gadr1sdd
11/20/95 kvj #1
11/29/95 kvj #4 -- renamed from gacdr.dd
12/08/95 kvj #7 -- editing.
12/18/95 kvj #9 -- earth location parameters
01/17/96 kvj #10 -- fields taken directly from the HRPT minor
frame.
02/01/96 kvj #11 -- see channel_3 as per Dave: <<...>>
02/07/96 kvj #14 -- adding definitions and notes for the GAC
frame telemetry items. Editing after own review.
02/12/96 kvj #15 -- test coeffcieints for the visible channels.
02/20/96 kvj #16 -- corrections as per jds.
This version given to HP for review.
02/21/96 kvj #17 -- editing: corrected bit count for RESYNC.
02/22/96 kvj #18 -- editing.
02/28/96 kvj #19 -- putting ref_sunlight flags into array.
02/29/96 kvj #20 -- minor editing. This draft going to Faye.
02/29/96 kvj #21 -- adding bit numbers for the PACS quality
indicators
05/06/96 kvj #22 -- corrected note 7.
configured 05/22/96 kvj
05/30/96 kvj #23 -- sorting & editing.
05/31/96 kvj #24 -- Fixed DO_NOT_USE_SCAN.
06/07/96 kvj #25 -- CLOCK_UPDATE flag.
06/19/96 kvj #26 -- corrected typeo and modified entry for
BIT_SLIPPAGE
07/01/96 kvj #27 -- corrected DO_NOT_USE_SCAN, CALIB_SCAN_-
PROBLEM_INDICATOR, and TIME_PROBLEM_INDICATOR.
07/05/96 kvj #28 -- modified DO_NOT_USE_SCAN, CALIB_SCAN_-
PROBLEM_INDICATOR, and CALIBRATION_ERROR.
07/08/96 kvj #29 -- update PSEUDO_NOISE_INDICATOR
reviewed 07/09/96 jds -- comments in <<..>>
07/29/96 kvj #30 -- corrections after review by jds. Also made
changes based on design review meetings.
reviewed 08/05/96 msg -- comments in <<..>>
08/05/96 kvj #31 -- changed CHANNEL3_INDICATOR to reflect
corrected Satellite to Ground Interface document.
08/06/96 kvj #32 -- corrections after review by msg.
reviewed 08/07/96 jds -- comments in <<..>>
08/12/96 kvj #33 -- corrections after reveiw by jds.
----------------------------------------------------------------
Data Dictionary: GAC 1b* Data Record
AD_CONVERTER_TEMPERATURE measures the output from a thermistor
located inside the A/D converter at the point of maximum
sensitivity to heat. {note 1}
BASEPLATE_TEMP measures the output of the thermistor located on
the baseplate. {note 1}
BIT_SLIPPAGE This indicator is based on a PACS-generated flag
passed to the PDP through the level 1a data set. A value of 1
indicates the condition occurred; a value of 0 indicates it did
not. (Bit 3 of word 2 of the PACS quality indicators.) {note 12}
BIT_SYNC_ERROR_COUNT A PACS-generated count of the number of
bits in error in the frame sync. (Bits 7 to 2 of word four of the PACS
quality indicators.) {note 12}
BIT_SYNC_STATUS This indicator is based on a PACS-generated
flag passed to the PDP through the level 1a data set. A value
of 0 implies bit sync lock. A value of 1 indicates that bit
sync lock dropped during the frame. (Bit 7 of word two of the
PACS quality indicators.) {note 12}
BLACKBODY_CH3B the output of a sample-and-hold circuit which
samples the IR Channel 3B data signal once each scan line as the
instrument views the black body calibration target. {note 1}
BLACKBODY_CH4 the output of a sample-and-hold circuit which
samples the IR Channel 4 data signal once each scan line as the
instrument views the black body calibration target. {note 1}
BLACKBODY_CH5 the output of a sample-and-hold circuit which
samples the IR Channel 5 data signal once each scan line as the
instrument views the black body calibration target. {note 1}
BLACKBODY_TEMPERATURE an array of 4 readings, one from each of
the 4 PRTs located on the black body calibration target. {note
1}
CALIBRATION_ERROR is a quality indicator normally set to 0 but
set to 1 by the PDP if the calibration coefficients reported on
this record are questionable or missing (i.e., if there is at
least one non-zero element in CALIB_SCAN_PROBLEM_INDICATOR).
