675
1 INTRODUCTION
ECDIS systems can integrate various types of
navigation information and display them on an
System Electronic Navigation Chart (SENC),
providing a platform for real-time navigational
decisions. In MSC Resolution 232(82), an amendment
regarding ECDIS performance standards released in
2006, it is emphasized that an ECDIS system must
provide an automatic Dead Reckoning
(DR)/Estimated Position (EP) facility for DR
navigation in the event of GPS failure, and to
crosscheck received positions by other means, such as
visual bearings (Radar, Line of Position [LOP],
celestial fix). Thus, celestial positioning has already
been listed as one of the assisted-positioning options
[1]. Generally, however, only positioning by plotting
LOP using the traditional Intercept Method (IM) is
provided while celestial positioning methods are not
yet implemented. GIS is an important base module for
visual display and processing of navigational
information in an ECDIS system, and eliminates some
of the restraints in previous positioning techniques
conducted on paper maps. Therefore, the positioning
approach proposed in this study was developed using
the ECDIS framework. The method was implemented
within a GIS module, and relies on fundamental
theories of celestial navigation (obtaining a fix by
plotting celestial circle of position - COP). Electronic
chart work is performed using the spatial data
processing and analysis capabilities found in GIS,
achieving outcomes that would be impossible with
paper chart work. The proposed method is simple,
fast, and accurate, and can avoid the cumbersome
work and inaccuracy of the traditional Intercept
Method (IM) or the complicated, and often obscured,
computation involved in numerical methods [2, 3, 4,
5, 6, 7]. The method can also provide a reference for
the development of next-generation electronic
Using GIS to Obtain Celestial Fix under the Framework
of an
ECDIS System
M.
C. Tsou
National Kaohsiung University of Science and Technology
, Cijin District, Kaohsiung City, Taiwan
ABSTRACT: This study proposes a simple method for obtaining a celestial fix, developed within a Geographic
Information System (GIS) under the framework of an ECDIS system. The underlying principle is dependent on
the most fundamental theory in celestial navigation; the circle of position (COP) of the celestial bodies is plotted
to find the fix. Through the spatial data processing, analysis, and visualization capabilities available in GIS, a
celestial fix may be obtained directly from plotting. This eliminates the limitations associated with finding the
fix manually using a paper map, but also avoids the cumbersome work and inaccuracy of the traditional
Intercept Method (IM) or the complicated, and often obscured, computation involved in numerical methods.
The proposed method is simple and accurate, and it applies to problems involving two or more celestial bodies
and high-altitude observations. It provides a reference for the development of a celestial positioning module in
an ECDIS system, and could also be integrated into an educational program on electronic celestial navigation.
http://www.transnav.eu
the
International Journal
on Marine Navigation
and Safety
of Sea Transportation
Volume 14
Number 3
September 2020
DOI: 10.12716/1001.14.03.
20
676
celestial navigation modules that improve the visual-
fixing functions in ECDIS systems.
2 POSITIONING BY PLOTTING COP
The principle of the COP fix is quite simple, and can
be carried out provided one can plot the COP directly
onto a paper chart. According to the relationship
between celestial and Earth coordinates, in which
they are each other’s projections, an observer’s COP is
the projection of the circle of zenith distance onto the
surface of the Earth. The center of the COP is the
Geographic Position (GP) of the celestial body. The
point of intersection closest to the estimated vessel
position is the observed vessel position (Figure 1).
Figure 1. The principle of COP positioning[4]
3 LIMITATIONS OF TRADITIONAL COP
POSITIONING
Although the principle of obtaining the position fix by
plotting COP is simple, it is difficult to use in practice
due to the following reasons:
1 The radius of COP is too large when plotted on a
typical paper chart.
2 As the measured latitude becomes further from the
equator, there is more distortion of the circle at
higher latitudes (Figure 2).
3 If a globe were used for direct positioning, the
diameter of the globe would need to be very large
to achieve the required accuracy. This is clearly
impractical.
In view of these limitations, the graphical
positioning method using COP can only be applied to
high-altitude observations for celestial bodies with an
altitude greater than 87°. However, such cases are
rare in practice, and therefore this case has very
limited value.
Figure 2. The projected curve of a COP, with the same
diameter, at various latitudes
4 REVISITING GRAPHICAL POSITIONING USING
COP
Digital storage and processing in ECDIS systems has
removed some limitations associated with paper
charts. When displaying vector data, for example, the
entire world or any part thereof may be displayed at
any scale provided adequate computer memory is
available. Maps covering different regions match
perfectly at the boundaries, and may therefore be
viewed as a single contiguous map with no limit on
map size. Moreover, on an ECDIS system, within
which GIS plays a key role, geodetic datum, and
projection can be changed to plot projected curve of
COP easily. For COP positioning, a general chart with
a small scale or an electronic plotting sheet covering
the whole world is enough. Upon completion, the
results can be overlaid on a chart with a larger scale
for display without compromising the accuracy.
