Earth Observing 1: General Questions and Answers

By Earth Resources Observation and Science (EROS) Center

Questions that have been asked of Earth Observing 1.

General

What is EO-1's orbit?

EO-1's descending equatorial crossing time is ~10:00a.m. The satellite is flying in formation with Landsat 7, thus on the World-wide Referencing System-2 (WRS-2). The acquisition from Landsat and EO-1 will be within 1 minute of each other. This orbit is very useful for cross comparisons of the instrument on both spacecrafts.

What is the geographic coverage of an ALI or Hyperion scene?

Below is a visual comparison to Landsat 7.

What does the browse represent for ALI and Hyperion?

For Hyperion, browses are created from the Level 0 (raw) data. Due to this, vertical lines, zero data values from sensor artifacts may appear in the browse. Note that this anomaly is corrected during Level 1 processing. Hyperion browses are the monochromatic; VNIR band 40. Each band is linearly stretched between the 1% and 97% histogram values. The data is also reduced by a factor of 4 in each direction and jpeg compressed with a quality of 75%.

ALI browses are generated from the Level 1 data and displayed as RGB (4:3:1). Each band is linearly stretched between the 1% and 97% histogram values. The data is also reduced by a factor of 8 in each direction and jpeg compressed with a quality of 75%.

What is the entity ID naming convention?

EO1HPPPRRRYYYYDDDXXXPL

EO1-Satellite

H-Hyperion Sensor

PPP-Target WRS path

RRR-Target WRS row

YYYY-Year of acquisition

DDD-Julian day of acquisition

X-Hyperion 0=off; 1=on

X-ALI 0=off; 1=on

X-AC 0=off; 1=on

P-Pointing mode

L-Scene length

DAR

How many Data Acquisition Requests (DARs) does the spacecraft image daily?

Currently, approximately 12 Data Collection Events (DCEs) can be scheduled per day. EO-1 was designed as a technology demonstration mission, thus there are various spacecraft constraints that limit the amount of data that can be collected.

What are the off-nadir pointing capabilities for EO-1 DAR collections?

The spacecraft is capable of pointing over one adjacent WRS (i.e.) Landsat, path in each direction from the spacecraft flight path. The off-nadir angle will vary depending upon the location of the target from the instrument flight (i.e. nadir) path.

When does distortion become a significant factor in off-nadir versus nadir collects?

There is no measurement of distortion available. For clarification, nadir generally refers to being within the spacecraft's flight path. For descending acquisitions (daylight) in the nadir path, the instruments are nominally imaging the eastern area of the path. The amount of distortion depends on where from nadir the target is located. If the target is located in the western area of the path, there will be less distortion imaging from the adjacent path to the west than from the nadir path. In other words, the spacecraft would manuever less to capture this angle than the western roll that would be required to image the same location from within the path. True nadir acquistions are very rare, less than 1% of the time. Only targets that are in the eastern part of the WRS path can be acquired at true nadir.

What is the geographic length of an EO-1 DAR collection?

A standard collection images for 8 seconds, and covers approximately 42 kilometers in the along-track direction. Acquisitions of 185 km can also be collected.

Data Products

Are there restrictions for the use of EO-1 data?

No, ALI and Hyperion data are in the public domain. Once a scene has been downloaded from the USGS, it can be redistributed as desired. Please see the Data Citation Policy for further information.

What if there is no data in the archive for my area of interest?

EO-1 is a tasking spacecraft, meaning the images acquired are driven by a schedule. This schedule is planned by considering user's Data Acquistion Requests (DARs).

What are the bands and spectral ranges for Hyperion data?

There are 220 unique spectral channels collected with a complete spectrum covering from 357 - 2576 nm. The Level 1 radiometric product has a total of 242 bands but only 198 bands are calibrated. Because of an overlap between VNIR and SWIR focal planes, there are only 196 unique channels. Calibrated channels are 8-57 for the VNIR, and 77-224 for the SWIR. The reason for not calibrating all 242 channels is mainly due to the detectors' low responsivity. The bands that are not calibrated are set to zero in those channels.

What product formats are available for Hyperion?

All Hyperion and Advanced Land Imager (ALI) data in the archive will be attempted to be processed to the Level 1Gst level of correction. If the scene fails the Level 1Gst processing level, it will be removed from the archive and will become unavailable.

Level 1Gst is terrain corrected and will be provided in 16-bit radiance values. The image data are available in Geographic Tagged Image-File Format (GeoTIFF) and is distributed at no charge download.

What are the differences between the earlier Thompson Ramo Woolridge (TRW) products and the new USGS

The USGS has created a new Hyperion product format. There are packaging and data differenced between the 2 sets of code explained below:

The USGS products are packaged in Hierarchical Data Format (HDF) with the datasets being image data, spectral center wavelengths, spectral bandwidths, gain coefficients and a flag mask. The previous TRW was image data with separate files for other ancillary details. The file naming conventions are different as well; the USGS utilizes an entitiy ID target versus the data capture time utilized by TRW.

Comparisons of the data show that the USGS products are very comparable to the TRW products. For most bands the radiometric differences are within 1%. Some differences to note are:

Dead detectors are not corrected or interpolated in USGS products, thus vertical stripes will be visible where these known dead detectors exist;
Radiometric differences may be greater because of the known dead detectors not being interpolated in USGS products.
An additional scan line in the USGS products accommodates for the SWIR shift
The first scan line in USGS products is not corrected for echo, since echo correction requires information from the preceding line;
For the spectral center wavelengths, USGS products represent means across all detectors for each band while the TRW products represented the wavelength at field of view pixel 128.

