IEEE Consumer Electronics Magazine - November/December 2021 - 24

Communications Technology
Figure 1. Types of laser ISLs.
velocities of satellites. Also, satellites in crossing
orbital planes or satellites in different orbits move
at high relative velocities and suffer from challenges
like Doppler shift, point-ahead angle (PAA),
and acquisition, tracking, and pointing (ATP).
Intraorbital plane ISLs and interorbital plane ISLs
with satellites in adjacent orbital planes are generally
stable. Interorbital plane ISLs with satellites in
crossing orbital planes aremake-break or intermittent
in nature and cannot last for long durations.
The small beam width is the main advantage
of FSO as it eliminates interference from neighbors.
However, in laser ISLs, the small beam
width turns into a disadvantage. Pointing a laser
beam at a moving satellite from another moving
satellite becomes a major challenge due to the
narrow beam divergence of the laser beam and
the different relative velocities of the satellites.
Hence, arises the demand for a very precise ATP
system on board a satellite platform for the laser
beam originating from one satellite to effectively
connect to another satellite. Suppose satellite A
receives a laser beacon from satellite B. Due to
their different relative velocities, A's response
signal to B has to be offset from B's original location
so that it is received by B at the appropriate
spatial and temporal location. This pointing offset
is referred to as PAA and is taken care of by
the ATP system. The different relative velocity of
the sender and the receiver result in a change in
the frequency of the received signal, which is
referred to as Doppler shift. This shift is present
for a pair of satellites in adjacent orbital planes
and is higher for two satellites in crossing orbital
planes or in different orbits. This has to be
addressed to establish a reliable laser ISL.5
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The high capacity and low latency requirements
of next-generation satellite networks
necessitate laser ISLs. In the absence of such
ISLs, a long-distance connection between two cities,
such as an intercontinental connection
between New York and London, must ping-pong
between ground stations and satellites. The
main properties of laser ISLs can be summarized
as follows: setup times for laser ISLs vary from a
few seconds to tens of seconds. These setup
times and the intermittent nature of laser ISLs
with satellites in crossing orbital planes or with
satellites in different orbits make such ISLs undesirable.
Laser ISLs are mainly constrained by visibility.
For example, the maximum ISL range for
an LEO satellite operating at an altitude of 550
km is 5014 km. Laser ISLs offer capacities in Gb/s
and interference is not an issue for these ISLs.6
Laser terminals for establishing laser ISLs will
be absolutely critical in the formulation of a
global communications network in space by
interconnecting hundreds of satellites in nextgeneration
satellite networks, such as the
upcoming LEO satellite constellations. Two such
terminals have already been developed by
Tesat11 while others are currently under development
by companies like Mynaric12 and General
Atomics.13 A summary of the capabilities of
these terminals in terms of their specifications is
given in Table 2.
SATELLITE CONSTELLATION BASICS
AND AN UPCOMING EXAMPLE
A uniform satellite constellation, where satellites
have circular orbits, can be defined by four
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