AxelGlobe Image Specification sheet

Change history

version revision date revised content remarks
1.00 2020-12-01 First version
1.10 2021-06-10 Reflect service update
1.20 2021-08-27 Add section 2.1. Revised definition of band/layer
1.30 2021-10-13 Changed File contents (section 3.2.3 and 3.3.3)
1.40 2021-12-13 Changed File contents (section 3.4)

1. Introduction

1.1. Purpose of this document

This document explains remote sensing image products taken by GRUS satellite operated by Axelspace. The specifications of this image product are subject to change without prior notice.

1.2. About AxelGlobe

AxelGlobe is the Earth observation infrastructure for a new era. It will have the ability to image the whole civilized world with a spatial resolution of 2.5 m every day. Its advantage is not just up-to-date information it will provide, but also in the collection of its past data. Comparing older images and studying their evolution can lead to invaluable insights and predictions for the future. As of June in 2021, there are five GRUS satellites that composes the AxelGlobe constellation, and the GRUS series will continue to be added in the future.

2. About GRUS satellite

GRUS is the name of the micro earth observation satellite that constitutes the AxelGlobe constellation. The GRUS satellite is equipped with two telescopes, which contributes to expanding the observation width. As a result, It has achieved both a high resolution of 2.5 m and a wide observation range of 55 km or more.

GRUS satellite illustration

The details of the GRUS satellite are described in Table 1.

Table 1. GRUS satellite system specifications

Item Contents
Number of satellite 5 (as of Aug. 2021)
Design life Over 5 years
Orbital altitude 585 km, Sun-synchronous sub-recurrent orbit
Equator passage time 10:40-11:00 (local time)
Spectral band Band ID Name Wavelength range (nm)
band0 Panchromatic 450-900 nm
band1 Blue 450-505 nm
band2 Green 515-585 nm
band3 Red 620-685 nm
band4 Red Edge 705-745 nm
band5 Near Infrared 770-900 nm
Ground resolution (at nadir)

Panchromatic: 2.5 m or less

Multispectral: 5.0 m or less

Swath 55 km or more
Longest collection length 1,000 km
Sensor bit depth 12-bit

2.1 Spectral bands, relative response and ESUN

GRUS RSR

GRUS RSR

Data

3. AxelGlobe image product specification

3.1. GRUS image product

A summary of GRUS satellite image products provided by Axelspace is outlined in Table 2.

These image products consist of AxelGlobe Cells, measuring about 5 km × 5 km per tile. This product is intended for applications that require accurate location information and map projection images. The image product is processed with sensor correction, geometric correction, and map projection. In addition to automatically extracting the ground control points (GCP) from the high positional accuracy base map using our unique matching technology, we also combine conventional methods for areas where the GCP extraction is difficult. In applying correction processing, the location accuracy of the product depends on the GCP used and varies from region to region.

In addition, mask data that indicates areas unsuitable for analysis are provided as an unusable data mask file(UDM file). The UDM marks pixels with missing data, areas falling outside the captured image boundary, or obscured by clouds. The UDM file is available in products produced on Oct. 14, 2020 or later.

Table 2. GRUS satellite image product types

Product name content
Multispectral Image Products (MSI) This product is processed with sensor and geometric correction. Pixel resolution is 2.5 m in panchromatic image and 5.0 m in multispectral image. Positional accuracy is aimed within 10-meter RMSE range.
Relative/absolute radiometric correction is also applied.
The pixel value for this data is obtained by scaling the reflectance at the top of the atmosphere to an unsigned 16-bit integer value.
True Color Image Products (TCI) This product is processed with sensor, geometric correction, and pansharpening. Pixel resolution is 2.5 m. Positional accuracy is aimed within 10-meter RMSE.
For visual enhancement purposes, this product has also been post-processed with color-correction technique. A pixel value is scaled into an unsigned 8-bit integer.

3.2. Multispectral Image Products (MSI)

Multispectral Image Products consist of panchromatic: has sensitivity in wide wavelength and 2.5m resolution, and multispectral: has sensitivity at different wavelength and 5 m resolution, bundled imaging products. In this image product, artefacts such as pattern noise caused by all sensors are corrected. The pixel value is then converted into Top-of-Atmosphere (TOA) reflectance values.

