GNSS Signal Structures
Tom Stansell
Stansell Consulting
Tom@Stansell.com
Bangkok, Thailand
23 January 2018
S t a n s e ll
C o n s u lt in g
RLIntroduction
It’s a pleasure to speak with you
this morning. What follows are
excerpts from three separate
presentations.
Regards,
Tom Stansell

S t a n s e ll
C o n s u lt in g
RL

Slide 2

Source Presentations

S t a n s e ll
C o n s u lt in g
RL

Slide 3

GNSS Modernization
and Interoperability
Tom Stansell
Stansell Consulting
Tom@Stansell.com

Slide 1

The Goal of Interoperability
 Ideal interoperability
allows navigation
with one signal
each from four ormore systems withno additional
receiver cost orcomplexity

Interoperable = Better Together than Separate
Slide 2

Main Benefits of Interoperability
More Satellites  Better Geometry  Improves: Satellite coverageNavigate where could not before Dilution of Precision

Accuracy is better everywhere Eliminates DOP holes (with open sky)

 RAIM*

Integrity checked everywhere, all the time Eliminates RAIM holes (with open sky) Phase ambiguity resolutionFor survey and machine control applications

 Accuracy
Allows higher elevation angle cutoff which reduces multipath, ionospheric,and tropospheric errors* Receiver Autonomous Integrity Monitoring
Slide 3

Spectrum of GNSS Signals

NAVICSlide 4

Slide 52018

Originally presented December 2008; Updated to current status and plans

GPS Signals Summary
CenterBand Frequency Signal Waveform
Notes
C/A
BPSK(1) Open Service
P(Y) BPSK(10)L1 1575.42 MHz
L1C
TMBOC Open Service, Separate Pilot and Data Channels
M
BOC(10,5)L21227.6 MHz

P(Y)
L2C
M

L51176.45 MHz

L5BPSK(10)BPSK(1) Open Service, Separate Pilot and Data Channels
BOC(10,5)
BPSK(10) Open Service, Separate Pilot and Data Channels

Slide 7

Slide 8

Galileo Signals Summary
BandCenterFrequency

E11575.42 MHz

E61278.75 MHzE51191.795 MHz E5a & E5b AltBOC(15,10) Open Service, Separate Pilot and Data Channels

Signals
E1 OS
PRS
CS
PRS

Waveform
Notes
CBOC
Open Service, Separate Pilot and Data Channels
BOC(15,2.5)
BPSK(5)BOC(10,5)

Commercial Service, Separate Pilot and Data Channels

Slide 10

Slide 11Note: Some signal changes
are being evaluatedSignal Plans
Slide 13

Future GLONASS Signal Spectrum

Slide 15

Signal Plans(From Several Presentations)

Slide 16

“QBOC”

GNSS Signals, Spectra,
and Receiver Fundamentals

1

2

3

Simple Pseudorandom Code Generator

Code length === 15
C/A Code length === 1023

4

Code Modulation Spreads the Spectrum

5Frequency Domain
6

GNSS Spectra
E5b/B2b

NAVIC

B3

E6

Note: NAVIC

L1, L2, & L5 are paramount, but also GLONASS, PRS, E5b, B3, & E67

Power Spectral Density (dBW/MHz)

GNSS L1 SpectrumFrequency (MHz)8

Receiver Signal Processing

910

27 Years with Just 3 GPS Signals
Direct civil access to C/A code
Indirect civil access by codelessand semi-codeless means

1978 to2005
11IIR-M Satellites Added Three More

Direct civil accessto L2C code

1978 to2005

2005

12

IIF Satellites Added L5

SafetyserviceinARNSband

1978 to2005

2005

2010

13

GPS III Will Add L1C

Better performance

1978 to2005

2005

2010

2018?

14

Modernized Signal Structures• The most important improvements in GNSS
signal structures since1978 have been adopted
for essentially every new and modernized signal– Including GPS, Galileo, BeiDou, and QZSS– Hopefully also for NAVIC and GLONASS CDMA• The improvements are (a) to have a data-lesspilot carrier and (b) to use Forward Error Control(FEC) to enhance data reception• There are many other variations, e.g.,– Binary Offset Carrier (BOC) combinations, spreadingcode structures, FEC techniques, power split between
data and pilot channels, symbol interleaving, etc.
– Each has a purpose, e.g., spectrum separation
15

Signal Structure, Interoperability, and Geometry

Tom Stansell
Consultant to the Aerospace Corporation
Tom@Stansell.com

23 January 2018
ICG Workshop, Bangkok, Thailand© The Aerospace Corporation 2015

Disclaimer

The views and opinions expressedherein are those of the author anddo not necessarily reflect the official
policy or position of The Aerospace
Corporation or of any agency of theU.S. government.
Portions of this work have been sponsoredby The Aerospace Corporation

