Sea navigation control process

A system for sea navigation which features a plurality of ships located in a single high density area (for example port) and a control center for this area using a common transmission channel. The ships are equipped to transmit data about their speed, heading and position, which are displayed on a panoramic screen fitted on all ships and in the control center. The control center has priority access to this common channel to send general interest messages or special messages to all or some of the equipped ships.

BACKGROUND OF THE INVENTION 
1) Field of Invention 
The present invention concerns a method of control over sea navigation. 
2) Description of Prior Act 
Despite all detection and control methods available at the present time, 
the safety of sea traffic in high traffic density zones, particularly 
coastal and port zones, is not always guaranteed. 
In order to guarantee safety, it is important to set up a secure 
communication system firstly between ships located in the same zone, and 
secondly between these ships and a maritime traffic control center 
monitoring this zone. 
At the present time there are various methods of communication between 
vehicles and/or for pinpointing vehicles. 
For land traffic, radiotelephone communications are used with coded 
destination addressing, but this type of system does not enable the user 
to pinpoint his correspondent. 
More complexed equipment (such as GEOSTAR/LOCSTAR) can be used for 
communications and for pinpointing; a central station communicates with a 
mobile station through two circuits each passing through a satellite. The 
forward-return time necessary for exchanging communications can be used to 
determine distances from the mobile to the two satellites, and therefore 
to pinpoint it. This system combines addressing and pinpointing but uses 
only satellite communications and demands long range communication links. 
For air traffic, secondary radar, particularly in S mode, can be used to 
communicate with an aircraft and simultaneously gives the position and 
identity. Like the GEOSTAR/LOCSTAR system, this radar combines addressing 
and pinpointing. Radar is badly adapted to the acquisition of addresses in 
a dense medium and requires complicated traffic management. 
The ADS (Automatic Dependence Surveillance) concept was introduced more 
recently, and can determine the position and identity of an aircraft 
(which itself transmits the necessary information) and communicate with 
it. This system is efficient, but it does not enable participants to 
communicate between themselves without passing through the control center. 
Concerning ships, the only communication method used worldwide at the 
present time is radiocommunication, generally in VHF, without addressing. 
Communications are set up on a predefined frequency, or channel, for each 
geographic area. These methods do not enable the user to correspond with a 
specific correspondent. 
Satellite communications (such as INMARSAT) are starting to be used, and 
their coverage is almost worldwide due to the number and position of 
usable satellites. Links are set up by sending a destination address code. 
French patents 2 601 168 and 2 661 536 use a system enabling ships to 
mutually locate each other, particularly in a dense environment (port 
zone, . . . ) and to safely and unambiguously communicate between 
themselves using addresses in order to prevent collisions. However, these 
systems do not enable a control center, for example a port control center, 
to start a communication with one or several specific ships, or with all 
ships located in its surveillance area. 
SUMMARY OF THE INVENTION 
An object of present invention is to provide a sea navigation control 
system for reducing risks of collision while allowing a control center to 
supervise all traffic within its surveillance area and thus to further 
reduce risks of collision. 
The control method according to the invention, by which each ship using it 
repetitively transmits messages containing data about its absolute 
geographic position, its heading and its speed to all ships concerned on a 
common channel, together with an arbitrary identification code acting as 
an address for message exchanges, and receives similar information from 
surrounding ships that it displays by symbols on a panoramic type screen, 
wherein the control is supervised by a control center equipped with 
communication resources using the common channel, which displays all 
information that it receives from all ships concerned within its 
surveillance zone on a screen, with all obstacles located in it, and has 
priority access to this common channel to address all or some of the ships 
concerned. According to one feature of the invention, in order that the 
common channel can be used optimally, the period at which messages are 
transmitted by ships is related to the dynamics of the ambient situation 
and, for each ship, is a function of at least one of the following 
criteria: its own speed, the speed of its neighbors, the distance from its 
neighbors, its own and its neighbors maneuverability, the time to be 
covered by this ship before reaching its closest point to a neighboring 
ship. 
