Navigation is comprised of a number of different processes. Some are done in a set order, some randomly, some almost constantly, others only infrequently. It is in choosing using these processes that an individual navigator’s experience and judgment are most crucial. Compounding this subject’s difficulty is the fact that there are no set rules regarding the optimum employment of navigational systems and techniques. Optimum use of navigational systems varies as a function of the type of vessel, the quality of the navigational equipment on board, and the experience and skill of the navigator and all the members of his team.
For the watch officer, ensuring the ship’s safety always takes priority over completing operational commitments and carrying out the ship’s routine. Navigation is his primary responsibility. Any ambiguity about the position of the vessel which constitutes a danger must be resolved immediately. The best policy is to prevent ambiguity by using all the tools available and continually checking different sources of position information to see that they agree. This includes the routine use of several different navigational techniques, both as operational checks and to maintain skills which might be needed in an emergency.
Any single navigational system constitutes a single point of failure, which must be backed up with another source to ensure the safety of the vessel.
It is also the navigator’s responsibility to ensure that he and all members of his team are properly trained and ready in all respects for their duties, and that he is familiar with the operation of all gear and systems for which he is responsible. He must also ensure that all digital and/or hardcopy charts and publications are updated with information from the Notice to Mariners, and that all essential navigational gear is in operating condition.
Navigating a vessel is a dynamic process. Schedules, missions, and weather often change. Planning a voyage is a process that begins well before the ship gets underway. Executing that plan does not end until the ship ties up at the pier at its final destination. While it is possible to over plan a voyage, it is a more serious error to under plan it. Carefully planning a route, preparing required charts and publications, and using various methods to monitor the ship’s position as the trip proceeds are fundamental to safe navigation and are the marks of a professional navigator.[1]
Possible replacement of Satellite Navigation section on Navigation page
Satellite navigation
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Global Navigation Satellite System or GNSS is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. A GNSS allow small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line of sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments. As of 2007, the United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS.
GPS concepts
editGPS measures distances between satellites in orbit and a receiver on Earth, and computes spheres of position from those distances.[2] The intersections of those spheres of position then determine the receiver’s position.[2]
The distance measurements described above are done by comparing timing signals generated simultaneously by the satellites’ and receiver’s internal clocks.[2] These signals, characterized by a special wave form known as the pseudo-random code, are generated in phase with each other.[2] The signal from the satellite arrives at the receiver following a time delay proportional to its distance traveled.[2] This time delay is detected by the phase shift between the received pseudorandom code and the code generated by the receiver.[2] Knowing the time required for the signal to reach the receiver from the satellite allows the receiver to calculate the distance from the satellite.[2] The receiver, therefore, must be located on a sphere centered at the satellite with a radius equal to this distance measurement. The intersection of three spheres of position yields two possible points of receiver position.[2] One of these points can be disregarded since it is hundreds of miles from the surface of the Earth.