3.2.1. Multispectral Image Product specification

The Multispectral Image Products contains the datasets shown in Table 3.

Table 3. Multispectral Image Product specification

Product attribute Content Description
Product configuration and format Panchromatic imagery file

16-bit panchromatic imagery data that is divided by AxelGlobe cell. GeoTIFF format or JPEG2000 format.

Layer assignment:
Layer1: band0 (Panchromatic)

Multispectral imagery file 16-bit multispectral imagery data that is divided by AxelGlobe cell. GeoTIFF format or JPEG2000 format.

Layer assignment:
Layer1: band1 (Blue)
Layer2: band2 (Green)
Layer3: band3 (Red)
Layer4: band4 (Red Edge)
Layer5: band5 (Near Infrared)

Unusable Data Mask (UDM) file

Binary image data that indicates the unusable region. GeoTIFF format or JPEG2000 format. The pixel resolution matches that of the product.

Layer assignment:
Layer1: No data region
Layer2: cloud covered region

Digital number assignment:

Layer1:
Invalid data area: 1
Valid data area: 0

Layer2:
Cloud area: 1
Cloud free area: 0

Metadata file Metadata for panchromatic imagery file, multispectral imagery file, and unusable data mask file. JSON format.
Product frame AxelGlobe Cell Our global grid system is based on the AxelGlobe Cell measuring about 5 km x 5 km per tile. A buffer of about 100m (overlap area with adjacent cells) is added to each of the four sides.
If the captured image does not fill the entire area of the cell, the unfilled area will be indicated with black pixels wherein No data value (0) is assigned.
Pixel resolution

2.5 m (Panchromatic)

5.0 m (Multispectral)

The actual ground sampling distance varies on the collection conditions (mainly due to the satellite roll angle), but a sampling process is applied to the specified pixel size when product is generated.
Bit depth 16-bit unsigned integer GRUS satellite images are stored in the onboard memory as up to 12 quantized bits. Relative DN values obtained directly from the sensor are converted to reflectance value at the top of atmosphere by radiometric correction, and the result is scaled (10,000 times) to a 16-bit dynamic range.
(* Refer to 3.2.4 Radiometric correction)
Resampling Cubic Convolution Performs resampling processing using the Cubic Convolution interpolation method, which uses the weighted average value calculated from the values of the 16 input cells around the output cell. This sampling method has the characteristic that the boundaries of data tend to be sharp.
Geometric correction sensor telemetry Internal detector geometric correction combining the virtual array with eight imagers
band-to-band registration Band-to-band registration correction used to rectify misalignment between bands
Geo-rectification In geo-rectification, the characteristic terrain and features are extracted and paired between the captured image and the existing satellite image data (base map). The extracted terrain and features are matched with the reference points of each base map corresponding to the reference points. Processing is performed to reduce the positional deviation of the entire image with respect to the base map.
Radiometric correction

Correction of relative value difference of radiometric characteristics between detectors

Undetected detector filling for null values from unresponsive detectors

Conversion to reflectance values at the top of atmosphere based on calibration factors

Scaling reflectance values at the top of atmosphere to 16-bit unsigned integer values (10,000 times)

Horizontal geodetic system WGS1984
Projection Universal Transverse Mercator projection (UTM) UTM is a transverse Mercator projection that divides the earth into longitude zones every 6° eastward from the 180° meridian and projects the central meridian as the central meridian from the spheroid directly into the plane for each longitude band. Zones are determined by the south-west corner coordinates at the start of capture.

3.2.2. File naming

Multispectral Image Products are named according to the following naming convention. The file name contains several components that help organize, categorize, and refer to the information contained in the product.

<SatelliteName>_<AcquisitionDatetime>_<ProductLevel>_<ImageType>_<CellID>.<Extention>

Table 4. File naming convention

File naming component Description
<SatelliteName> Satellite name abbreviation (e.g. : GRUS-1A = GRUS1A)
<AcquisitionDatetime> The collection start date and time in the yyyymmddhhmmss description format. (e.g.:Jan. 2, 2018 11:32:54 = 20180102113254)
<ProductLevel> L3A
<ImageType> Image type Description
Panchromatic PAN
Multispectral MSI
Unusable mask file for panchromatic PAN_UDM
Unusable mask file for multispectral MSI_UDM
<CellID> Unique ID of AxelGlobe Cell
<Extention> Extension of imagery file(e.g.:GeoTIFF:tif)

3.2.3. File contents

Details of the Multispectral Image Products are provided in the file contents shown below.

