Transcript Radio Navigation: Perspectives and Challenges

Fast TOC-less UTC Service
through Loran-C/Eurofix
Prof. dr. Durk van Willigen, Arthur Helwig,
Wouter Pelgrum & Gerard Offermans
Reelektronika b.v. / GAUSS Research Foundation
ILA 31 – Washington DC, October 27-30, 2002
1
Time: Which One?
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Time driven by atomic references:
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Time driven by rotation rate of the earth:
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BIPM (International Time Bureau)
TAI (International Atomic Time)
Loran = TAI – 10 seconds (fixed)
GPS = TAI – 19 seconds (fixed)
UTC, basically what the British want to see on their watch
Earth-driven UTC has a variable number of integer Leap
Seconds offset to atomic-driven TAI, Loran or GPS
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UTC = TAI – LS (Leap Seconds)
DTAI = TAI – UTC (Recommendation ITU-R TF.460-6)
On October 28th, 2002: DTAI = 32 seconds
Durk van Willigen et al – Reelektronika / GRF
2
How to Bring Time to Users?
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WWV, DCF77, HBG, …
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Worldwide, 1-ms class accuracy (sky waves), very
low-cost, robust
Sends UTC
GPS
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Worldwide, sub 1-µs class accuracy, low cost,
vulnerable
Sends SV-time
Receiver computes GPS time which can be
corrected into UTC
Durk van Willigen et al – Reelektronika / GRF
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How to Bring Time to Users?
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Loran-C
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Not worldwide, sub 1-µs class accuracy, medium
cost, robust
Sends no time info
User resolves Loran time from
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A priori known date/time and position
Computed TOC time
Phase comparison with Loran-C pulse
Maximum allowable a priori time error depends on
sophistication of receiver
Durk van Willigen et al – Reelektronika / GRF
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Time Transfer in 2 Steps
1. Synchronize user clock oscillator to source
clock

Time offset between user and source is stable
but is not equal to zero
2. Steer time offset between user and source
to zero, either by:
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Varying user clock frequency
Introducing time steps in PPS output
Do both (course and fine steering) to get fastest
acquisition of time
Durk van Willigen et al – Reelektronika / GRF
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How to Express Quality of Clocks?
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Frequency makes Time:
Pulse per second (PPS) error expressed in
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MTIE – Maximum Time Interval Error
TDEV – Deviation of Time Interval Error
MTIE & TDEV specified by
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ITU-T => G.811 specs
ANSI => Stratum-1 specs
Durk van Willigen et al – Reelektronika / GRF
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Frequency Requirements versus
Oscillator Specs
Basic diagram
downloaded from the
Internet
Caution: Clock-Loran
Transition Area
“Telecom
Synchronization with
GPS”
Hugo Frueauf, Zyfer Inc.
April 2002
[email protected]
Statum-1 is most
demanding spec in
telecommunication
Durk van Willigen et al – Reelektronika / GRF
Loran-C upper error
limit for DSP receivers
and accurate ASF
7
GPS Time Is Excellent But…
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Time is essential to keep modern society
going
GPS is attractive, accurate and low-cost
precise timing source worldwide
GPS is not very robust to interference
If GPS fails then three possible backups:
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Cesium clock: frequency only, unless set to UTC
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Rubidium clock: frequency only, unless set to UTC
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Excellent stability/month, 50 k$ class
Excellent stability/day; 1-2 k$ class
Loran-C: frequency and time
Durk van Willigen et al – Reelektronika / GRF
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TOC Concept

Start of 1st pulse of PCI for all Loran-C chains
coincided with the PPS (TAI) time tick on 1 January
1958 at 00:00:00 hrs
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PPS = Pulse Per Second
TOC = Time of Coincidence of start of Loran-C PCI
and a 1 PPS time tick
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TOC repeats every 500 … 1,000 seconds in the US,
Russia and Asia
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TOC repeats every 5,000 – 10,000 seconds in Europe !!
Durk van Willigen et al – Reelektronika / GRF
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TOC Difficulties

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Large Time to First Fix of UTC time
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USA, Russia and Asia: GRI/10 s (8 – 16 min)
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NELS: GRI/100 s (1.4 – 2.8 hr)
Time and Day at first fix must be known better than
within one GRI (40-100 ms) for single chain receivers
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Multi-chain tracking reduces TTFF and a-priori time
requirement through wide-laning significantly
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Does not provide Year and Day of year
Durk van Willigen et al – Reelektronika / GRF
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Eurofix Offers Simple Solution

