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Earth Syst. Sci. Data, 10, 1417–1425, 2018
https://doi.org/10.5194/essd-10-1417-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Gridded Satellite (GridSat) GOES and CONUS data
Kenneth R. Knapp1 and Scott L. Wilkins2
1NOAA/NESDIS/National Centers for Environmental Information, Asheville, NC 28801, USA
2Cooperative Institute for Climate and Satellites – North Carolina (CICS-NC),
North Carolina State University, Asheville, NC 28801, USA
Correspondence: Kenneth R. Knapp (ken.knapp@noaa.gov)
Received: 6 March 2018 – Discussion started: 27 April 2018
Revised: 17 July 2018 – Accepted: 18 July 2018 – Published: 6 August 2018
Abstract. The Geostationary Operational Environmental Satellite (GOES) series is operated by the US Na-
tional Oceanographic and Atmospheric Administration (NOAA). While in operation since the mid-1970s, the
current series (GOES 8–15) has been operational since 1994. This document describes the Gridded Satel-
lite (GridSat) data, which provide GOES data in a modern format. Four steps describe the conversion of original
GOES data to GridSat data: (1) temporal resampling to produce files with evenly spaced time steps, (2) spatial
remapping to produce evenly spaced gridded data (0.04◦ latitude), (3) calibrating the original data and storing
brightness temperatures for infrared (IR) channels and reflectance for the visible channel, and (4) calculating
spatial variability to provide extra information that can help identify clouds. The GridSat data are provided
on two separate domains: GridSat-GOES provides hourly data for the Western Hemisphere (spanning the en-
tire GOES domain) and GridSat-CONUS covers the contiguous US (CONUS) every 15 min (dataset reference:
https://doi.org/10.7289/V5HM56GM).
1 Introduction
The National Oceanographic and Atmospheric Administra-
tion (NOAA) has used the Geostationary Operational En-
vironmental Satellite (GOES) series since 1975 to monitor
weather and other environmental conditions. While the se-
ries name has remained the same, the satellites providing the
data have seen stepwise increases in capabilities. The origi-
nal system was a spin-stabilized system. The system received
a significant upgrade in 1994 with the launch of GOES 8 (the
GOES-I to GOES-M series), providing a five-channel imager
(Menzel and Purdom, 1994). The recent launch (in 2016)
of GOES 16 is another substantial increase in capabilities
(Schmit et al., 2017). The satellites have been maintained at
two primary locations: GOES West near 135◦ W, which ob-
serves the Eastern Pacific and western North America, and
GOES East near 75◦ W, providing coverage of North and
South America and much of the Atlantic Ocean.
Gridded Satellite (GridSat) is produced as a means to facil-
itate access to GOES data. There are two domains provided:
a 15 min domain provided over the contiguous US called
GridSat-CONUS and an hourly domain that spans the West-
ern Hemisphere to cover the entire GOES domain, called
GridSat-GOES. In the following, we use the term GridSat-
GOES to describe both these datasets, since their only differ-
ence is the temporal resolution and spatial coverage.
1.1 Purpose for gridded GOES data
Since the launch of the GOES-I to GOES-M series in 1994,
worldwide computing capabilities have significantly im-
proved and changed the landscape of how the data are trans-
mitted, accessed, and used. When first launched, most GOES
processing was conducted using the Man computer Interac-
tive Data Access System (McIDAS, Lazzara et al., 1999)
and data were transferred via satellite broadcast or high den-
sity tapes. Thus, data were used primarily for weather re-
search and forecasts. About 25 years later, the data can be
accessed with a variety of computers and software, primarily
transferred over the internet and used for a myriad of pur-
poses. Thus, the demand has significantly increased while
the support of GOES formats has remained stagnant. Prior
Published by Copernicus Publications.
1418 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data
Table 1. Summary of GOES data access for AREA files.
CLASS AREA files Gridded GOES
Format AREA netCDF-4
Supported software languages 1 (McIDAS) > 30 (supported at Unidata)
Navigation Possible using thousands of lines of code Stored directly in file
Calibration IR: information available at NESDIS site Stored directly in file
Vis: available in scientific journals
Temporal availability Variable (1 to 30 min) Regular (CONUS: 15 min, GOES: 1 h)
Volume (1 month of data) ∼ 563 GB ∼ 45 GB
Table 2. Comparison of GridSat-GOES and GridSat-CONUS with similar currently available datasets.
Globally merged IR1 GridSat-B1 CDR GridSat-GOES GridSat-CONUS
Spatial coverage Global (60◦ N–60◦ S) Global (70◦ N–70◦ S) Western Hemisphere Contiguous US
Temporal coverage 1998–present 1980–present 1994–present 1994–present
Spatial resolution ∼ 4 km ∼ 8 km ∼ 4 km (0.04◦) ∼ 4 km (0.04◦)
Temporal resolution 30 min 3 hourly Hourly 15 min
Spectral coverage 1 (IR) 3 (IR, WV, VIS) 82 (6 channels and 2 others) 82 (6 channels and 2 others)
Climate quality intercalibration None IR None None
Volume (1 month) 45 GB 12 GB 45 GB 12 GB
1 Janowiak et al. (2001); 2 see Sect. 4.1 for a description of the channels in the variable list.
to GridSat-GOES, the data were available from the NCEI
archive as follows.
– Raw GVAR data – a very complex binary format in-
tended for transmission of satellite data via satellite up-
links and downlinks. This can be read by a very limited
set of software. The target audience are satellite experts
with access to large computing resources.
– AREA format – a format developed for McIDAS to pro-
cess satellite data. This is well documented, but still re-
quires a thorough knowledge of satellite data to extend
to other applications. The target audience are those with
access to McIDAS software (e.g., academia).
– netCDF format – the netCDF file provided by the
archive does not follow accepted standards in storing
data and it provides no information on converting val-
ues to reflectance and brightness temperatures. There is
also no obvious way to renavigate imagery. Therefore,
while the target audience may be scientific users, these
issues make the data provided by the archive less useful
for that audience.
– TIFF and JPEG – these formats are solely for imaging.
The data provided have map overlays. The target audi-
ence are the general public looking for satellite images
for qualitative analysis, not quantitative studies.
What is lacking is a format that can be used by the general
scientific community, such that it can easily be incorporated
into other processes and used for analysis in a wide range of
applications.
