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    Re: US. Navy fix accuracy and Interval.
    From: Jeremy C
    Date: 2013 Mar 26, 07:08 -0700

    Bryon, you Navy guys and your yards! I love it when I hear on the radio: "US NAVY WARSHIP to the ship 1200 yards on my port bow." It is funny because most sailors in the world don't know what a yard is. We tend to talk meters, cables (1 cable = 0.1 nm), and NM's.

    But, back to the matter at hand: Navigational accuracy and timing. Electronic Chart Display and informations Systems (ECDIS) are now required on a sliding scale for the merchant ships of the world. Just last week I was in a class for this system. We spoke extensively about navigational accuracy.

    There is no regulation or standard for absolute cross track error or positional accuracy in the regulations. The key is to be within your pre-determined (by the master) "safe areas of navigation." These areas should be clear of dangerous navigational hazards and deviation from the plan will sure to be a finding in any accident investigation. In the end, the Master decides how much error from the track line he can allow.

    In some cases more allowable XTE is better since you often have to deviate from your intended track due to traffic, especially in the termination of traffic schemes. Fishing boats and yachts love to hang out right on your track line. If you allow yourself only 50 meters, you will most likely be in waters you haven't properly vetted and that will get you in trouble during an investigation.

    The next question is how close can we get? If you are using DGPS, like nearly all ships, your accuracy is going to be within 10 meters. You can say that your positional accuracy is within a ship width. DGPS is good to about 200nm from the transmitter site. Pure GPS will get you within 30 meters for "deep sea" navigation. With the introduction of "L2" recievers using both GPS and GLONASS, accuracy is within a meter. So with properly set up and equipped ECDIS systems, you can know your position within a shipwidth at all times.

    As far as timing is concerned. There is a regulation for this. The ECDIS shall log the position (and other data) at least once per minute. Typically we set the ECDIS to drop a position on the chart every minute but log position every 10 seconds or so. Standard GPS and DGPS receivers calculate position every second. My latest L2 reciever has a 10 Hz position rate so it gives me a GPS/GLONASS position every 0.1 seconds which can be logged as well.

    Positional checks are also part of the equation. This is done near land, mostly via radar overlay or manually comparing a range and bearing of a radar conspicious object to it's range and bearing on the ECDIS with it's GPS position. We also look at the GPS receivers HDOP to judge the confidence of our position.

    I will note that there is even a visual bearing device available that interfaces with the ECDIS and gyro systems so that you can take visual LOP's and have them plotted directly on the ECDIS. Sadly I doubt I will ever see one on my ship.

    Quite frankly, these days knowing where I am isn't the issue. I basically know where I am at all times. My biggest issue is that the charts aren't accurate or detailed enough. Truely, the weakest link in navigation these days is the hydrographic data available to me.

    Jeremy
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