gnss technology advances in a multi-constellation
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Pratap Misra
Sogei Workshop IIGNSS Technology Advances in a Multi-Constellation Fr amework
Rome, September 25-26, 2014
Source: International Committee on GNSS (ICG)
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© 2014 Pratap Misra
Modest Beginning
“ The mission of this [GPS] program is to:1. Drop 5 bombs in the same hole2. Build a cheap set that navigates (<$10,000),
and don’t you forget it!”
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Program Manager’s Office, circa 1975
Prof. Brad Parkinson
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Modest Expectations
• Performance Specifications:– Position: 10 m
– Velocity: 0.1 m/s
– Time: 100 ns
to unlimited number of users instantaneously, continuously, in all weather, all over
• Not clear if the system could be built– Uncertainty about the clock technology
• Number of receivers estimated as ~27,000– Uncertainty about users and markets
• Price of a receiver estimated as ~$10 000– Uncertainty about price
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What’s Changed?Electronics kept getting smaller and cheaper
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TI 4100
Rockwell Collins’ GDM
1977 1982
Magellan NAV 1000
1988
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What’s Changed?Recognition of the Value of Position Coordinates
5Dominant User Community: 1 Billion-plus Cellphone Users
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What’s Changed? How Wars are Fought
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The strike was carried out by a single B-2 at night after flying from Whiteman AFB nonstop
US Air Force photo
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Why Galileo?
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In combination with GPS, the higher number of satellites available to the user will offer:
•Higher Precision•Higher Availability•Better Coverage
for positioning, navigation, and time (PNT)
“Galileo will help Europe maintain and develop its know-how in the space, receivers and applications sectors, securing economic revenues and jobs … worth up to 90 billion euros over the first twenty years”
This is too important a technology to allow to pass by
http://ec.europa.eu/enterprise/policies/satnav/galileo/why/index_en.htm
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© 2014 Pratap Misra
Multi-GNSS NavigationOutline
• It All Started 40 years Ago• What’s Changed? Everything!
– Technology, How Wars are Fought, How We Live
• GNSSs Under Development:– Recent Progress & Setbacks
• What We Want from GNSS? – Performance Metrics & Service Requirements
• Multi-GNSS Navigation: Strength in Redundancy– Coexistence & Cooperation
• Why can’t they use the same time scale and coordinate frame?
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© 2014 Pratap Misra
GNSS Status
• 75 working satellites in early 2014– GPS: 30-plus– GLONASS: 24-plus– Galileo: 4– BeiDou: 16
• Launches Scheduled in 2015: GPS IIFs; Galileo FOCs; GLONASS-K
• Only GPS is operational and in > 1 billion mobile devices –market expected to grow to 5 billion
• 4 operational systems with 120 satellites in 2020
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Source: ICG-7 presentation by Dr. Lu Xiaochun
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GPS• Maintained a constellation of 30+ operational satellites• Launched 7 Block IIFs• Modernization: GPS III & OCX• SIS URE < 1 m
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GPS IIISource: Lockheed-Martin
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7 satellites lost due to faulty launches in 4 years!
Source: ROSCOSMOS
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Galileo
• Enormous Progress in deployment of ground segment• Successful completion of IOV• First position fix using only Galileo satellites (2013)• Design validation of FOC satellites
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One of first two FOC satellite (Image: ESA)
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19Photo: ESA Image: ESA
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© 2014 Pratap Misra
Multi-GNSS NavigationOutline
• It All Started 40 years Ago• What’s Changed? Everything!
– Technology, How Wars are Fought, How We Live
• GNSSs Under Development:– Recent Progress & Setbacks
• What We Want from GNSS? – Performance Metrics & Service Requirements
• Multi-GNSS Navigation: Strength in Redundancy– Coexistence & Cooperation
• Why can’t they use the same time scale and coordinate frame?
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© 2014 Pratap Misra
Performance MetricsMy System is Better than Yours
• Accuracy• Availability of Service• Integrity (or Reliability)
– Can I count on my position estimate to be ‘correct’?
• Robustness– Can the system withstand ‘small’ disturbances?
• Security
• Challenge: – Deliver service like a utility: water or power supply
– Rigorous, constant maintenance: No screw ups!
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GNSS Users’ Perspective
23Based on GPS SPS PS, 2008; and T. Powell briefing to ICG, 2007
Interface Specification (IS)
Performance Standard(PS)
Signal-in-Space (SIS) Interface
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© 2014 Pratap Misra
Multi-GNSS NavigationOutline
• It All Started 40 years Ago• What’s Changed? Everything!
– Technology, How Wars are Fought, How We Live
• GNSSs Under Development:– Recent Progress & Setbacks
• What We Want from GNSS? – Performance Metrics & Service Requirements
• Multi-GNSS Navigation: Strength in Redundancy– Coexistence: Compatibility– Cooperation: Interoperability
• Why can’t they use the same time scale and coordinate frame?