Table 1 contrasts the access to GOES data via McIDAS
area files and the GridSat-GOES formats. The AREA file for-
mat is fully supported by one software application, the navi-
gation (outside of McIDAS) requires more than 1000 lines of
code, and the calibration information (outside of McIDAS) is
not directly available in the AREA files, but requires access-
ing satellite-specific coefficients from another source. Lastly,
the temporal resolution of the files can vary from 1 to 30 min,
with a complex scan schedule that varies depending on the
scan mode selected for the day (e.g., when severe weather is
expected, forecasters can request 1 min rapid scans of a re-
gion, at the expense of other regional scans). Conversely, the
GridSat-GOES format provides calibrated and mapped data
that has been temporally sampled to the nearest 15 min (for
CONUS) and 1 h (for GOES). The format – netCDF-4 – is
supported by Unidata, which provides information on how
to use netCDF in multiple languages and applications.
1.2 Relationship to other available datasets
GridSat-GOES and GridSat-CONUS are most similar to two
other datasets: the globally merged IR (infrared) dataset
(Janowiak et al., 2001) and the GridSat-B1 Climate Data
Record (CDR) (Knapp et al., 2011). Various details of the
datasets are compared in Table 2. The global merged data
has provided information for precipitation processing since
the beginning of global meteorological geostationary satel-
lite coverage in 1998, but only provides information from one
channel: the infrared window. The GridSat CDR expands on
this by providing intercalibrated IR brightness temperatures
with a longer period of record (1980–present). The GridSat-
Earth Syst. Sci. Data, 10, 1417–1425, 2018 www.earth-syst-sci-data.net/10/1417/2018/
K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data 1419
GOES datasets focus on providing access to complete GOES
data in a new, updated format. It provides access to all GOES
Imager channels. Since it does not expand to other satellites,
it is mostly limited to the Western Hemisphere.
2 GOES data processing
The effort to produce GridSat data required processing
GOES area files retrieved from NOAA CLASS (Compre-
hensive Large Array-data Stewardship System). In so doing,
we ordered and downloaded 294 TB of data in about 10 mil-
lion files. This represents the entire archive of GOES-8 to
GOES-15 data for 1994–2016. Processing these AREA files
required several steps in order to produce the final GridSat
products. The first step was to resolve the issue of duplicate
files in the archive. This occurs when multiple files are avail-
able for one scan time. Next, specific scan times were se-
lected. While the GOES scan schedule is a complex system
of various scan schedules taking image scans (i.e., regions)
at different times, the GridSat data provide grids at regular
times. Image scans are then navigated, which involves deter-
mining the location of each pixel and, conversely, determin-
ing which image pixel is closest to a given Earth location.
Once navigated, the scans are converted from digital counts
with a 10 bit depth (hence they range from 0 to 1023) to geo-
physical units, either reflectance or brightness temperature, a
process called calibration (note that visible data are not pro-
vided as reflectance factor, which would have a correction for
the illumination geometry). Lastly, two other variables which
provide information on the spatial uniformity of the images
are calculated and stored in the resulting GridSat file. More
details on these procedures are provided below.
2.1 Resolving duplicate files
Files were originally archived and stored on tapes. Each time
a tape is read, there is the possibility that the file has slightly
different contents. When this occurs, both files are retained
and are differentiated by a trailing hyphen and number (e.g.,
“−1”). This produced duplicate files of the same image.
Duplicate files are prevalent in the GOES archive. They
exist for 1994 through 2003 and can be up to 30 % of the files
for any given year. Because some files have more than one
duplicate, it is not unusual to find files that are repeated 3 or
4 times. The most repeated file is “goes08.1995.060.174513”
(from GOES 8 on 1 March 1995 at 17:45:13 UTC), which
has six files for the single scan. The total number of extra
files in the archive exceeds 64 000 files; for context, that is
more than the number of GOES files archived for 1997. In
essence, there is the equivalent of an extra satellite year in
the archive.
The reprocessing effort set up an objective algorithm to
reconcile these duplicate files. Files were compared scan line
by scan line. First, files that were identical were resolved sim-
ply by renaming or removing files. Where scan lines differed,
the scan lines that were more highly correlated with neigh-
boring scan lines were selected. This led to selecting one of
the multiple files as the best; however, some cases resulted in
a completely new file (when best scans were selected from
different files). This process resolved all duplicated files.
2.2 Temporal sampling
The GOES Imager produces about 120 images per day; the
exact number varies by scan schedule. Given the wide va-
riety of scans and times, the GridSat-GOES and GridSat-
CONUS sectors store data at nominal 60 and 15 min inter-
vals, respectively. That is, a sector that begins at 17:45 UTC
will be included in the 17:45 UTC CONUS grid as well as the
18:00 UTC GOES grid. Thus, users do not need to be aware
of the scan schedule, or understand where each is taken, in
order to access and process GridSat-GOES data.
Furthermore, the hourly processing allows GridSat-GOES
to merge images from various scans. The GOES scan sched-
ule provides a full disk scan once every 3 h. However, the var-
ious scans in the intermediate times, when merged, produce
something that can approximate full disk coverage. Figure 1
provides a demonstration of this capability. Figure 1a shows
an 18:00 UTC image, for which a full disk scan was avail-
able. However, in Fig. 1b, the sectors from multiple sepa-
rate scans (Northern Hemisphere Extended, Southern Hemi-
sphere, etc.) are merged on the same grid, thus providing a
nearly full disk image.
The GridSat-GOES dataset provides a variable (called
delta_time) that allows the calculation of the actual scan time
of each pixel by storing the difference in time between the
GridSat image time (17:45 and 18:00 UTC in the previous
example) and the time of the actual instrument scan as de-
rived from the scan line information in the AREA files. This
allows for calculation of accurate solar illumination angles.
2.3 Navigation
Once a grid time is selected, the image pixels nearest each
grid cell are selected through sampling. No averaging of the
observations is performed. Sampling maintains the pixel bit
depth, radiative characteristics, and spatial resolution. Data
were navigated following the algorithms provided for GOES
data by NESDIS (1998). Some navigation errors will be
present in the remapped data from the original data; navi-
gation accuracy for GOES is about 4 km during daylight and
6 km at night (Ellrod et al., 1998).
Since data are stored on a 0.04◦ equal angle grid, simply
displaying the data without transform, as in Fig. 1, provides
an equirectangular map projection.
www.earth-syst-sci-data.net/10/1417/2018/ Earth Syst. Sci. Data, 10, 1417–1425, 2018
1420 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data
Figure 1. Sample of infrared window GridSat-GOES images for 18:00 and 19:00 UTC. (a) The 18:00 UTC image provided by a full disk
scan that began at 17:45 UTC. (b) The 19:00 UTC image is a combination of three separate scans.
2.4 Calibration
Infrared channel data are calibrated following documenta-
tion from the NOAA STAR web page (Weinreb et al., 1997,
2017). Scan lines are calibrated separately and converted to
a brightness temperature. The brightness temperatures are
stored in two-byte integers using data packing (e.g., CF con-
vention scale_factor and add_offsets). Brightness tempera-
ture noise is about 0.1 K at 300 K (Ellrod et al., 1998), which
was improved to approximately 0.05 K at 300 K for imagers
on GOES-13 to GOES-15 imagers (Zou et al., 2015).