CALIBRATION_TARGET_DATA {note 9} radiometric counts from each
of channels 3, 4, & 5 taken as the AVHRR scanned the calibration
target. There are 10 observations for each of the three
channels. {note 11}
CALIB_CHAN_PROBLEM_INDICATOR a 2-dimensional array of Boolean
codes -- one column for each of the IR channels -- indicating
the occurrence (1--> yes, 0 --> no) of the channel specific
conditions that follow:
+
(1) this channel is not calibrated. Notice that the
indicator for channel 3 will be on when 3A is
active.
(2) the calibration for this channel is
questionable
(3) insufficient black body counts this scan
(4) insufficient space view counts this scan
(5) spare
(6) some bad black body counts this scan
(7) some bad space view counts this scan
(8) spare
CALIB_SCAN_PROBLEM_INDICATOR an array of Boolean codes
indicating the occurrence ( 1--> yes, 0 --> no) of the conditions
given below. These indicators apply to the scan line as a
whole. Indicators for specific channel conditions are given by
the CALIB_CHAN_PROBLEM_INDICATOR.
+
(1) there was no successful IR calibration
for this scan.
(2) marginal IR calibration (i.e., there were
marginal or failed calibrations for some
channels).
(3) no calibration due to bad or missing
PRT data.
(4) calibrated but with marginal PRT data
(5) some uncalibrated channels for this
scan line.
(6) no visible calibration
CHANNEL3_INDICATOR indicates which of channel 3A and 3B is
active or that the state is uncertain. Uncertainty will occur
at the transition point between channels and will usually last
for two scan lines. The indicator can take on the following
values:
+
-1 --> unknown
1 --> channel 3A
0 --> channel 3B
CLOCK_DRIFT_CORRECTION indicates whether the scan time given in
this record has been corrected for the drift in the spacecraft
clock. A value of 0 indicates no correction has been made, and
a value of 1, that the time in TIME_OF_DAY_OF_SCAN has been
corrected for drift.
CLOCK_DRIFT_DELTA the difference between the time reported on
this data record and that reported by the spacecraft. The time
reported on the data record has been corrected by the PDP for
the drift of the spacecraft's clock. If the CLOCK_DRIFT_DELTA
is added to TIME_OF_DAY_OF_SCAN the time reported by the
spacecraft will result.
CLOCK_UPDATE the clock on-board the spacecraft has been updated
on command from SOCC just prior to the current scan line. If
this flag is set on (1), an update was expected and actually
detected in the data. This flag will almost always be off (0).
COOLER_HEAT flag from the digital B data in TIP indicating the
status of the cooler heater. 1 --> on; 0 --> off.
COOLER_HOUSING_TEMPERATURE measures the output from a thermistor
located in the radiant cooler housing. {note 1}
DATA_GAP_INDICATOR Normally zero, if this indicator is set to 1
on a given record, that record is the first one following a data
gap. This indicator is controlled by the PDP, which sets the
indicator based on the patterns of time codes it finds in the
level 1a data.
DAY_OF_YEAR_OF_SCAN the day of the year on which the scan
began, set by the PDP based upon the date and time information
provided in the data stream by the spacecraft.
DETECTOR4_BIAS_VOLTAGE measures a voltage directly proportional
to the regulated -12 volt DC which supplies the Channel 4 IR
detector bias current. {note 1}
DETECTOR5_BIAS_VOLTAGE measures a voltage directly proportional
to the regulated -12 volt DC which supplies the Channel 5 IR
detector bias current. {note 1}
DO_NOT_USE_SCAN is a general quality indicator set by the PDP.
If its value is 1, the scan line should be treated by the
product processors as though it were missing. This flag will be
set if the P-N indicator is on, or if the calibration is
not performed for some other reason, or if the earth location
information is missing or erroneous, or if the scan contains a
corrupt time code and a valid time can not be inferred.
EARTH_LOCATION_ERROR Normally zero, this indicator indicates the
occurrence of a problem in earth location.
EARTH_LOCATION_PROBLEM_INDICATOR
+
(1) not earth-located because of bad time
(2) questionable earth location because of
questionable time code
(3) questionable earth location because of marginal
agreement with reasonableness check
(4) earth location probably bad -- fails
reasonableness check.
EARTH_SHIELD flag from the digital B data in TIP indicating the
status of the earth shield. 1 --> deploy; 0 --> disable.
