ASF Measurement and Processing Techniques, to allow Harbour Navigation at High Accuracy with ...

79 Pages·2010·1.56 MB·English

by  Trinity House

ASF Measurement and Processing Techniques, to
allow Harbour Navigation at High Accuracy with
eLoran.
Chris Hargreaves
Dissertation submitted to The University of Nottingham in partial fulfilment for the
degree of Master of Science in Navigation Technology.
September 2010

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Abstract
To navigate a ship along an approach channel to enter a harbour a mariner
must be able to fix their position to a high degree of accuracy. The International
Maritime Organisation (IMO) have set out a list of requirements, which must be
met by any electronic positioning system during the various voyage phases
including port approach.
The ability of Global Navigation Satellite Systems (GNSS) to provide high
accuracy with high reliability is well known, and as such the Global Positioning
System (GPS) has become ubiquitous as the system of choice for position
fixing, in the maritime world and beyond. The General Lighthouse Authorities of
the UK and Ireland have expressed their concern that an over reliance on any
one system can leave critical infrastructure vulnerable if this system’s service is
disrupted or denied for any reason. As such they have argued for the
establishment of Enhanced Long Range Navigation (e-Loran) to be used in
parallel with, and as a backup to, GNSS for Position Navigation and Timing
(PNT).
This report describes the efforts of the GLAs to measure and process Loran
Additional Secondary Factors (ASF) in order to obtain sub-10m (95%) accuracy
from eLoran. Software applications have been written in MatLab™ to aid the
gathering and processing of eLoran data, and the current state-of-the-art in ASF
measurement and processing is described.
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Contents

1 Introduction and Overview ....................................................................................... 5
1.1 The GLAs and the Case for eLoran ................................................................ 5
1.2 Outline of MSC ............................................................................................... 5
2 History and Background .......................................................................................... 6
2.1 A Brief History of Loran ................................................................................... 6
2.2 Loran-C ........................................................................................................... 6
2.3 The Northwest European Loran System (NELS) ............................................ 9
2.4 Towards eLoran .............................................................................................. 9
2.5 International Maritime Organisation (IMO) .................................................... 11
2.6 Realisation of eLoran .................................................................................... 12
3 Additional Secondary Factors (ASF) ..................................................................... 15
3.1 Theory ........................................................................................................... 15
3.2 Short History of ASF ..................................................................................... 17
3.2.1 Pre-NELS .................................................................................................. 17
3.2.2 ASF for NELS ........................................................................................... 18
3.2.3 ASF for the FAA eLoran Evaluation .......................................................... 19
3.3 ASF Today .................................................................................................... 20
3.4 ASF Measurement ........................................................................................ 21
3.5 ASF Measurement Equipment ...................................................................... 23
3.6 ASF Measurement Errors ............................................................................. 27
3.7 Summary ...................................................................................................... 30
4 ASF Processing ..................................................................................................... 32
4.1 ASF Measurement Campaigns ..................................................................... 32
4.1.1 Equipment ................................................................................................. 32
4.1.2 Sea Trials .................................................................................................. 33
4.1.3 ASF-Unit Calibration at Lowestoft ............................................................. 33
4.1.4 eLoran Assessment in the Orkney Islands ............................................... 34
4.1.5 GPS Jamming Trial in Newcastle ............................................................. 36
4.2 ASF Error Mitigation and Minimisation ......................................................... 37
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4.2.1 Local Clock De-Synchronisation ............................................................... 37
4.2.2 Removal of Temporal Variations .............................................................. 39
4.2.3 Setting up a Differential-Loran Reference Station .................................... 41
4.2.4 Processing Differential-Corrections .......................................................... 42
4.2.5 Other Error Sources and Conclusion ........................................................ 43
4.3 Producing an ASF Grid ................................................................................. 44
4.3.1 The Basic ASF Grid .................................................................................. 45
4.3.2 The Interpolated ASF Grid ........................................................................ 48
4.3.3 Error Statistics .......................................................................................... 56
4.3.4 Standard Deviation of Residuals .............................................................. 59
4.3.5 Summary of ASF Processing .................................................................... 60
4.4 GLA eLoran Position-Solution Algorithm ...................................................... 61
5 GLA eLoran Survey Software ................................................................................ 65
5.1 Survey Mode ................................................................................................. 65
5.2 Validation Mode ............................................................................................ 69
6 Summary and Conclusions .................................................................................... 75
6.1 Summary ...................................................................................................... 75
6.2 Conclusions - ASF Measurement Technique ............................................... 75
6.3 Conclusions - ASF Gridding Technique ........................................................ 76
References ................................................................................................................... 78

