Avionics News July 2023 - 29

" The PNTAB concerns apply to both civil and military
use of GPS, " said Dana Goward, a retired U.S. Coast Guard
captain and military pilot who led the $1.3 billion per year
navigation authority. He now leads the RNT Foundation, a
nonprofit that works to promote and protect resilient navigation
and timing, including GPS.
" The Department of Defense has several efforts underway
to improve GPS reception, anti-jamming capability and use
by the military, and it is developing military-only systems for
use in the future, " Goward said. " But there is nothing coming
down the pike that will benefit or protect civil users. GPS is a
military system, and the government has issued an executive
order telling civil users they have to fend for themselves. "
Goward is a representative to the PNT
board who speaks for the RNT Foundation.
Adversaries
are able to deny GPS
service to America while many GPS
devices in use have little or no protection
against disruption. The RNT Foundation
stresses that GPS and PNT modernization
requires a holistic systems approach,
including protecting signals, toughening
receivers and augmenting services. The
needed technologies are mature and
readily available, but U.S. government
leadership is lacking, and the industry
can't fill gaps on its own without government
leadership and permission.
GPS is also falling behind Galileo
and BeiDou in providing high-accuracy
services and authentication that the
signal being received is in fact GPS,
according to the PNTAB. One risk associated
with being overtaken is that GPS
had almost no impact on users who had access to the other satnav
satellites, according to Scott. He began his career designing
GPS systems at Texas Instruments when he was 19 and
then moved into the cellular phone industry before becoming
a consultant.
Many instances of
serious GPS interference
have been reported,
particularly to aircraft.
There are wellestablished
techniques
such as multi-element
antennas that would
virtually eliminate
the threat of such
interference, but the U.S.
government does not
allow manufacturers to
produce and sell them.
might lose its status as the primary GNSS used in most GNSS
chips. The board recommends the U.S. government should
develop and implement GPS high-accuracy and resilience
service for internet users with performance comparable to
other GNSS providers. Goward said if another GNSS system
comes to be seen as more robust and reliable than GPS, then
that service could be prioritized over GPS in receiver chips.
Every receiver that ingests multiple GNSS signals has a way
of prioritizing one over another when there are differences.
The world is moving now to the use of multiple GNSS
signals with the availability, at the moment, of healthy satnav
satellites numbering 48 for BeiDou, 30 for GPS, 23 for Galileo
and 23 for Glonass, according to Logan Scott, a systems
signal processing expert.
In July 2019, Galileo went off the air for a week, and it
Goward noted that using multiple GNSS constellations for
navigation is better than just having one but won't provide the
resilience and reliability needed for some applications. " The
European Union recognized this several years ago saying that
GNSS alone is insufficient for safety-critical applications, "
Goward said. " This is because all GNSS operate in the same
frequency band and can all be easily, and often are, jammed
at the same time. And, since they are space-based, they are
all impacted by solar weather that can
degrade or disrupt signals. "
Avidyne, headquartered in Melbourne,
Florida, and a maker of
integrated avionics for general aviation,
solely supports GPS, but its president
and chief executive officer, Dan
Schwinn, thinks it is a valid concept to
use other GNSS to reduce the impact of
GPS interference.
Avidyne's GPS was designed when
the company was concerned about adjacent
band interference from spectrum
repurposed from satellite to terrestrial
use by Lightsquared (now Ligado). The
PNTAB is still concerned that Ligado is
trying to broadcast on an adjacent band.
Without clear specifications, Schwinn
said the company's GPS was designed
to have a higher level of adjacent band
interference rejection than is required in
case this became an issue. So Avidyne's
system is more tolerant of out-of-band interference than other
general aviation GPS systems, according to Schwinn. But Avidyne
did not include mitigation of in-band interference, which
is where some of the interference problems are now being
reported. " We continue to be interested in options for mitigating
various types of GPS interference, " Schwinn said.
While the powers that be in Washington, D.C., sort through
ways to protect and strengthen the GPS system, most members
of the Aircraft Electronics Association depend on it for
their livelihood but probably don't have time to keep track of
what groups like the PNTAB are recommending.
Jason Moorefield, the avionics manager at Freedom
Aviation Inc. in Lynchburg, Virginia, helps maintain Liberty
avionics news * july 2023 29
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