EARTH_SHIELD_POSITION a measurement from which the status of
the radiant cooler earth shield can be determined. That status
can be any of the following: fully closed, partly open, and
fully open. {note 1}
ELECTRONICS_CURRENT measures the DC current load on the +28
volt bus (pins 1 and 2 on j3). This is directly proportional to
the DC current into the power converters for the remaining
electronics not including the motor power supply. {note 1}
ELECTRONICS_TELEMETRY flag from the digital B data in TIP
indicating the status of the electronics/telemetry. 1 --> on; 0
--> off.
ELECTRONICS_TEMPERATURE measures the output of the thermistor
located inside the electronics box. {note 1}
FLYWHEELING_DETECTED This indicator is based on a
PACS-generated flag passed to the PDP through the level 1a data
set. A value of 1 indicates the condition occurred; a value of
0 indicates it did not. (Bit 4 of word 2 of the PACS quality
indicators.) {note 12}
FRAME_SYNC {note 9} The synchronization words used by ground
processing to locate the beginning of a GAC frame.
FRAME_SYNC_ERROR This indicator is based on a PACS-generated
flag passed to the PDP through the level 1a data set. A value
of 0 indicates that no errors in the frame sync word were
detected, a value of 1 that at least 1 error occurred. (Bit 6 of
word 2 of the PACS quality indicators.) {note 12}
FRAME_SYNC_LOCK This indicator is based on a PACS-generated
flag passed to the PDP through the level 1a data set. A value
of 0 indicates frame sync lock, a value of 1, that frame sync
previously dropped lock. (Bit 5 of word 2 of the PACS quality
indicators.) {note 12}
GDTIME Geometric Data Time taken from the TIP CPU telemetry.
HEIGHT distance in km from the satellite to the earth
ellipsoid, measured along a line drawn normal to the ellipsoid.
{note 5}
ID_AVHRR {note 9} The ID field from the GAC frame {note 6}
used to construct this GAC record. The two 10-bit words have
been stored in this array in the same order as they appeared in
the GAC frame. {note 7}
+
Element 1:
bits meaning
----- -----------------------------------------
15-10 spare (all 0s)
9 0 --> internal sync; 1 --> AVHRR sync
8-7 00 --> GAC frame; 01 --> HRPT minor frame
#1; 02 --> minor frame #2; 03 --> minor
frame #3.
6-3 spacecraft address
2 0 --> frame stable; 1 --> frame sync
occurred
1 0 --> pseudonoise; 1 --> normal AVHRR
input
0 1 --> channel 3A is active; 0 -->
channel 3B is active. {note 8}
+
Element 2: spare. The least significant 10 bits of this
element will contain the second ID word as it was sent from the
spacecraft.
IR_CALIBRATION_COEFF_PRIMARY The calibration coefficients
computed by the PDP for the IR channels using the primary
calibration algorithm. These are the calibration coeffieints
that should be used to convert radiometric counts from channels
3B, 4, and 5 in the current scan line to radiance. (See
OBSERVATIONS.)
IR_CALIBRATION_COEFF_SECONDARY The calibration coefficients
computed by the PDP for the IR channels using a secondary
calibration algorithm. These coeffieints are experimental and
most users should ignore them.
LAT_LON_DEGREES The geodetic latitude and longitude, in that
order, of 51 points along the scan line {note 4}. The
latitude's range is +/- 90 degrees, with 0 at the equator and
positive angles to the north. The longitude is measured from
Greenwich, with positive values to the east; its range is +/-
180 degrees.
MOTOR_CURRENT measures the DC current load on the +28 volt
(Motor) bus (pins 3 and 4 on j3). This is directly proportional
to the DC current into the scan motor power supply. {note 1}
MOTOR_HOUSING_TEMPERATURE measures the output from a thermistor
located on the scan motor housing. {note 1}
MOTOR_TELEMETRY flag from the digital B data in TIP indicating
the status of the scan motor/telemetry. 1 --> on; 0 --> off.
NUMBER_OF_EL_POINTS the number of earth locations provided per
record. For the AVHRR instruments, this will always be 51.
OBSERVATIONS The radiometric counts reported by the insturment
in all five channels at each of 409 fields of view.
ORBIT_NODE This field indicates the direction of the spacecraft
at the time of the scan. A value of zero indicates the
spacecraft is heading north (ascending), a value of one that it
is going south (descending).
PATCH_CONTROL flag from the digital B data in TIP indicating the
status of the patch control. 1 --> on; 0 --> off.
