following years, so a smaller GSD can be expected. Varies with off nadir angle too. 

  4. RPC definition 


Last update: 2024-02-19