▁obtained ▁by ▁obser ving ▁the ▁body ▁as ▁it ▁moved ▁in ▁its ▁observation ▁arc , ▁relative ▁to ▁the ▁fixed ▁stars , ▁using ▁an ▁optical ▁teles cope . ▁This ▁corresponds ▁to ▁knowing ▁the ▁object ' s ▁relative ▁direction ▁in ▁space , ▁measured ▁from ▁the ▁observer , ▁but ▁without ▁knowledge ▁of ▁the ▁distance ▁of ▁the ▁object , ▁i . e . ▁the ▁result ant ▁measurement ▁contains ▁only ▁direction ▁information , ▁like ▁a ▁unit ▁vector . ▁ ▁With ▁rad ar , ▁relative ▁distance ▁measurements ▁( by ▁timing ▁of ▁the ▁rad ar ▁echo ) ▁and ▁relative ▁velocity ▁measurements ▁( by ▁meas uring ▁the ▁Do pp ler ▁effect ▁of ▁the ▁rad ar ▁echo ) ▁are ▁possible ▁using ▁radio ▁teles cop es . ▁However , ▁the ▁returned ▁signal ▁strength ▁from ▁rad ar ▁decre ases ▁rapidly , ▁as ▁the ▁inverse ▁fourth ▁power ▁of ▁the ▁range ▁to ▁the ▁object . ▁This ▁generally ▁limits ▁rad ar ▁observations ▁to ▁objects ▁relatively ▁near ▁the ▁Earth , ▁such ▁as ▁artificial ▁sat ell ites ▁and ▁Near - E arth ▁objects . ▁Lar ger ▁a pert ures ▁permit ▁tracking ▁of ▁trans pond ers ▁on ▁inter plan et ary ▁space craft , ▁and ▁natural ▁bodies , ▁throughout ▁the ▁solar ▁system . ▁ ▁Vari ous ▁space ▁ag encies ▁and ▁commercial ▁prov iders ▁operate ▁tracking ▁networks ▁to ▁provide ▁these ▁observations . ▁ ▁See ▁Category : De ep ▁Space ▁Network ▁for ▁a ▁partial ▁listing . ▁ ▁Space - based ▁tracking ▁of ▁sat ell ites ▁is ▁also ▁regularly ▁performed . ▁ ▁See ▁List ▁of ▁radio ▁teles cop es # Space - based ▁and