PATCH_POWER measures the DC voltage being applied to the control
heater on the radiant cooler patch which contains the IR
detector. {note 1}
PATCH_TEMPERATURE a PRT reading from radiant cooler patch which
contains the IR detector. The amplifier gain/offset is adjusted
for the patch temperature range. {note 1}
PATCH_TEMPERATURE_EXTENDED a PRT reading from radiant cooler
patch which contains the IR detector. The amplifier gain/offset
is adjusted for the extended temperature range of the patch.
{note 1}
PITCH_ERROR_CORRECTION the value provided in the TIP's CPU
parameter APITCH. It represents the Euler error angle. See the
Programming and Control Handbook (Reduction of CPU Telemetry)
for instructions for the conversion of this integer value to an
angular measure.
PSEUDO_NOISE_INDICATOR This indicator is normally set to 0 but
is set to 1 by the PDP if bit 1 {note 7} of the first ID word in
the GAC frame has been set to 0 by the MIRP. This indicates
that the GAC data is manufactured and should not be used. No
calibration will occur, and DO_NOT_USE_SCAN will contain 1.
PWTIP$AC the "passive wheel test in progress" indicator from
the TIP CPU telemetry. The two-byte hex code given in the
telemetry has been stored as a one-byte indicator that can be
interpreted as follows:
+
0 --> normal (no test)
1 --> yaw axis test inprogress
2 --> roll axis test in progress
3 --> pitch axis test in progress
-1 --> attitude monitoring locked out
+
(reference: KLM Telemetry List, Figure 27)
RADIATOR_TEMPERATURE PRT reading from the first (warmer) stage
of the radiant cooler. {note 1}
REFERENCE_VOLTAGE measures a DC voltage proportional to the
+6.4 volt reference voltage source in the electronics. {note 1}
REF_SUNLIGHT_DETECTED This field indicates whether the PDP's
calibration module detected the gain anomaly {note 2} in one of
the IR channels. The field is an array of three indicators, one
for each of the IR channels in the natural order. A value of -1
implies that the checks designed to detect the gain anomaly
could not be performed; a value of 0 implies the checks were
made, and the anomaly was not detected; 1 implies the anomaly
was detected.
RELATIVE_AZIMUTH_ANGLE the angle between the projections onto
the tangent plane of lines drawn from the viewed point to the
sun and from the viewed point to the satellite. It is measured
from sun to satellite and has a range of +/- 180 degrees; angles
eastward from the sun are positive. {note 4}
RESYNC This indicator is normally zero but is set to 1 by the
PDP if the resync (AKA rephase) flag in the telemetry header (ID
AVHRR word, bit 2 {note 7}) has been set to 1 by the MIRP. The
flag is repeated for 12 HRPT minor frames (4 GAC frames) after a
resync has occurred. {note 3}
ROLL_ERROR_CORRECTION the value provided in the TIP's CPU
parameter AROLL. It represents the Euler error angle. See the
Programming and Control Handbook (Reduction of CPU Telemetry)
for instructions for the conversion of this integer value to an
angular measure.
SATELLITE_ZENITH_ANGLE (aka local zenith angle) the angle from a
line drawn from the viewed point to the satellite and one from
the viewed point normal to its tangent plane. The range of
satellite zenith angle is 0 to 90 degrees. {note 4}
SCAN_LINE_NUMBER the relative scan number within the period of
the 1b data set, starting with 1. Scan_line_number will take
into account missing scans and thus attempt to give an accurate
count of the number of scan lines generated by the spacecraft.
Set by the PDP.
SCAN_MOTOR flag from the digital B data in TIP indicating the
status of the scan motor mode. 1 --> high power; 0 --> low
power.
SELECT_STATUS_3A_3B flag from the digital B data in TIP
indicating which of channels 3A and 3B is active. 1 --> 3A; 0
--> 3B.
SMODE the attitude control submode word from the TIP CPU
telemetry. The two-byte hex code given in the telemetry has
been stored as a one-byte indicator that can be interpreted as
follows:
+
0 --> nominal
1 --> rate nulling
2 --> yaw gyrocompass
3 --> search
4 --> coast
+
(reference: KLM Telemetry List, Figure 27)
SOLAR_ZENITH_ANGLE the angle between a line drawn from the
viewed point to the sun and one from the viewed point normal to
its tangent plane. The range of solar zenith angle is 0 to
180 degrees. {note 4}
SPACE_DATA {note 9} radiometric counts from each of the acitve
AVHRR channels (1 - 5) as the instrument scanned space.
