The Forever Young Global Navigation Satellite System
GPS is Starting to Show its True Value
GPS is no longer confined to car navigation systems
but is equally at home on cellular phones,
tablets and digital still cameras.
SMS check-in (notifying a friend of your whereabouts),
generation of detailed life logs
(recording your day-to-day activities)
and other activities that use positional information
are greatly changing communications and our lives.
Now more than 40 years after its development,
GPS is again subject to rapid progress showing that
it still has plenty of potential for growth.
Global Positioning System (GPS)
The Global Positioning System is a Global Navigation Satellite System (GNSS) managed by the United States Government. GPS
development started in 1973 and in December 1993 was declared operational for civilian users. As of 2012, the system is operated
using 31 satellites. Other GNSS's include GLONASS, a Russian system, Galileo, a system run by the EU and the European Space
Agency, and Beidou, a Chinese positioning system. There are also a number of reinforcing and complementary systems. For
example, Japan is building a quasi-zenith satellite system. (See the last page)
■Figure 1 Structure of a Satellite System
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■Figure 2 Method for Generating GPS Signals
50 bps
Navigation
message
(Time and orbital
information)
1.023 MHz
Navigation messages, PN codes (pseudo-random
pulses) and L1 carrier (carrier wave) are multiplied PN code
before transmission.
The generated signals are synchronized to
accurate time generated by an atomic clock
(rubidium and caesium).
L1 carrier
Accuracy: 1×10^-5 ppm
Spread spectrum
BPSK modulation
1575.42 MHz
Transmission signal
■Figure 3
Navigation Message
Data rate = 50 bps, 1-bit length = 20 ms
(20 PN code cycles)
1 Frame = 5 subframes
1 subframe = 300 bits, 6 seconds
1 cycle = 25 frames, 12.5 minutes
Orbital information: Ephemeris (precise satellite orbit)
and almanac (status information for each satellite)
What Does a GPS Receiver Do?
Many end users of GPS devices are under
the misconception that they directly receive
positional information on where they
are from the GPS satellite. The signals
transmitted from a GPS satellite contain
only the satellite's position (orbit), the time
the signal was sent and other data. The GPS
receiver uses the difference between the time
the signal was sent and when it was received
to calculate the distance between itself and
the satellite.
Knowing the positions of the three satellites
and the distances to the receiver from three
satellites enables triangulation to calculate
receiver position as shown in figure 1.
Still the clock in the GPS receiver is not
as accurate as the atomic clock used by
the GPS satellites (a deviation of 1 second
per hundreds of thousands of years). For
example, if the reception of the signal is off
by a mere 1 ms (1 millionth of a second), it
will introduce a distance error of 300 m. For
that reason, a signal from a fourth satellite is
used for time corrections.
Method for Generating
GPS Signals
Let's take a closer look at the signals a GPS
satellite transmits (figure 2).
Pseudo-random code (PN code) called C/A
code is used to encode the time and orbital
data "the navigation message" necessary
for calculating positions. This transmission
Clock correction data
from a satellite
Subframe 1
300 bits (6 s)
Preamble and clock information (synchronized data)
Ephemeris
Almanac
(precise satellite orbital data)
(orbital data assigned each received satellite)
Subframe 2
Subframe 3
Frame
Subframe 4
Subframe 5
1500 bits (30 s)
∗ It takes 30 seconds to acquire 1 frame.
technique is referred to as spread spectrum.
Spread spectrum is also called CDMA, a
transmission technique used by cellular
phones. The advantage of spreading the
frequency band of the signal and its energy
is that multiple communications can
be performed in the same band without
interference.
Each GPS satellite uses a unique PN code.
The receiver can therefore identify which
satellite transmitted the signal.
Finally, the spread-spectrum signal and the
high frequency carrier are multiplied to
generate BPSK modulation. Information
modulates the carrier wave by changing the
phase of the carrier wave.
A GPS receiver that receives the carrier
wave reversed the procedure for signal
generation in a satellite (demodulate the
BPSK modulation signal and perform
spread-spectrum despreading) to obtain the
navigation message and calculate the present
position.
GPS Objectives –– Positioning
Time and Signal Strengthening
Once the receiver receives one complete
frame (1500 bits) of a navigation message,
it can determine the orbit of the satellite (see
figure 3). Since the data rate of a navigation
message is 50 bps, it takes 30 seconds to
receive one frame.