There is no on-board visible calibration reference for
any GOES satellite prior to GOES 16, so some correc-
tion is necessary given the decay of the instrument gain
(Knapp and Vonder Haar, 2000). Therefore, visible channel
data are calibrated in GridSat-GOES following Inamdar and
Knapp (2015) to provide a temporally stable set of visible
observations.
2.5 Visible and infrared window variability
Lastly, two extra variables are provided: IR and visible chan-
nel variability. For each pixel that was mapped to a grid cell,
the spatial variability is provided. The spatial variability is
calculated as the standard deviation of the 3 × 3 pixels cen-
tered on the selected pixel. This is calculated at the original
satellite resolution. Thus, for the visible channel, which is
available at 1 km pixels, it provides a measure of sub-grid
cell variability. For the IR, whose original resolution is near
4 km, the calculation provides a measure of the grid cell vari-
ability. Both of these measures are related to cloud cover and
can be used to determine the presence of clouds (Rossow and
Garder, 1993).
3 Future plans
Many possible improvements exist. Future improvements
will depend on user needs. Possible improvements include
the following:
– Expansion to 1980–1994 – GridSat-GOES is currently
limited to the GOES-I to GOES-M series of satellites
(GOES 8 through GOES 15). The dataset can be ex-
panded back in time to provide information from the
previous series (GOES 1–7). While it would have fewer
channels, it could maintain similar temporal and spatial
resolutions.
– Expansion to GOES 16 and beyond – the newest se-
ries of GOES has the Advanced Baseline Imager (ABI),
which increases the number of channels to 16, increases
the spatial resolution to 2 km for most channels, and the
temporal resolution of full disk scans is 15 min (Schmit
et al., 2005, 2017). Nonetheless, there will continue to
be demand for this long-term GridSat series at a reduced
volume. Expanding GridSat-GOES to GOES 16 could
provide a solution for users needing large spans of time.
– Cloud information – including cloud information (prob-
ability, temperature, optical thickness, etc.) is possible
and can be provided in future versions. While increasing
the volume of the data, it would increase the usefulness
to many users.
– Increase spatial resolution – visible data are available
at 1 km spatial resolution and there has been some de-
mand for that. However, it would significantly increase
the volume (thus decreasing the usefulness of the data).
One viable option would be to include the 1 km channel
data only for the CONUS sector.
Earth Syst. Sci. Data, 10, 1417–1425, 2018 www.earth-syst-sci-data.net/10/1417/2018/
K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data 1421
– Merge GOES East and GOES West – data are presently
provided separately for GOES East and GOES West. It
is possible to merge the projections to one value, but
doing so would impact data usefulness. In particular, the
visible channel would be affected by including satellites
in the same scene that have different viewing geome-
tries.
– Climate quality calibration – presently, we are apply-
ing the operational brightness temperature corrections.
There are higher quality calibrations that could be de-
rived. While the visible channel calibration already in-
cludes values that provide longer-term stability, im-
provements could be made.
This is a short list of changes that could be made to further
improve the utility of the data. User feedback will be used to
prioritize these tasks and resources applied to those that are
in highest demand.
4 Data availability
The data are available from the following links: GridSat-
CONUS, https://www.ncei.noaa.gov/data/gridsat-goes/
access/conus (Knapp, 2017); and GridSat-GOES, https:
//www.ncei.noaa.gov/data/gridsat-goes/access/goes (Knapp,
2017). The web page https://www.ncdc.noaa.gov/gridsat/
(Knapp, 2018) provides additional GridSat-GOES informa-
tion and allows user registration. Table 4 provides a list of
files (for GridSat-GOES) provided by each satellite for each
year. The most files possible in a year is 8760 files at 24 files
per day (and 8784 in a leap year). The variation in numbers
shows gaps in the record: large deviations from 8760 exist
in the 1990s with more stable and relatively fewer missing
files in the more recent years. This shows the progression
of the GOES East position from GOES 8 to 12 to 13; a
similar progression is apparent for GOES West. There are
also numerous GridSat-GOES files for GOES 14; however,
it was not listed in Table 3 since it never assumes a role as
either GOES East or West for a significant time. The entire
GridSat-GOES period of record for all satellites is 24 TB,
which is a factor of 10 smaller than the full-resolution
archive (342 TB). The GridSat-CONUS period of record
totals about 5 TB of data.
4.1 Variables
The following variables are provided in GridSat-GOES and
GridSat-CONUS netCDF files. They are listed with a sum-
mary of how they could be used. A sample set of images is
provided in Fig. 2 for GridSat-CONUS.
– Lat – the latitude coordinate variable (in ◦ N). This pro-
vides the ability to map the data.
– Lon – the longitude coordinate variable (in ◦ W), which
also provides the ability to map the data.
Table 3. List of duplicate file numbers by year. The percentages are
relative to the number of scans in the year.
Year Number of scans
with duplicate files
1994 448 (3 %)
1995 26 137 (41 %)
1996 15 830 (22 %)
1997 4743 (7.6 %)
1998 5 (0.01 %)
1999 37 (0.04 %)
2000 25 (0.02 %)
2001 41 (0.04 %)
2002 0 (0 %)
2003 153 (0.15 %)
– Time – nominal time for the GridSat data (in units of
“days since 1 January 1970 00:00:00 UTC”).
– Lat_bounds – from the CF convention, these provide the
spatial bounds of the grid cells used.
– Lon_bounds – from the CF convention, these provide
the spatial bounds of the grid cells used.
– Time_bounds – from the CF convention, these provide
the temporal bounds of the temporal sampling.
– Satlat – the latitude of the subsatellite point (in ◦ N).
This can be used in conjunction with satlon to calcu-
late the viewing angles (e.g., satellite view zenith and
azimuth angles).
– Satlon – the longitude of the subsatellite point (in ◦ W).
– Satrad – distance (in km) of the satellite from the center
of the Earth.
– Filename – filenames for the source archive AREA files
included in the GridSat file, to provide lineage.
– Delta_time – provides the difference of the actual scan
time (in minutes) from the optimal grid time, used for
calculations of solar geometry.
– Ch1 – reflectance (or scaled radiance) for the 0.6 µm
band. The visible channel provides observations of
clouds, Earth’s surface, and aerosols.
– Ch2 – brightness temperature of the 3.9 µm channel.
This wavelength measures both reflected sunlight as
well as the Earth’s radiance and provides information
on cloud microphysics and other surface information.
