**GVAR for the XGOHI Missions**[^1]

This technical note provides a concise description and serves as a
request for user feedback relating to the adaptations made to standard
GVAR for Extended GOES High Inclination (XGOHI) missions. GVAR telemetry
is organized into blocks 0 through 11. Block 0 is a documentation block
corresponding to the start of an Imager scan. Each Block 0 is followed
by blocks 1 through 10 containing the Imager radiometric data. Sounder
data and other documentation data are contained in Block 11s. A full
description of GVAR is found in DRL 504-02 Part 1, Revision 2, Section
3.0, "GVAR Transmission Format"[^2].

GVAR for XGOHI is essentially the same as standard GVAR; however,
several spare words in standard GVAR have been utilized to provide XGOHI
users with additional information. Most users will neither need nor care
about this additional information. For such users, XGOHI GVAR is
completely interchangeable with standard GVAR. Users with critical
timing requirements and users concerned with the spatial response
(sharpness/blurriness) or radiometric noise after resampling may be
interested in the additional data provided in the XGOHI GVAR. Two types
of additional information are provided in each Block 0:

1.  Documentation of the resampling configuration of the Enhanced Sensor
    Data Processing System (ESPS) that created the XGOHI GVAR; and

2.  Time-tag correction coefficients that allow users to more precisely
    time-tag GVAR pixels.

The additional XGOHI GVAR parameters and their addresses in the GVAR
stream are listed in Table 1.

**Table 1. Additional XGOHI Content in GVAR Block 0.**

+---------------+--------------------+-------+------------------------+
| Word(s)\*     | Content/Mnemonic   |       | Description            |
+---------------+--------------------+-------+------------------------+
| 7267          | XGOHI_ON           | bit   | Resampling Status Flag |
|               |                    | s\*\* |                        |
|               | (logical)          |       | (bit 0 = 1 if          |
|               |                    | 0     | resampling is on)      |
|               | XGOHI_Kernel_ID    |       |                        |
|               |                    | 2-7   | ID of the resampling   |
|               | (integer 0 -- 63)  |       | kernel suite           |
+---------------+--------------------+-------+------------------------+
| 7271-7274     | XGOHI_P0 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *P~0~*                 |
+---------------+--------------------+-------+------------------------+
| 7275-7278     | XGOHI_P1 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *P~1~*                 |
+---------------+--------------------+-------+------------------------+
| 7279-7282     | XGOHI_P2 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *P~2~*                 |
+---------------+--------------------+-------+------------------------+
| 7283-7286     | XGOHI_P3 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *P~3~*                 |
+---------------+--------------------+-------+------------------------+
| 7287-7290     | XGOHI_P4 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *P~4~*                 |
+---------------+--------------------+-------+------------------------+
| 7291-7294     | XGOHI_Q0 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *Q~0~*                 |
+---------------+--------------------+-------+------------------------+
| 7295-7298     | XGOHI_Q1 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *Q~1~*                 |
+---------------+--------------------+-------+------------------------+
| 7299-7302     | XGOHI_Q2 (real     |       | Time Tag Coefficient   |
|               | number)            |       | *Q~2~*                 |
+---------------+--------------------+-------+------------------------+

\*Counting from one.

\*\*bit 0 is the most significant bit (msb) of an 8-bit Block-0 word

It is also worth noting that standard GVAR contains certain fields such
as scan direction and detector number that lack meaning after
resampling. These fields are filled with sensible values (*i.e.*, the
values that would apply were resampling "off") but are not meaningful
for XGOHI.

**Resampling Configuration**

The GOES Imager and Sounder can operate with on-board Image Motion
Compensation (IMC) "on" or "off" and the ESPS can operate with
resampling "on" or "off". Apparent image motion caused by orbit and
attitude motion of the spacecraft is compensated either by IMC or
resampling but not both. On-board IMC is enabled independently for the
Imager and Sounder, but resampling is possible for only the Imager. When
on-board Imager IMC is enabled or resampling is enabled, the images are
fixed-gridded; *i.e.*, there is a fixed and standard relationship
between image line and pixel number and geographic location as defined
in the [Earth Location Users Guide
(ELUG)[^3]](http://www.osd.noaa.gov/gvar/documents/Earth_Location_Users_Guide.pdf).
Fixed-gridding enables movie loops to be created where the ground is
fixed and only clouds move. When on-board IMC is disabled and resampling
is disabled, the GVAR is said to be dynamically gridded; *i.e.*, there
is a time varying relationship between image line and pixel number and
geographic location that depends on the spacecraft orbit and attitude in
a complicated way. The ground in movie loops created from dynamically
gridded data will be unstable without additional ground processing.

