Transmitting from Space: ESA’s Global Transmission Services Experiment

Note: this article originally appeared in the March 2001 issue of ESA’s “On Station” newsletter.

Introduction

In 1996, following the successful Euromir-95
mission, ESA negotiated with RSC-Energia the
Euromir Extension mission. Euromir-E should
have taken place towards the end of 1997, and
reused much of ESA’s
equipment already
onboard Mir. However, the
Progress collision in June
1997 destroyed these plans
– the vast majority of ESA’s
equipment became inaccessible inside the
damaged Spektr module.

Instead, ESA and RSC-Energia agreed at the
end of 1997 to devote the financial resources
from Euromir-E to Western Europe’s first
scientific endeavour on the International Space
Station (ISS). At this stage, GTS was an
experiment proposed by the German Steinbeis
Transferzentrum Raumfahrtsysteme (TZR) and
was highly recommended by the ESA Peer
Groups. Following an in-house feasibility
assessment, GTS stepped in to replace
Euromir-E.

The GTS Antenna Unit installed on Zvezda

Time-signal stations on Earth usually transmit
their information at long wavelengths. Watches
synchronise themselves with these signals by
activating their receivers typically once per day
or after turn-on. In Germany, the signal is
broadcast by the Physikalisch-Technische
Bundesanstalt (PTB), in Braunschweig, using a
simple encoding scheme to transfer both time
information and synchronisation impulses on a
77 kHz carrier. Although this scheme is quite
efficient and simple, it has several
disadvantages:

• the time signal is only Europe-wide, within
  2000 km;
  
• moving between countries requires different
  receivers and decoders;
  
• the low data rate requires lengthy, good
  receiving conditions.

To overcome these limitations, GTS will
transmit a UTC time signal from the Space
Station via an external antenna with a broad
antenna pattern. The Station’s orbit means that
this signal is transmitted over almost the whole
Earth each day. The signal can be received at
any location several times daily, for 5-12
minutes at a time, strong enough for even
small wristwatches. GTS also broadcasts the
Station’s current orbital position, so clocks
‘know’ their local time zone without user
interaction.

Further main scientific objectives are:

• to verify the performance and accuracy of a
  time signal transmitted to the Earth’s surface
  from low orbit under real space operational
  conditions;
  
• to measure the signal quality and data rates
  on the ground;
  
• to measure disturbing effects such as
  Doppler shifts, multi-path reflections,
  shadowing and elevation effects.

The flight model of the GTS Electronics Unit.

GTS essentially consists of two major elements
in the Station’s Russian Zvezda module: the
Electronics Unit (EU) and the Antenna Unit
(AU).

The EU accommodates all the elements for
the experiment, signal generation and signal
distribution. In addition, it provides the
necessary command and control interfaces
with the Station’s Russian Segment for GTS
data exchange, telemetry transfer, GTS
telecommanding and data receipt from
Zvezda’s systems, such as orbital data.

A main EU feature is the ultra-stable quartz
oscillator (USO). This oscillator is the core of the
local time generator, and provides an accuracy
of up to 10^-13. If synchronised almost every orbit
with an atomic clock on the ground – such as
the one at PTB – GTS generates an onboard
time-signal reference of unprecedented
accuracy for the Station’s own applications. GTS
provides this signal to Zvezda’s systems and
other scientific onboard users via a pulse and
frequency output.

Being only 350x300x210 mm in size, the EU
will be housed in the Zvezda module, behind
Panel 408.

The EU generates the GTS signal on two
frequencies – 400.1 MHz and 1.4 GHz – for
transmission to Earth by the AU. The
transmitter includes a digitally controlled
frequency synthesiser that allows testing of
various frequencies and modulation
techniques. Specific modulation techniques
will be applied in order to avoid interference
with other ground-to-space links aboard the
Station.

The antenna has specially shaped radiators
to create a near-constant signal strength on
the ground down to 15° above the horizon for
the receiver during the Station’s pass. The UHF
main beam uses a 4-element phased array. The
beam can be rotated in eight steps with an
angle of 70° relative to nadir using beam-forming
electronics located on the back of the
antenna plate. The power and command
signals are routed together with the RF signals
through two coaxial cables. For extended
services, requiring larger bandwidths than a
time signal, the L-Band (1.4 GHz) with its
dedicated antenna on the AU is used.

In December 1998, the AU’s hardware
development, integration and test activities
ended with its installation on an EVA handrail
on Zvezda, along with the RF cables. Zvezda
and the AU were safely launched into orbit in
July 2000.

The GTS Antenna Unit on the Zvezda module.

Experiment Operation Summary

Data and time information destined for GTS
will be transmitted from the experimenter’s site
in Stuttgart, Germany and directly received by
GTS. The ground station will supply GTS with
the synchronisation marks and pre-calculated
ground track information needed to minimise
the active time for the ground receivers. The
GTS internal clock is adjusted and/or
synchronised using this dedicated uplink. In
order to maintain this clock’s very high
accuracy, this adjustment is required at least
once per day.

The Mission Control Centre in Moscow

(MCC-M) will control the on/off of the GTS
Electronics Unit. In addition, a dedicated
capability controlled by MCC-M allows
individual transmitter switching in specific
operational cases.

