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Watchkeeping, Errors of Instruments

Q. What is the correction you apply to the barometer reading?
The pressure at sea level is more than the pressure indicated by barometer. In a range involving a limited distance above sea level, it may be assumed that the pressure increases by 0.1 millibars per meter. In addition to this, index error is applied. The instrument is calibrated by the meteorological department. The difference is usually referred to as index error.

Q. How would you know that the atmospheric pressure has fallen say 5 millibars below the normal?
The normal pressure for the area and the month is given in the routing chart or the sailing directions. However, before comparing with the given pressure, the pressure from aneroid barometer must be corrected for the height and index error.

Q. What corrections you must apply to the soundings obtained by echo sounder?
Echo sounder gives soundings below keel. Assuming that the vessel is on even keel and at the speed of ship squat is say ‘s’ meters, the present depth of water = draft + s + soundings. But since, this is equal to the chart datum + height of tide. Thus, to find the charted sounding for present position, the following calculation is done:
Charted sounding = (draft + sinkage due squat + under keel sounding) – Height of tide.

Q. What error can come in shallow waters, where the transmission and reception transducers are separated by some distance?
If the two transducers located at some distance apart; one being used for transmission and the other for reception, the total distance traveled by the pulse will be more, giving rise to an error called Pythagoras error.

The distance between the two transducers is 2x. θ is not constant and will depend on depth. The error therefore will be more for shallow depths where more accuracy is needed.

Q. What is the estimated error in a GPS position?
The probable error in distance = UERE x GDOP meters.
GPS receivers give a figure called UERE (in meters) or User’s Equivalent Range Error. A user’s equivalent range error in distance found from the known possible errors due ionospheric, etc causes. The GDOP figure size is inversely proportional to the size of diamond formed by lines meeting the satellites used and the position on earth. The separation of satellite used is very important. The wider the angular separation between the used satellite the more accurate the fix. This is represented by a low value of GDOP.

Q. What is the error in distance caused due unknown compass error in the terrestrial bearings? The error in distance caused due unknown compass error in the terrestrial bearings is given by the following formula:

Q. How do you confirm the accuracy of the VRM on radar?
Error may be determined in the following ways:

  1. The VRM is synchronized with fixed range rings. The range shown should be the range indicated by fixed range rings. For a given range scale say 6 miles, where each ring is separated from adjacent one by one mile, the fixed rings are likely to show correct ranges.
  2. Using radar, when position is found using 3 shore object, which are spread over 1800, the three arcs may be undersized or oversized. Thus, the error of VRM is determined to be negative or positive respectively.

Q. What is the relationship between the latitude error and longitude error as used in the noon fix calculations?
The relationship used is longitude error = latitude error x ‘c’ correction

Q. What is satellite clock offset?
This error is the mean of average error in the four atomic clocks on board the SV as compared to the GPS time. The error in the atomic clocks is monitored by the monitor stations and the master control station. Any error is found by earth based control stations and is transmitted in the navigation message sent by the SVs.

Q. What is User clock Bias?
This error is caused due to an error in the shipboard receiver clock. The shipboard receiver clock may have good stability for a short period of time or at any instant but this is not good enough for the system which deals with atomic clocks and nano seconds. The measuring time error however, will be same for all the satellites used. This error can be used as an additional unknown, which can be resolved by using an additional satellite. This concept is similar to using an extra distance to find error in VRM on radar.

Q. How many satellites do you need for an earth based receiver?
The earth is used as one of the position spheres. Thus, additionally the distances from two satellites giving two more position spheres should be enough. There is however, an unknown user’s clock bias in the simultaneous observations. This causes an extra unknown and hence an extra satellite to resolve it.

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