EULA_en.txt                                             # License agreement (English)
EULA_jp.txt                                             # License agreement (Japanese)
GRUS1A_20200811011052
├─ GRUS1A_20200811011052_L3A_MSI_N42092354.tif          # Multispectral imagery
├─ GRUS1A_20200811011052_L3A_MSI_N42092355.tif          # Multispectral imagery
├─ GRUS1A_20200811011052_L3A_MSI_metadata.json          # Meta data for multispectral imagery
│
├─ GRUS1A_20200811011052_L3A_MSI_UDM_N42092354.tif      # UDM for multispectral imagery
├─ GRUS1A_20200811011052_L3A_MSI_UDM_N42092355.tif      # UDM for multispectral imagery
├─ GRUS1A_20200811011052_L3A_MSI_UDM_metadata.json      # Meta data for UDM of multispectral imagery
│
├─ GRUS1A_20200811011052_L3A_PAN_N42092354.tif          # Panchromatic imagery
├─ GRUS1A_20200811011052_L3A_PAN_N42092355.tif          # Panchromatic imagery
├─ GRUS1A_20200811011052_L3A_PAN_metadata.json          # Meta data for panchromatic imagery
│
├─ GRUS1A_20200811011052_L3A_PAN_UDM_N42092355.tif      # UDM for panchromatic imagery
├─ GRUS1A_20200811011052_L3A_PAN_UDM_N42092354.tif      # UDM for panchromatic imagery
└─ GRUS1A_20200811011052_L3A_PAN_UDM_metadata.json      # Meta data for UDM for panchromatic

3.2.4. Radiometric correction

To ensure the radiometric quality of GRUS satellite image products, we regularly observe calibration sites around the world and perform relative/absolute radiometric calibration.

All GRUS satellite images are collected in 12-bit and stored in the satellite's onboard memory as an image data with up to 12 quantized bits. It is scaled to a 16-bit dynamic range by radiometric correction in the ground processing system. With this, the relative pixel DN value obtained directly from the sensor is converted to the reflectance at the top of the atmosphere (TOA) by absolute radiometric correction and stored as a 16-bit value after scaling.

The pixel value of the Multispectral Image Products can be converted to the TOA reflectance by the following formula.

      REF(i)=DN(i)* ScaleFactor(i)
      where ScaleFactor(i) = 0.0001

 where
 i: band id
 REF:TOA reflectance
 DN:pixel value

Besides, TOA reflectance(REF) can be converted to the TOA radiance by the following formula:

      RAD(i) = REF(i) * (ESUN * cos(90 - solar_elevation_angle)) / (PI * (sun_earth_distance^2))
 where
 i: band id
 REF: TOA reflectance
 RAD: TOA radiance

3.3. True Color Image Products (TCI)

The True Color Image Product is a product consisting of AxelGlobe Cell, measuring about 5 km × 5 km per tile. This product is intended for applications that require accurate location information and map projection images. The True Color Image Product is processed with sensor correction, geometric correction, and map projection. In addition to automatically extracting the ground control points (GCP) from the high positional accuracy base map using our unique matching technology, we also combine conventional methods for areas where the GCP extraction is difficult. In applying correction processing, the location accuracy of the product depends on the GCP used and varies from region to region.

True Color Image Products are tone-corrected and pansharpened imagery products optimized for visual applications. This image product corrects artefacts such as pattern noise caused by all sensors while pixel values are corrected for visual enhancement purpose. A file that indicates areas unsuitable for analysis is attached as a usable data mask file (UDM file). The UDM marks pixels with missing data, fall outside the captured image boundary, or are obscured by clouds.

3.3.1. True Color Image Product Specification

The True Color Image Product contains the corresponding datasets delineated in Table 5.

Table 5. True Color Image Product specification

Product attribute Content Description
Product configuration and format Pan-sharpened imagery file

8-bit color-scaled pan-sharpened imagery data that is divided by AxelGlobe cell. GeoTIFF format or JPEG2000 format.