– Ch3 – brightness temperature of the 6.7 µm channel,
which is also called the water vapor channel. The chan-
nel provides information on the distribution and move-
ment of water vapor in the upper atmosphere. The at-
www.earth-syst-sci-data.net/10/1417/2018/ Earth Syst. Sci. Data, 10, 1417–1425, 2018
1422 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data
Figure 2. Sample images produced from GridSat-CONUS for 19 August 1999 at 17:45 UTC showing all eight channels. Since this is
GOES 8, there is no channel 6 data; central wavelengths are provided in Sect. 4.1. Visible data are enhanced to show detail across the
dynamic range.
mosphere at this wavelength is opaque and the channel
rarely sees the surface.
– Ch4 – brightness temperature of the 11 µm channel,
generally called the IR window channel. It provides in-
formation on the radiant temperature on the surface or
cloud top.
– Ch5 – brightness temperature of the 12 µm channel.
This channel provides information on the amount of at-
mospheric contamination in the 11 µm, which is why it
is often called the split window. This channel is empty
for GOES-12 to GOES-15 satellites because it is not in-
cluded on those satellites.
– Ch6 – brightness temperature of the 13 µm channel.
This channel provides information on cloud cover and
cloud height. This channel is empty for GOES-8 to
GOES-11 satellites.
– Ch1v – spatial standard deviation of the channel 1 re-
flectance. This provides a measure of sub-grid cell vari-
ability because the calculation is made on the original
satellite data, then sampled to the grid cells. Useful in
identifying clouds (Rossow and Garder, 1993).
– Ch4v – spatial standard deviation of the channel 4 (in-
frared window) brightness temperatures, which is useful
for identifying clouds.
As mentioned in Sect. 3, it could be possible to expand this
variable list as deemed necessary to meet user needs.
Earth Syst. Sci. Data, 10, 1417–1425, 2018 www.earth-syst-sci-data.net/10/1417/2018/
K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data 1423
Table 4. Number of files per satellite per year for GridSat-GOES.
GOES East GOES West
Year GOES 8 GOES 12 GOES 13 GOES 9 GOES 10 GOES 11 GOES 15
1994 2316
1995 8351 2324
1996 8143 6794
1997 7209 5903
1998 8418 4526 3816
1999 8534 8617
2000 8581 8692
2001 8547 8655
2002 8557 8655
2003 2109 6511 8662
2004 8610 8655
2005 8585 8664
2006 8581 4075 4586
2007 8277 8647
2008 8204 8676
2009 8287 8353
2010 2449 6268 8664
2011 8759 8040 609
2012 8179 8695
2013 8280 8757
2014 8755 8231
2015 8759 8753
2016 8779 8782
Figure 3. Spatial coverages of the 5375×3750 pixel domain of GridSat-GOES (a) and the 1500×650 pixel domain of GridSat-CONUS (b).
4.2 Coverages
The coverages of GridSat-GOES and GridSat-CONUS are
shown in Fig. 3. GridSat-GOES spans the Western Hemi-
sphere; in fact, it extends into the Eastern Hemisphere, span-
ning 150 to 5◦ E and spanning 75◦ S to 75◦ N. The GridSat-
CONUS spans 125 to 65◦ W and 25 to 50◦ N. Both sectors
are provided at 0.04◦, which is approximately 4 km near the
Equator.
4.3 Uncertainty
Menzel and Purdom (1994) provide information on GOES
calibration accuracy based on prelaunch design. Post-launch
analysis has deemed IR calibration accurate to within 0.15 to
0.25 K (Ellrod et al., 1998; Wang et al., 2011). The visi-
ble calibration was provided by Inamdar and Knapp (2015),
where the uncertainty is about 3 % (in scaled reflectance
units).
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1424 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data
4.4 Caveats
While the GridSat-GOES dataset is suitable for scientific and
research use, some caveats should be considered prior to us-
age. First, users should become familiar with the limitations
and quality of the source data: GOES Imager data (Ellrod et
al., 1998; Goodman et al., 2018).
The IR calibration is not yet climate quality. We presently
use the operational calibration without adjustment. There are,
however, some issues that reduce the ability to use GridSat-
GOES for climate use. For instance, the midnight black body
calibration correction was implemented by NOAA in 2003.
Therefore, IR calibration is lower quality near the satel-
lite local midnight prior to 1 April 2003. Also, the GSICS
(Global Space-based Inter-Calibration System) community
has worked on IR calibration adjustments (Goldberg et al.,
2011), which are also not included in this initial version.
Users should also be aware of the limitations of the tempo-
ral and spatial sampling. The full-resolution visible data are
sampled to the 0.04◦ grid spacing. Similarly, full-resolution
temporal scans (e.g., 1 min rapid scans) are used only when
they provide an observation closest to the optimal grid time.
Therefore, users requiring high spatial or temporal sam-
pling will need to access GOES data using another method.
Furthermore, no adjustments were made for parallax error,
which are cloud location errors at high view zenith angles
(Vicente et al., 2002).
Another limitation of the temporal sampling is the gaps
that occur when data are not available. Three primary causes
for the gaps are scan schedules, eclipse keep-out zones,
and archive gaps. The scan schedule causes gaps mostly
in the CONUS grid when the satellite is scanning the
full disk (which takes longer than 15 min). For example
for GOES East, there is no 18:00 UTC GridSat-CONUS
file since it is still doing a full disk scan that started at
17:45 UTC. These gaps generally occur at regular intervals
and are usually limited to one missed CONUS grid. The
keep-out zone refers to when the satellite is near the local
midnight and has the potential to look directly at the Sun.
Since this would permanently damage the satellite sensor and
optics, a keep-out zone was implemented to reduce the pos-
sibility of damage. These are times near local midnight near
the March and September equinoxes. The resulting gaps are
daily and last a few hours. The GOES-13 to GOES-15 series
includes an improved design that allows for shorter keep-out
zones. Another limitation is the archive itself. There are some
gaps in the archive where scans exist in other archives for
one reason or another. These gaps can be longer than those
caused by the scan schedule, often exceeding a day.
A complete list of known caveats and issues is maintained
on the GridSat-GOES website.
5 Conclusions
The GridSat-GOES and GridSat-CONUS datasets provide
GOES data to a wide audience in a standard format. Since
the tasks of calibrating and navigating have been performed
during production, data access is greatly simplified for the
user. The temporal granularity structure is also much sim-
pler, using fixed grid times rather than the complex and vari-
able heritage scan times. The data are then made available
in the widely supported netCDF-4 format. Future improve-
ments and versions will be driven by user requirements.
Author contributions. KRK produced the GridSat data and
wrote the manuscript. SLW ordered and managed the GOES data
downloaded from NOAA CLASS.