5 CELESTIAL NAVIGATION FIX BASED ON GIS.
GIS has been widely applied in a variety of fields.
For example, Traffic Geographic Information Systems
(GIS-T) are an extension of GIS technology into the
realm of transportation, integrating GIS with various
kinds of traffic information analysis and processing
techniques. Marine Geographic Information Systems
(MGIS) are an important application of GIS in the
management of the marine environment and ocean
resource data. Both systems have their own areas of
expertise and technologies. Provided it can be
integrated into Marine Traffic GIS (MTGIS), the new
system can be applied in marine traffic and ocean
surveying. ECDIS can be viewed as such an example.
6 THE INTEGRATION OF GIS WITH THE
NAVIGATION INFORMATION SYSTEM
Figure 3 demonstrates the framework of a Navigation
Information System (NIS). The ECDIS system, as the
core of the NIS, manages spatial data (GIS) and
attribute data (Management Information System
MIS), and provides a platform for integration of
navigation data and decision-making. Most common
features, such as navigation route planning (e.g.,
checking safety contours and critical area) and
677
position monitoring (e.g., XTE alerts, waypoint alerts,
and anchor watch) are derived from the geo-
processing capabilities in MTGIS. It is also shown in
Figure 4 that only a very small portion of GIS
functionality is used in ECDIS at present. To develop
more intelligent NIS in the future, the capabilities of
MTGIS must be expanded.
Figure 3. The relationship between GIS and ECDIS in the
NIS framework [9]
7 IMPLEMENTATION OF GIS IN CELESTIAL
POSITIONING
GIS software ArcGIS 10.5 released by ESRI is
employed in this study; this software package
includes complete GIS functionality and has been
widely applied in geographical information related
research. There is no extra data required other than
the general astronomical observation data. The GIS
functions used are also basic functions that may be
found in any normal GIS software package.
7.1 Input Data
To plot COP of a celestial body in the ECDIS system,
the required input data are a map, the Greenwich
Hour Angle (GHA) and Declination (Dec), and the
observed altitude (Ho) of the body. A general chart
with a small scale or an electronic plotting sheet
covering the whole world may be used. The results
may subsequently be overlaid onto any electronic
navigation chart. Dec corresponds to the latitude of
the GP of the celestial body. GHA must be converted
to represent the longitude of GP. Ho is used to
calculate the radius of COP with the unit of nautical
miles. DR position is used to determine the vessel
position.
7.2 Main Functions in GIS and Operating Procedure
To make the execution automatic, the ModelBuilder
module in ArcGIS was used. The procedure is shown
in Figure 4. The main GIS functions involved in the
process are:
1 Defining the Geodetic Datum: The default geodetic
datum in ECDIS is WGS84, which assumes that the
earth is an ellipsoid with a semi-major axis of
6378137 meters and semi-minor axis of 6356752.3
meters. Both the celestial sphere and the Earth are
considered as perfect spheres in celestial
navigation. The coordinates between the two are
each other’s projections. In astronomical
positioning, COP plotted with respect to the
geodetic datum (WGS84) in an ECDIS system is
slightly different to the COP plotted by assuming
the earth as a perfect sphere. Therefore, the former
datum cannot be directly applied in celestial
positioning. To adjust the geodetic datum, the
average radius of earth (6371000 meters) is taken
as the geodetic datum of Earth as a perfect sphere.
All subsequent distance calculation is based on
this datum.
2 Buffer: In this study, it is proposed that the buffer
function may be used to construct the COP of the
celestial body. The circle is centered at its GP, and
radius is the co-altitude (90°Ho). In the GIS
environment, different distance units can be used
as radius of the buffer ring, and previously
arduous tasks (such as plotting COP on Mercator
paper chart in high latitude regions or for large
areas) may be easily achieved. In the construction
of COP, radius of the buffer ring must be set as the
co-altitude in nautical miles.
3 Intersections: Locate the intersection of spatial
entities; the intersections of the resultant COPs are
the possible vessel positions.
4 Spatial Query: As there may be more than one
point of intersection between COP, DR position is
needed for further determination. In this study, the
spatial query function is used to search for the
point of intersection near the DR position and to
determine vessel position. The query operator is
set to ‘within a distance’. The radius for the search
is user-defined; however, 60 NM is generally
sufficient. In a two-body fix, a single point is
obtained from this operation, and this point is the
celestial fix of the vessel; in case of a multi-body
fix, a further processing step is required .
5 Mean Center: In a multi-body fix, a group of
intersection points in the proximity of the DR
vessel position is obtained via a spatial query from
multiple COP intersections. The mean center
function calculates the center of this group; this is
the most probable position (refer to Figure 5). This
operation is equivalent to finding vessel position
from the cocked hat region between the points of
intersection.
Figure 4. The procedure used to obtain a multi-body
celestial fix in GIS