Further objects and advantages of the present invention will be apparent 
from the following description of a preferred embodiment as illustrated in 
the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Each ship participating in the anti-collision system in the invention is 
equipped with a device such as that shown schematically on N on FIG. 1, 
and will be referred to throughout the following description as the 
"equipped ship". 
Device N shown in FIG. 1 includes a transmitter 1 discontinuously 
transmitting messages at a very low average load factor (defined as a 
ratio between the transmission time and non-transmission time), of the 
order of 10.sup.-4 to 10.sup.-5. The transmission power and frequency are 
chosen so as to limit the transmitter range to 1 to a few tens of 
kilometers. The limitation may be due to the curvature of the earth if a 
transmission frequency is chosen for which propagation is done by direct 
line of sight, for example a frequency in the UHF band (several tens of 
MHz) or higher, but without exceeding the X band so that propagation 
remains practically insensitive to meteorological conditions. The device 
comprising elements 1, 4, 5 and 6 may also be a VHF transmitter-receiver 
ordinarily used on ships, together with an appropriate modem. However in 
this case the load factor will be a little higher due to the low 
throughput of available modems. The transmitter frequency F.sub.o is the 
same for all transmitters and receivers in the system. 
Transmitter 1 is connected through a switch 2 to an antenna 3 designed for 
omnidirectional transmission in the horizontal plane. 
Transmitter 1 is also connected to a modulator 4. This modulator 4 
generates a binary "word" containing all information to be transmitted and 
transposes it into a signal modulating the transmitter 1. The modulation 
shape is of the pulse type so as to enable the total lack of transmission 
outside the time during which the message is transmitted. However the 
invention method does not impose the type of information modulation; each 
binary element may be coded using any known coding technique, for example 
such as pulse position coding, or coding by phase shift (PSK). 
The transmitted message contains the following information: 
--the ship's coordinates, preferably in latitude and longitude, for example 
each coded on twenty two binary elements. These coordinates are provided 
by the ship's radio navigation system. Ships are generally equipped with 
radionavigation equipment permanently, precisely and reliably giving their 
absolute geographic position. The precision required by the anti-collision 
process in the invention is of the order of 100 meters. For example, the 
radionavigation system known under the "GPS" name satisfies these 
conditions: 
--the ship's speed and heading, this information is generally available on 
all ships, at least in analogue form, which simply has to be converted 
into digital form. This information may be coded with sufficient precision 
by 6 and 8 binary elements respectively; 
--possibly (if the standard requires it), the heading change, coded on 2 
binary elements; turn to port or turn to starboard. This information may 
be provided automatically by any known rotation direction readout device, 
activated at the start of the maneuver. The standard may also contain 
enriched anticipated information containing more than the heading change 
alone, particularly the value of the future heading but this would require 
that it be manually input (by keyboard), and there are risks that the 
operator would forget to input it. 
--A call sign or identification code is described further in more detail 
below. 
It is beneficial if this information is preceded, using a conventional 
technique in message transmission, by a preamble for initializing some 
receiver circuits. It is also beneficial if this information is 
complemented by binary elements forming an end of message symbol, and if 
it is considered that permanent repetition of messages is not sufficient 
to eliminate all errors, binary error correction elements may be added 
(for example binary parity elements). 
As mentioned above, if the ship is equipped with a GPS type radionavigation 
receiver, this receiver could supply most information mentioned above with 
a precision very much better than that necessary for the system in the 
invention. In this case, for each item of information we could neglect the 
superfluous lowest order binary elements and keep only those which are 
considered to be significant and provide the necessary and sufficient 
precision to implement the method in the invention as described above. 
Thus the length of the transmitted message is about at least 100 binary 
elements. If the passband assigned to the system is of the order of 
several megahertz, the message will be transmitted in several tens of 
microseconds. 