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1 Introduction and Overview
1.1 The GLAs and the Case for eLoran
The General Lighthouse Authorities of the UK and Ireland (the GLAs) have
argued the case that reliance on GNSS as a sole means of Position, Navigation
and Timing (PNT) can leave the UK’s critical infrastructure vulnerable. The
GLAs have demonstrated the effects of GPS jamming [1] on a typical modern
ship with an integrated bridge system, concentrating on the effect of GPS denial
on navigation. To mitigate the risk of GPS disruption, the GLAs have argued
that an alternative, dissimilar source of accurate PNT should be established as
a national backup to GNSS. eLoran is seen as the best available system which
can potentially meet all the requirements of the navigation and timing user
communities, and as such forms a core part of the GLAs’ strategy [2].
In light of recent developments in the USA, the GLAs have not altered their
stance on eLoran. It is, by design, a regional system, and can be used to
augment GNSS in much the same way as all regional augmentations such as
marine DGPS or EGNOS.

1.2 Outline of MSC
The work presented here comprises the efforts made by the candidate in
assisting the delivery of high-accuracy eLoran suitable for use by the mariner to
navigate the coasts and harbour approaches of the British Isles. The provision
of eLoran at 10m (95%) accuracy requires the accurate measurement of the
Loran signal propagation delay Additional Secondary Factors (ASF), and
appropriate mitigation at the user’s receiver. The candidate has furthered the
development of the data acquisition and processing techniques required to
measure, assess and distribute quality-assured ASF data. In addition, custom
software applications have been written to oversee the survey of ASF along a
harbour approach, and validate the gathered data. This software’s operation is
demonstrated with an example of an ASF survey conducted around the Orkney
Islands in 2009.

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2 History and Background
2.1 A Brief History of Loran
Loran stands for Long Range Navigation and has existed in several forms over
the years. These incarnations are similar in that they have all been high-power;
terrestrial; pulsed; radio-navigation systems.
The first system, Loran-A was developed during the Second World War as a
navigation system for the US military. Loran-A operated at 1.95 MHz over a 400
mile range and provided very poor positioning accuracy. Improvements to the
signal-specification increased the accuracy of Loran-A, the improved signal was
broadcast under the name of Loran-B.
Developments in low-frequency radio-navigation led to the development of
Loran-C, which began broadcasting in 1958, and eventually took over from
Loran-A and B.

2.2 Loran-C
Loran-C is a terrestrial radio-navigation system, which consists of a number of
high-power radio transmitters operating at a centre frequency of 100kHz. These
transmitters are organised into Chains, with a single Master station and several
Secondary stations in each Chain. The Master station transmits groups of nine,
precisely timed and shaped pulses. Figure 2-1 illustrates the signal format.

Figure 2-1 – The Loran pulse shape
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The leading edge of the pulse is precisely defined (1.1). The shape of the pulse
envelope is used to identify the Standard Zero Crossing (SZC), this is the point
of the signal that is tracked by a Loran receiver and used to make Time-of-
Arrival measurements.
2(t-t)
i(t)= A(t-t)2exp sin(0.2pt) (1.1)
 65 

Each Master (M) pulse group transmission is followed in sequential order by a
group of eight pulses from each of the Secondary stations in the same chain.
The delay between Master and Secondary transmissions is termed the
Emission Delay (ED), and is precisely controlled by the transmitter. Once all of
the Secondary stations have transmitted, the cycle begins again with the next
Master station transmission. The time between successive Master station group
transmissions is referred to as the Group Repetition Interval (GRI), this is shown
in Figure 2-2.