The GTS continuously transmits time and
and information on the Station’s position along
its ground track. These data are complemented
by GTS housekeeping telemetry. Equipment
status and housekeeping data will also be
transmitted via a dedicated telemetry interface
to MCC-M. The GTS time signal can be issued
via the dedicated data interfaces to the
Station’s data management system to serve
as the reference time signal for the whole
Station.

In order to maximise the scientific return
GTS will operate autonomously and preferably
continuously. Intervention, such as the
switching of transmitters, is normally
performed by MCC-M and not the crew.

Interrupting the experiment runs (power
shut-off, transmitter switching) needs to be
avoided. Reactivation after any shutdown of
the experiment costs 8-10 days in clock
recalibration before the measurements can run
normally. GTS is designed to operate
continuously for the 2 years agreed with
RSC-Energia.

Status and Planning

The GTS Electronics Unit completed Final Flight
Acceptance in mid-March, aiming for launch to
the Station aboard a Progress ferry towards the
end of April. It is envisaged that the
Expedition-3 crew will install the EU and
activate GTS around the beginning of July
2001. Then, after functional verification, GTS
can be declared operational towards the end of
that month.

GTS Applications

At present, two concrete applications are
foreseen:

• time signals for worldwide clock
  synchronisation,
  
• secure code transmission for anti-theft
  devices that can deactivate a stolen car and
  its keys.

It could be argued that the clock application
could also be implemented using Global
Positioning System signals. However, GPS has
major drawbacks:

• no local time zone information (political
  and/or geographical) is provided;
  
• no daylight savings time information is
  generated;
  
• without L-band signals, wristwatches cannot
  use the service;
  
• GPS is a military system and can deliberately
  broadcast erroneous signals if required.

In order to minimise the computational
effort and power consumption in wristwatches,
GTS continuously transmits the local time for
the Station’s current ground spot. The receivers
detect the moment of the closest Station
approach by measuring the signal’s Doppler
shift. The ambiguity of a left or right passage is
resolved using the rotating beam information.
From these data, the receiving clock
determines the correct time data packet for its
current position. Via the EU’s computer,
changes in time zones, daylight savings and
UTC time adjustments can be easily
implemented through commands from the
controlling ground station.

Compared with GPS, the Station is an ideal
platform for instant repairs and, in comparison
with satellites, offers rapid replacement of
faulty hardware via the frequent upload
opportunities and crew availability. This
approach offers simpler design solutions, which
lower development cost and save time.
The anti-theft application demands a secure
signal to receivers small enough to be
integrated into car keys. Such a signal must be:

• difficult to imitate, so that it cannot be faked
  with a transmitter on the ground;
  
• non-repeating, so that it cannot be recorded
  and replayed to simulate a true signal;
  
• fail-safe, i.e. the hardware must be
  redundant and easy to service.

The combination of non-linear pseudo noise
spectrum signals, the large Doppler shift and
the orbit information form a fingerprint that
cannot be faked by car crackers on the ground.

The Commercial Potential of GTS

From the beginning, GTS was conceived as a
commercial application. It has followed not
only a new approach to developing space
hardware for new services, but also an
approach on how future operations costs can
be financed through a ‘service provider
company’, founded by the development and
research partners and financed through selling
licences and specific services to end users.

The commercial structure of the GTS
Development Project can be summarised as:

• the main industrial partner in the ground
  segment is Fortis Uhren GmbH, who
  developed the hardware for the experiment
  on their own and who will make the
  required ground tests using the Station
  signals, thereby implementing the standard
  GTS services that are the core of the
  commercial utilisation;
  
• TZR developed the space segment
  (supported by DLR) and is coordinating the
  scientific utilisation of GTS;
  
• the flight opportunity, covering integration
  and launch, the operation in space during
  the 2-year experimental phase, and
  associated ground infrastructure services
  (e.g. data exchange with MCC-M) is provided
  by ESA.

GTS development costs share..

GTS aims at near-term commercialisation
and is therefore financed by industry to a
significant extent. The ground segment
required the development of new receiver
chips and their integration into wristwatches
and car electronics. The total development
budget is about EUR6.5 million, of which 53% is
public funding (ESA and German national) and
47% is industrial funding.

During the 2-year initial experiment phase,
the GTS hardware will be operated by TZR, the
scientific coordinator. Plans are already taking
shape to found a GTS Service Company, with
the partners participating as ‘shareholders’.
Commercial utilisation during this initial phase
will then be coordinated through the Service
Company. As ESA is providing the flight
opportunity and
operations, as well
stimulating
Station commercialisation,
it will
participate in the
company.

For the future,
apart from
managing GTS
licensing and
providing
continued
services, the Service Company will most likely
also have to establish contracts with the
Station Partners in order to ensure continued
GTS services on the ISS, make hardware
upgrades and issue and manage licences to
end-users to refinance the operational costs of
the hardware or invest in future developments.
The main GTS ground station will be
operated by the Service Company. It will also
take responsibility for user interfacing, data
processing, transmission to and from the GTS
hardware (e.g. in the case of car theft
protection activation) and provide marketing
and promotion services.