Specification for SHIROP data format
1.

Super Low Altitude Test Satellite (SLATS), Tsubame
JAXA proceeds technical studies for a satellite continually orbiting lower than 300 km, so called very low Earthorbit (VLEO), which has not demonstrated in the past. One of the benefits of the very low altitude satellite is thatthe high-resolution imaging and the small-aperture optical system are available simultaneously. In the VLEO, on theother hand, atmospheric drag and the density of atomic oxygen, which causes material deterioration, are about1000 times larger than those in the typical orbit for the earth observation satellites. Because most of earth
observation satellites require a very accurate attitude control as well as the long-period operation, there had beensignificant technical difficulties to operate those in VLEO.
The goal of Super Low Altitude Test Satellite (SLATS), Tsubame, is to demonstrate a possibility for the satellite
operation in VLEO. SLATS was launched by H-IIA vehicle alongside GCOM-C on December 23rd, 2017 fromTanegashima space center. After launch, it had lowered the altitude mainly by aerobraking, and it started theoperation to keep a certain altitude lower than 300km using an ion engine from April 2nd, 2019. SLATS had
successfully demonstrated the 6-stage operations at the altitude range of 271.1 to 181.1 km. At the same time, thehigh-resolution satellite images were taken, and those qualities have been proved. In addition, SLATS had made thelong-term measurements for the first time for atmospheric drag and the AO deterioration of exposed material.
The all planned missions were fully achieved, and SLATS completed its operation in October 1st, 2019.
For more details:

https://global.jaxa.jp/projects/sat/slats/Fig. 1. SLATS Overview

Fig. 2 Altitude profiles for SLATS operation
2.

Small and High Resolution Optical Sensor: SHIROP
Small and high-resolution optical sensor (SHIROP) onboard SLATS is the sensor aiming to test the improvementof ground sampling distance by the VLEO operation. The specifications of SHIROP are summarized in Table 1, andthe picture of SHIROP are shown in Figure 3
Table 1 Specification for SHIROP
Item

Specification

Optical design

Cassegrain telescope with correcting lens system

Clear aperture>20 cmDetector

Area CCDImaging

Single-shot, TDI imaging (max 64 stage)

Wavelength

Panchromatic (visible）

IFOV

2.7 µrad (CT/AT)

FOV

17.7 mrad (CT)×11.8 mrad (AT)

Altitude range

500km ～ 160km

Imaging period

Variable in above altitude range

Quantization bit rate

12bit（extend to15bit on ground）

Total MTF

> 0.08

Timing control for imaging

Realtime, reserve by time, autonomous by latitude, sequential imaging

Weight19.4kg（Optics 16.9kg, Electronics 1.9kg）

Size

Optics 270×540×270 mm, Electronics 125×225×95 mm

Pwer consumption<33W including heaters

Figure 1 Picture of the SHIROP optics

2.1.

Wavelength range
0.48 ～ 0.70 µm Panchromatic

2.2.

GSD (IFOV)IFOV : 2.7 µrad
GSD 73 cm at altitude of 271 km at nadir
GSD 59 cm at altitude of 217 km at nadir
GSD 49 cm at altitude of 181 km at nadirOver the entire operation period, SLATS has gradually lowered its orbit. The GSD gets improved as thealtitude becomes lower. The relation of the altitude and the GSD at nadir is shown in Figure 4.

Figure 2 GSD as a function of altitude

2.3.

Pointing
SHIROP can take an image at the range of +/-45 deg along the cross track with the center of the nadir. TheGSD gets worse as the pointing angle gets large. It can be roughly estimated as (GSD for pointing) = (GSDof the nadir) / (angle of pointing)
Position accuracy (Just for reference)The position accuracy is not guaranteed since SLATS is categorized asa technological test mission, but some kinds of evaluation has been performed.* Note that this is just forreference, the evaluation results are below:
In a north-south direction： about 30 m
In a west-east direction： about 52 m* Based on the geometrical evaluation using the 35 SHIROP images including the GCP points constructed
for the ALSOS mission.

2.4.

Quantization bit rate
The quantization bit rate for the SHIROP image data and the image product are 15- and 16-bit, respectively.

3.

Specification for image products

3.1.

File format
Geotiff format (16-bit)

3.2.

Size of scene
The size of a scene is 6576 x 4384 pixels, where the pixel size is corresponding to the GSD of the picture.

3.3.

Process levels
The two types of image products are provided as shown in Table 2.
Table 2 Class of product image
Process level

Process details

Level 1Ba) Radiometric correction and geometric calibration have done, but geometricalprojection has NOT been processed.
b) Radiometric correction and geometric correction (orthorectification) havebeen processed. The cubic convolution (CC) method is employed for theresampling for geometric projection.

3.4.

Compression
The provided image data are compressed by the process shown in Table 3.
Table 3 Types image compression process
Types

Details

Uncompressed (Raw)

No compression processed

Compressed (Comp)

JPEG compression processed. A 12-iit image is converted to an 8-bit image*and JPEG compressed with onboard computer, then it is extended to a15-bit image on ground.
*The data of 98% levels according to the histogram for 12-bit image are
extracted for the next resampling process to a 8-bit image.

3.5.

Definition of file name
1. Level 1B-a)：Radiometric correction image
Timetttttttttt_Numxx_SCENEyy_zzzzzzz#_0_active_UTMnns_NMI_****.tif
tttttttttt: Satellite time of imaging (seconds since at 0:00:00 on January 1st, 2013)
xx: Sequence number in the date of imaging (starts 01)
yy: Scene number for saving (00～63)zzzzzzz#: Method of timing control for imaging (autonomous or reserve), and the sequence number forsequent imaging (1or2)
Autonomous by latitude：RSVLAT
Reserve by time：RSVTIME
nn: UTM Zone number (01～60)
s: The Northern/Southern Hemisphere (N/S)****: Compression process, Uncompressed (Raw) or compressed (Comp)e.g.) Time0200460630_Num01_SCENE37_RSVLAT1_0_active_UTM04N_NMI_Raw.tif
2. Level 1B-b)

Radiometric correction and orthorectification image

Timetttttttttt_Numxx_SCENEyy_zzzzzzz#_0_active_UTMnns_ORI_****.tif
tttttttttt: Satellite time of imaging (seconds since at 0:00:00 on January 1st, 2013)
xx: Sequence number in the date of imaging (starts 01)
yy: Scene number for saving (00～63)zzzzzzz#: Method of timing control for imaging (autonomous or reserve), and the sequence numberfor sequent imaging (1or2)
RSVLAT Autonomous by latitude：RSVLAT
Reserve by time：RSVTIMEnn: UTM Zone number (01～60)
s: The Northern/Southern Hemisphere (N/S)****: Compression process, Uncompressed (Raw) or compressed (Comp)e.g.) Time0184014742_Num02_SCENE31_RSVLAT1_0_active_UTM19N_ORI_Comp.tif