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Navigation with GPS+GLONASScirca 1990
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• Unknown time offset between GPS Time and GLONASS Time
• Unknown GLONASS coordinate frame (SGS 85 )
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GPS & GLONASS Position Estimates*1-Minute Samples, 15 June 1996
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Transformation Between WGS 84 & SGS 85 (1992)
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• Satellite Positions in SGS 85: Broadcast; recorded by a GLONASS receiver• Satellite Positions in WGS 84: Computed by radar tracking
Source: Lincoln Laboratory Journal, 1993
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GPS & GLONASS Position Estimates*1-Minute Samples, 15 June 1996
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Coexistence & Cooperationamong GNSSs
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• Coexistence: CompatibilityLiving peacefully with others despite fundamental disagreements
• Cooperation: InteroperabilityWorking together for a common purpose
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GNSS Spectral Map
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GNSS Compatibility: Coexistence
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1990
2010
Source: Dr. John Betz, MITRE
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Multi-Constellation Navigation
• Each GNSS requires a self-consistent
- Time Scale- Coordinate Frame
• Inter-operability requires simple, known transformations
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Why can’t they use the same time scale?
• All are ‘tied to’ Coordinated Universal Time (UTC)– Civil time standard
– Computed monthly (at BIPM)
– Local real-time realizations UTC(k)
|UTC – UTC(k)| < 100 ns
– |GPST – UTC(USNO)| < 1 µµµµs modulo 1 second– |GLONASS Time – UTC(SU)| < 1 µµµµs – |BeiDou Time – UTC(NTSC)| < 1 µµµµs modulo 1 second
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GNSS Times
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10 s 20 s
• GPS Time• Galileo Time
GLONASS Time
BeiDou Time
UTCFlows uniformly except for leap seconds
|GNSST – UTC(k)| ≈ 10 ns – 1 µs
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Coordinate SystemsThe Language of Positioning
• Once regional coordinate frames were adequate
– NAD 27, European Datum, Tokyo Datum, Everest Datum, …
• A GNSS require a global coordinate frame
– US DoD developed WGS 60, 66, 72, 84
– US National Standard: NAD 83
• Q: Why not define coordinates once and for all?
• A: Coordinates change from hour to hour, day to day, … coordinates and velocities
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Global Coordinate SystemConventional Terrestrial Reference System
THEORY
•Earth-centered, earth-fixed •Cartesian coordinate frame for (x, y, z)
– Origin at center of mass
– z-axis through average pole of rotation 1900 -1905
– x-axis on equatorial plane
• Ellipsoid of revolution for ( λ, ϕ, h) – to fit the shape of the earth (geoid,
i.e., equi-potential surface extending MSL)
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z
x
λ
φ
h
(x, y, z)
Reference
Meridian
.
y
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Global Coordinate Frame(s )
REALIZATION
•Adopt (and refine) coordinates of a set of points•Fit the x-, y-, and z-axes by least-squares adjustment
• Ellipsoid of revolution for ( λ, ϕ, h) – to fit the shape of the earth
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+(x1, y1, z1)
+(x3, y3, z3)
xy
+(x2, y2, z2)
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Why Can’t They Use the same Coordinate Frame?
• GPS: WGS 84 (US DoD)
• Maintained through adopted coordinates of 15 GPS Tracking stations using GPS measurements
– WGS 84 (G730) -- 1994 , (G873) -- 1997, (G1150) --2002, (G1674) 2012 (G for GPS; nnnn: GPS Week Number)
• WGS 84 = ITRF2000 (at centimeter level)
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Source: Z. Altamimi
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Why Can’t They Use the same Coordinate Frame?
• All are tied to ITRF, international standard defined and maintained by IERS
• Realizations:
– ITRF88, -89, …, -2000, -2005
– Positions and velocities of a set of sites using GPS, VLBI, SLR, LLR, DORIS
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Source: Z. Altamimi
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Why Can’t They Use the same Coordinate Frame?
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90.07
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8
18ITRF PZ
x x cm
y y cm
z z cm−
− = + + +
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Multi-Constellation NavigationWhere the Users Are
42Sourve: Dr. Frank van Diggelen, Broadcom
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Drivers of Multi-Constellation Navigation
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Drivers of Multi-Constellation Navigation
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Multi-Constellation NavigationGLONASS Suffers System- Wide Outage
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Source: Russian Federal Space Agency website
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Live Test During GLONASS OutageStrength in Redundancy
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Source: Dr. Frank van Diggelen, Broadcom
Receiver: X GPS+GLONASS
Receiver: Y GPS+GLONASS+BeiDOU+ QZSS
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Multi-Constellation NavigationSummary
• The challenge is to build, operate, and maintain GNSSs without drama
• Receiver design part already moving along nicely
• Uncertainty about user acceptance
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NANUs & NAGUs
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NANU?
NAGU?
NABU? NAQU?
Notice Advisory to Navstar Users
Notice Advisory to GLONASS Users
Notice Advisory to Galileo Users