One frame contains the time information,
precise orbital information of a satellite
(ephemeris) and rough orbital information
for all GPS satellites (almanac). Orbital
information on all satellites is required to
help the receiver search for satellites that can
be used in triangulation. Since each frame
carries 1/25 of the almanac data, it takes 25
times 30 seconds or 12.5 minutes to retrieve
all the data.
Almanac data has a validity of several
months, which means that acquired data can
be reused. The search methods have been
optimised to reduce signal acquisition times.
The problem is that ephemeris information is
indispensable to accurate orbital calculation.
This information and the time information
takes at least 18 seconds to obtain but
may take 30 seconds depending on start
of reception. If ephemeris is obtained, the
position can be calculated in 2 to 3 seconds
or even in a single second if circumstances
are favorable, but ephemeris data is valid
only between two to four hours.
To speed calculation of time to first fix
(TTFF) or position, cellular phones and
other communication devices now often use
A-GPS (assisted GPS) to obtain satellite
position and time information from external
servers. Technology allowing receivers to
predict orbital information has also been
developed.
Noise countermeasures are another major
GPS issue. As stated in a detailed discussion
below, GPS signals are extremely weak
and improving their tolerance to in-device
noise is a major challenge when integrating
GPS units in digital still cameras and other
devices.
Creating New Developments in GPS, a Profound Technology
With the launch of the first GPS satellite
in 1978, Sony started GPS development
and succeeded in triangulation using four
satellites by the end of the year.
Sony released the "NVX-1", our first
navigation device, in 1992 before the formal
start of GPS operation in 1993 when the
number of satellites had been increased to 24.
With a view to bring the GPS business that
had so far revolved around car navigation
systems into a wider world, engineers
in charge of developing ICs for GPS
applications were moved to our laboratory in
the spring of 2006.
Five years later in 2011, the engineers were
moved back to the Semiconductor Business
Group. The purpose of this move was to
bring our cumulative expertise and energy
to bear on a GPS market that is undergoing
dramatic changes.
■Figure 4 GPS Signal Strength
Signal strength [dBm]
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Thermal noise
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Kenichi Nakano
Manabu Nitta
Hideki Awata
Manager
Section 8
Communication LSI
Product Department
Analog LSI Business
Division
Semiconductor
Business Group
Sony Corporation
Section 8
Communication LSI
Product Department
Analog LSI Business
Division
Semiconductor
Business Group
Sony Corporation
Section 8
Communication LSI
Product Department
Analog LSI Business
Division
Semiconductor
Business Group
Sony Corporation
∗ Positions they held as of end of July, 2012.
–– Opening up New Possibilities
used. At least, there was no concept like
check-in in our minds.
the hard way that noise could not be taken
lightly.
Nakano: After Sony reoriented the GPS
Nakano: The idea of how your positional
data can be used has greatly changed,
I think. As cellular phones started taking
off, some people complained that they
disliked the idea that they c ould be
contacted anywhere. Now, it has become
an indispensable communication method.
People are using GPS to tell each other
where they are making it possible to meet
up with friends who happen to be in their
neighborhood and this may develop into
communication methods that bring us back
to the real world. Great changes are afoot.
Even so, many GPS issues such as power
consumption, sensitivity, time to first fix still
need to be solved.
Nitta: As this figure (figure 4) shows, GPS
–– Strength of an All-around
Research Approach
Awata: The signal waveform around the
used in such devices, we had no clear
idea.
N it ta: E xc e p t a s a l o g of yo ur ow n
activities, we had no clue how it would be
Awata: When we starting integrating GPS
ICs into portable terminals, we learned
business from the car navigation field, two
of us here and other GPS engineers were
sent to the laboratory to hone our skills.
Those efforts are what brought us to where
we are today. I am the present project
leader and I set great store by a group of
people who have experienced both the
joys and hurdles of GPS development.
A w a t a : A t t h e l a b o r a t o r y, w e w e r e
prepared for development not limited to the
car navigation field. Of course, we were
proud of the achievements we had made in
the car navigation field, but we had to look
ahead to the next goal.
Nitta: From the star t, we focused on
cellular phones and digital still cameras.
Awata: But how positional data would be
signals are extremely weak. GPS signal
strength is -130 dBm or 30 dB lower than
cellular phone signals (GSM ), which
means that they are 1/1000 the strength
of a cellular phone signal. When it comes
to Bluetooth and Wi-Fi, the difference is
even greater; the GPS signal has only 1
millionth of the strength of such signals.
