IEEE - Aerospace and Electronic Systems - September 2023 - 27

Liang et al.
in different positions during the flight for reliable data
delivery to the earth station. As the space station moves
into the next satellite's range of coverage, the link delay
of each state is also different due to the different link path
taken. The data link paths may switch between the onehop
relay to the two-hop relay when needed for contact.
DIFFERENT SCENARIOS OF LINK DISRUPTIONS
BETWEEN SPACE STATION TO EARTH STATIONS
The periodic movement of space station and the resulted
end-to-end link disruption can be predicted. However, the
link disruption may also occur randomly due to the other
factors such as nonalignment ofcontact windows, weather,
or Sun storm. These disruption scenarios generally occur in
two different cases: 1) link disruption occurred over the
downlink from the space station to the ground station; and
2) link disruption occurred over the uplink from the ground
station to the space station. Figure 2 illustrates the transmission
scenarios for an LTP block transmission having a link
disruption experienced in both cases.
CASE 1: LINK DISRUPTION OVER DOWNLINK FROM
SPACE STATION TO GROUND STATION
In space communications, a primary goal is to have scientific
data successfully delivered (from spacecraft) over the
downlink to the earth ground station. The space station
communication is not exceptional. In other words, a link
disruption over a downlink from space station to an earth
ground station mainly affects the transmission of the data
sent from the space station. With respect to BP/LTP, it
affects the transmission ofLTP segments ofa block, either
regular data segment(s) and/or a CP segment.
Figure 2(a) illustrates the transmission scenario for an
LTP block transmission having a link disruption experienced
during transmission ofdata segments ofan LTP block. When
a link disruption event occurs, the transmission of all the segments
that are already sent out (prior to their delivery at the
receiver) are failed. That is, they are lost over the data link.
The receiver only receives those data segments that are delivered
prior to the link disruption, and the rest segments of the
block (including the last one, CP segment) are lost. Because
of the failed delivery of the CP, the receiver is not triggered
for transmission ofanRS.As a result, the CP segment is resent
at the sender upon expiration of the RTO timer. Assumed the
link is resumed when the CP segment is resent, then the resent
CP is successfully delivered triggering the receiver for the
delivery status. The receiver responds to the sender with an
RS reporting the lost segments of the block that need to be
resent. Then, the lost data segments are retransmitted.
CASE 2: DISRUPTION OVER UPLINK FROM GROUND
STATION TO SPACE STATION
The reversing uplink from ground station to space station
is mainly for transmission ofACK segments. This implies
Figure 2.
LTP block transmission scenarios having a link disruption experienced. (a) Disruption occurred during block transmission over a downlink.
(b) Disruption occurred during transmission ofan RS over uplink.
SEPTEMBER 2023
IEEE A&E SYSTEMS MAGAZINE
27

IEEE - Aerospace and Electronic Systems - September 2023

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