Basics


	checkSun Open Spectrum Galileo/GPS controlled Solar Tracking Freeware/GPL context

Galileo/GPS Timesignal for 2-axis Solar Tracking

Specified as NMEA 0183 (ASCII) GPS signals are
available worldwide. The purpose here is to use these
signals for solar tracking: Time/Date for the position of
the sun and geogr. coordinates to adjust the global
data to retrieve relevant local facts for tracking. (Galileo
starts 2008).

UTC stands for global “Mean Solar Time” as a virtual
circle around the equator, centred at the earth’s axis (also
any parallel to it!) with identical segments of 15° per hour
(and 1° per 4 time-minutes.) The left upper image shows
the 0° Greenwich-Meridian and the UTC 15°-hour
meridians. The zenith sunray travels at the equator at
equinox (21March and 22Sept.) It cycles yearly sine-
wise between the tropics at 23,5°N/S. (Please check
animations at the bottom).

At 40°N/075°W there is the slanted green horizontal plane
for that position with a red parallel to the earth’s axis and
a green plumb-line pointing to the center of the earth.
(Sunrays arrive in a parrallel way everywhere, that is from
the same direction on the north- and southpole)

The daily count of UTC starts with the sun over the date
line 180°E with 00:00 hrs. A 090°E sun position then
corresponds to UTC 06:00 The lower left image shows
UTC 14:00 (at 030°W) and the ticks leading to it.

The earth’s travel aound the sun is not a perfect circle like UTC, but has an elliptical shape. This
means that the sun is sometimes a little ahead or behind the (virtual, meridional) UTC position.
This difference is called “Analemma” (www.analemma.com), cycling between ±16 Minutes of
Time and repeating each year with a ~10Sec deviation in a 4 year cycle. For 21March the table
shows -7’, in the image shown as A= -7. Applying the analemma to the UTC time figure gives us
the global “True Solar Time”, corresponding to the real travel of the sun.

To adapt to local conditions at 075°W this global “True Solar Time” needs to be correctd for
the Time Difference to UTC (that relates to the Greenwich maridian).75°W div. by 15° per hour
results in -5hrs. So the Local “True Solar Time” for UTC14:00 is 08:53 at 075°W. For east-
longitudes, on the other hand adjustments are added to the global “True Solar Time”.

    Steering the tracker with the Local “True Solar Time” converted to an “Angular
    Tracking Radial” in a 360° -in-24 Hour clock-wise ircle” should allow a tracking
    tolerance of <than ½° - in a rotation parallel to the axis of the earth - for 1st axis
    tracking so far. (It’s 15° per hour and 1° per 4 Minutes to convert).


	The graphics below should provide an impression for a 1st-axis “plumb-line” installation at 40°N/ 21June solostice. The hourly zenith-sunrays for each 15° around the northern tropic are projected from the green Vertecal (plumb-line) onto the Horizontal Plane at 40°N. The resulting azimuth pattern along the horizon has an hourly spacing of suntravel with changing angle intervals.

2nd (vertical) Axis Considerations

If the 1st axis is rotated around a parallel to the earth’s axis, the 2nd (vertical) axis has little to
do. Only ±47° a Year is required for precise tracking, managed either by mechanical linkage
and gear reduction or guided by Galileo/GPS Date information in accordance with declination
values of the sun between the tropics ( - to be deposited in microcontroller memory). For a Tracker
operated with just one axis only (operating parellel to the earth’s axis instead around the plumb-
line) - the maximum vertical deflection of the module towards the sun is 23,5° at solstice only.


				Year


		Equinox


6 Aug.


		21June


			6 Nov.


	21Dec.


			Characteristics of Parallel Sunlight Travel. Please klick an icon for a small animation.







				*