SynchroNet is a GNSS based time and frequency transfer system that allows
to exploit high accuracy synchronization of accurate clocks
To solve this problem GNSS based
techniques have been preferred over other
methods for many reasons, among which:
• availability of the GNSS
navigation signals
• performances
• diagnostic information about
GNSS system health.
GNSS based synchronization could be
carried out by mean of the CommonView
technique which exploits navigation
signals broadcasted by GNSS SVs (Space
Vehicles) which are in a condition of “common view” from the
observation points of the
two sites to synchronize.
Figure 3: SynchroNet network topology
A schematic
representation of
this method and the
related process is
given in Figure 1.
The CommonView
method is very simple in
its theoretical formulation
but requires that a number
of issues are carefully
assessed and resolved.
From the sketch above,
can be identified at least two such issues:
• Delays compensation
• Data exchange
Delays to be compensated are mainly
due to the propagation in the atmosphere
(i.e. Ionosphere and Troposphere). For
each segment a delay factor is computed
using mathematical models. The residuals
of delay corrections are among the
limiting factors for the Common View
synchronization performances.
While Delays affect only the
synchronization computation, Data
Exchange affects, also, other aspects that
system designers have to take into account:
• Data acquisition process methodology
• Data types and formats
• Data and Identity Spoofing
• Data preprocessing and
quality assessment
• HW equipment status monitoring.
Synchronet Overview
SynchroNet is a distributed
approach to synchronization with
centralised monitoring and control
facilities (see Figure 2).
To provide a synchronization system
that deals with all issues described in
the previous paragraph, SynchroNet
implements a networked infrastructure
around the core time transfer algorithms
and distributes the synchronization
process over a hierarchic network
with hierarchic network nodes roles.
This approach allows to distribute and
keep balanced the processing load and
limits the propagation of failures.
In the SynchroNet system three
kinds of nodes are present:
• The Control Centre
• The MRT’s
• The SyN’s
Figure 3 provide an overview of
the SynchroNet network topology
involving each kind of node.
The Control Centre provides centralised
monitoring of each node of the network in terms of equipment health status,
connection link and security and
synchronization performances. The
CC is also responsible for network
management allowing the supervisor to
change the network topology (add a new
node, remove a node, assign a node to a
different MRT) and reconfigure per node
or network-wide parameters (acquisition
periods, performances thresholds, etc)
Figure 4: Preliminary analysis tool for optimal baseline determination
Figure 5: Number of satellites in common view for different baselines
MRTs are the distributed time references
and form the hierarchic synchronization
backbone of the system. Each MRT
computes synchronization of the allocated
SyNs and receives the synchronization
parameters from higher level MRTs.
SyNs are the leaves of the graph
representing the SynchroNet network;
they collect GNSS observables and
send them to the controlling MRT
which, in turn, computes and returns
the synchronization parameters and the
performance status information. The SyNs
are, computationally, passive elements
for what concerns synchronization.
SynchroNet is characterized, at each
level, by a strong modularity of its
components also at node level.
Each SynchroNet node can be seen as
a functional logical entity composed by
many sub modules interacting with each
other by mean of well defined interfaces and protocols; this
means that a single
node deployment may
span many physical/
virtual machines.
Each sub module
is well defined in
terms of pre and
post conditions and
follows the principle
of OOD (Object
oriented Design)
providing information
hiding and interface
based asynchronous
and concurrent
access to the
implemented service.
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