OLSG Report_Final_06_05_12 - Interagency Operations Advisory ...

Optical Link Study Group (OLSG) Final Report
IOAG.T.OLSG.2012.V1
Cryo-cooled (approx. 150 K) short-wave infrared (SWIR) HgCdTe avalanche
photodiodes (APD) (small, few-pixel array) with future upgrade to single photoncounting
detectors (e.g., super-conducting nano-wire technology) [TRL 3-5]
Receiver signal-chain using M-ary PPM (M=2,4,16,32,…) (de-)modulation and
appropriate decoding and DTN implementation [TRL 7]
Environmental/weather monitoring system with spatially resolved real-time cloud
cover forecast on short time scales (hours) [TRL 5]
Initial pointing, acquisition, and tracking is performed entirely in the optical domain
relatively easily, given that satellites around L2 are Sun-illuminated and plainly visible with a
1 m class telescope and commercial high-performance CCD cameras. First, the (wideenough)
beacon is pointed to the acquired satellite position (with suitable point-ahead, if
necessary, from flight dynamics data), followed by the acquisition of the downlink beam.
Then, closed-loop tracking of the downlink beam can be performed using position
information from the small communications receiver array.
3.5.4 CFLOS Analysis
The L2 Scenario was the first mission scenario analyzed by the OLSG, and the analysis
methodology has evolved using the following specific assumptions:
1. Simulations considered only those opportunities (i.e., days) where there is a
contiguous 3-hour period of CFLOS to downlink one day’s science data from the
buffer. Thus, no partial credit is given (data is sent or lost in entire day increments),
and each day without at least one contiguous 3-hour CFLOS window equals one day’s
data loss. This situation corresponds to the most penalizing (and unrealistic)
assumption.
2. Simulations considered the single largest contiguous period of CFLOS, even if it is less
than 3 hours (however only with 1-hour granularity, since that is the temporal
resolution of available cloud data). Partial credit is given if even a fraction (i.e., 1/3,
2/3 or all) of a day’s data can be downlinked within the corresponding CFLOS
window (1, 2 or 3 hours wide). This situation also corresponds to a somewhat
penalizing assumption, since only one window is considered, even if it is followed by
another window after a short interruption (the hourly granularity poses a
fundamental limitation in our simulations).
3. Simulations considered the aggregate hours of all available CFLOS windows in a day
until all of the stored data could be downlinked. As an illustrative case, the entire
buffer (after 3 consecutive days without a downlink possibility) could be downlinked
completely if 9 hours of accumulated CFLOS (still only with hourly granularity, i.e.,
data downlinked in increments not smaller than 1/3 day) were available on the
fourth day.
Note that while the optical data rate advantage is to be substantiated by realistic link
budget calculations, the former does not enter into the CFLOS analysis as an absolute value.
The analysis is sufficiently defined by the requirement that one day’s mission data can be
downlinked in three hours.
Based on the L2 Scenario described above, LNOT was used to determine the PDT for several
site configurations based on a six-year period from 2005-2010. The calculation method for
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