Sea Surface Temperature Global Area Coverage (GAC) Processing

Appendix A: Calibration and Navigation correction factors

Most end users will not need to be concerned with the details of the 1st stage in processing of raw sensor data but may find this section useful in gaining an understanding of the calibrations and navigation provided with the final product Tables of the calibration and navigation factors are given in at the end of this appendix as a courtesy to to those users who may be processing raw AVHRR sensor data. The methods used to derive these factors (unit conversion section 1.1, navigation section 1.2) and how they are applied during the ingestion process are briefly described in this appendix.

A 1.1 Calibration and counts-to-radiance conversion

During ingestion, the raw AVHRR digital counts are converted to calibrated radiance values. This is achieved during the calibration stage.

A1.1.1 Visible and near-IR channels

The initial counts-to-radiance conversion for the visible and near-IR channels (channels 1 and 2) is a simple linear relationship. The slope and intercept for this conversion are fixed for each satellite, and are based on pre-launch calibration coefficients. Following the conversion to radiance, values are corrected for in-orbit sensor degradation and inter-satellite calibration.

The calibration/correction procedure is two tiered. The first level, relative degradation rate, accounts for the daily in-orbit degradation of the sensor. The AVHRR radiometers degrade in orbit, initially because of outgassing and launch-associated contamination, and subsequently due to continued exposure to the harsh space environment. It is therefore necessary to compensate for the in-orbit degradation to obtain correct values of the sensed radiance at the top of the atmosphere. The absence of on-board calibration devices for the visible and near-IR channels requires a vicarious calibration. The relative degradation rate is established by viewing an Earth scene, and comparing sensor measurements to its response on the day of launch of the spacecraft. The calibration scene chosen is the southeastern part of the Libyan Desert (21°-23°N latitude; 28°-29°E longitude). It is assumed that this desert site is a radiometrically stable target, and that sensor decay is exponential in time. The degradation rate is estimated by monitoring the desert target over time. This estimate is then used to correct the AVHRR radiance values to the sensor's response on the day of launch.

After the visible and near-IR channels have been corrected for in-orbit degradation, a second calibration factor is applied to correct for differences between AVHRR instruments aboard the various NOAA spacecraft. This second level of the calibration establishes inter-satellite linkages to ensure the quality and continuity of the long-term records. The AVHRR on NOAA-9 was chosen as the standard to determine the inter-satellite calibration factors. NOAA-9 was chosen as the standard because the post-launch performance of this radiometer has been studied extensively, which resulted in NOAA-9 having an absolute calibration. The absolute calibration of NOAA-9 was obtained by measurements of the upwelled radiance emitted by a target in White Sands, New Mexico. These calibration measurements included simultaneously observation by the AVHRR aboard NOAA-9 and a by well-calibrated radiometer aboard an aircraft under identical conditions of illumination and observation. The calibration factor obtained from the White Sands data set enabled the NOAA-9 radiance values to be rendered absolute, thus NOAA-9 was chosen as the standard for comparison of all other AVHRR sensors.

To obtain the inter-satellite calibration factors a matchup dataset was assembled of measurements made by each AVHRR sensor (NOAA-7, -9, -11, -14) over the Libyan Desert calibration site. NOAA-9 measurement conditions were used as the criteria for inclusion in the matchup dataset. A matchup of a given satellite's (NOAA-7, -11,or -14) observations of the Libyan Desert site was required to (a) be within 1 degree of the corresponding solar and satellite zenith angles of the NOAA-9 observation and (b) have been made during the same month of the year. The inter-satellite calibration factor was then determined from the matchup database for each individual AVHRR by regression of the degradation-corrected radiance values of the particular AVHRR against the absolute radiance values obtained by NOAA9 over the Libyan Desert for the same month and viewing geometry. The application of the inter-satellite calibration factor (slope of regression) links all AVHRRs to the NOAA-9 absolute calibration.

A 1.1.2 Infrared channels 3,4 and 5

Count to radiance conversion of the infrared AVHRR channels is more complicated than for the visible and near-IR channels. This is due to the presence of onboard blackbody calibration devices, and to the corrections required by the non linearities in the response functions of the channels 4 and 5. A detailed description of the count to radiance conversion of the infrared channels is beyond the scope of this document, and readers are referred to the NOAA-KLM Users Guide, sections 7.1.2.2 and 7.1.2.3 for details. Please note that this points to a draft version of the NOAA-KLM Users Guide which is presently awaiting final approval.