Broadcast concept: At the next beep time is …
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Eurofix: At the end of the Eurofix type 4 message date
and time at the start of next Master pulse is …
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Maximum update rate of Day/Time can be up to once per
30 GRI, i.e. 1.5 to 3 seconds
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If user position and ASF accurately known, and highquality receivers used, time error relative to Loran-C
station time can meet or exceed Stratum-1 specifications
Durk van Willigen et al – Reelektronika / GRF
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TOC-less UTC Concept
Eurofix Type 4.1 Message of 30 GRI’s says
Next pulse starts at [Year, Day, Time]
1
2
3
1 GRI
30
1
2
Next Eurofix Message of 30 GRI’s
Master and Secondary stations broadcast Type 4.1 Eurofix Messages containing
nominal Year, Date and Time at which next pulse of Master station starts
Eurofix Message Type 4.2 contains Leap Seconds and Precise Time Offsets from
nominal transmission time values. It does not broadcast hours, days and years
User must add ED to Master station value to find Year, Date and Time at which next
pulse of Secondary station starts
Durk van Willigen et al – Reelektronika / GRF
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Pulse Start Times
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The start of all emitted Master pulses theoretically coincides with 10 µs
time ticks in Europe, and with 100 µs time ticks elsewhere
So worldwide, start times of master pulse transmission can uniquely be
expressed in 10 µs units
Nominal Emission Delays of secondary stations do normally not match this
10 µs time tick frame outside Europe
Outside Europe user can find start time of secondary stations by adding
nominal ED to Master start time
If ED is SAM controlled, then SAM correction values must also be provided
to user. This transfers SAM into TOE control if user has access to SAM
correction data (RTCM PAPER 9-2001/SC104-246 by Bill Roland)
In Europe all secondary stations are TOE controlled, and ED’s are
multiples of 100 µs. User needs no additional ED information
Durk van Willigen et al – Reelektronika / GRF
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Type 4.1 Message Format
Bit
Item
Unit
Range
1-4
Message Type
1
16
5-6
Message sub-type
1
4
7-36
Time at Master
10 µs
3,600 s
37-50
Hour & date of year
1 hour
8,760 hours
50-55
Year
1
64 years
56
Spare
Durk van Willigen et al – Reelektronika / GRF
(1 hour)
(1 year)
(2000-2063)
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Type 4.2 Message Format
Bit
Item
Unit
Range
1-4
5-6
Message Type
Message sub-type
1
1
7-36
Time at Master
10 µs
16
4
3,600 s
37-44
Leap Seconds
1s
256 s
10 ns
10 µs
(DTAI = TAI - UTC)
45-54
Precise Time
55-56
Spare
Durk van Willigen et al – Reelektronika / GRF
(1 hour)
15
Time Error Budget
Total error = transmitter + propagation + receiver
 Pr ovider  Clk 2  TX 2  ED 2
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AS F  Diu rn al