On-board IMC will be disabled when the satellite orbital inclination
reaches about 2° where the dynamic range for on-board IMC saturates,
therefore, the Imager will normally be operated with on-board IMC "on"
and on-ground resampling "off" at low inclination and on-board IMC "off"
and on-ground resampling "on" at high inclination. When operation is
transitioned to XGOHI, Imager data will normally be transmitted in
fixed-grid mode except for times when resampling is turned off. Sounder
data will be transmitted in dynamic-gridding mode with the geolocation
of each Sounding being transmitted in GVAR. Table 2 summarizes the
routine operational states of the system.

**Table 2. States for IMC and Resampling for Normal and XGOHI
Missions.**

+---------+---------+---------+----------+-------+-------+----------+
| **Mi    | *       | **I     | **Imager | **So  | **IS  | **XG     |
| ssion** | *Inclin | mager** | Resa     | under | CAN** | OHI_ON** |
|         | ation** |         | mpling** | IMC** |       |          |
| *       |         | **IMC** |          |       | **bit | **bit    |
| *Type** |         |         |          |       | 8**   | 0**      |
+---------+---------+---------+----------+-------+-------+----------+
| Normal  | Low     | On      | Off      | Off   | 1     | 0        |
+---------+---------+---------+----------+-------+-------+----------+
| XGOHI   | High    | Off     | On       | Off   | 1     | 1        |
+---------+---------+---------+----------+-------+-------+----------+

On-board Imager IMC status is provided by bit 8 of GVAR Block 0 words
3-6 (mnemonic ISCAN, 32 bits). Normally, the IMC status bit is set to 1
or 0 depending on whether or not IMC is enabled for the Imager on-board.
For XGOHI, this bit has a slightly different meaning: it is set to "1"
or "0" depending on whether or not the GVAR is fixed or dynamically
gridded. ESPS resampling status is provided by bit 0 of the resampling
status word (mnemonic XGOHI_ON, 8 bits). The resampling status bit will
be set to "1" when resampling is "on" and the Imager IMC status bit will
also be set to "1" to indicate that Imager IMC is being applied
on-ground during resampling.

The ESPS supports up to 64 resampling suite definitions. Each suite is a
complete definition of the resampling algorithm applied to each Imager
channel. The active resampling suite ID is reported in bits 2-7 of the
resampling status word. NOAA maintains a [GEOSS in the Americas/GOES-10
Information and Schedules
webpage[^4]](http://www.ssd.noaa.gov/PS/SATS/GOES/TEN/) that contains
information pertaining to XGOHI including a description of the
resampling configuration corresponding to each suite ID.

**Time Tag Corrections**

Applications that are very sensitive to the time tagging of individual
pixels can use the time-tag correction coefficients included in each
GVAR Block 0 to improve the precision of pixel time tags. There are
eight time-tag correction coefficients included in each Block 0 for this
purpose. Time-tag imprecision can be reduced from as much as a minute
without correction to less than five seconds when the time-tag
correction coefficients are used. Correction to a finer time resolution
does not make much sense because resampled pixels are inherently
multi-temporal weighted averages of a few raw detector samples.

It is important to first understand how time tagging works with standard
GVAR. Each Block 0 includes three header and trailer times corresponding
to "current", "lagging", and "old-lagging" values. The header time tags
the start of a scan and the trailer time tags the end of a scan. The
terms "current", "lagging", and "old-lagging" respectively refer to the
most recent header/trailer time, its predecessor, and its predecessor's
predecessor. Detector samples in the blocks 1 to 10 that follow a Block
0 form a swath of GVAR data. They may be drawn from one of the three
scans: "current", "lagging", or "old-lagging". GVAR word 15 in blocks
1-10 (mnemonic LLAG in DRL 504-02) identifies which of the three choices
applies to that block: LLAG = 0 for current, LLAG = 1 for lagging, and
LLAG = 2 for old-lagging. The appropriate time-tag mnemonic is then
selected according to Table 3.

