Supplementary memorandum submitted by
the Royal Astronomical Society (SAGE 04a)
SPACE WEATHER EFFECTS ON THE GALILEO SATELLITE
NAVIGATION SYSTEM
1. The memorandum is submitted in response
to a Committee request for further comments on the resilience
of Galileo to space weather.
2. The European satellite navigation system,
Galileo, is being developed to complement the Global Positioning
System (GPS), which is owned and operated by the US Department
of Defence. Galileo is one of a number of non-US systems satellite
navigation systems; others include the GLONASS system operated
by Russia and Compass or Beidou-2 system being developed by China.
3. All these satellite navigation systems,
including Galileo, exploit the same scientific principles—namely
the reception of suitable radio signals from a network of spacecraft
can be used to determine the location of the satnav device and
give a precise measure of the current time. Any satnav device
must receive signals from a minimum of four spacecraft to obtain
a precise location and time.
4. The Galileo programme is applying lessons
learned from GPS to develop its system so that it will deliver
more accurate location data than is provided by the existing GPS
service. This includes increased resilience against the day-to-day
effects of space weather, eg by incorporating more sophisticated
models of those effects.
5. The existence of complementary systems,
such as Galileo and GPS, will provide a level of redundancy against
random failures of any one system, eg through the use of satnav
devices that can simultaneously use the complementary systems.
However, it does not provide redundancy against extreme space
weather events because those events are global phenomenon—and
thus have the potential to induce many simultaneous failures across
all these systems.
6. This potential for simultaneous failures
arises because these systems operate on the same scientific principles.
Thus they are all vulnerable to the same space weather effects:
— Damage to satellite sub-systems by radiation
or electrical charging.
— Solar radio bursts interfering with the
satnav signal.
— Disruption of the satnav signal as it
passes through Earth's upper atmosphere.
7. Space weather damage to these satellites
is treated very seriously. The GPS satellites are military systems
and thus are thought to incorporate a high level of protection
against both hostile human interference and space weather. The
Galileo programme also aims to build its operational spacecraft
so they incorporate good protection against space weather. The
two Galileo test spacecraft now in orbit carry a number of radiation
monitors, several of which are UK-built. The data from these monitors
are now available to researchers via agreements with the Galileo
programme. This will help to build up UK and other European understanding
of the radiation and charging environment that Galileo will face.
The EU FP7 programme has just funded several research projects
in this area and these include significant UK participation and
leadership.
8. Solar radio interference with satnav
systems was well demonstrated by an intense solar event at around
19:30 UTC on 6 December 2006. This caused widespread temporary
failure of GPS receivers across North and South America. Fortunately,
the event occurred well after sunset in the UK, so no effects
were recorded here. Future solar events, occurring during UK daytime,
could disrupt the reception of the weak signals from both GPS
and Galileo. The direct impact would be a brief (10 mins) loss
of satnav signals, but we do not yet understand the wider economic
and societal impact from simultaneous loss of many satnav systems.
9. The radio signals from GPS and Galileo
are very slightly delayed (compared to travel at the speed of
light) as they cross Earth's upper atmosphere. Satnav receivers
must correct for this delay to give a precise position. During
severe space weather events the corrections may be become inaccurate,
so it is important (a) to warn satnav users of this inaccuracy
via the "integrity flags" included in signals sent to
satnav receivers and (b) for users to then switch to backup navigation
systems.
10. The radio signals from GPS and Galileo
are also subject to scintillation due to turbulence in Earth's
upper atmosphere (this is the radio equivalent of the twinkling
of stars due to turbulence in the lower atmosphere). Severe scintillation
can cause receivers to lose satnav signals—and thus loss
of position and time data. However, these bad conditions are usually
confined to polar regions and to equatorial regions (the latter
especially around dusk). During severe space weather events the
aurora borealis (Northern Lights) may expand from the polar regions
and cover the UK. In those circumstances, we should expect severe
scintillation and loss of satnav signals over the UK.
11. The risk of losing satnav signals (both
GPS and Galileo) to natural interference (solar radio bursts and
scintillation) is very dependent on receiver design. There is
considerable scope for engineering mitigation of these effects
by good design. It is important to raise and maintain market awareness
of this so that vendors are encouraged to provide high quality
equipment and users recognise the need to buy such equipment when
they have critical requirements for accuracy.
12. It is important to complement with satnav
with other navigation systems that have different response to
space weather. An excellent UK example is the e-LORAN system being
developed by Babcock (formerly VT Communications) under contract
to the General Lighthouse Authorities. This uses a ground-based
radio system operating at very different frequencies to GPS and
Galileo.
Mike Hapgood
Royal Astronomical Society
24 November 2010
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