In principle, the radiance sensed by an AVHRR IR channel is a function of the temperature of the target (Earth) and the spectral response of the channel. Laboratory calibration of channels 4 and 5 indicated that the response of the detectors is nonlinear. This non-linearity depends both on the AVHRR operating temperature and on the scene radiance as a function of output counts.

The procedure to convert AVHRR counts to radiance for the infrared channels is a three-step process. The first step is a linear transformation of counts to radiance, based on the onboard blackbody calibration devices (space view and sensor baseplate). The second step applies a non-linearity correction factor (derived from pre-launch calibration data) to correct radiance values for channels 4 and 5 only. The third and final step involves the use of lookup tables to convert radiance to temperature as a function of the sensor's operating temperature.

A 1.2 Navigation, clock, and attitude correction

The major sources of error in geo-locating AVHRR data are (a) drift in the spacecraft clock (which causes errors in the estimated along-track position), and (b) uncertainty errors in spacecraft and sensor attitude.

Clock Correction

To minimize error in the along track position estimated by the orbital model a satellite a clock correction factor is applied to the time code imbedded in each piece. The method used to determine these clock correction factors is presented below. The clock aboard a given satellite drifts continually at a relatively constant rate (e.g., for NOAA-14, ~9msday-1) compared to the reference clock on Earth. Because of this drift, the NOAA/NESDIS Satellite Operation Control Center periodically sends a command to the satellite to reset the on-board clock to a new baseline thereby eliminating the accumulation of a large time offset error between the Earth and satellite clocks. To correct for clock drift between these resets, correction factors were determined from a database of satellite clock time and Earth time offsets collected at the RSMAS HRPT receiving station. During HRPT transmission, both the satellite clock (used to create the embedded time code in each piece) and the Earth clock are simultaneously available. The clock correction bias was determined by (1) visual examination of the Earth/satellite clock differences collected in the database to locate the precise magnitude and timing of clock resets performed by the Satellite Operation Control Center and (2) recorded time differences between the identified reset periods were then filtered to remove spurious noise, and regressed against the corresponding satellite time to determine the clock drift correction. These drift corrections were then applied to all data time-stamped during a given reset period.

The tables below contain a listing of the clock offsets immediately before and after resets and the corresponding embedded time code. A table is shown for each NOAA spacecraft.

Table A.1.a. Clock offsets for NOAA-7

Start Date(YYDDD) Start Time (HHMMSS) Offset End Date (YYDDD) End Time (HHMMSS) Offset
81179 000000 0.1 82003 235959 0.8
82004 000000 0.8 82053 235959 1.1
82054 000000 -0.5 82181 235959 0.1
82182 000000 -0.6 83025 235959 0.3
83026 000000 -0.1 83181 235959 0.5
83182 000000 1.5 83193 235959 1.6
83194 000000 -0.7 84081 235959 0.3
84082 000000 -1.1 84342 235959 -0.1
84343 000000 -0.9 84366 235959 -0.8
85001 000000 0.0 85107 235959 0.4
85108 000000 0.2 85136 235959 0.3
85137 000000 -1.1 85181 235959 -1.0
85182 000000 -0.0 85304 235959 0.4
85305 000000 -0.3 86148 235959 0.3
86149 000000 -1.3 86158 235959 -1.3