Figure 2-2 – Example of a Loran chain with three Secondary stations (X, Y
and Z)

Each Chain is named after the Master station and is identified by its GRI,
quoted as a multiple of 10μs. For example, the transmitter at Lessay in Northern
France has a repetition interval of 67310μs, and is the Master of the GRI 6731.
Secondary stations are identified by the letters W; X; Y; Z, depending on the
order they transmit. The GLAs operate a station at Anthorn in Cumbria, which is
the third station to transmit in the Lessay chain, and is given the designation
Anthorn 6731Y.
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With the Loran-C system, a user obtains their position by measuring time
differences (TD) between the reception of a Master (identified by its ninth pulse)
and a Secondary. Each TD measurement places the user on a hyperbolic line
of position, and at least two such time-difference measurements are needed to
obtain a fix. Originally these hyperbolic TD lines were overlaid onto navigation
charts and plotted by hand. An experienced navigator would expect to be able
to fix their position with an absolute accuracy of ¼ of a nautical mile with Loran-
C using these hyperbolic charts.

Figure 2-3 – Hyperbolic lines of position (LOP) for an example Master-
Secondary pair. Image reproduced from [3]

In a hyperbolic system it is not necessary to precisely control the timing of each
Master transmission, rather it is the stability of the Emission Delay (ED)
between the Master and Secondary transmissions that is important. Timing of
the Secondary transmissions was originally controlled by reception of the
Master’s transmission at the Secondary, with the Secondary station transmitting
a certain time following Master signal reception.
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In recent years development work has been carried out, particularly in Europe
and the USA, to improve the Loran-C system, in an attempt to bring it into line
with modern navigational requirements.
2.3 The Northwest European Loran System (NELS)
The Northwest European Loran System (NELS) Consortium was established in
1994 and consisted of several National Operating Agencies (NOAs), Norway;
France; Denmark; Germany; Ireland. Norway was also the host of the
Coordinating Agency Office (CAO) – the organisation that was in overall charge
of NELS.
NELS began the process of upgrading the existing Loran-C infrastructure to
improve its performance. In particular the work included pioneering Time-of-
Emission (TOE) control, whereby the transmission of the Loran signal at each
site is maintained relative to UTC. This means all Loran signals can be related
to a common time-reference, or ‘paper clock’. This improvement allows a user
to compare pseudo-range measurements from all available Loran transmitters
(the all-in-view concept) irrespective of the chain’s GRI or the Master /
Secondary relationships. A position fix can then be derived by using Least
Squares, or a similar technique. This all-in-view method overcomes many of the
limitations of TD hyperbolic positioning, and also enables Loran to be an
independent source of UTC time.
NELS also reassigned GRIs to improve Loran performance in the noisy
European radio environment, developed coverage prediction techniques and
produced maps of the signal propagation delays, the latter are discussed in
Chapter 3

2.4 Towards eLoran
On 10th September 2001 the Volpe Transportation Centre in the USA published
a report [4] into GPS vulnerability and the reliance on GPS of US critical
infrastructure. The timing and prescience of such a report into the potential
impact of an attack on the country’s critical infrastructure led to increased
concern about the vulnerability of GPS and the need for a national backup
system. A study was carried out to investigate whether Loran-C could
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potentially act as a backup and complementary Position Navigation and Timing
(PNT) system for maritime, aviation and timing [5].
This study involved a large number of organisations including academia,
consultancies and the US Coast Guard (USCG), under the sponsorship of the
Federal Aviation Administration (FAA). This study was divided along two lines.
The Loran Integrity Performance Panel (LORIPP) was concerned with
identifying whether the system could meet the stringent integrity requirement for
the duration of an aircraft Non-Precision Approach (NPA). The Loran Accuracy
Performance Panel (LORAPP) was not so much interested in integrity but
whether the system could meet the accuracy requirement of 8-20m (95%) for
the Harbour Entrance and Approach (HEA) phase of a voyage. These two sets
of requirements are outlined in Table 1.

Navigation Accuracy Availability Continuity Integrity
Phase (95%) Risk
Definition of ¼M 99.7% 99.7% 10-5
Loran Capability (460m)
(US FRP)
FAA NPA 300m 99.9% to 99.9% to 99.99% 10-7
Requirements 99.99% (over 150 s)
USCG HEA 8-20m 99.7% to 99.85% to 99.97% 10-5
Requirements 99.9% (over 3 hours)
Table 1 – US performance requirements for eLoran, the most stringent
requirements are shown in bold.

The results of this study indicated that if Loran-C was significantly improved it
could meet the requirements of both the FAA and USCG. The improvements,
illustrated in Table 2, meant sweeping changes to Loran-C of such magnitude
that it was appropriate to rename the system enhanced-Loran, or e-Loran.

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