If each equipped ship transmits such a message at a period of about one 
second, the traffic load induced on the system by a ship is between 
10.sup.-4 and 10.sup.-5. For example, if a hundred ships are 
simultaneously present in a single geographic area (such as a port), the 
traffic load on the system is only 10.sup.-2 to 10.sup.-3 which guarantees 
a good probability that messages will not be mutually scrambled. Note also 
that this is a relatively severe case, since orders of magnitudes of 
maneuvering time for ships to avoid collision are measured in minutes, and 
the message repetition period could be significantly increased thus 
reducing the probability of mutual scrambling. In particular, these 
comments make it possible to consider the use of a standard VHF channel 
and its associated modem as a communication channel. Separations between 
channels are usually 25 or 50 KHz, which severely reduces the transmission 
speed compared with the rates mentioned above. The duration of a message 
will then be measured in tens of milliseconds. About 100 ships 
simultaneously present would result in a 10% load if the average period 
was 10 seconds, or 1.7% if the period was increased to 1 minute. These 
values remain acceptable to give a satisfactory probability of 
non-collision between messages, particularly when taking account of the 
adaptations proposed below. 
It will be beneficial to randomize the time at which each message is 
transmitted, such that mutual scrambling remains possible, due to the fact 
that transmissions from the various ships are not synchronized. Thus for 
the first example mentioned above for a repetition period of one second, 
this value would only be a mean statistical value and the actual value 
would have a wide dispersion. The result is that any scrambled message 
received from a given ship would not be scrambled repeatedly. Also, the 
high redundancy of messages sent (for a period of about 1 second, any one 
message will be repeated several times before a significant change in the 
heading and/or speed and/or geographic position) will mean that the 
received scrambled message can be ignored. 
In order to better adapt the device in the invention to zones with high 
ship density and/or to use less efficient standard equipment (VHF 
transmitter-receiver with modem as described above), the number of 
messages transmitted by some participants may be reduced. This can be done 
by relating the transmission period of their messages to the dynamics of 
the surrounding situation. Thus a slow and/or isolated ship could transmit 
less frequently than the average, whereas a ship that is moving faster 
and/or is close to other ships would transmit more frequently. Parameters 
determining the message transmission repetition frequency are, for each 
ship, its speed, the speed of its neighbors, their distance from the ship 
in question, and possibly the maneuverability of these ships. These 
parameters may be combined into a single parameter which is the 
theoretical time that the ship in question would take to reach the closest 
point to its neighboring ship if it does not change its heading or its 
speed. The period at which messages are transmitted may thus vary as a 
function of these parameters from a few seconds to a few minutes. 
Message transmission instants are preferably at random, in order to avoid 
the use of a complex call management system. Since the throughput through 
the single common channel in the system is limited, the invention 
optimizes its use by means of a "slotted Aloha" type technique, and for 
example uses synchronization signals obtained from signals captured by GPS 
receivers and transmitted by a pinpointing satellite, in order to 
synchronize the entire network. Since recovery of synchronization signals 
is a technique well known on land, it will not be described here. Starting 
from these synchronization signals, time is broken down into equal basic 
periods with a duration slightly longer than the message duration, 
possibly increased by the propagation time for the maximum system range. 
Each participant places each of its messages within a period selected at 
random. This thus reduces the risk of collision between messages 
transmitted by different ships; either they are located in different 
periods, or they fully overlap. In the latter case they will be 
incomprehensible and will be ignored, and since the next repetition of 
each will be also at random, the probability of a new overlap is extremely 
low. 
Another simpler possibility of collecting synchronization could be as 
follows: any ship may be considered as being either within the 
radioelectric range of another participant or a control center, or outside 
its range. In order to determine this, before its first transmission, a 
ship should listen for a time at least equal to the time defined as the 
longest period in the system. If it receives nothing during this time, it 
knows it is isolated and starts to transmit on the lowest recurrence (in 
accordance with what is acceptable based on the criteria defining 
intervals without synchronization). Its period may then change when the 
environment changes (arrival of participants within radioelectric range). 
It remains without synchronization as long as received messages are from 
ships and not from a control center. Synchronization is only useful in 
very dense zones, generally justifying the presence of a control center. 