≈100 ns ??
≈100-150 ns ??
Megapulse, NELS, USCG
Prof. Last,
USCG, NELS
 Receiver  Skywave2  CRI 2  CWI 2  Noise2  Clk 2
≈1-10 ns
Locus, Reelektronika, Detectis, …
Durk van Willigen et al – Reelektronika / GRF
16
TD-Observations of Sylt
TD of 7499M - 6731Z
0.02
Sylt-Reeuwijk = 388 km
0.01
TD [us]
0
-0.01
-0.02
-0.03
-0.04
10 m Integration Time
30 m Integration Time
-0.05
0
Durk van Willigen et al – Reelektronika / GRF
5
10
time [hours]
15
20
17
Dual-rate Sylt TIE
Time Interval Error over 100 sec
400
Time Interval
200
100 sec
0
-200
0
5
10
15
Time Interval Error over 1000 sec
20
25
0
5
10
15
Time Interval Error over 10000 sec
20
25
0
5
20
25
200
100
1,000 sec
0
-100
-200
400
10,000 sec
[ns]
200
0
-200
10
15
time [hours]
Durk van Willigen et al – Reelektronika / GRF
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TDEV of dual-rate Sylt TIE
Durk van Willigen et al – Reelektronika / GRF
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Time Steps at Sylt
TD of 7499M - 6731Z
0.02
Sylt-Reeuwijk = 388 km
0.01
TD [us]
0
-0.01
-0.02
-0.03
10 m Integration Time
30 m Integration Time
Transmitter Time Steps
-0.04
-0.05
0
Durk van Willigen et al – Reelektronika / GRF
5
10
time [hours]
15
20
20
Discrete TX Time Control Optimal?
TD of 7499M
- 6731Z,- 6731Z
compensated
TD of 7499M
TD
0.028
0.02
24-Hrs X-rate TD in
Reeuwijk at 388 km
from Sylt
0.016
0.01
4
00
2
-0.01
-0.01
TD [us]
TD [ns]
Single-station dualrate offers canceling
of ASF, diurnal, and
receiver clock
effects
1800 s integration time
0
-0.02
-0.02
-2
-0.03
-0.03
-4
Reelektronika
LORADD Receiver
-8
-0.05
-0.05
00
Durk van Willigen et al – Reelektronika / GRF
10 m Integration Time
3010mmIntegration
IntegrationTime
Time
Transmitter
Time
Steps
30 m Integration Time
-0.04
-0.04
-6
55
10
time [hours]
15
20
21
TDEV LPA-less Dual-rate Sylt TIE
Time Interval Error over 100 sec
50
Time Interval
0
100 sec
-50
-100
0
5
10
15
Time Interval Error over 1000 sec
20
25
0
5
10
15
Time Interval Error over 10000 sec
20
25
0
5
20
25
100
50
1,000 sec
0
-50
100
10,000 sec
[ns]
50
0
-50
-100
10
15
time [hours]
Durk van Willigen et al – Reelektronika / GRF
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LPA-less Dual-rate Sylt TIE
Durk van Willigen et al – Reelektronika / GRF
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Emission Time Accuracy?
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Europe (NELS)
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TOE of all stations better than 100 ns (1) relative to UTC
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ED better than 30 ns (1)
Outside Europe (USA, Russia, Asia, …)
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Master stations better than < 100 ns relative to UTC (1)
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Secondary stations better than < 140 ns relative to UTC (1)
Durk van Willigen et al – Reelektronika / GRF
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Loran-C/Chayka Station Tasks
1. Extract date and time from Loran time
2. Compute date/time at start of first pulse of
Master station after end of next Eurofix
type 4 message to be broadcast
3. Generate content of next Eurofix type 4
message to be broadcast by Master or
Secondary station
Durk van Willigen et al – Reelektronika / GRF
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Low-End Timing Receiver Issues
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Single-chip receiver design
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Needs only one single Loran-C station
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Environment dictates selection of E-field or H-field
antenna
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Simple Xtal clock performs coasting
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Propagation time not anticipated
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Accuracy better than 1 GRI (<100 ms)
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Competition from other time sources
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Europe: DCF77 clocks (Mainflingen / 77.5 kHz)
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Elsewhere: ?
Durk van Willigen et al – Reelektronika / GRF
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High-End Timing Receiver Issues

Control of signal-delay from antenna to analog-to-digital converter
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Skywave, CW and X-rate interference
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Re-radiation errors
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ASF model errors and unknown diurnal effects
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SNR of received Loran-C/Chayka signal
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Determines clock-to-Loran-C transition region
Tracking-loop bandwidth optimization
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Determines clock-to-Loran-C transition region
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Robust Stratum-1 performance with Rubidium or OCXO clock
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Just about 5 % of Cesium clock price (≈50 k$)
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GPS is the only true competitor in this class of accuracy
Durk van Willigen et al – Reelektronika / GRF
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Conclusions

NELS approved proposal to implement TOC-less UTC
message transmissions on the four Eurofix stations at Bø,
Værlandet, Sylt and Lessay
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This service will offer robust and precise time (1 µs class)
in Europe which can easily be expanded to the USA, Asia
and Russia (Chayka)
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Message type conform Recommendation ITU-R M.589-3
and RTCM SC-104, version 2.3

Open signal structure is business invitation to
industry
Durk van Willigen et al – Reelektronika / GRF
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