Any Four Will Do 1

Interchangeability and CARS

Bradford ParkinsonStanford University

Any Four Will Do 22

What’s Next?

04/19/23

Any Four Will Do 304/19/23 3

GLONASS – (Russia)

PNT to Explode with Opportunities

Galileo(European)GPS

(USA)

Compass (Beidou China)

QZSS(Japan)

The Five Design Keys (& Barriers to Entry)

1. The CDMA signal (PRN or Spread spectrum)

2. Van-Allen qualified atomic clocks

3. Orbit prediction to a few meters (URE) in 20,000 miles of travel

4. Spacecraft that lasted about ten years (cost of ownership)

5. User Equipment that could (eventually) be miniaturized (<$)

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International Augmentations

Differential GPS Networks

International GNSS Service Global Differential GPS System

Space-Based Augmentation Systems

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New Signals are On the Way

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Summary - Spectrum of Modernized GPS Signals

L5ARNS/RNSS Band

L1ARNS/RNSS Band

L2RNSS Band

P(Y)

C/A

L2C

M

L5

L1C

Earlier GPSDual Frequency w/ Semi-codeless P(Y)

Block IIR-MLaunch 2005Dual Frequency L1 C/A & L2C

Block IIF/IIR-M+Launch 2008Three Frequency L1 C/A, L2C, & L5

Block IIILaunch 2013L1C, L2C, L5, & L1 C/A Code

L5

New!

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GPS L2 GPS L1

1164

MH

z

Galileo E1 L1 E2Galileo E6Galileo E5A Galileo E5B

GPS L5 GLONASSG2

GLONASS G1

1188

MH

z

1214

MH

z12

17M

Hz

1239

MH

z

1260

MH

z12

61M

Hz

1300

MH

z

1559

MH

z

1563

MH

z

1587

MH

z

1591

MH

z15

93M

Hz

1610

MH

z

RNSS Band

Upper L-Band

RNSS Band

1151

MH

z

1237

MH

z

ARNS Band ARNS Band

RNSS BandRNSS Band

GLONASSG3 ?

Compass B6

B1Compass B5B

B2

Coming: a Plethora of Signals

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Latest Signal Verified – L5

8

• L5 spectrum captured by Stanford 45.7m dish antenna. • Antenna gain as high as 52 dB, • L5 signal high above the noise floor in frequency.

• Clear L5 spectrum - both main lobe and two side lobes

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Levels of Cooperation - New Systems:Can’t we just get along?

• Degrees of “Getting Along”– Compatible

• Lets not hurt each other

– Interoperable• I’ll try yours, you can try mine

– Interchangeable• Any four will do

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GPS and Galileo/Compass/GLONASS-

The Users Viewpoint• Real Measure: Interchangeability “Mix and Match” with the same

ranging accuracy

– Enabled by L1C - defined, implemented, and operable – including all

details

– Seamless WAAS/EGNOS/+ ?

– True clock Synchronization (Common Clock) and common grid

• Payoff – Availability, Accuracy and Robustness for

Worldwide Users

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Achieving Interchangeability

• Simplest: Common Clock and Geodetic System– Probably Implies a common Control System– Nationalistic Resistance

• A New Proposal – Cross-Augmentation Reference System (CARS)– Each Satellite Broadcasts WAAS like

Corrections to allow it to be seamlessly operated as a part of the “other” constellation

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Three levels of “WAAS-like Corrections”

• Correction for ephemeris and specific satellite clock– Implies frequent upload capability

• Correction for satellite clock alone– Geodetic correction assumed constant

• Intersystem time calibration (sub-nano-second)

– Assumed very slowly changing

In addition some form of Integrity Message could be included

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GPS/Galileo CARS Example– Cross Augmentation Reference System

• GPS Monitor Stations track Galileo satellites (L1C)– Form WAAS-like Corrections for GPS satellites to

operate as Galileo Ranging sources (Clock and Position – Include integrity?)

– Include corrections as part of the GPS message – Result: GPS satellite is seamlessly useable as a part

of the Galileo Constellation

ANY Four Satellites will do: A step beyond Interoperable-CARS is truly “interchangeable”

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CARS : GPS/Galileo ExampleFirst Step: Monitor the “other” constellation signals

GPS Control Segment

GPS

GPS GPS

GPS

Galileo

Galileo

Galileo

Galileo

• GPS Reference Receivers Monitor all L1C signals

• GPS Master Control Calculates a WAAS/EGNOS type correction for each GPS satellite (or for inter-system time) –for operation in the Galileo Constellation04/19/23

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GPS Control Segment

GPS

GPS GPS

GPS

Galileo

Galileo

Galileo

Galileo

CARS : GPS/Galileo ExampleSecond Step: Send Augmentation Corrections to Each “own” satellite to allow it to be “seen” (used) as a member of the other constellation

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• Thus, any GPS satellite can operate as full member of the other constellation

• Note:

• Corrections are slowly changing (low data rate)

• Each GPS satellite either

• only broadcasts its own correction to the Galileo reference

• Broadcasts inter-system time difference and time-difference rate

• Each constellation controls this use

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And Galileo would do the same for GPS (etc. for other GNSS Systems)

• Advantages:– Satellites are truly Interchangeable– Avoids issue of which time or which Geodetic

System– Very Low Rate message, low overhead– Very efficient – corrections broadcast only for the

satellite being used– Each Satellite Provider has some control over use of

their system– User can have highly accurate PNT in coordinates of

their choosing

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Unintended Consequences?

• Each Constellation forced to be smaller?– Still need core capability to operate alone?– Military cannot use for “Fire” clearance – Sky

impaired warrior not helped– Next Gen (FAA) still needs MOPS – May take

a long time with Galileo

• Misleading Information– Cross Augmentation Bit in message?

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Implementation of CARS

• Bi-Lateral Agreements

– Could be done experimentally on a unilateral basis

• Technical Working Group to adapt WAAS or EGNOS-

like format for message

– Which level and frequency of update?

– Include Integrity?

– Ionospheric correction probably not necessary

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Conclusion

• We are entering an era with incredible

opportunities in Worldwide PNT

• With a concerted effort of cooperation, all

users should receive benefits:

economic, safety and convenience

• The ultimate goal is interchangeability

– CARS would enable this

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Notes

• Time only: Phi, Phidot, and tzero

• System to system time only

• Saves a calibrating satellite

• Unintended consequences– Need less satellites?

• Military• Next Gen

• Limited Upload Capability