Table A.1.b. Clock offsets for NOAA-9

Start Date (YYDDD) Start Time (HHMMSS) Offset End Date (YYDDD) End Time (HHMMSS) Offset
86014 183754 -0.53 86051 090207 -0.80
86051 184443 0.45 86190 092344 -0.76
86190 190707 0.46 86308 195123 -0.80
86309 081339 0.47 87048 210941 -0.84
87049 075241 0.40 87139 195622 -0.85
87140 081825 0.39 87216 210921 -0.75
87217 075309 0.48 87293 204300 -0.77
87294 090756 0.48 87365 225918 -0.79
88001 004020 0.21 88061 234258 -0.89
88062 012352 0.36 88125 000338 -0.87
88125 014432 0.39 88187 224121 -0.87
88188 002232 0.40 88250 225846 -0.93
88251 022040 0.32 88306 224954 -0.89
88307 003033 0.35 88363 001536 -0.88
88363 015925 0.37 89023 110528 -0.25
89024 091318 0.75 89094 111829 -0.91
89095 035705 0.31 89143 031928 -0.85
89144 030810 0.36 89192 111852 -0.79
89193 035610 0.46 89242 232725 -0.80
89243 024905 0.45 89290 035408 -0.79
89291 020143 0.45 89338 101232 -0.81
89339 025034 0.41 89365 212451 -0.30
90001 023815 0.69 90058 102159 -0.87
90059 030319 0.36 90107 105326 -0.99
90108 014914 0.25 90149 221415 -0.93
90150 033528 0.31 90191 222735 -0.85
90192 033935 0.36 90233 110326 -0.81
90234 034148 0.40 90275 110402 -0.79
90276 020138 0.42 90314 095839 -0.72
90320 232628 0.33 90365 224854 -1.03
91001 022659 -0.07 91022 214259 -0.71
91023 030313 0.52 91064 231826 -0.76
91065 025737 0.47 91106 044021 -0.82
91107 024821 0.39 91142 223525 -0.72
91142 224834 0.16 91176 034520 -0.88
91177 033300 0.31 91211 031322 -0.78
91213 043140 0.39 91231 225648 -0.21
91270 042933 -0.24 91288 224028 -0.86
91289 053517 0.36 91331 000157 -1.03
91331 051428 0.44 92007 233938 -0.95
92008 063157 0.52 92049 231312 -0.90
92050 042700 0.58 92090 195517 -0.84
92091 055256 0.75 92133 221715 -0.77
92134 065146 0.70 92182 234047 -0.99
92183 134449 -0.01 92207 120034 -0.87
92211 124842 0.45 92236 141404 -0.42
92246 233007 0.71 92287 231141 -0.78
92288 060512 0.68 92329 143633 -0.79
92330 002139 0.69 93005 234707 -0.87
93006 064126 0.63 93047 231538 -0.91
93048 060831 0.57 93089 175433 -0.96
93090 002050 0.52 93136 193915 -1.26
93137 020351 0.25 93166 195650 -0.88
93167 004310 0.62 93182 060334 0.00
93182 131313 1.02 93213 181559 -0.18
93243 011321 1.22 93292 194948 -0.72
93293 021545 0.77 93341 210417 -1.15
93343 013534 0.83 94025 203714 -1.04
94026 012315 0.42 94060 194824 -0.95
94061 072640 0.53 94101 020148 -1.03
94103 013547 0.35 94137 200139 -1.02
94138 022803 0.48 94181 235541 -1.28
94182 030236 0.20 94209 193554 -0.94
94210 020305 0.07 94235 204323 -1.01
94236 012744 0.53 94277 214407 -1.15
94278 022908 0.30 94305 204327 -0.82
94306 012723 0.67 94347 150149 -1.06
94348 004630 0.42 95019 202513 -1.11
95020 010905 0.41 95052 195749 -0.97
95053 004426 0.50 95087 190612 -0.90
95088 013146 0.56 95115 212721 -0.62
95116 021019 0.89 95150 203430 -0.57
95151 030113 0.91 95201 193323 -1.24
95202 020016 0.74 95214 200458 0.21