Nakano: GPS signal level is even lower
than thermal noise. Cellular phones have
solid electromagnetic interference (EMI)
countermeasures in place to protect
them from interference. Integrating GPS
in digital still cameras and other devices
not usually thought of as communication
devices is a more formidable problem.
antenna after reception shows noise only
and it is virtually impossible to see the
GPS signal.
Nakano: Despreading a received spread-
Field Trials of Complementary GPS Technologies at “Abashiri Prison”
Use of a Sony GPS receiver module showed that positioning is possible in locations where it is normally difficult to get a signal.
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MICHIBIKI, a Quasi-zenith Satellite
MICHIBIKI is a GPS satellite that the Japan
Aerospace Exploration Agency (JAXA)
launched in September 2010. MICHIBIKI stays
close to zenith around 8 hours per day. It will
complement the GPS system to provide positional information accurate down to 1 meter.
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Indoor Messaging System "IMES"
This is a system proposed by JAXA. Three
dimensional data (identifies what room on
what floor) prerecorded by the transmitter is
sent using the same system as GPS.
Abashiri Prison where a visitor is confronted with numerous facilities
spread out over a vast area is the ideal location to conduct a field test
of a seamless positioning system. In the test, more than 100 local
university students, parents and children took part in a stamp rally to
verify the effect and issues of the new positional technology.
This environment proved tougher than
imagined as signals were reflected of the
prison gratings. For that reason, this field
test was extremely useful in finding the
issues that will have to be bridged before
commercialization. (Kenichi Nakano)
spectrum signal produces peaks and
through synchronous summation, that is
overlapping the signal by itself cancels
random noise. This process eventually
reconstitutes the original signal.
is essential, we also pay attention to ease
of use for set manufacturers.
GPS that are still going strong 40 years
after development.
Nitta: It is repeated interaction with set
Nitta: Despreading and synchronous
summation are standard procedures, but
each manufacturer tweaks them in subtly
different ways.
Nakano: That our core members returned
from the laboratory with an improved skill
set has plainly worked in our favor.
Awata: Although the basic concept of
satellites that just transmit signals has not
changed, there is still plenty of room for
technology improvements and refinements
at the receiving end. We must come up
with new development as new applications
of the technology appear. And that is
what is so exciting about this profound
technology.
Awata: We also had to develop noise
countermeasures –– and we were aware
from the start that noise countermeasures
would become an area where
differentiation would arise.
N i t t a : W e t h o u g h t t h a t a F o u r i e rconversion block for obtaining the GPS
signal could also be used to measure
noise. This concept led to building a
special block for removing noise in the
CXD5400 ∗ GPS receiver module. Then we
added a function for outputting the result of
noise measurements.
Awata: Visualization allowed us to analyze
the noise from a set in operation on a PC
monitor and determine what type of noise
is at work.
Nitta: If the signal before and after noise
removal could be compared, the effect of
removal would also be plain to see.
Awata: While a noise monitoring function
designers that leads to such functionality
and helps to form a proposal.
∗: See the New Products section in CX-NEWS,
Volume 65.
–– Technology that
Fires up an Engineer
Nakano: That GPS is more about
o bt aining ac c urate time infor mat ion
rather than just positional information is
not generally known even though GPS
clocks are phenomenally more acuurate
than radio clocks that are synchronized to
terrestrial time signals.
Awata: When you start up a car navigation
system, you get very accurate time data,
but few people are aware of this.
Nakano: I am sure we will see applications
that employ time information. It will
become possible to simultaneously shoot
photos at a high degree of accuracy at
dif ferent locations around the globe.
Anyway, there are few technologies like
Nitta: Yes, it is certainly profound. And it
is thanks to a solid basic design that all
this has become possible. This provides
us with room for coming up with new
applications.
Awata: Our stint of thorough GPS study at
the laboratory was a great opportunity.
Nitta: We were allowed to conduct a lot of
trial and error experimentation and it was
really exciting.
Naka no: Ye s, a n d i t s t i l l i s exc i t i n g
(laughs).
Awata: And GPS will go on to become
bigger.
Nakano: The members are all headed in
the same direction and our morale is high
so I am sure we can look forward to a
future with positive results.