Competing interests. The authors declare that they have no con-
flict of interest.
Acknowledgements. We would like to acknowledge Jes-
sica Matthews and Anand Inamdar of CICS-NC for helping make
this GOES reprocessing possible. This dataset would not have been
produced without the computational support of CICS-NC. This
work was supported by NOAA through the Cooperative Institute
for Climate and Satellites – North Carolina under cooperative
agreement NA14NES432003.
Edited by: David Carlson
Reviewed by: three anonymous referees
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Knapp, wilkins 2018 - gridded satellite (grid sat) goes and conus data-annotated

  • 1. Earth Syst. Sci. Data, 10, 1417–1425, 2018 https://doi.org/10.5194/essd-10-1417-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Gridded Satellite (GridSat) GOES and CONUS data Kenneth R. Knapp1 and Scott L. Wilkins2 1NOAA/NESDIS/National Centers for Environmental Information, Asheville, NC 28801, USA 2Cooperative Institute for Climate and Satellites – North Carolina (CICS-NC), North Carolina State University, Asheville, NC 28801, USA Correspondence: Kenneth R. Knapp (ken.knapp@noaa.gov) Received: 6 March 2018 – Discussion started: 27 April 2018 Revised: 17 July 2018 – Accepted: 18 July 2018 – Published: 6 August 2018 Abstract. The Geostationary Operational Environmental Satellite (GOES) series is operated by the US Na- tional Oceanographic and Atmospheric Administration (NOAA). While in operation since the mid-1970s, the current series (GOES 8–15) has been operational since 1994. This document describes the Gridded Satel- lite (GridSat) data, which provide GOES data in a modern format. Four steps describe the conversion of original GOES data to GridSat data: (1) temporal resampling to produce files with evenly spaced time steps, (2) spatial remapping to produce evenly spaced gridded data (0.04◦ latitude), (3) calibrating the original data and storing brightness temperatures for infrared (IR) channels and reflectance for the visible channel, and (4) calculating spatial variability to provide extra information that can help identify clouds. The GridSat data are provided on two separate domains: GridSat-GOES provides hourly data for the Western Hemisphere (spanning the en- tire GOES domain) and GridSat-CONUS covers the contiguous US (CONUS) every 15 min (dataset reference: https://doi.org/10.7289/V5HM56GM). 1 Introduction The National Oceanographic and Atmospheric Administra- tion (NOAA) has used the Geostationary Operational En- vironmental Satellite (GOES) series since 1975 to monitor weather and other environmental conditions. While the se- ries name has remained the same, the satellites providing the data have seen stepwise increases in capabilities. The origi- nal system was a spin-stabilized system. The system received a significant upgrade in 1994 with the launch of GOES 8 (the GOES-I to GOES-M series), providing a five-channel imager (Menzel and Purdom, 1994). The recent launch (in 2016) of GOES 16 is another substantial increase in capabilities (Schmit et al., 2017). The satellites have been maintained at two primary locations: GOES West near 135◦ W, which ob- serves the Eastern Pacific and western North America, and GOES East near 75◦ W, providing coverage of North and South America and much of the Atlantic Ocean. Gridded Satellite (GridSat) is produced as a means to facil- itate access to GOES data. There are two domains provided: a 15 min domain provided over the contiguous US called GridSat-CONUS and an hourly domain that spans the West- ern Hemisphere to cover the entire GOES domain, called GridSat-GOES. In the following, we use the term GridSat- GOES to describe both these datasets, since their only differ- ence is the temporal resolution and spatial coverage. 1.1 Purpose for gridded GOES data Since the launch of the GOES-I to GOES-M series in 1994, worldwide computing capabilities have significantly im- proved and changed the landscape of how the data are trans- mitted, accessed, and used. When first launched, most GOES processing was conducted using the Man computer Interac- tive Data Access System (McIDAS, Lazzara et al., 1999) and data were transferred via satellite broadcast or high den- sity tapes. Thus, data were used primarily for weather re- search and forecasts. About 25 years later, the data can be accessed with a variety of computers and software, primarily transferred over the internet and used for a myriad of pur- poses. Thus, the demand has significantly increased while the support of GOES formats has remained stagnant. Prior Published by Copernicus Publications.
  • 2. 1418 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data Table 1. Summary of GOES data access for AREA files. CLASS AREA files Gridded GOES Format AREA netCDF-4 Supported software languages 1 (McIDAS) > 30 (supported at Unidata) Navigation Possible using thousands of lines of code Stored directly in file Calibration IR: information available at NESDIS site Stored directly in file Vis: available in scientific journals Temporal availability Variable (1 to 30 min) Regular (CONUS: 15 min, GOES: 1 h) Volume (1 month of data) ∼ 563 GB ∼ 45 GB Table 2. Comparison of GridSat-GOES and GridSat-CONUS with similar currently available datasets. Globally merged IR1 GridSat-B1 CDR GridSat-GOES GridSat-CONUS Spatial coverage Global (60◦ N–60◦ S) Global (70◦ N–70◦ S) Western Hemisphere Contiguous US Temporal coverage 1998–present 1980–present 1994–present 1994–present Spatial resolution ∼ 4 km ∼ 8 km ∼ 4 km (0.04◦) ∼ 4 km (0.04◦) Temporal resolution 30 min 3 hourly Hourly 15 min Spectral coverage 1 (IR) 3 (IR, WV, VIS) 82 (6 channels and 2 others) 82 (6 channels and 2 others) Climate quality intercalibration None IR None None Volume (1 month) 45 GB 12 GB 45 GB 12 GB 1 Janowiak et al. (2001); 2 see Sect. 4.1 for a description of the channels in the variable list. to GridSat-GOES, the data were available from the NCEI archive as follows. – Raw GVAR data – a very complex binary format in- tended for transmission of satellite data via satellite up- links and downlinks. This can be read by a very limited set of software. The target audience are satellite experts with access to large computing resources. – AREA format – a format developed for McIDAS to pro- cess satellite data. This is well documented, but still re- quires a thorough knowledge of satellite data to extend to other applications. The target audience are those with access to McIDAS software (e.g., academia). – netCDF format – the netCDF file provided by the archive does not follow accepted standards in storing data and it provides no information on converting val- ues to reflectance and brightness temperatures. There is also no obvious way to renavigate imagery. Therefore, while the target audience may be scientific users, these issues make the data provided by the archive less useful for that audience. – TIFF and JPEG – these formats are solely for imaging. The data provided have map overlays. The target audi- ence are the general public looking for satellite images for qualitative analysis, not quantitative studies. What is lacking is a format that can be used by the general scientific community, such that it can easily be incorporated into other processes and used for analysis in a wide range of applications. Table 1 contrasts the access to GOES data via McIDAS area files and the GridSat-GOES formats. The AREA file for- mat is fully supported by one software application, the navi- gation (outside of McIDAS) requires more than 1000 lines of code, and the calibration information (outside of McIDAS) is not directly available in the AREA files, but requires access- ing satellite-specific coefficients from another source. Lastly, the temporal resolution of the files can vary from 1 to 30 min, with a complex scan schedule that varies depending on the scan mode selected for the day (e.g., when severe weather is expected, forecasters can request 1 min rapid scans of a re- gion, at the expense of other regional scans). Conversely, the GridSat-GOES format provides calibrated and mapped data that has been temporally sampled to the nearest 15 min (for CONUS) and 1 h (for GOES). The format – netCDF-4 – is supported by Unidata, which provides information on how to use netCDF in multiple languages and applications. 