63 
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
Processing of raw digital NOAA-AVHRR data 
for sea- and land applications 
G.J.Prangsma & J.N.Roozekrans 
Koninklijk Nederlands Meteorologisch Instituut (KNMI), De Bili, Netherlands 
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ABSTRACT: A project has been started at the KNMI to develop algorithms for digital processing of data 
of the Advanced Very High Resolution Radiometer (AVHRR) in an automatic way. Digital processing gives 
the opportunity to obtain quantitative data of the Earth-surface. For the conversion of the raw AVHRR- 
data into true parameters of the Earth-surface many steps have to be taken. The processing scheme is 
presented and the results of verification studies are discussed. 
1. INTRODUCTION 
The current generation of operational polar 
orbiting meteorological satellites is capable of 
producing a wealth of quantitative data on the 
state of the Earth surface and the overlying 
atmosphere. 
Recently many applications (see e.g. Kerr et al., 
1983; Muasher and Ince, 1984; Kerr et al, 1984; 
Harries et al., 1983) have been shown feasible on a 
routine basis, but all operational use of 
satellite-based remotely sensed data relies heavily 
on a suite of (semi-) automatic processing steps to 
convert the raw data into usefull quantitative 
information. 
In this paper we will present the automatic 
processing scheme as currently implemented at KNMI. 
After a short description of the nature of the data 
used (section 2) in section 3 an exposition is 
given of the various processing steps. 
Preliminary results of this processing are 
presented in section 4. 
Conclusions reached so far are given in section 5. 
2. NOAA - POLAR ORBITING SATELLITES 
The Advanced Very High Resolution Radiometer 
(AVHRR) carried on the TIROS-N/NOAA series of 
polar-orbiting meteorological satellites is a 
multichannel instrument with tv/o thermal IR- 
channels (3 + 4) centred at about 3-7 pm and 
11 pm on the NOAA-6 and -8 satellites and in 
addition a thermal channel (5), centred at about 
12 pm, on the NOAA-7 and -9. The AVHRR also 
contains a visible channel 1 (0.58 - 0.68 pm) and a 
near-infrared channel 2 (0.73 - 1.10 pm). The 
spatial resolution of the AVHRR is 1.1 km at nadir 
increasing to about 3 km at the maximum scan-angle 
of 55°. Radiometric resolution is 10 bits (1024 
steps). Two overhead passes per satellite per day 
are received. Currently two satellites are in 
operational use: N0AA-6 (overhead passes at + 8.00 
and + 20.00 hours, local time) and N0AA-9 ( + 3.00 
and + 15.00 hours). NOAA-10 is scheduled for launch 
in June 1986. 
The data used for this study have been obtained 
from the archive maintained at Dundee University on 
computer compatible tapes. 
3. PROCESSING PROCEDURES 
The automatic processing scheme for the conversion 
of the raw data into true earth-surface parameters 
involves the following steps: 
1.Unravelling of the Dundee-tape format, 
extraction of calibration information and de 
multiplexing of the 4 (resp. 5) AVHRR channels. 
2. Calibrations of the radiometric data and 
derivation of brightness temperatures and top-of- 
atmosphere albedos. 
3. Cloud-detection, land/sea discrimination. 
4. Geometric correction and navigation. 
3.1. Reading Dundee-tapes 
Dedicated software has been developed to read the 
complex format of the Dundee tapes. The data on the 
tape are in 10 bit words packed 3 words into 4 
bytes. The four (resp. five) AVHRR-channels are 
multiplexed per scanline and must be separated. 
The input-module is designed to extract the 
individual 10-bit datawords and store these in 
separate files: on for the header/calibration data 
and one for each AVHRR-channel. 
3.2. Calibration of thermal infrared channels 
In-flight calibration of the IR-channels of the 
AVHRR is possible because the instrument output is 
a linear function of the input radiant energy. 
During every scan-line, the instrument views cold 
space (zero radiance) and its housing (+ 290K); The 
housing portion of the instrument has been designed 
to be a blackbody target for in-orbit-instrument 
calibration. Four Platimum Resistance Thermometers 
(PRT's) are embedded in the housing and monitor the 
temperature of the target. By determining the 
instrument output while looking at the known warm 
target and cold space (also in the data format) it 
is possible to ascertain the linear relation 
between the raw counts (DN) and radiance (L) (L = 
A*DN+B) for every scanline. To save computertime 
the calibration constants are determined every 
100 t scanline using calibration data, averaged 
over 100 scanlines. 
The calculated radiances in the two IR channels 4 
and 5 are subsequently corrected for slight non- 
linearities using a pre-flight defined correction 
table (NOAA, 1979). To convert the radiances into 
brightness temperatures (T b ) the Planck radiation 
law is used, knowing the pre-flight normalised 
response function for a given channel (see NOAA, 
1979). Since this approach is very computertime 
consuming, Singh (1984) suggested a more simple 
procedure, which was found to be as accurate as the 
Planck formula: 
T b = ~[Ln(L)-Al’ A and B bei - n 8 determined once 
for every channel and satellite.