If some received messages are transmitted from a control center, their 
origin will be used as synchronization in order to initiate a "slotted 
Aloha" type procedure. This synchronization is approximate but is 
acceptable here since propagation times are low compared with the message 
duration. If an absolute synchronization becomes necessary, it could 
easily be done. Each participant would simply need to correct the above 
coarse synchronization by the propagation time between the control center 
and itself. This time can be calculated from the known geographic 
positions of the 2 partners. 
Outside the short transmission times from transmitter 1, inverter 2 
connects antenna 3 to a receiver 5 tuned to the system common frequency. 
Receiver 5 is connected to a data demodulator 6 extracting information 
from the received signal, carrying out the reverse operations to those 
carried out in the modulator 4. This modulator is also connected to a data 
input device 4A such as a keyboard. 
Demodulator 6 is connected through a screen management device 7 to a 
display screen 8. For example, elements 7 and 8 could be a microcomputer 
and its display monitor. These elements 7 and 8 may be used together with 
a device 7A to display the identification code of one or several 
surrounding ships. 
The purpose of screen 8 is to present an operator with the entire 
environment of his ship by using information received from surrounding 
equipped ships, and information received from his own equipment. FIG. 2 
shows a non-restrictive example of information that could be displayed on 
screen 8. This information could be displayed in a form similar to that on 
a panoramic radar screen. 
According to the example in FIG. 2, screen 8 displays the various ships 
(for example 10, 11 and 12) as large light spots, with his own ship 
(reference 13) being of a different color and/or shape and/or brightness 
from those of the other ships. For example, different shapes and/or colors 
of spots may correspond to different types of ships. Each spot 
representing a ship is extended by a segment or a straight line 
representing the corresponding ship's speed vector. The length of this 
vector is proportional to the speed of the ship, and its orientation 
defines the heading of this ship. It would also be beneficial to represent 
information about the heading change close to the speed vector by a 
different color point or line, either at the left or right depending on 
the direction change. The general presentation of screen 8 may be made 
with the North at the top of the screen, but it would also be beneficial 
to have the top of the screen aligned with the prow of the ship, such that 
the line of travel of this ship is then fixed. The speed vector of each 
ship may be an absolute speed, or according to an alternative, a speed 
relative to the speed of the ship 13 (whose own speed vector will then be 
zero), the various relative speed vectors of the other ships then being 
determined by vector composition of their own speed and the speed of ship 
13. The point showing the ship 13 may be located at the center of the 
screen or may be off centered in a direction opposite to its own speed 
vector in order to give priority to a "forward view". 
It would be beneficial to display the identification code (10A, 11A and 
12A) close to the point representing each other ship (10, 11 and 12 
respectively on FIG. 2). 
It would also be beneficial for each equipped ship to contain a radar 
enabling it to detect surrounding non-equipped ships or with 
non-functioning equipment, and fixed obstacles (rocks, coastline, etc.). 
FIG. 2 shows two echoes 14 and 15 each representing non-equipped ships and 
a coastline 16. Echoes 14 and 15 will preferably be displayed in a shape 
and/or color different from those of points 10 to 13 so that the operator 
can immediately realize that they show non-equipped ships or ships with 
non-functioning equipment, and the absence of the corresponding speed 
vector does not mean that the speed of this ship is zero. 
All transformed coordinates, vectors and possibly information obtained from 
the onboard radar, are done by the control device 7, by a known method, 
the construction of which will be well understood by those skilled in the 
art when reading this description. 
Moreover, fixed data stored in a mass memory can also be supplied to the 
control device 7. The screen may also display cartographic data such as 
coasts, buoys, lighthouses, etc. 
According to a beneficial alternative of the invention, ship equipment also 
contains a radio call recognition circuit 9 connected firstly to the 
demodulator output 6, and secondly to a data input keyboard (not shown but 
the function of which could be carried out by 4A), on which the operator 
enters the call sign (which is usefully an identification code such as 
that described below) of the ship with which he wishes to enter into 
contact, this call sign being immediately sent to demodulator 4 and built 
into the message periodically transmitted by transmitter 1. Circuit 9 may 
be a simple comparator in the called ship, comparing the call sign 
received from the calling ship with its own call sign, and initiating an 
audible and/or visual alarm when finding that they are equal. Obviously 
the message received by the called ship contains the calling ship's call 
sign, which may be displayed on screen 8 on the called ship. For example 
it could be displayed in plain text (alphanumeric call sign) in a corner 
of the screen. According to one beneficial alternative, a symbol would 
appear close to the point representing the calling ship (such as one of 
the points 10 to 12) instead of or in addition to this display, or the 
point itself may be modified; for example the symbol could be a circle 
surrounding the point representing the calling ship, and/or this point 
could flash or be displayed highlighted. 
