10/7/2015 1 Innovative Solutions International Satellite Navigation Division ION NTM 01 Capabilities of the WAAS and EGNOS For Time Transfer SBAS, an Alternate

04/21/23 1

InnovativeSolutionsInternational

Satellite Navigation Division

ION NTM 01

Capabilities of the WAAS and EGNOSFor Time Transfer

SBAS, an Alternate to GPS for Time

Bill KlepczynskiInnovative Solutions International

CGSIC29 March 2001

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Outline Timekeeping Introduction & Terminology

Time Distribution

Time Transfer

Time Synchronization

Time Coordination

SBAS Capabilities in Time Distribution

SBAS Capabilities in Time Transfer

Conclusions

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TimekeepingIntroduction

Navigation systems have always been related and associated with timekeeping ever since Harrison developed his No. 4.

WAAS and EGNOS are the latest in the evolution of navigation systems. They are Satellite Based Augmentation Systems (SBAS) to GPS.

SBAS have some interesting properties that make them potential leading contributors to timekeeping even in their early stages of development.

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TimekeepingIntroduction

Timekeeping is concerned with Time Distribution and Time Transfer.

Time Distribution = Satellite and 1 User

User interested in getting time

Time Transfer = Satellite and 2 Users

Users interested in comparing clocks

Both Time Distribution and Time Transfer imply Time Synchronization and Time Coordination

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Time Synchronization

In Synchronization, users utilize information gained from their observations and from other users to physically set their clock to some reference time scale.

GOAL - Users’ clocks are set running identically to some reference clock.

Information to check the synchronized clock needs to be gathered at regular intervals.

Systems may run off from reference, but the degree of run off depends on precision and accuracy of updates and quality of clock.

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Time Coordination

In Coordination, users only note the difference between their local clock and some reference time scale.

Users then use this information to relate other measurements to the reference time scale.

Again, an information link must be maintained to verify and check the behavior of the users local reference clock with respect to the reference time scale.

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Time Distribution

How does clock on wall get its time?

Navigation system distributes its reference time scale (GPS Time) and, sometimes, its relation to other time scales (UTC).

Users can set their clocks to the navigation system reference time or simply note the difference.

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Time Transfer

User works with one or several other users.

Each users note the difference between their local clocks and the navigation reference time scale.

Users must make their observations at the same instant of time and use the same satellite in order to minimize common errors.

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SBAS and Timekeeping

WAAS or a Satellite Based Augmentation System (SBAS) can contribute in all timekeeping areas

SBAS still in its early stages of development

Will show current capabilities of an SBAS

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Advantages of SBAS in Timekeeping - I

Main advantage of an SBAS is that it remains at approximately same point in sky, i.e., at same elevation and azimuth. Can use high gain antenna, such as Feed Horn or Dish (see earlier paper).

2 users can simultaneously track phase of GEO for long periods of time. This allows detection of pathological behavior in timed systems in real time (see earlier paper).

Can make observations throughout the night when ionosphere is at a minimum of activity !

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Advantages of SBAS in Timekeeping - II

A world wide timed GNSS (network of SBAS) will dramatically change some traditional timing concepts.

Can do continuous “code - carrier phase” tracking.

Several different satellites that can be used.

Several different upload stations

Signals are under independent control from GPS

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Current GPS Data

From USNO Time Service web site, one can get recent GPS timing data for comparison with WAAS and EGNOS data.

Data is obtained with a single frequency timing receiver.

Klobuchar Ionospheric model is basis for data reduction.

All satellites are plotted on the graph.

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USNO MC - GPS Time(all satellites) JAN 2001

= 7.7 ns.

-30

-20

-10

0

10

20

30

40

0 1 2 3 4 5 6 7 8

Days

na

no

se

co

nd

s

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Time Transfer between 2 Labs

NRC USNO

WAAS

EGNOSUSNO - WAAS = XNRC - WAAS = AUSNO- NRC = X - A

USNO - EGNOS = YNRC - EGNOS = BUSNO- NRC = Y - B

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NRC - USNO (6/06/2000) using WAAS, midnight, 30

minutes, relative, s.e.= 1.3 ns

-10

-5

0

5

10

289.82 289.83 289.84 289.85 289.86 289.87 289.88 289.89 289.9 289.91

days

ns.

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NRC - USNO (6/06/2000) using EGNOS, midnight, 30

minutes, relative, s.e. = 2.1 ns.

-10

-5

0

5

10

289.82 289.83 289.84 289.85 289.86 289.87 289.88 289.89 289.9 289.91

days

ns.

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Time Transfer between 2 SBAS

NRC USNO

WAAS

EGNOSUSNO - WAAS = XUSNO - EGNOS = YWAAS - EGNOS = Y - X

NRC - WAAS = ANRC - EGNOS = BWAAS - EGNOS = B - A

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WAAS - EGNOS (06/06/2000)using USNO, 30 minutes,

s.e. = 1.2 ns.

-8

-6

-4

-2

0

2

4

6

8

290.042 290.043 290.044 290.045 290.046 290.047 290.048 290.049 290.050 290.051

days

ns.

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WAAS - EGNOS (06/06/2000)using NRC, 30 minutes,

s.e. = 1.0 ns.

-8

-6

-4

-2

0

2

4

6

8

290.039 290.040 290.041 290.042 290.043 290.044 290.045 290.046 290.047

days

ns.

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Discussion

Keep in mind that WAAS is semi-operational

EGNOS does not transmit continually

EGNOS transmissions are for testing different segments of the program

There are some problems with early available software that will be overcome with time.

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Conclusions

Even though the data presented here is selective, an SBAS appears to be a significant improvement over GPS.

Several different timekeeping labs could be continuously tracking several GEOs, thus allowing for better coordination of different national time scales. Data seems to indicate that sub-nanosecond time transfer should be achievable between laboratories on a continuous basis.

Several stations, with the proper choice antenna, can track several different GEOs. This would allow either synchronization or coordination of different SBAS. Data seem to indicate that nanosecond level coordination between SBAS is achievable.

Documents

10/7/2015 1 Innovative Solutions International Satellite Navigation Division ION NTM 01 Capabilities of the WAAS and EGNOS For Time Transfer SBAS, an Alternate