Layer assignment:
Layer1: band3 (Red)
Layer2: band2 (Green)
Layer3: band1 (Blue)

Unusable Data Mask (UDM) file

Binary image data that is indicative of the unusable region. GeoTIFF format or JPEG2000 format.The pixel resolution matches that of the product.

Layer assignment:
Layer1: No data region
Layer2: cloud covered region

Digital number assignment:

Layer1:
Invalid data area: 1
Valid data area: 0

Layer2:
Cloud area: 1
Cloud free area: 0

Metadata file Metadata for pan-sharpened imagery file and unusable data mask file. JSON format
Product frame AxelGlobe Cell Our global grid system is based on the AxelGlobe Cell measuring about 5 km x 5 km per tile. A buffer of about 100m (overlap area with adjacent cells) is added to each of the four sides.
If the captured image does not fill the entire area of the cell, the unfilled area will be indicated with black pixels wherein No data value (0) is assigned.
Pixel resolution 2.5 m The actual ground sampling distance varies on the collection conditions (mainly due to the satellite roll angle), but a sampling process is applied to the specified pixel size when product is generated.
Bit depth 8-bit unsigned integer GRUS satellite images are stored in the onboard memory as up to 12 quantized bits. Relative DN values obtained directly from the sensor are tone-scaled to 8-bit for visual purpose.
Resampling Cubic Convolution Performs resampling processing using the Cubic Convolution interpolation method, which uses the weighted average value calculated from the values of the 16 input cells around the output cell. This sampling method has the characteristic that the boundaries of data tend to be sharp.
Geometric correction sensor model Optical distortion correction due to sensor optics
band-to-band registration Band-to-band registration correction used to rectify misalignment between bands
Geo-rectification In geo-rectification, the characteristic terrain and features are extracted and paired between the captured image and the existing satellite image data (base map). The extracted terrain and features are matched with the reference points of each base map corresponding to the reference points. Processing is performed to reduce the positional deviation of the entire image with respect to the base map.
Radiometric correction

Correction of relative value difference of radiometric characteristics between detectors

Undetected detector filling for null values from unresponsive detectors

Scaling a 12-bit relative DN value to an 8-bit unsigned integer value by gamma correction

Enhancement Dark Object Subtraction (DOS) The dark pixels in the image are used to reduce noise such as water vapor and dust in the atmosphere.
(Applies to images taken after July 7, 2020)
Horizontal geodetic system WGS1984
Projection Universal Transverse Mercator projection (UTM) UTM is a transverse Mercator projection that divides the earth into longitude zones every 6° eastward from the 180° meridian and projects the central meridian as the central meridian from the spheroid directly into the plane for each longitude band.
Zones are determined by the south-west corner coordinates at the start of capture.

3.3.2 File naming

True Color Image Products are named according to the following naming convention. The file name contains several components that help organize, categorize, and refer to the information contained in the product.

<SatelliteName>_<AcquisitionDatetime>_<ProductLevel>_<ImageType>_<CellID>.<Extention>

Table 6. File naming convention

File naming component Description
<SatelliteName> Satellite name abbreviation (e.g. : GRUS-1A = GRUS1A)
<AcquisitionDatetime> The collection start date and time in the yyyymmddhhmmss description format. (e.g.:Jan. 2, 2018 11:32:54 = 20180102113254)
<ProductLevel> L3A
<ImageType> Image type Description
Pan-sharpen PSM
Unusable mask file for Pan-sharpen PSM_UDM
<CellID> Unique ID of AxelGlobe Cell
<Extention> Extension of imagery file(e.g.:GeoTIFF:tif)

3.3.3. File contents

Details of the True Color Image Products are provided in the file contents shown below.

EULA_en.txt                                             # License agreement (English)
EULA_jp.txt                                             # License agreement (Japanese)
GRUS1A_20200811011052                                   
├─ GRUS1A_20200811011052_L3A_PSM_N42092354.tif          # Pan-sharpened imagery
├─ GRUS1A_20200811011052_L3A_PSM_N42092355.tif          # Pan-sharpened imagery
├─ GRUS1A_20200811011052_L3A_PSM_metadata.json          # Pan-sharpened imageryMeta data
│
├─ GRUS1A_20200811011052_L3A_PSM_UDM_N42092354.tif      # UDM for pan-sharpened imagery
├─ GRUS1A_20200811011052_L3A_PSM_UDM_N42092355.tif      # UDM for pan-sharpened imagery
└─ GRUS1A_20200811011052_L3A_PSM_UDM_metadata.json      # Meta data for UDM for pan-sharpened imagery

