Worldwide VLF standard frequency and time signal broadcasting

R ece nt studies a nd m eas urements h ave shown t h at t he ph ase stabi li ty of t he sig na ls in t he VLF region is ve ry much hi gher t h an in the HF s pectrum. This fact, a lo ng wi t h its excelle nt coverage cha racteristics, h as caused co nsider a ble interest in e mployin g this medium fo r t he wide distribu t io n of standard frequ en cies a nd t ime r eference. Basic limitatio ns in stability of the received signa ls are disc ussed, in clu din g p at h phase di stortio n, cfl,rri c r-tonoise and en velope delay variation s as r elated to precise synchro ni zation of clocks, a nd hi ghl y acc urate frequ en cy calibrations.