Hyperion products

What are the files contained in an ALI Level 1 product?

An example of the products distributed with an ALI L1 product:

entityID.MET

Metadata file (site location, start/stop time, etc)

Order#.SUM

Order summary of files

entityID.M1R

First Sensor Chip Assembly (SCA)

entityID.M2R

Second SCA

entityID.M3R

Third SCA

entityID.M4R

Fourth SCA

2002_001_acs.hdf

This file and the following files are related to Level 0 processing and are useful to the instrument teams for calibration and validation.

2002_001_ali.hdf

2002_001_eff.hdf

2002_001_gps.hdf

2002_001_mis.hdf

2002_001_wrp.hdf

2002_001_xbd.hdf

What are the bands and spectral ranges for ALI?

ALI was built to provide vital information for the next Landsat mission. The ALI bands are thus very similar to Landsat, but note that ALI does not have a thermal band.

The table below shows a comparison between the ALI and ETM+ bands and spectral ranges.

Why are there 4 image data files within the Level 1 ALI product?

The Level 1 radiometric processing produces four strips of image data recorded by the instrument's Sensor Chip Assemblies (SCAs). The four files can be mosaicked together from left to right, starting with the .M4R file through the .M1R file. The mosaicked file will represent geographic region of 37 kilometers in width. Please note that since this is a radiometric product only, geometric errors may exist along the mosaic lines. The absence of georeferencing in the data is also the reason the files must be mosaicked by pixel-based mosaicking.

The information needed for mosaicking will be contained within each image file's HDF dataset attributes 1-8, called the "Number of Cross Track Pixels". This number reflects the number of columns (samples) of the strip of image. This is the offset in the X (column) for each strip from its adjacent strip. There is an overlap between the four SCAs of approximately 10 pixels for the multispectral bands and approximately 30 pixels for the panchromatic band, which is the reason for the odd-numbered offsets in the sample direction. The offset for the Y (row) direction will always be 1 or 0, depending on what the software expects. The HDF dataset attribute 1-9, called the "Number of Along Track Pixels," reflects the number of rows (lines) of image data. The four strips should be similar, but not exact, in size, both in rows and in columns.

Example

"Number of Cross Track Pixels" is 315

"Number of Along Track Pixels" is 2740

The following offsets are reflected for the 4 strips:

.M4R offsets y=1, x=1

.M3R offsets y=1, x=316

.M2R offsets y=1, x=631

.M1R offsets y=1, x=946

Total mosaic size would be 2740 rows by 1260 columns.

Also note that the Panchromatic (PAN) band will need approximately 3X the offset, due to its 10-meter resolution in comparison to the 30-meter resolution bands.

What product formats are available for ALI?

Level 1Gst is terrain corrected and will be provided in 16-bit radiance values. The image data are available in Geographic Tagged Image-File Format (GeoTIFF) and is distributed at no charge download.

What are known ALI anomalies?

Leaky detectors (2) with cross talk are present buy are constrained to < 1% by the Level 1 correction algorithm. Stray light can exist for scenes with high contrast; dim targets (i.e. small lakes) within bright backgrounds.

Data Conversion

How are the radiance values (L) determined within the Hyperion bands?

The digital values of the Level 1 product are 16-biy radiances and stored as a 16-bit signed integer. The SWIR bands have a scaling factor of 80 and the VNIR bands have a scaling factor of 40 applied. The units are W/m2 SRµm.

VNIR L = Digital Number/40

SWIR L = Digital Number/80

What is the signal-to-noise ratio (SNR) for Hyperion?

The SNR for Hyperion is 190 to 40 as te wavelengths increase.

How do you convert radiance to reflectance?

For relatively clear Hyperion and ALI scenes, a reduction in between-scene variability can be achieved through a normalization for solar irradiance by converting spectral radiance to planetary reflectance or albedo. This combined surface and atmospheric reflectance of the Earth is computed with the following formula:

Formula
Earth to Sun table
ALI table

Download hyperion irradiance band information.

What is the flag mask?

Three different items are represented by the mask: saturated, dead and flat pixels. The pixels in the flag mask can have several values:

0 = Normal data

1 = Saturated pixel

2 = Dead detector (unresponsive to light, with zero values)

3 = Flat detector (dead but register a constant value)

4 = Fill value (overflow, value exceeds calibration range)

What is the signal-to-noise ratio (SNR) for ALI?

The SNR is much improved in relation to the Landsat 7 ETM+; comparisons show five times better SNR.

What is the conversion to get from ALI Level 1 to a Level 0?

Theoretically it should be possible to get to L0 using the formula. However, the calibration coefficients were applied before the leaky pixel correction. Because the leaky pixels corrections vary from scene to scene, the response should be customized for each.

The formula for converting to Level 0:

Level 0 = (L1R/(response x scaling factor)) + offset

Please note that due to leaky pixel correction applied after the calibration process it is not possible to get back to a true Level 0 product.

Data Import

What does it mean when my data has a .tar.gz or .tgz extension?

Your data will need to be unzipped and untarred before you can use the data files. UNIX systems should have the "gunzip" and "tar" commands available for uncompressing and accessing the data. For PC users, free software can be downloaded from the web; unfortunately, we cannot recommend any type of software. Check your PC, you may already have the appropriate software.