**Table 3. Time Tag Fields Selected from Block 0 by LLAG.**

  --------------- -------------- ------------- -------------- -------------
  **LLAG from     **Header**                   **Trailer**    
  Block 1 to 10                                               
  (word 15)**                                                 

                  **Mnemonic**   **Block-0     **Mnemonic**   **Block-0
                                 Words**                      Words**

  0 (Current)     TCHED          31-38         TCTRL          39-46

  1 (Lagging)     TLHED          47-54         TLTRL          55-62

  2 (Old-Lagging) TOHED          5571-5578     TOTRL          5579--5586
  --------------- -------------- ------------- -------------- -------------

Note: Times are given in Binary Coded Decimal (BCD) format:
'YYYYDDDHHMMSSmmm'. Each byte codes a digit of YYYY = year, DDD =
day-of-year, HH = hour, MM = minute, SS = second, mmm = millisecond.
Times are Universal Time Coordinated (UTC). Day counting begins with 1.

The simplest approach to standard GVAR pixel time tagging is to select
the appropriate header or trailer time as the time tag for pixels in the
GVAR swath. Such an approach is precise to about a second; but, it does
not work well for XGOHI because detector samples are taken from many raw
scan lines to form the pixels in an XGOHI GVAR swath; consequently, the
underlying sampling time varies across a GVAR swath. The time-tag
correction coefficients describe this variation. Let *x* be the pixel
number relative to the westernmost edge of the frame divided by the
total number of pixels across the frame; therefore, *x* = 0 at the
westernmost edge and *x* = 1 at the easternmost edge of the frame. To
compute the precise time tag of an XGOHI pixel in the *n*-th GVAR swath,
one computes *x* for that pixel and evaluates the rational polynomial
function

![](media/image1.wmf). (1)

If the denominator evaluates to zero then no time-tag correction is
possible and the user should set ![](media/image2.wmf). The time-tag
correction coefficients *P* and *Q* are found in the Block 0 of the
*n*-th GVAR swath at the addresses defined in Table 1. The value of
![](media/image3.wmf) rounded to the nearest integer counts a number of
scans ahead (![](media/image4.wmf)) or behind (![](media/image5.wmf)) a
scan time tagged in the Block 0 of the *n*-th GVAR swath. It represents
the offset indexing the scan that would contain the underlying raw
detector samples. The Block 0 of GVAR swath number *n+*round(∆*n*) would
then contain the appropriate time tag for the pixel in question. It
should be used for precision time tagging of the pixel instead of a time
tag from the Block 0 for the *n*-th swath. For example: if *n* is the
GVAR swath containing the resampled pixel and LLAG = 1 for the block
1-10 containing that pixel then TT = TLTRL(*n*) would be the trailer
time that would be used for standard GVAR time tagging. However, for
XGOHI, the trailer time should really be drawn from a different Block 0;
*i.e.*, TT = TLTRL(*n+*round(∆*n*)). The user should of course check
that *n+*round(∆*n*) does not extend beyond the bounds of the current
frame. If it does then the pixel in question is likely a fill pixel and
the user should set ![](media/image6.wmf).

When ∆*n* \> 0, a user would normally need to wait until round(∆*n*)
more scans have been acquired (approximately the same number of seconds)
to apply the XGOHI time-tag correction algorithm; but, a quick estimate
may be immediately obtained by extrapolating the growth in TLTRL with
scan line. Smooth extrapolation will sometimes be interrupted by space
looks. Even these may be accounted for knowing the time of the last
space look and the interval between space looks.

**Contact Information for Feedback**

  -----------------------------------------------------------------------
  If you have questions not answered by the information provided here or
  via the GVAR Download Center files, you can contact the following:\
  Bashar Al-Mallah at <bashar.al-mallah@noaa.gov>

  -----------------------------------------------------------------------

[^1]: Prepared by Carr Astronautics and Nortel Government Solutions on
    NOAA contract CM1301-6CT-0010.

[^2]: <http://www.osd.noaa.gov/gvar/gvardownload.htm>, current on 3
    August 2007.

[^3]: <http://www.osd.noaa.gov/gvar/gvardownload.htm>, current on 3^rd^
    August, 2007.

[^4]: <http://www.ssd.noaa.gov/PS/SATS/GOES/TEN/>, contact:
    <Brian.Hughes@noaa.gov>.