3.4. Metadata

The description of product metadata is shown in Table 7

Table 7. Metadata Description

Section Field 1 Field 2 Data type Description
Company
companyName string Company name
companyLocation string Main office location
contact string Contact number
copyright string Copyright
processorVersion string Software version of processor
bundlerVersion string Software version of data delivery
processDate string Product processing date
EOMetadata
satelliteID string Unique ID assigned to the satellite
satelliteName string Official satellite name
orbitType string Types of satellite orbit
orbitInclinationAngle double Inclination angle of satellite orbit
orbitDirection string Direction the satellite in orbit is heading at the time of image acquisition
captureID string Unique ID corresponding to a capture event
acquisitionDateTime
acquisitionStartDateTime date Capture start date and time of the capture event.
Describe it in ISO8601 extended format with delimiter
acquisitionEndDateTime date Capture end date and time of the capture event.
Describe it in ISO8601 extended format with delimiter
satelliteOffNadirAngleNominal double Nominal value of the off-nadir (degree) of the satellite at the time of the capture event
satelliteElevationAngleNominal double Nominal value of the elevation angle (degree) of the satellite at the time of the capture event
satelliteAzimuthAngleNominal double Nominal value of the azimuth angle (degree) of the satellite at the time of the capture event
solarElevationAngleNominal double Nominal value of the sun elevation angle(degree) at the collection event
solarAzimuthAngleNominal double Nominal value of the sun azimuth angle at the capture event
panchromaticRowGSDNominal double Nominal value of ground sampling distance (GSD) at the row direction at the panchromatic sensor.
Unit: meter
panchromaticColumnGSDNominal double Nominal value of ground sampling distance (GSD) at the column direction at the panchromatic sensor.
Unit: meter
multispectralRowGSDNominal double Nominal value of ground sampling distance (GSD) at the row direction at the multispectral sensor.
Unit: meter
multispectralColumnGSDNominal double Nominal value of ground sampling distance (GSD) at the column direction at the multispectral sensor.
Unit: meter
GSDUnits string Unit of GSD
resamplingAlgorithm string Resampling method used in production Processing: Cubic Convolution
radiometricCalibrationApplied Boolean Yes, No
earthSunDistance double Distance between the center of the sun and the center of the earth
Unit: Astronomical Unit[AU]
absoluteGain
Panchromatic
Blue
Green
Red
Red Edge
Near Infrared
double Gain parameter when converting the observed DN value into the TOA radiance [W⋅sr-1 ⋅m-2]
Listed for each band
ESUN
Panchromatic
Blue
Green
Red
Red Edge
Near Infrared
double Extra-terrestrial solar irradiation
Unit: [mW/m2]
productMetadata
pansharpen Boolean Yes, No
bitsPerPixel string 1U, 8U, 16U
compressionType string NONE / DEFLATE / JPEG2000
outputFormat string GeoTIFF/JPEG2000
spatialReferenceSystem
EPSGCode Integer EPSG code related to image geodetic system and projection
geodeticDatum String geodetic datum
projection String projection
projectionZone String projection zone
layerConfiguration
layerX String

[Satellite image data]
Describes the band allocation in the Xth layer

[Other attached data]
Describes the data type in the Xth layer

valueInterpretation For the following items, only unusable mask data are described
layerX String Described the pixel value interpretation in the Xth layer
imageTileMetadata Repeat for the number of image files
imageName string image file name
cellID string Unique ID of AxelGlobe Cell
imageLocation Tile range information units are based on product projection
type string Polygon
coordinates list Describes the coordinate values (X coordinate, Y coordinate) that make up the tile boundary in the order of bottom left, top left, top right, bottom right, bottom left
imageLocationUnits string Coordinates units
rowGSD double The pixel size in the row direction of the tile. Units are based on product projection
columnGSD double The pixel size in the column direction of the tile. Units are based on product projection
GSDUnits string Unit of GSD
numberRows Integer Number of lines (pixels) in the image
numberColumns Integer Number of columns (pixels) in the image
numberBands Integer Number of bands in the image
cloudCoverPercentage double Cloud cover rate
Unit : %
only satellite image data are described