According to another alternative of the invention, the screen management 
device 7 may be combined with a "mouse" type device commonly used with 
microcomputers, this device producing a mobile marker 17 on screen 8, for 
example in the shape of a cross. When this marker is superimposed on a 
symbol representing a ship that the operator wants to call by radio, the 
operator presses or "clicks" the "mouse" start button. This command is 
processed by device 7 which generates the corresponding call sign 
(symbolized by the broken line 18) and sends it to modulator 4. In 
producing this call sign, device 7 stores call signs received from all 
neighboring ships (displayed on screen 8), sets up a relation between the 
point on which the marker 17 stopped and the corresponding call sign, and 
sends this call sign. These functions controlled by device 7 are easy to 
implement for the expert, and will not be described in more detail. 
Obviously the "mouse" may be used to acknowledge the call in the called 
ship, and possibly to open a radio link. The use of the "mouse" prevents 
possible errors in the two ships (caller and called ships) due to 
inputting a wrong call sign on the keyboard. 
A ship operator may use the mouse or the 4A keyboard to input and/or modify 
the "identification code" of his own ship. 
This identification code may be arbitrary. It does not need to be taken 
from a lexicon, and cannot be used to genuinely identify its user. This 
code is a binary number without any meaning other than as an address in 
the exchange of messages as described in detail below. 
However it will be useful if it is standardized as follows: 
--some of the binary elements in this number could be assigned to 
identification of the ship type. This information is useful for 
coordination of maneuvers of ships close to each other. 
--one of the binary elements of the number could be used to indicate if the 
code is taken from a lexicon (some ships may wish to identify themselves, 
or in any case not have any reason to hide their identity) or if it has no 
specific meaning. 
--the rest of the code, if it is not taken from a lexicon, contains enough 
binary elements such that the probability of accidentally using two 
identical codes within the same zone is negligible, for example about 16 
binary elements. 
--According to a first alternative, the choice of the rest of the code 
would be left to the user. However this solution may have disadvantages; 
mischievous use of another user's code and a more frequent use of some 
simplified codes increasing the risks that two codes would be identical in 
the same geographic area. 
--According to a second beneficial alternative, the rest of the code would 
be chosen independently by a processor 4B, connected to modulator 4 and 
circuit 9. This processor 4B could generate a pseudo-random sequence when 
it is started up. 
--If the user wants to avoid permanent identification, the processor 4B 
could periodically change the pseudo-random sequence. The processor could 
make this change during a period of inactivity (absence of any reception 
during a large number of successive periods). 
--Obviously, if processor 4B detects accidental use by another user 
(detection through circuit 9) of the code sent by its transmitter 1, it 
could immediately change its own code, or at least the pseudo-random 
sequence that it generates. 
When a communication has been set up between two ships, and each has 
received the message from the other described in detail with reference to 
FIG. 2 and used to display the corresponding data on screen 8, these ships 
can exchange other message types, beneficially replacing a phone link. 
These other types of messages may in particular concern maneuvering 
intentions of these two ships. In order to reduce congestion on the 
transmission channel and to facilitate understanding of these messages, 
they are coded using a lexicon containing the list of commonly used 
messages (words and/or phrases) for all possible maneuver types such as: 
intention to maintain heading, to turn to port or to starboard, waiting 
for tug, broken down, etc. . . Obviously each ship operator has a 
translation of the lexicon in his own language. A code can also be 
provided for a "request for phone transmission" in the relatively unlikely 
case that one or more of the operators wish to transmit a message not 
appearing in the lexicon. The frequency to be used for this phone link 
could also be stated. Obviously, three or more ships may participate in 
this exchange of coded messages at the same time; due to their short 
length (for example only 8 binary elements are necessary to code 256 
different messages) there is little risk of simultaneous transmission by 
several ships. Messages may be repeated several times at random intervals 
in order to reduce the risks of simultaneous transmissions. 
