On-Board Data Processing 
The sensor's data stream will be formatted and 
the spacecraft's time code and ephemeris data in- 
corporated into the format prior to recording or 
transmitting data. The attitude of the spacecraft 
(pitch, roll and yaw) will be reported with a within- 
limits flag. 
Data Downlinks 
Three downlinks are currently planned for 
SeaWiFS data. One would be a stored-data 
downlink that would provide the global area cov- 
erage and local area coverage data to the EOSAT 
ground receiving facility. These data would be 
downlinked directly on the X-band. 
Another downlink would convey real-time, 
high-resolution LAC data at a frequency compati- 
ble with currently operating high-resolution picture 
transmission (HRPT) receiving stations and 
planned naval AN/SMQ-11 stations (L-band or S- 
band). The baseline direct-downlink data format is 
similar to the TIROS-N HRPT frame format. It pro- 
vides the proper synchronization blocks for the 
hardware while still transmitting all of the required 
data at an effective data rate of 665 kbps. 
The third downlink would convey real-time, 
low-resolution GAC data to subscribers on a UHF 
frequency. These data would be transmitted at 
56 kbps, allowing some subscribers (e.g., moder- 
ately sized ships at sea that cannot mount large, 
tracking, dish antennas) to use relatively inexpen- 
sive, commercially available receivers and fixed 
receiving antennas. 
The data for real-time transmission will be pro- 
vided on a subscription basis. They will be en- 
crypted by the SeaWiFS Data Formatter to protect 
their commercial value; decoding keys will be fur- 
nished to subscribers in a manner similar to those 
furnished to subscribers of television services. No 
tape-recorded data will be available on either 
real-time downlink. 
The Implementation Panel recognized that re- 
xX ception of high-resolution LAC data is also desired 
by users restricted to unsophisticated receiving 
stations. However, transmitting high-resolution 
data to a fixed antenna would require much great- 
er power in the spacecraft transmitter than is 
available on the currently envisioned Landsat 
spacecraft. Adding the required power would not 
only be costly, but also might exceed the weight 
limitations of the spacecraft. One potential solu- 
tion to this problem would be to process the 
1.13-km recorded data immediately upon receipt 
by the ground-receiving site, redistributing the 
data products via radio facsimile or telecommuni- 
cations links. 
The desire of some users to have near-real- 
time access to a global data set presents a great- 
er problem, since the current data-handling plans 
for the tape-recorded global data set do not allow 
real-time access. However, a study is being un- 
dertaken to determine the feasibility and cost of 
rerouting the X-band, downlinked global data set 
via communication links to the processing facilities 
of commercial and operational users. 
Data Products For Research Users 
The following data products will be produced 
at NASA/GSFC within 8 to 10 days of data acquisi- 
tion: 
© The Level-1 data product will contain the total 
radiances for visible and near-infrared bands 
and brightness temperatures for thermal 
bands. 
The Level-2 product is a product derived from 
Level-1 data containing chlorophyll-a, other 
pigments, water-leaving radiances, and sea- 
surface temperature. 
Level 3 is a daily mosaic product for nine Level- 
2 products on an 18-km grid with statistics in- 
cluding the number of pixels, number of days, 
and sums of squares for each grid element. 
Weekly composites will be made of the geo- 
physical data. 
