IEEE - Aerospace and Electronic Systems - January 2023 - 10

Another Golden Anniversary: NAVSTAR, the Global Positioning System
GLONASS and system development commenced in 1978.
The GLONASS designers chose frequency division multiple
access instead ofCDMA likely due to better native analog
electronics capability rather than digital. GLONASS
has been plagued by a wide variety of challenges over the
decades. The current Russian government is committed to
sustaining GLONASS but accuracy outside of the former
Soviet Union is significantly worse than GPS.
The European Union jumped into the satnav game in
the late 1990s and the result is the constellation known as
Galileo. Galileo started initial services on 15 December
2016, and as of the time of this writing (2022) is nearly at
full operational capability. Galileo provides performance
on par with GPS. Unlike GPS, Galileo has no military
mission. The People's Republic of China commenced
development of their own satnav also in the late 1990s
and their system, known as Beidou, achieved operational
capability over China in 2020. Coverage has been optimized
over China and availability outside of China currently
can be spotty. When sufficient satellites are visible,
performance is comparable to GPS and Galileo. The multiple
satnav constellations are collectively referred to as
global navigation satellite systems (GNSS).
Data collected by Ohio University shows the relative perFigure
6.
Magellan Nav 1000 Plus. This is the second generation (released
in 1991) of the world's first civilian handheld GPS receiver.
[Photo by the author.]
In 1996, the Federal Communications Commission
(FCC) enacted a mandate requiring that at a future date all
cellular service providers must be capable of providing
the location of a cell phone in the event of an emergency
call. In the U.S. the emergency number is 911 and the
FCC requirement became known as the E-911 mandate
( " E " for enhanced). A wave of research and development
was spawned by the mandate with one result being the
fact that virtually every cell phone produced today has an
embedded satnav receiver. The total number of GPS
receivers in the world is, thus, in the billions with civil
receivers dwarfing the population of military receivers.
When produced in volume, the receiver chips cost the
phone manufacturers a few dollars each. For the average
civil user, the cost of the receiver is five orders of magnitude
less than the cost of the initial prototypes.
ONE SKY, MULTIPLE CONSTELLATIONS
The Soviet Union did not sit idly by while GPS was being
developed. While the operational side ofthe Air Force was
attempting to kill the program, the Soviets did not take any
chances. The Soviet (now Russian) satnav is called
10
formance ofthe four constellations (see Figure 7). Horizontal
position error heat maps are displayed (darker colors means
more data points) for data collected at Athens, OH, USA on
14 August 2022. For each constellation, position was computed
using single-frequency measurements and no augmentation
system corrections (more on augmentations shortly).
The Republic of India has deployed a regional satnav
known as the Indian Regional Navigation Satellite System
or Navigation with Indian Constellation (NavIC). NavIC
consists of three geostationary satellites and four geosynchronous
satellites (two pairs with figure-eight ground
tracks). NavIC became operational in 2018 with a coverage
area that fully encompasses the Middle East, Far East,
and most of the Indian Ocean.
AUGMENTATIONS
In the late 1980s, the FAA recognized that GPS by itself,
although sufficient in terms of accuracy, was not sufficient
in terms of integrity. Integrity, in this context, is a measure
of the degree of trust one can put in a system. An integrity
failure occurs when a navigation system provides
" hazardously misleading information. " In other words, the
system is providing highly erroneous data and there is no
warning given. The FAA had decades of experience with
monitor systems that guaranteed the integrity of its terrestrial-based
navigation and landing systems, and a similar
system for GPS was needed. The challenge, of course, was
that GPS had a global coverage volume. Even monitoring
the continental United States would be challenging.
IEEE A&E SYSTEMS MAGAZINE
JANUARY 2023
IEEE - Aerospace and Electronic Systems - January 2023

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