1.2 Relationship to other available datasets GridSat-GOES and GridSat-CONUS are most similar to two other datasets: the globally merged IR (infrared) dataset (Janowiak et al., 2001) and the GridSat-B1 Climate Data Record (CDR) (Knapp et al., 2011). Various details of the datasets are compared in Table 2. The global merged data has provided information for precipitation processing since the beginning of global meteorological geostationary satel- lite coverage in 1998, but only provides information from one channel: the infrared window. The GridSat CDR expands on this by providing intercalibrated IR brightness temperatures with a longer period of record (1980–present). The GridSat- Earth Syst. Sci. Data, 10, 1417–1425, 2018 www.earth-syst-sci-data.net/10/1417/2018/
  • 3. K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data 1419 GOES datasets focus on providing access to complete GOES data in a new, updated format. It provides access to all GOES Imager channels. Since it does not expand to other satellites, it is mostly limited to the Western Hemisphere. 2 GOES data processing The effort to produce GridSat data required processing GOES area files retrieved from NOAA CLASS (Compre- hensive Large Array-data Stewardship System). In so doing, we ordered and downloaded 294 TB of data in about 10 mil- lion files. This represents the entire archive of GOES-8 to GOES-15 data for 1994–2016. Processing these AREA files required several steps in order to produce the final GridSat products. The first step was to resolve the issue of duplicate files in the archive. This occurs when multiple files are avail- able for one scan time. Next, specific scan times were se- lected. While the GOES scan schedule is a complex system of various scan schedules taking image scans (i.e., regions) at different times, the GridSat data provide grids at regular times. Image scans are then navigated, which involves deter- mining the location of each pixel and, conversely, determin- ing which image pixel is closest to a given Earth location. Once navigated, the scans are converted from digital counts with a 10 bit depth (hence they range from 0 to 1023) to geo- physical units, either reflectance or brightness temperature, a process called calibration (note that visible data are not pro- vided as reflectance factor, which would have a correction for the illumination geometry). Lastly, two other variables which provide information on the spatial uniformity of the images are calculated and stored in the resulting GridSat file. More details on these procedures are provided below. 2.1 Resolving duplicate files Files were originally archived and stored on tapes. Each time a tape is read, there is the possibility that the file has slightly different contents. When this occurs, both files are retained and are differentiated by a trailing hyphen and number (e.g., “−1”). This produced duplicate files of the same image. Duplicate files are prevalent in the GOES archive. They exist for 1994 through 2003 and can be up to 30 % of the files for any given year. Because some files have more than one duplicate, it is not unusual to find files that are repeated 3 or 4 times. The most repeated file is “goes08.1995.060.174513” (from GOES 8 on 1 March 1995 at 17:45:13 UTC), which has six files for the single scan. The total number of extra files in the archive exceeds 64 000 files; for context, that is more than the number of GOES files archived for 1997. In essence, there is the equivalent of an extra satellite year in the archive. The reprocessing effort set up an objective algorithm to reconcile these duplicate files. Files were compared scan line by scan line. First, files that were identical were resolved sim- ply by renaming or removing files. Where scan lines differed, the scan lines that were more highly correlated with neigh- boring scan lines were selected. This led to selecting one of the multiple files as the best; however, some cases resulted in a completely new file (when best scans were selected from different files). This process resolved all duplicated files. 2.2 Temporal sampling The GOES Imager produces about 120 images per day; the exact number varies by scan schedule. Given the wide va- riety of scans and times, the GridSat-GOES and GridSat- CONUS sectors store data at nominal 60 and 15 min inter- vals, respectively. That is, a sector that begins at 17:45 UTC will be included in the 17:45 UTC CONUS grid as well as the 18:00 UTC GOES grid. Thus, users do not need to be aware of the scan schedule, or understand where each is taken, in order to access and process GridSat-GOES data. Furthermore, the hourly processing allows GridSat-GOES to merge images from various scans. The GOES scan sched- ule provides a full disk scan once every 3 h. However, the var- ious scans in the intermediate times, when merged, produce something that can approximate full disk coverage. Figure 1 provides a demonstration of this capability. Figure 1a shows an 18:00 UTC image, for which a full disk scan was avail- able. However, in Fig. 1b, the sectors from multiple sepa- rate scans (Northern Hemisphere Extended, Southern Hemi- sphere, etc.) are merged on the same grid, thus providing a nearly full disk image. The GridSat-GOES dataset provides a variable (called delta_time) that allows the calculation of the actual scan time of each pixel by storing the difference in time between the GridSat image time (17:45 and 18:00 UTC in the previous example) and the time of the actual instrument scan as de- rived from the scan line information in the AREA files. This allows for calculation of accurate solar illumination angles. 2.3 Navigation Once a grid time is selected, the image pixels nearest each grid cell are selected through sampling. No averaging of the observations is performed. Sampling maintains the pixel bit depth, radiative characteristics, and spatial resolution. Data were navigated following the algorithms provided for GOES data by NESDIS (1998). Some navigation errors will be present in the remapped data from the original data; navi- gation accuracy for GOES is about 4 km during daylight and 6 km at night (Ellrod et al., 1998). Since data are stored on a 0.04◦ equal angle grid, simply displaying the data without transform, as in Fig. 1, provides an equirectangular map projection. www.earth-syst-sci-data.net/10/1417/2018/ Earth Syst. Sci. Data, 10, 1417–1425, 2018