Codes for coded messages may be displayed on the monitor 7A or screen 8. 
According to one beneficial alternative, the translation of these codes is 
displayed in plain text using a character generator, the manufacture of 
which is straightforward for an expert. Similarly in order to avoid the 
need to browse through a lexicon, the keyboard 4A could be replaced by a 
display device, for example a pulldown menu or icon type screen showing 
all available messages grouped in types. A pointing device, such as a 
"mouse", could be used to select the required message and to immediately 
send the corresponding code. 
Device C also shown on FIG. 1 is used in the control center. This device C 
is similar to devices N used on ships, with the main difference that it 
does not receive information about its own latitude, longitude, speed and 
heading/heading change, since it is fixed. However, its fixed position 
must be transmitted. Obviously its screen 7A may be larger and have a 
better resolution than that used on ships, since it must be able to 
display all symbols corresponding to the ships and its surveillance zone. 
It may also be equipped with several screens each displaying part of its 
surveillance zone. Finally, antenna 3 on device C may be an antenna with a 
high vertical directivity in order to generate a fiat beam on the horizon, 
and in azimuth in order to avoid transmitting towards land. 
The control center displays the same type of information as the ships on 
its own screen 8, using the information transmitted by them. Obviously 
since this center is fixed, its own coordinates and the orientation of the 
image displayed on the screen are fixed. The symbol representing it is 
displayed on the screen as a function of the layout of the displayed zone. 
Obviously the speed vector of each ship displayed on its screen can only 
be the absolute speed. 
Messages transmitted by the control center may be addressed to all ships 
that it is monitoring, or to some of them or to only one of them. It will 
preferably have priority access to the common channel. For example, 
messages that it sends, or the first message in a series, will thus 
contain a preamble of several bits. This preamble may indicate the nature 
of the information that follows: either a "conventional" message similar 
to messages sent by ships, or a general interest message. In the first 
case the information may contain the center identification, the 
identification of one or several selected ships to be contacted, and 
orders or questions to be sent to them, possibly extracted from a lexicon 
as described above. 
In the second case, transmitted information may for example be the 
distribution of a map, weather information, information about port 
activities . . . In most cases these general interest messages may be very 
much longer than "conventional" messages, and their period may be very 
low. 
In order to reduce risks of overlapping between long general interest 
messages and conventional ship messages, the invention assigns longer 
transmission capabilities to general interest messages. 
According to a first method of construction, general interest messages are 
preceded by one or several special "conventional" messages indicating that 
a general interest message will be broadcast for a given time period 
starting from a given time. For example, this information may be taken 
from a lexicon available to all ships. It would be beneficial for the time 
interval to be a multiple of the "slotted Aloha" base period. All ships 
must stop transmitting during this period. This process may be automated 
by integrating a detection circuit into ship demodulators to detect these 
special conventional messages and to decode the duration of the general 
interest messages, and the detection circuit may disable its transmitter 
or prevent the reversing switch from being put into the transmitter 
position during this period. 
According to another method of making the invention, one "slotted Aloha" 
period out of n is reserved for the central station, and all other ships 
maintain silence during this period. Ships from outside the zone covered 
by the control center gradually synchronize with the "slotted Aloha" and 
remain in the network listening position until they recognize the period 
reserved for the control center. For example, in order to facilitate this 
recognition, the control center could transmit a special code during these 
reserved periods when it does not have any messages to transmit. 
When the network uses a lexicon, one of the messages in this lexicon could 
mean that the transmitting ship wants to change to voice communication on 
a predefined frequency, or on a frequency coded in this message. 
Infrequent and short voice communications may be set up using only one 
transmitter-receiver, normally used for transmitting the messages 
described above, and temporarily busy for voice communications.