STATUS_CHANNEL1 flag from the digital B data in TIP indicating
the status of the channel 1. 1 --> on; 0 --> off.
STATUS_CHANNEL2 flag from the digital B data in TIP indicating
the status of the channel 2. 1 --> on; 0 --> off.
STATUS_CHANNEL3A flag from the digital B data in TIP indicating
the status of the channel 3A. 1 --> on; 0 --> off.
STATUS_CHANNEL3B flag from the digital B data in TIP indicating
the status of the channel 3B. 1 --> on; 0 --> off.
STATUS_CHANNEL4 flag from the digital B data in TIP indicating
the status of the channel 4. 1 --> on; 0 --> off.
STATUS_CHANNEL5 flag from the digital B data in TIP indicating
the status of the channel 5. 1 --> on; 0 --> off.
SUBBLOCK_NUMBER The AVHRR calibration is performed upon
subblocks composed of 5 scan lines. This field contains the
number of the subblock to which this particular scan line
belongs.
SYNC_DELTA {note 9} this word may be interpreted as follows:
bits 15-10 are spare. Bit 9 is flag: 0 --> AVHRR sync early, 1
--> AVHRR sync late. Bits 8-0 give a 9-bit binary count of
0.9984 MHz periods. {note 7}
TELEMETRY {note 9} The first five elements contain the ramp
calibration data for the five active channels; the next three
elements contain a reference value or counts from one of the
PRTs measuring the temperature of the calibration target {note
10}. The ninth element is the channel 3 patch temperature, and
the 10th is spare. It contains the same bit pattern as sent from
the spacecraft.
TELEMETRY_LOCK flag from the digital B data in TIP indicating
the status of the telemetry lock. 1 --> locked on; 0 --> not
locked on.
TIME_CODE {note 9} The time code field from the GAC frame
unpacked into two 4-byte integers. The first is the day count
and the second is the millisecond count.
TIME_ERROR is a quality indicator normally set to 0 but set to
1 by the PDP if the time reported on this record is questionable
or missing. Also see TIME_PROBLEM_INDICATOR. If this scan
marks the first such error, its record number is recorded in
TIME_SEQUENCE_ERROR_RECORD of the header record.
TIME_OF_DAY_OF_SCAN the time of day in milliseconds of the start
of the scan. Set by the PDP based upon the date and time
information provided in the data stream by the spacecraft. (See
CLOCK_DRIFT_DELTA.)
TIME_PROBLEM_INDICATOR provides an array of boolean codes
indicating the occurrence (or not) of the following conditions:
+
(1) Time received from the s/c was
obviously bad. The time on this scan
has been inferred from those on nearby
scan lines.
(2) Bad time received from s/c and can't
infer one reliably.
(3) Pattern of scan times changed. May be
related to clock update.
(4) Spare.
TIP_ATTITUDE_CORRECTION indicates whether the earth location
fields in this record where corrected for deviations in
spacecraft attitude. A value of 1 indicates the correction has
been made; a value of 0 indicates it has not. Currently this
field will always be set to 0.
TIP_PARITY_MINOR_FRAME1 This indicator is based on a
PACS-generated flag passed to the PDP through the level 1a data
set. A value of 1 indicates the condition occurred; a value of
0 indicates it did not. (Bit 7 of word 3 of the PACS quality
indicators.) {note 12}
TIP_PARITY_MINOR_FRAME2 This indicator is based on a
PACS-generated flag passed to the PDP through the level 1a data
set. A value of 1 indicates the condition occurred; a value of
0 indicates it did not. (Bit 6 of word 3 of the PACS quality
indicators.) {note 12}
TIP_PARITY_MINOR_FRAME3 This indicator is based on a
PACS-generated flag passed to the PDP through the level 1a data
set. A value of 1 indicates the condition occurred; a value of
0 indicates it did not. (Bit 5 of word 3 of the PACS quality
indicators.) {note 12}
TIP_PARITY_MINOR_FRAME4 This indicator is based on a
PACS-generated flag passed to the PDP through the level 1a data
set. A value of 1 indicates the condition occurred; a value of
0 indicates it did not. (Bit 4 of word 3 of the PACS quality
indicators.) {note 12}
TIP_PARITY_MINOR_FRAME5 This indicator is based on a
PACS-generated flag passed to the PDP through the level 1a data
set. A value of 1 indicates the condition occurred; a value of
0 indicates it did not. (Bit 3 of word 3 of the PACS quality
indicators.) {note 12}
VIS_CALIBRATION_COEFF_ACTIVE The currently recommended
calibration coefficients for the AVHRR's visible channels 1, 2 &
3A. The calibration curves are bilinear and five coefficients
are given for each of the 3 channels: the intercept and slope of
the first linear segment, followed by the intercept and slope of
the second segment, followed, finally, by the cross over point
-- the value of counts below which segment one is used and above
which segment 2 is used. The field is organized as an array the
columns of which correspond to the channels, and the rows to
coefficients in the order described above.
