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Technologies: Skyplot

These utilities plot the location of GPS satellites in the sky for a given latitude, longitude, and time. These applets work well in Netscape version 6, Microsoft Internet Explorer version 5, and appletviewer.

Simple Skyplot

Here's How It Works:

• The outermost circle in the plot represents the horizon. The innermost circle represents a 60 degree elevation, the remaining circle represents a 30 degree elevation.
• The center of the skyplot is 90 degree elevation, or directly overhead.
• North is at the top of the skyplot, east to the right, south at the bottom, and west to the left.
• Satellites are plotted by their PRN number;
• The color of the satellite PRN number is black if it is above both the horizon and the user-selected elevation mask, and red if it is above the horizon but below the elevation mask.
• To select a time other than the current time, de-select the "Current Time" checkbox and select the desired time using the GPS Week and GPS Second scrollbars.
• The latitude and longitude and elevation mask can also be selected.
• The PDOP is calculated for all satellites above the elevation mask.
• A PDOP of -1 indicates that the PDOP is infinite.
• The Inmarsats are not included in the PDOP calculation.

Advanced Skyplot

Here's How It Works:

• The outermost circle in the plot represents the horizon;
  the innermost circle represents a 60 degree elevation, the remaining circle represents a 30 degree elevation.
• The center of the skyplot is 90 degree elevation, or directly overhead.
• North is at the top of the skyplot, east to the right, south at the bottom, and west to the left.
• Satellites are plotted by their PRN number;
• A larger font means positive elevation, a smaller font means negative elevation.
• So for example, a large-font 22 plotted on the innermost circle 45 degrees clockwise from the top means that PRN 22 is 60 degrees above the horizon to the northeast.
• The color of the satellite PRN number is green if it is below the horizon, black if it is above both the horizon and the user-selected elevation mask, and red if it is above the horizon but below the elevation mask.
• To select a time other than the current time, de-select the "Current Time" checkbox and select the desired time using the GPS Week and GPS Second scrollbars.
• The latitude and longitude and elevation mask can also be selected.
• The PDOP is calculated for all satellites above the elevation mask.
• A PDOP of -1 indicates that the PDOP is infinite.
• The "P" and "W" on the skyplot indicate the Pacific and Atlantic-West Inmarsats respectively. These are the satellites which broadcast the WAAS differential correction signals.
• The Inmarsats are not included in the PDOP calculation.