  • 4. 1420 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data Figure 1. Sample of infrared window GridSat-GOES images for 18:00 and 19:00 UTC. (a) The 18:00 UTC image provided by a full disk scan that began at 17:45 UTC. (b) The 19:00 UTC image is a combination of three separate scans. 2.4 Calibration Infrared channel data are calibrated following documenta- tion from the NOAA STAR web page (Weinreb et al., 1997, 2017). Scan lines are calibrated separately and converted to a brightness temperature. The brightness temperatures are stored in two-byte integers using data packing (e.g., CF con- vention scale_factor and add_offsets). Brightness tempera- ture noise is about 0.1 K at 300 K (Ellrod et al., 1998), which was improved to approximately 0.05 K at 300 K for imagers on GOES-13 to GOES-15 imagers (Zou et al., 2015). There is no on-board visible calibration reference for any GOES satellite prior to GOES 16, so some correc- tion is necessary given the decay of the instrument gain (Knapp and Vonder Haar, 2000). Therefore, visible channel data are calibrated in GridSat-GOES following Inamdar and Knapp (2015) to provide a temporally stable set of visible observations. 2.5 Visible and infrared window variability Lastly, two extra variables are provided: IR and visible chan- nel variability. For each pixel that was mapped to a grid cell, the spatial variability is provided. The spatial variability is calculated as the standard deviation of the 3 × 3 pixels cen- tered on the selected pixel. This is calculated at the original satellite resolution. Thus, for the visible channel, which is available at 1 km pixels, it provides a measure of sub-grid cell variability. For the IR, whose original resolution is near 4 km, the calculation provides a measure of the grid cell vari- ability. Both of these measures are related to cloud cover and can be used to determine the presence of clouds (Rossow and Garder, 1993). 3 Future plans Many possible improvements exist. Future improvements will depend on user needs. Possible improvements include the following: – Expansion to 1980–1994 – GridSat-GOES is currently limited to the GOES-I to GOES-M series of satellites (GOES 8 through GOES 15). The dataset can be ex- panded back in time to provide information from the previous series (GOES 1–7). While it would have fewer channels, it could maintain similar temporal and spatial resolutions. – Expansion to GOES 16 and beyond – the newest se- ries of GOES has the Advanced Baseline Imager (ABI), which increases the number of channels to 16, increases the spatial resolution to 2 km for most channels, and the temporal resolution of full disk scans is 15 min (Schmit et al., 2005, 2017). Nonetheless, there will continue to be demand for this long-term GridSat series at a reduced volume. Expanding GridSat-GOES to GOES 16 could provide a solution for users needing large spans of time. – Cloud information – including cloud information (prob- ability, temperature, optical thickness, etc.) is possible and can be provided in future versions. While increasing the volume of the data, it would increase the usefulness to many users. – Increase spatial resolution – visible data are available at 1 km spatial resolution and there has been some de- mand for that. However, it would significantly increase the volume (thus decreasing the usefulness of the data). One viable option would be to include the 1 km channel data only for the CONUS sector. Earth Syst. Sci. Data, 10, 1417–1425, 2018 www.earth-syst-sci-data.net/10/1417/2018/
  • 5. K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data 1421 – Merge GOES East and GOES West – data are presently provided separately for GOES East and GOES West. It is possible to merge the projections to one value, but doing so would impact data usefulness. In particular, the visible channel would be affected by including satellites in the same scene that have different viewing geome- tries. – Climate quality calibration – presently, we are apply- ing the operational brightness temperature corrections. There are higher quality calibrations that could be de- rived. While the visible channel calibration already in- cludes values that provide longer-term stability, im- provements could be made. This is a short list of changes that could be made to further improve the utility of the data. User feedback will be used to prioritize these tasks and resources applied to those that are in highest demand. 4 Data availability The data are available from the following links: GridSat- CONUS, https://www.ncei.noaa.gov/data/gridsat-goes/ access/conus (Knapp, 2017); and GridSat-GOES, https: //www.ncei.noaa.gov/data/gridsat-goes/access/goes (Knapp, 2017). The web page https://www.ncdc.noaa.gov/gridsat/ (Knapp, 2018) provides additional GridSat-GOES informa- tion and allows user registration. Table 4 provides a list of files (for GridSat-GOES) provided by each satellite for each year. The most files possible in a year is 8760 files at 24 files per day (and 8784 in a leap year). The variation in numbers shows gaps in the record: large deviations from 8760 exist in the 1990s with more stable and relatively fewer missing files in the more recent years. This shows the progression of the GOES East position from GOES 8 to 12 to 13; a similar progression is apparent for GOES West. There are also numerous GridSat-GOES files for GOES 14; however, it was not listed in Table 3 since it never assumes a role as either GOES East or West for a significant time. The entire GridSat-GOES period of record for all satellites is 24 TB, which is a factor of 10 smaller than the full-resolution archive (342 TB). The GridSat-CONUS period of record totals about 5 TB of data. 4.1 Variables The following variables are provided in GridSat-GOES and GridSat-CONUS netCDF files. They are listed with a sum- mary of how they could be used. A sample set of images is provided in Fig. 2 for GridSat-CONUS. – Lat – the latitude coordinate variable (in ◦ N). This pro- vides the ability to map the data. – Lon – the longitude coordinate variable (in ◦ W), which also provides the ability to map the data. Table 3. List of duplicate file numbers by year. The percentages are relative to the number of scans in the year. Year Number of scans with duplicate files 1994 448 (3 %) 1995 26 137 (41 %) 1996 15 830 (22 %) 1997 4743 (7.6 %) 1998 5 (0.01 %) 1999 37 (0.04 %) 2000 25 (0.02 %) 2001 41 (0.04 %) 2002 0 (0 %) 2003 153 (0.15 %) – Time – nominal time for the GridSat data (in units of “days since 1 January 1970 00:00:00 UTC”). – Lat_bounds – from the CF convention, these provide the spatial bounds of the grid cells used. – Lon_bounds – from the CF convention, these provide the spatial bounds of the grid cells used. – Time_bounds – from the CF convention, these provide the temporal bounds of the temporal sampling. – Satlat – the latitude of the subsatellite point (in ◦ N). This can be used in conjunction with satlon to calcu- late the viewing angles (e.g., satellite view zenith and azimuth angles). – Satlon – the longitude of the subsatellite point (in ◦ W). – Satrad – distance (in km) of the satellite from the center of the Earth. – Filename – filenames for the source archive AREA files included in the GridSat file, to provide lineage. – Delta_time – provides the difference of the actual scan time (in minutes) from the optimal grid time, used for calculations of solar geometry. – Ch1 – reflectance (or scaled radiance) for the 0.6 µm band. The visible channel provides observations of clouds, Earth’s surface, and aerosols. – Ch2 – brightness temperature of the 3.9 µm channel. This wavelength measures both reflected sunlight as well as the Earth’s radiance and provides information on cloud microphysics and other surface information. – Ch3 – brightness temperature of the 6.7 µm channel, which is also called the water vapor channel. The chan- nel provides information on the distribution and move- ment of water vapor in the upper atmosphere. The at- www.earth-syst-sci-data.net/10/1417/2018/ Earth Syst. Sci. Data, 10, 1417–1425, 2018
