Question

Which one of the coordinate pairs represents the satellite orbit in the skyplot of the GNSS constellations?

• A. Elevation, Azimuth
• B. Right ascension, Declination
• C. Altitude, Declination
• D. Zenith angle, Azimuth

Hint:

A skyplot is drawn in the local horizon system centered on the receiver, think about which two angles, one vertical and one horizontal, define a direction in that system.

Answer 1

The Correct Option is A

Step 1: Understand what a skyplot shows.
A GNSS skyplot is a polar diagram, drawn at a fixed receiver location, that shows the paths traced by the tracked satellites across the visible sky over a period of observation. It is drawn in the topocentric horizon coordinate system, the natural local system centered on the observer, whose reference plane is the local horizon and whose pole is the local zenith.

Step 2: Identify the two coordinates used.
In the horizon system a satellite's position is fixed by two angles: the elevation angle \(E\), measured from the local horizon plane upward to the satellite, running from \(0^\circ\) at the horizon to \(90^\circ\) at the zenith, and the azimuth angle \(A\), measured in the horizontal plane clockwise from geographic north to the vertical plane containing the satellite, running from \(0^\circ\) to \(360^\circ\). On the plot, azimuth is the angular coordinate around the circle and elevation is drawn along the radius, with the zenith (\(E = 90^\circ\)) placed at the center and the horizon (\(E = 0^\circ\)) at the outer rim, so a satellite rising, tracking overhead and setting traces a curved path from the rim inward and back to the rim.

Step 3: Reject the other coordinate pairs.
Right ascension and declination belong to the equatorial celestial coordinate system, referenced to the Earth's rotation axis and the vernal equinox, which is used for star catalogues and is not centered on, or oriented to, the local horizon of the receiver, so it cannot describe a locally observed satellite track directly. Altitude paired with declination mixes a horizon-system quantity with an equatorial-system quantity, which are two different reference frames, such a mixed pair is not a valid single coordinate system and is never used for a skyplot. Zenith angle is simply the complement of elevation, \(Z = 90^\circ - E\), a graph could technically be redrawn using it, but the named, standard coordinate pair that defines the horizon system used to plot the skyplot is elevation and azimuth, not zenith angle and azimuth.

Step 4: Conclusion.
The skyplot of GNSS satellites is plotted using the topocentric horizon system, so the correct coordinate pair is elevation and azimuth.

\[ \boxed{\text{Elevation, Azimuth}} \]