▁a ▁certain ▁set ▁of ▁specific ations . ▁These ▁specific ations ▁are ▁listed ▁in ▁the ▁table ▁below . ▁ ▁If ▁an ▁ir rad iance ▁sensor ▁is ▁placed ▁in ▁PO A , ▁it ▁must ▁be ▁placed ▁at ▁the ▁same ▁t ilt ▁angle ▁as ▁the ▁P V ▁module , ▁either ▁by ▁att aching ▁it ▁to ▁the ▁module ▁itself ▁or ▁with ▁an ▁extra ▁platform ▁or ▁arm ▁at ▁the ▁same ▁t ilt ▁level . ▁Check ing ▁if ▁the ▁sensor ▁is ▁properly ▁aligned ▁can ▁be ▁done ▁with ▁port able ▁t ilt ▁sens ors ▁or ▁with ▁an ▁integrated ▁t ilt ▁sensor . ▁ ▁Main ten ance ▁ ▁The ▁standard ▁also ▁spec ifies ▁a ▁required ▁maintenance ▁schedule ▁per ▁accuracy ▁class . ▁Class ▁C ▁sens ors ▁require ▁maintenance ▁per ▁manufact urer ' s ▁requirement . ▁Class ▁B ▁sens ors ▁need ▁to ▁be ▁re - cal ibr ated ▁every ▁ 2 ▁years ▁and ▁require ▁a ▁he ater ▁to ▁prevent ▁precip itation ▁or ▁cond ens ation . ▁Class ▁A ▁sens ors ▁need ▁to ▁be ▁re - cal ibr ated ▁once ▁per ▁year , ▁require ▁clean ing ▁once ▁per ▁week , ▁require ▁a ▁he ater ▁and ▁require ▁vent ilation ▁( for ▁therm op ile ▁py ran om eters ). ▁ ▁Sat ellite ▁remote ▁sens ing ▁of ▁ir rad iance ▁ ▁P V ▁performance ▁can ▁also ▁be ▁estimated ▁by ▁satellite ▁remote ▁sens ing . ▁These ▁measurements ▁are ▁indirect ▁because ▁the ▁sat ell ites ▁measure ▁the ▁solar ▁rad iance ▁reflected ▁off ▁the ▁earth ▁surface . ▁In ▁addition , ▁the ▁rad iance ▁is ▁filtered ▁by ▁the ▁spectral ▁absor