VIS_CALIBRATION_COEFF_HISTORIC The calibration coefficients
that were in effect at the launch of the spacecraft. The field
is in the same format as is VIS_CALIBRATION_COEFF_ACTIVE.
VIS_CALIBRATION_COEFF_TEST These coefficeints are for
experimental purposes and should not be used to produce
products.
VOLTAGE_STATUS flag from the digital B data in TIP indicating
the status of the voltage calibrate. 1 --> on; 0 --> off.
YAW_ERROR_CORRECTION the value provided in the TIP's CPU
parameter AYAW. It represents the Euler error angle. See the
Programming and Control Handbook (Reduction of CPU Telemetry)
for instructions for the conversion of this integer value to an
angular measure.
YEAR_OF_SCAN the year in which the scan began. It will contain
the year as a four-digit number. It is set by the PDP based upon
the date and time information provided in the data stream by the
spacecraft.
----------------------------------------------------------------
Notes ++++
note 1 These data items come from the TIP analog telemetry as
specified in the AVHRR's UIIS (Section 3.1.5.4 and Table 11) and
the KLM Telemetry List (Table II).
note 2 The gain anomaly results from sunlight reflecting off
the instrument's calibration target, effectively invalidating
the assumption that the target is a black body. This problem is
expected to be eliminated with NOAA-K (NOAA-15) and later
spacecraft, but logic to handle the anomaly, should it occur, is
included.
note 3 See NOAA-KLM System Design Report Vol. 2; General
Electric; 30 August 1990; pg. 4.8-54.
note 4 This data item is an array of these angles, each of which
is associated with one of 51 GAC FOVs starting with the fifth
and then every eighth, i.e., 5, 13, ..., 405.
note 5 "The ellipsoid is a mathematically tractable
approximation to the geoid, which is an equipotential surface at
mean sea level. Although the geoid is mathematically defined,
its form makes it impractical for calculations. The maximum
departure of the ellipsoid from the geoid is approximately
plus/minus 65 meters. (The ellipsoid used by NOAA since spring
of 1984 is WGS-72.) Ref. [Irv Ruff from memo from Emily Harrod
to Geof Goodrum.]
note 6 See sheets 17-18 of Satellite to Ground Interface
(NOAA-K,L,M); 6/13/95; Martin Marietta Astro Space; IS3287200;
note 7 The bit numbering convention used in the Satellite to
Ground Interface (STGI) is different from the one used here and
to describe the 1b formats: The STGI uses bit 1 to signify the
most significant bit; we use n-1, where n is the number of bits
in the word; they use n for the least significant bit; we use 0.
note 8 The channel 3A/3B indicator is unreliable at the
transition from one channel to the other: The switch occurs on
command from an on-board computer and is not synchronized in any
way with the scan. Thus the first scan that indicates that a
transition occurred or the scan that immediately preceded it
might have had both channels active during different parts of
the scan. Also see the description of CHANNEL3_INDICATOR above.
note 9 These data correspond to similarly named words in the GAC
frames and HRPT minor frame formats described in the Satellite
to Ground Interface (NOAA-K,L,M). Here the 10 bit words found
in the GAC telemetry have been stored in two-byte integers.
note 10 Each GAC frame corresponds to one scan line and
contains either reference values in these three words or three
measurements from one of the 4 PRTs connected to the AVHRR's
calibration target (aka black body); i.e, the same PRT is
sampled three times. It takes 5 scan lines to acquire complete
knowldege of the current temperature of the calibration target.
Four of the scan lines will provide measurements from the four
PRTs, and the fifth will provide a reference from which may be
inferred the order of the PRTs on the other scans. (Notice that
the STGI and others refer to these three words as AVHRR Chan #n
Target Temp, where n = 3, 4, & 5. This is misleading: these
measurements have nothing to do with the radiometric channels.)
note 11 The STGI refers to these observations as "Back Scan".
note 12 Bit 0 is the least significant bit and word 1 is most
significant word.