Eliminating the need for a second ground 
station offers two main advantages. First, the 
spacecraft can employ existing communications 
equipment for the playback data, reducing the 
amount of equipment on the spacecraft and re- 
quiring less operating power.'' Second, the glo- 
bal data set would be available at a single loca- 
tion at the EOSAT ground receiving station, and 
there would be no delays in data access due to 
shipping from a second, remote facility. The Sys- 
tem Integration Study is examining tradeoffs be- 
tween increased spacecraft tape recorder ca- 
pacity and the possibility of an additional ground 
receiving station, including the power required 
for nighttime data playback. 
Data Downlinks 
Current plans are for SeaWiFS to have three 
data downlinks. The first would be a stored-data 
link that would provide 40 minutes of global area 
coverage (GAC) and 10 minutes of local area 
coverage (LAC) to the EOSAT ground receiving 
facility. These data would be downlinked directly 
on the X-band. EOSAT will collect these data on 
some form of computer-compatible media and 
make them available to NASA for transfer to the 
NASA/GSFC ground receiving facility. 
The second data downlink would convey 
real-time, high-resolution LAC data, downlinked 
at 665 kbps, at a frequency compatible with cur- 
rently operating HRPT receiving stations and 
planned naval AN/SMQ-11 stations (probably L- 
band or S-band). The third data link would con- 
vey real-time, low-resolution GAC data to sub- 
scribers on a UHF frequency. These data would 
be transmitted at 56 kbps, allowing some sub- 
Mt is important to note that one of the key spacecraft limitations 
encountered in accommodating both the SeaWiFS mission and 
ETM mission on Landsat-6 lies in the power subsystem. These 
power constraints will affect mission operations, particularly 
with respect to nighttime operations. Some compromises in the 
ETM and SeaWiFS task profiles may be necessary to accom- 
modate the SeaWiFS mission. However, if the required compro- 
mises are unacceptable, expansion of spacecraft power re- 
sources will be addressed. 
scribers (e.g., moderately sized ships at sea that 
Cannot mount large, trainable, dish antennas) to 
use relatively inexpensive, commercially available 
receivers and fixed receiving antennas. 
The direct data downlinks will be provided on 
a subscription basis. No tape-recorded data 
would be available on either real-time link, and 
the real-time data will be encrypted to protect 
their commercial value. A key will probably be 
used for encryption and each subscriber will be 
supplied with a suitable key. 
The Implementation Panel recognized that re- 
ception of high-resolution LAC data is also de- 
sired by users restricted to unsophisticated re- 
ceiving stations. However, transmitting high- 
resolution data to a fixed antenna would require 
much greater 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 po- 
tential solution to this problem would be to pro- 
cess the 1.13-km data immediately upon receipt 
by the ground-receiving site, redistributing the 
data products via radio facsimile or telecommuni- 
cations links. 
The desire of some operational users to have 
near-real-time access to a global data set 
presents a greater problem, since the current 
data-handling plans for the tape-recorded global 
data set do not allow real-time access. A study is 
being undertaken to determine the feasibility of 
rerouting the X-band, downlinked global data set 
via communication links to the processing facili- 
ties of commercial and operational users. How- 
ever, this strategy is likely to be quite costly. 
Ground-Station Data Processing 
for Research Users 
Global data for research will be acquired 
from the EOSAT ground station on computer- 
compatible media and routed to a government 
ground-processing site. As currently envisioned, 
NASA will serve as the agent for processing and 
distributing these data. 
IMPLEMENTATION PANEL 
59 