Table A.1.c. Clock offsets for NOAA-11

Start Date (YYDDD) Start Time (HHMMSS) Offset End Date (YYDDD) End Time (HHMMSS) Offset
88270 175315 0.10 88362 225730 0.55
88363 003825 -0.45 89108 223157 0.55
89109 001247 -1.21 89283 223149 0.92
89284 001246 -0.99 90001 005132 0.19
90001 062225 -0.81 90086 225343 0.51
90087 003437 -0.48 90157 232813 0.67
90158 010906 -0.33 90200 185021 0.40
90206 004808 0.68 90210 000412 0.76
90214 063112 0.10 90233 230231 0.53
90234 004321 -0.47 90296 231326 0.66
90297 005426 -0.35 90345 223316 0.54
90346 072437 -0.44 91037 234244 0.63
91038 012353 -0.38 91078 224447 0.42
91079 002546 -0.59 91127 233159 0.36
91128 011307 -0.64 91176 204252 0.35
91177 001500 -0.67 91232 233429 0.47
91233 011521 -0.54 91281 223014 0.44
91282 001116 -0.57 91330 230237 0.45
91331 004335 -0.56 92014 233119 0.47
92015 011217 -0.53 92056 233822 0.36
92057 011908 -0.64 92105 235823 0.46
92106 013910 -0.55 92147 235749 0.38
92148 013925 -0.63 92210 230706 0.74
92211 004718 -0.26 92252 230538 0.70
92253 004621 -0.32 92308 215437 0.96
92309 082622 -0.05 92336 225954 0.59
92337 003847 -0.42 93012 225434 0.56
93013 003543 -0.46 93047 223341 0.36
93048 001415 -0.67 93089 222657 0.33
93090 014845 -0.68 93138 223524 0.47
93139 001555 -0.55 93181 235524 0.51
93182 013608 -0.50 93234 231252 0.78
93235 005344 -0.21 93271 221249 0.68
93272 000413 -0.33 93313 235403 0.70
93314 013043 -0.33 93355 234331 0.73
93356 012432 -0.28 94032 233238 0.77
94033 011326 -0.24 94067 230533 0.64
94068 004655 -0.38 94109 224303 0.69
94110 003436 -0.34 94144 221719 0.57
94145 000747 -0.43 94181 230407 0.50
94182 005611 -1.51 94209 173715 -0.81
94209 204026 0.23 94221 232635 0.54
94222 010716 -0.45 94256 224431 0.45
94257 003856 -0.59 94298 223205 0.53
94299 002843 -0.47 94340 221659 0.65
94341 001032 -0.38 95012 164514 0.60
95013 194752 -0.36 95052 231636 0.67
95053 010842 -0.33 95094 225950 0.77
95095 005202 -0.23 95208 163447 2.81
95208 163450 2.81 96004 165442 7.36

Table A.1.d. Clock offsets for NOAA-14

Start Date (YYDDD) Start Time (HHMMSS) Offset End Date (YYDDD) End Time (HHMMSS) Offset
95013 223633 -0.37 95213 233350 0.59
95214 011409 -0.93 95365 225835 0.04
95365 211723 0.53 96079 234708 1.07
96080 012736 -0.43 96222 230133 0.59
96223 004223 0.07 96289 223559 0.58
96290 001652 -0.93 97126 223140 0.69
97127 001236 0.18 97182 001344 0.64
97182 015422 1.63 97196 230953 1.75
97197 005044 0.25 97231 232830 0.55
97232 010911 0.03 97302 000502 0.63
97302 014537 -0.90 98111 234827 0.55
98112 012906 0.04 98181 223817 0.62
98181 001812 -0.88 98342 230000 0.45
98343 000000 -0.05 98365 202823 0.14
98365 220000 -0.34 99131 205700 0.72
99132 002800 0.22 99222 235900 0.97
99223 010000 -0.52 99310 235900 0.15
99311 000000 -0.65 00018 235900 -0.02
00019 000000 -0.50 00238 235900 0.70

Attitude Corrections

After clock correction, a nominal attitude correction is then applied to minimize the uncertainty in regard to the direction in which the spacecraft is pointing. The nominal attitude correction applied was determined by averaging the absolute attitude of the spacecraft over many geographic locations and times along the orbital track. The method used to determine the absolute attitude of the spacecraft involves matching a digital coastal outline to a given image and recording the amount of pitch, yaw, and roll required to make the outline and land coincide. This method has the advantage that it can be performed over small geographical distances and is similar to other techniques which rely on widely separated geographical control points to anchor the navigation. These correction factors are given in Table A.2. The resultant navigation information, output by the SECTOR procedure for each piece, provides the mapping parameters needed to convert between the satellite perspective of pixel and scan line, and Earth-based latitude and longitude coordinates.

Table A.2. Nominal attitude correction parameters. A=ascending D=descending

Satellite

Pass

Tilt

Roll

Yaw
NOAA-9
A
 0.145 0.000 0.000
D
 -0.145 0.000 0.000
NOAA-11
A
 -0.210 -0.108 0.000
D
 0.210 0.108 0.000
NOAA-14
A
 0.000 0.000 0.000
D
 0.000 0.000 0.000

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