2© The Aerospace Corporation 2015

The Most Important Ingredient• Only Navigation by Satellite can provide excellent Geometry– Continuous, worldwide, four dimensional, with excellent accuracy– GDOP, Geometric Dilution of Precision, and its important children:• PDOP, HDOP, VDOP, and TDOP
– Although the satellite signals may be weak, the geometry is strong• No terrestrial navigation aid delivers “the most important ingredient”• Do users need better geometry than GPS alone can provide?
• The answer is a definite “YES” as demonstrated by:– Widespread use of GLONASS in products from consumer mobile
phones to commercial survey and machine control products• In spite of the difficulty of using GLONASS FDMA with GPS CDMA
– Plus widespread development of receivers to use all available GNSS

• Aircraft at altitude and ships at sea may not need more than GPS– But integrity by A-RAIM requires many more satellites• Users subject to signal blockage or outage do need more satellites• Thus, the second most important ingredient is signal interoperability– Enabling the best geometry by using every interoperable satellite signal
3© The Aerospace Corporation 2015

Signal Structure and Interoperability Considerations• Interoperability is in the eye of the beholder• For example, L1C and E1 OS have identical center frequencies andidentical spectra, but almost everything else is different
Spreading
Pilot
CodeSpreading
Channel Data
Pilot
ForwardOverlay Message
LengthCodewithPower Power SymbolError
CodeFrameSignal (chips)
Duration Modulation BOC(6,1) Percent Percent Rate
Bit Rate Correction Duration Length
10,230
10 ms
TMBOC
Pilot
25%
75% 100 SPS 50 BPS
LDPC18 sec
18 sec
L1C
4,092
4 ms
CBOC
Both
50%
50% 250 SPS 125 BPS Convolutional 100 ms 720 sec
E1 OS• Receivers will handle the differences and hide them from the user
– The user will experience better performance due to more satellites• However, different types of receivers will take advantage of some ofthe signal differences between systems– Identical center frequency is important for high precision receivers andfor bandwidth limited GNSS antennas on aircraft– Many receivers will use GPS L1 C/A for fast signal acquisition but theother signal structures for navigation, positioning, and timing

4© The Aerospace Corporation 2015

Predicting the Future• If there are three global interoperable GNSS constellations in 2020
– GPS, Galileo, and BeiDou, with a total of 72 to 90 operational satellites1.

Use of GLONASS FDMA will decrease for precision applications– The current demand for more satellites will be satisfied by interoperable
CDMA signals, leaving little demand for the more difficult FDMA signals

2.

Users will not say “this is my GNSS” or “this is my BeiDou”– There will be few if any GPS-only or BeiDou-only or Galileo-only receivers– Users won’t know and they won’t care where the signals originate– They will just enjoy the better performance provided by better geometry– And they probably will continue to call their device a “GPS” (sorry!)3.

Special, unique, or “orphan” signals will be little used
– Use of GPS L2C will decline because no other GNSS provides it– The standard dual-frequency pair will become 1575.42 and 1176.45 MHz– E5b and B2b will be little used, whereas E5a and B2a will be widely used
• A lively discussion topic!

5© The Aerospace Corporation 2015

Future Decrease in High Precision FDMA Use• A pure “time delay” Δt is characterized by a linear slope of phaseversus frequency• However, a bandpass filter mustPhase / f   / ( / t )  t

rapidly attenuate signals outside
the bandpass region• This introduces nonlinearities inFrequency
phase versus frequency, especiallyat the band edges• In high precision applications it is desirable for every signal fromevery satellite to experience the same nonlinearities so there are notime delay differences between signals due to receiver filtering• This will be true if every signal has the same center frequency• Because this is not true for GLONASS FDMA signals, very carefulcalibration of each channel is required for near-precision results• This is why high precision use of GLONASS FDMA will likely
decrease substantially with deployment of Galileo and BeiDou6© The Aerospace Corporation 2015

Growth Continues and Should Accelerate• Application growth is fueled primarily by the private sector– Heavily regulated products, e.g., for aviation and the military, are slow tochange and generally lag in innovation (sad but true)• Factors that encourage innovation and application growth:– Competition, Moore’s law, opportunity, fear, and the profit motive• What in the future will stimulate growth:– Much better GNSS geometry improves availability, continuity, integrity,and accuracy, especially in difficult environments• Urban canyons, real canyons, open pit mining, even aviation– A-RAIM will become practical and begin to displace SBAS use– Ambiguity resolution for Real Time Kinematic (RTK) in survey andmachine control will become almost instantaneous and more reliable• Improved vertical accuracy will displace some laser plane requirements• Alternate means to communicate message parameters will promote“instant navigation” for all applications (push to navigate)

7© The Aerospace Corporation 2015