Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 29)

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(6) near vehicle parking areas; and
(7) where heat is exhausted by vehicles or buildings.
Conversely, BSRN stations m ust be located where facilities exist, preferably on a full-tim e 24-hour
basis. Ideally, the site should be co-located with a synoptic station and within 50 km of an upper
air station.
3.1.2 Horizon
The ideal site for the m easurem ent of solar and terrestrial radiation for m eteorological purposes is
one that has a com pletely flat horizon. The W MO Guide to Meteorological Instrum ents and
Methods of Observation (W MO No. 8) recom m ends that if possible no obstruction should be
present, particularly within the azim uth range of sunrise and sunset over the year (see Annex I for
a solar position algorithm ). In cases where obstructions do occur, the instrum ent should be
located where these subtend an elevation angle of less than 5° to m inim ize their effects. The total
diffuse radiation received by a surface from elevation angles of less than 5° accounts for only
about 1% of the total global radiation. The determ ination of the change in radiation fluxes with
respect to changing clim ate, and the use of surface m easurem ents to test and ground-truth
satellite retrieval algorithm s do not require strict adherence to this guideline when distant
topography is considered. In the latter case, m easurem ents in areas of com plex topography are
required to determ ine the capabilities of the retrievals.
W hile the distant horizon m ay be influenced by topography, the local horizon should be as clear
as practically possible. A distance of 12 tim es the height of any object to the location of the sensor
will ensure that the elevation of the object is less than 5° above the horizon. The site should be
located such that all objects are to the poleward side of the installation and do not interfere with
the direct beam radiation at any tim e during the year. The instrum ents should be rem oved, as far
as is practical, from any highly reflective objects. W here a site is to be developed in a built-up
area, the sensors can be located on the roof of a building to overcom e problem s with the local
horizon.
W hile antennas and other slender objects should be avoided, their effect is m inim al and can be
endured if they are less than 1° wide, and do not block the direct beam radiation during any tim e
of the year.
3.1.3 Latitude, Longitude, Elevation
A detailed description of the m easurem ent site and its surroundings is probably one of the m ost
significant pieces of m etadata provided to other researchers. It is of utm ost im portance to describe
the site and its surroundings, not only in term s of latitude, longitude and elevation, but also with
respect to the topography and land use surrounding the m easurem ent location. One m ust
consider this description in term s of the pixel size of present-day satellite m easurem ents and the
potential for influences on the radiation regim e due to m ultiple scattering.
The first and forem ost inform ation required are the geographic coordinates of the site: latitude,
longitude and elevation above m ean sea level (m sl). These norm ally can be obtained from high-
quality topographic m aps obtained through the m apping agencies of national governm ents. The
BSRN archive records this inform ation in a floating point form at with three decim al places. This is
equivalent to an accuracy of approxim ately 3.5 seconds of arc, or about 108 m etres in latitude
and 76 m etres in longitude at 45°. To obtain such accuracies, a m ap with a scale of better than
1:100000 is required. The latitude and longitude should be recorded in decim al degrees, North
and East positive with both the South Pole and 180° W defined as zero. For exam ple, a station
located in the Northern Hem isphere and east of Greenwich, such as Potsdam , Germ any (52 N, 13
E) would be encoded 142.000, 193.000, while for a sim ilar latitude, but in the Canadian prairies
(52 N , 105 W ) the location would be encoded 142.000, 75.000. This is for consistency with the
Archive station-to-archive file form at.
Elevation can also be read from topographic m aps, norm ally to within 5 m etres. More accurate
m easurem ents require site surveys. The Archive records the elevation to within 1 m etre.