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Chapter 5 - Determining Your Position
Table 2: GPS status messages (continued)
Message Meaning
TRACKING
ACQUIRE one or more satellites are being tracked trying to acquire satellites
SKY SEARCH searching for satellites with no previous information
Viewing GPS satellite status
To display more information about signals received from the
GPS satellites, press the SAT INFO menu key on the GPS
POSITION screen. The GPS SATELLITES screen is displayed showing the received Signal-to-Noise Ratio (SNR) for each satellite (the higher the ratio, the better the quality of the satellite signals), and a satellite map indicating where the satellites are currently located in the sky. Satellites are identified by their PRN number, a two-digit number assigned by the U.S. government. GPS satellites are represented by circles; WAAS satellites are represented by hexagons.
The center of the satellite map represents the center of the sky, and the outer (or last) ring is the horizon. The view is looking down from above, with East to your right and West to your left.
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The following GPS data is also displayed:
958 Operator’s Manual, Rev. A
Using DGPS
Chapter 5 - Determining Your Position
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ACCURACY: The estimated accuracy--in feet--of the GPS system to be expected at the current time. Your position readings should be more accurate than this estimate 95 percent of the time.
HDOP: This is a technical measure of the quality of your fix. HDOP (Horizontal Dilution of Precision) is calculated from the satellites’ current positions. You’ll usually see an
HDOP value displayed immediately after the 958 is turned on, and before it’s ready to navigate. HDOP can range from an ideal value of slightly less than one, up to poor values of 10 or more. Any value less than two indicates excellent satellite configuration.
When using the 958 with optional radiobeacon DGPS , you’ll usually see position measurements accurate to about one to three meters, speed measurements accurate to about 0.1 knot, and COG measurements accurate to about 0.5 degree. DGPS corrections remove most of the errors caused by atmospheric variations.
NOTE:
When you’re using DGPS as your navigation source, your position coordinates, as calculated by the 958, may actually exceed the accuracy of some charts.
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Chapter 5 - Determining Your Position
When the DGPS indicator is displayed, it means the 958 is using this higher level of accuracy. When you require this additional accuracy, be sure to check often to see that either the DGPS or WAAS indicators are present.
Displaying DGPS status
To display more information about received DGPS corrections, press the DGPS INFO menu key on the GPS POSITION screen.
The DGPS STATUS screen is displayed, showing the DGPS transmitter currently used.
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The upper part of the screen shows the beacon’s name, frequency, baud rate, and the beacon’s distance and bearing from your vessel. The lower part of the DGPS STATUS screen shows three bar graphs:
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SNR: The Signal-to-Noise Ratio is a good measure of overall signal quality, and should be as high as possible. Values of 15 and higher are best; anything below 10 could indicate poor reception.
SIGNAL: This represents the actual strength of the received signal. This value should be as high as possible, and it varies with distance from the differential transmitter.
NOISE: The NOISE level measures the static caused by lightning and other atmospheric effects, and should be
958 Operator’s Manual, Rev. A
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