Watchkeeping - GPS

Question 1

How many satellites are needed to provide an accurate GPS signal?

Answer 1

4 Satellites above the horizon for a 3D fix.

Question 2

What does HDOP refer to?

Answer 2

Horizontal Dilution of Position which is in reference to the quality of data received which directly correlates with the positioning of satellites
available. The GPS will display a value on the unit, the greatest being 1.

Question 3

What errors exist in the GPS system and can they be corrected by the navigator?

Answer 3

• Multipath errors which refer to the transmitting signals from satellites being reflected from local objects rather than in a direct line to the receiver.
• Satellite clock error which is an error in time within the satellite system which is corrected by ground level control.
• Orbit errors occur when the satellite drifts slights from its predicted trajectory, this is monitored and corrected by ground level systems.
• Geometric Dilution of Position which refers to the positioning of the satellites in respect to the vessel. Signal received from satellites
  which are close together will be weaker than that of those further apart.
• Ionospheric and Tropospheric delay caused by free electrons in the atmosphere which slow down and interfere with the propogation
  of signals from satellite to receiver.
• Plotting on a chart which is not WGS84 datum.

Question 4

What is the difference between GPS and DGPS?

Answer 4

DGPS employs a fixed position base station which corrects for the error in satellite signal this is then transmitted to the ships unit and is applied to the satellite signal received by the ship.