Page 70 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed
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Chapter 3: Search and Survey 49
the receiver. This increases the travel time of the signal, thereby
causing errors (impossible to correct).
3. Receiver clock errors. The receiver’s built-in clock is not as accurate
as the atomic clocks onboard the GPS satellites resulting in very slight
timing errors (can be improved using more sophisticated equipment).
4. Orbital errors, also known as ephemeris errors. These are inaccura- cies in the satellite’s reported location (probably not possible to
correct).
5. Number of satellites visible. The more satellites that are visible to a
GPS receiver the greater the accuracy (possible to correct manually).
6. Satellite geometry or shading. This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight
grouping (possible to correct manually).
7. Intentional degradation of the satellite signal. SA is an intentional
degradation of the signal once imposed by the U.S. Department of Defense (now switched off and currently irrelevant).
4. Projections
One of the most important issues relating to the use of the GPS is under- standing the projection method and the datum that is used for this. Many people quote GPS positions, and when one asks the fundamental question, “What datum did you use?,” they have no idea to what you are referring. When the SA was on and positions were not particularly accurate, this issue probably did not matter. Today with GPS precision, it is very important. This whole projection method issue relates to the problem that the surface of the Earth is not flat, so to produce a map of its surface requires projection of the curved surface onto a flat surface. Nor is the surface of the earth spherical, it is in fact a complex ellipsoid (sometimes referred to as a spheroid, although this is not generally correct term). This ellipsoid is projected onto a flat surface to produce a map. Different ellipsoids apply in different areas and a GPS generally has a bewildering array of ellipsoids built into the system. All that is necessary, if one wishes to work with a GPS and an exist- ing chart, is to look up ellipsoid or datum for the chart and set the GPS to this datum. The coordinates on the GPS will then plot correctly on the map. The map is a projection and by its nature a projection is a compromise. In the early days of navigation globes were often taken onboard ships to sim- plify navigational problems. It is not possible to make a map or a chart of a three-dimensional surface like an ellipsoid. Some projections, therefore, attempt to maintain the correct length of the meridians, while distorting



















































































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