Theoretically, then, only three time measurements are required to obtain a fix from GPS.[2] In practice, however, a fourth measurement is required to obtain an accurate position from GPS.[2] This is due to receiver clock error.[2] Timing signals travel from the satellite to the receiver at the speed of light; even extremely slight timing errors between the clocks on the satellite and in the receiver will lead to tremendous range errors. The satellite’s atomic clock is accurate to 10-9 seconds; installing a clock that accurate on a receiver would make the receiver prohibitively expensive.[2] Therefore, receiver clock accuracy is sacrificed, and an additional satellite timing measurement is made.[2] The fix error caused by the inaccuracies in the receiver clock is reduced by simultaneously subtracting a constant timing error from four satellite timing measurements until a pinpoint fix is reached.[2]
Assuming that the satellite clocks are perfectly synchronized and the receiver clock’s error is constant, the subtraction of that constant error from the resulting distance determinations will reduce the fix error until a “pinpoint” position is obtained.[2] It is important to note here that the number of lines of position required to employ this technique is a function of the number of lines of position required to obtain a fix.[2] GPS determines position in three dimensions; the presence of receiver clock error adds an additional unknown.[2] Therefore, four timing measurements are required to solve for the resulting four unknown.[2]
GPS use
editIn order for the GPS receiver to navigate, it has to track satellite signals, make pseudorange measurements, and collect navigation data.[3]
A typical satellite tracking sequence begins with the receiver determining which satellites are available for it to track.[3] Satellite visibility is determined by user-entered predictions of position, velocity, and time, and by almanac information stored internal to the receiver.[3] If no stored almanac information exists, then the receiver must attempt to locate and lock onto the signal from any satellite in view.[3]
When the receiver is locked onto a satellite, it can demodulate the navigation message and read the almanac information about all the other satellites in the constellation.[3]
A carrier tracking loop tracks the carrier frequency while a code tracking loop tracks the C/A and P code signals.[3] The two tracking loops operate together in an iterative process to acquire and track satellite signals.[3]
The receiver’s carrier tracking loop will locally generate an L1 carrier frequency which differs from the satellite produced L1 frequency due to a Doppler shift in the received frequency.[3] This Doppler offset is proportional to the relative velocity along the line of sight between the satellite and the receiver, subject to a receiver frequency bias.[3]
The carrier tracking loop adjusts the frequency of the receiver-generated frequency until it matches the incoming frequency.[4] This determines the relative velocity between the satellite and the receiver.[4] The GPS receiver uses this relative velocity to calculate the velocity of the receiver.[4]
This velocity is then used to aid the code tracking loop.[4] The code tracking loop is used to make pseudorange measurements between the GPS receiver and the satellites.[4] The receiver’s tracking loop will generate a replica of the targeted satellite’s C/A code with estimated ranging delay.[4]
In order to match the received signal with the internally generated replica, two things must be done:
- The center frequency of the replica must be adjusted to be the same as the center frequency of the received signal;[4] and
- the phase of the replica code must be lined up with the phase of the received code.[4]
The center frequency of the replica is set by using the Doppler-estimated output of the carrier tracking loop.[4] The receiver will then slew the code loop generated C/A code though a millisecond search window to correlate with the received C/A code and obtain C/A tracking.[4]
Once the carrier tracking loop and the code tracking loop have locked onto the received signal and the C/A code has been stripped from the carrier, the navigation message is demodulated and read.[4] This gives the receiver other information crucial to a pseudorange measurement.[4] The navigation message also gives the receiver the handover word, the code that allows a GPS receiver to shift from C/A code tracking to P code tracking.[4]
The handover word is required due to the long phase (seven days) of the P code signal.[4] The C/A code repeats every millisecond, allowing for a relatively small search window[4]
The seven day repeat period of the P code requires that the receiver be given the approximate P code phase to narrow its search window to a manageable time.[4] The handover word provides this P code phase information. The handover word is repeated every subframe in a 30 bit long block of data in the navigation message. It is repeated in the second 30 second data block of each subframe.[4] For some receivers, this handover word is unnecessary; they can acquire the P code directly.[4] This normally requires the receiver to have a clock whose accuracy approaches that of an atomic clock.[4] Since this greatly increases the cost of the receiver, most receivers for non-military marine use do not have this capability.[4]
Once the receiver has acquired the satellite signals from four GPS satellites, achieved carrier and code tracking, and has read the navigation message, the receiver is ready to begin making pseudorange measurements. Recall that these measurements are termed pseudorange because a receiver clock offset makes them inaccurate; that is, they do not represent the true range from the satellite, only a range biased by a receiver clock error.[4] This clock bias introduces a fourth unknown into the system of equations for which the GPS receiver must solve (the other three being the x coordinate, y coordinate, and z coordinate of the receiver position).[4] Recall from the discussion in Article 1101 that the receiver solves this clock bias problem by making a fourth pseudorange measurement, resulting in a fourth equation to allow solving for the fourth unknown.[4] Once the four equations are solved, the receiver has an estimate of the receiver’s position in three dimensions and of GPS time.[4]
The receiver then converts this position into coordinates referenced to an Earth model based on the World Geodetic System (1984).[4]