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)

© 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!”

2

Program Manager’s Office, circa 1975

Prof. Brad Parkinson


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

3


What’s Changed?Electronics kept getting smaller and cheaper

4

TI 4100

Rockwell Collins’ GDM

1977 1982

Magellan NAV 1000

1988

What’s Changed?Recognition of the Value of Position Coordinates

5Dominant User Community: 1 Billion-plus Cellphone Users

What’s Changed? How Wars are Fought

6

The strike was carried out by a single B-2 at night after flying from Whiteman AFB nonstop

US Air Force photo

Why Galileo?

7

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


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?

8


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

9

Source: ICG-7 presentation by Dr. Lu Xiaochun


GPS• Maintained a constellation of 30+ operational satellites• Launched 7 Block IIFs• Modernization: GPS III & OCX• SIS URE < 1 m

10

GPS IIISource: Lockheed-Martin

11

12

13

14

15

16

7 satellites lost due to faulty launches in 4 years!

Source: ROSCOSMOS


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

17

One of first two FOC satellite (Image: ESA)

18

19Photo: ESA Image: ESA

20







• Multi-GNSS Navigation: Strength in Redundancy– Coexistence & Cooperation


21


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!

22

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

24







• Multi-GNSS Navigation: Strength in Redundancy– Coexistence: Compatibility– Cooperation: Interoperability


25

Navigation with GPS+GLONASScirca 1990

26

• Unknown time offset between GPS Time and GLONASS Time

• Unknown GLONASS coordinate frame (SGS 85 )

27

GPS & GLONASS Position Estimates*1-Minute Samples, 15 June 1996

Transformation Between WGS 84 & SGS 85 (1992)

28

• 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

29

GPS & GLONASS Position Estimates*1-Minute Samples, 15 June 1996


Coexistence & Cooperationamong GNSSs

30

• Coexistence: CompatibilityLiving peacefully with others despite fundamental disagreements

• Cooperation: InteroperabilityWorking together for a common purpose

31

GNSS Spectral Map

GNSS Compatibility: Coexistence

32

1990

2010

Source: Dr. John Betz, MITRE


Multi-Constellation Navigation

• Each GNSS requires a self-consistent

- Time Scale- Coordinate Frame

• Inter-operability requires simple, known transformations

33


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

34

GNSS Times

35

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


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

36


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)

37

z

x

λ

φ

h

(x, y, z)

Reference

Meridian

.

y


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

38

+(x1, y1, z1)

+(x3, y3, z3)

xy

+(x2, y2, z2)


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)

39

Source: Z. Altamimi



• 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

40

Source: Z. Altamimi



41

90.07

36

8

18ITRF PZ

x x cm

y y cm

z z cm−

− = + + +

Multi-Constellation NavigationWhere the Users Are

42Sourve: Dr. Frank van Diggelen, Broadcom

Drivers of Multi-Constellation Navigation

43

Drivers of Multi-Constellation Navigation

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Multi-Constellation NavigationGLONASS Suffers System- Wide Outage

45

Source: Russian Federal Space Agency website

Live Test During GLONASS OutageStrength in Redundancy

46

Source: Dr. Frank van Diggelen, Broadcom

Receiver: X GPS+GLONASS

Receiver: Y GPS+GLONASS+BeiDOU+ QZSS


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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48

NANUs & NAGUs

49

NANU?

NAGU?

NABU? NAQU?

Notice Advisory to Navstar Users

Notice Advisory to GLONASS Users

Notice Advisory to Galileo Users

Documents

GNSS Technology Advances in a Multi-Constellation