Research Users' Requirements 
Satisfactory data processing and delivery is of 
the highest priority to the success of the SeaWiFS 
mission. Data processing and delivery for re- 
search users should meet the following require- 
ments: 
@ Global coverage at 4.5 km resolution with a 2- 
day revisit cycle 
® Quick-look data products for selected areas 
within 12 hours of collection for near-real- 
time use in positioning of research vessels 
©@ Level-1 imagery available in 7 days. Level-2 
imagery for both high- and low-resolution 
data, available to the user within 10 days of 
observation.’ 
Crossing Time 
Landsats 1 through 5 have been maintained 
in a sun-synchronous, 705-km orbit with a 9:30 
a.m. equatorial crossing time, principally because 
the main function of their sensors is to observe 
land masses. However, the observation of ocean 
color depends critically on having sunlight pene- 
trate into the water where it is absorbed and 
backscattered into the sensor. For this reason, 
ideally, observations should be as near noon 
(solar zenith = 0°) as possible, but the research 
community agrees that there will be adequate wa- 
ter penetration by sunlight and that viable atmo- 
spheric correction will be possible if the equatorial 
crossing time is no earlier than 10:30 a.m. and no 
later than 1:30 p.m. This implies that the Landsat- 
6 crossing time be moved from its current 9:30 
a.m. baseline to at least 10:30 a.m.® 
Duty Cycle 
The SeaWiFS instrument is designed to pro- 
vide world coverage of the ocean basins, and 
7 Level-1 imagery is raw data in a reformatted structure with 
calibration and navigation information within the header. 
Level-2 imagery is geophysical data, not gridded nor resam- 
pled. 
SYNTHESIS OF REQUIREMENTS 
many commercial, operational, and research ap- 
plications will require data from the Mediterrane- 
an Sea, Central Atlantic, Pacific, and Indian 
oceans and regions in the Arctic and Antarctic. 
Selected regions will probably be routinely cov- 
ered according to schedule while other regions 
will be viewed on an ad hoc basis. A 100% duty 
cycle would be ideal, since sea-surface tempera- 
ture data could be acquired during nighttime, 
giving a swath of visible and infrared imagery 
during the day and a swath of infrared data at 
night. However, it is recognized that satellite 
power restrictions may not permit a full duty cy- 
cle. The capability of the satellite in this regard is 
discussed in Section 5. 
Mission Life 
In order to conduct the in-situ field work ne- 
cessary to exploit the research value of ocean- 
color observations from space, experiments must 
be staged in a variety of seasons and conditions. 
Logistically, the spacecraft mission must last at 
least 3 years to encompass typical events, e.g., 
the occurrence of the El Nifo phenomenon. In 
addition, even the coarsest values for ranges in 
interannual productivity are not known for most 
ocean areas. Hence, a minimum of 3 years is 
necessary to identify its scale of variability. The 
nominal lifetime of Landsat-6 is 5 years, and this 
extension in time is highly desirable. In this re- 
gard, the instrument's calibration must be con- 
stant or identifiable during the mission to allow 
valid comparisons of ocean values observed at 
different times. 
Requirements Summary 
Table 5 summarizes the goals for the Sea- 
WiFS system set forth by the SeaWiFS Working 
Group. 
8 this accords with the recommendations of the Geology and 
Evapotranspiration/Botany panels of the Thermal Infrared 
Working Group (Putnam 1986) that the equatorial crossing 
time of Landsat-6 be as close as possible to noon without ex- 
cessive loss of imagery due to cloud cover, i.e., about 10:30 
am 
45 