  • 6. 1422 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data Figure 2. Sample images produced from GridSat-CONUS for 19 August 1999 at 17:45 UTC showing all eight channels. Since this is GOES 8, there is no channel 6 data; central wavelengths are provided in Sect. 4.1. Visible data are enhanced to show detail across the dynamic range. mosphere at this wavelength is opaque and the channel rarely sees the surface. – Ch4 – brightness temperature of the 11 µm channel, generally called the IR window channel. It provides in- formation on the radiant temperature on the surface or cloud top. – Ch5 – brightness temperature of the 12 µm channel. This channel provides information on the amount of at- mospheric contamination in the 11 µm, which is why it is often called the split window. This channel is empty for GOES-12 to GOES-15 satellites because it is not in- cluded on those satellites. – Ch6 – brightness temperature of the 13 µm channel. This channel provides information on cloud cover and cloud height. This channel is empty for GOES-8 to GOES-11 satellites. – Ch1v – spatial standard deviation of the channel 1 re- flectance. This provides a measure of sub-grid cell vari- ability because the calculation is made on the original satellite data, then sampled to the grid cells. Useful in identifying clouds (Rossow and Garder, 1993). – Ch4v – spatial standard deviation of the channel 4 (in- frared window) brightness temperatures, which is useful for identifying clouds. As mentioned in Sect. 3, it could be possible to expand this variable list as deemed necessary to meet user needs. Earth Syst. Sci. Data, 10, 1417–1425, 2018 www.earth-syst-sci-data.net/10/1417/2018/
  • 7. K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data 1423 Table 4. Number of files per satellite per year for GridSat-GOES. GOES East GOES West Year GOES 8 GOES 12 GOES 13 GOES 9 GOES 10 GOES 11 GOES 15 1994 2316 1995 8351 2324 1996 8143 6794 1997 7209 5903 1998 8418 4526 3816 1999 8534 8617 2000 8581 8692 2001 8547 8655 2002 8557 8655 2003 2109 6511 8662 2004 8610 8655 2005 8585 8664 2006 8581 4075 4586 2007 8277 8647 2008 8204 8676 2009 8287 8353 2010 2449 6268 8664 2011 8759 8040 609 2012 8179 8695 2013 8280 8757 2014 8755 8231 2015 8759 8753 2016 8779 8782 Figure 3. Spatial coverages of the 5375×3750 pixel domain of GridSat-GOES (a) and the 1500×650 pixel domain of GridSat-CONUS (b). 4.2 Coverages The coverages of GridSat-GOES and GridSat-CONUS are shown in Fig. 3. GridSat-GOES spans the Western Hemi- sphere; in fact, it extends into the Eastern Hemisphere, span- ning 150 to 5◦ E and spanning 75◦ S to 75◦ N. The GridSat- CONUS spans 125 to 65◦ W and 25 to 50◦ N. Both sectors are provided at 0.04◦, which is approximately 4 km near the Equator. 4.3 Uncertainty Menzel and Purdom (1994) provide information on GOES calibration accuracy based on prelaunch design. Post-launch analysis has deemed IR calibration accurate to within 0.15 to 0.25 K (Ellrod et al., 1998; Wang et al., 2011). The visi- ble calibration was provided by Inamdar and Knapp (2015), where the uncertainty is about 3 % (in scaled reflectance units). www.earth-syst-sci-data.net/10/1417/2018/ Earth Syst. Sci. Data, 10, 1417–1425, 2018
  • 8. 1424 K. R. Knapp and S. L. Wilkins: Gridded Satellite (GridSat) GOES and CONUS data 4.4 Caveats While the GridSat-GOES dataset is suitable for scientific and research use, some caveats should be considered prior to us- age. First, users should become familiar with the limitations and quality of the source data: GOES Imager data (Ellrod et al., 1998; Goodman et al., 2018). The IR calibration is not yet climate quality. We presently use the operational calibration without adjustment. There are, however, some issues that reduce the ability to use GridSat- GOES for climate use. For instance, the midnight black body calibration correction was implemented by NOAA in 2003. Therefore, IR calibration is lower quality near the satel- lite local midnight prior to 1 April 2003. Also, the GSICS (Global Space-based Inter-Calibration System) community has worked on IR calibration adjustments (Goldberg et al., 2011), which are also not included in this initial version. Users should also be aware of the limitations of the tempo- ral and spatial sampling. The full-resolution visible data are sampled to the 0.04◦ grid spacing. Similarly, full-resolution temporal scans (e.g., 1 min rapid scans) are used only when they provide an observation closest to the optimal grid time. Therefore, users requiring high spatial or temporal sam- pling will need to access GOES data using another method. Furthermore, no adjustments were made for parallax error, which are cloud location errors at high view zenith angles (Vicente et al., 2002). Another limitation of the temporal sampling is the gaps that occur when data are not available. Three primary causes for the gaps are scan schedules, eclipse keep-out zones, and archive gaps. The scan schedule causes gaps mostly in the CONUS grid when the satellite is scanning the full disk (which takes longer than 15 min). For example for GOES East, there is no 18:00 UTC GridSat-CONUS file since it is still doing a full disk scan that started at 17:45 UTC. These gaps generally occur at regular intervals and are usually limited to one missed CONUS grid. The keep-out zone refers to when the satellite is near the local midnight and has the potential to look directly at the Sun. Since this would permanently damage the satellite sensor and optics, a keep-out zone was implemented to reduce the pos- sibility of damage. These are times near local midnight near the March and September equinoxes. The resulting gaps are daily and last a few hours. The GOES-13 to GOES-15 series includes an improved design that allows for shorter keep-out zones. Another limitation is the archive itself. There are some gaps in the archive where scans exist in other archives for one reason or another. These gaps can be longer than those caused by the scan schedule, often exceeding a day. A complete list of known caveats and issues is maintained on the GridSat-GOES website. 5 Conclusions The GridSat-GOES and GridSat-CONUS datasets provide GOES data to a wide audience in a standard format. Since the tasks of calibrating and navigating have been performed during production, data access is greatly simplified for the user. The temporal granularity structure is also much sim- pler, using fixed grid times rather than the complex and vari- able heritage scan times. The data are then made available in the widely supported netCDF-4 format. Future improve- ments and versions will be driven by user requirements. Author contributions. KRK produced the GridSat data and wrote the manuscript. SLW ordered and managed the GOES data downloaded from NOAA CLASS. Competing interests. The authors declare that they have no con- flict of interest. Acknowledgements. We would like to acknowledge Jes- sica Matthews and Anand Inamdar of CICS-NC for helping make this GOES reprocessing possible. 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