Nearly all transmitters for ELF, VLF, LF and MF and most HVDC schemes have extensive grounding systems, which can be seen sometimes on satellite pictures.
Here are some examples:
HVDC
http://wikimapia.org/#lang=de&lat=48...25742&z=16&m=b : Grounding electrode of HVDC Hulunbeir-Liaoning
http://wikimapia.org/#lang=de&lat=28...28320&z=17&m=b : Grounding electrode of HVDC Ballia-Bhiwadi
http://wikimapia.org/#lang=de&lat=45...51766&z=16&m=b : Grounding electrode of HVDC Quebec-New England
http://wikimapia.org/#lang=de&lat=49...15228&z=17&m=h : Grounding electrode of HVDC Volgograd-Donbass
http://wikimapia.org/#lang=de&lat=57...97116&z=18&m=b : Grounding electrode of HVDC Gotland
http://wikimapia.org/#lang=de&lat=39...68352&z=17&m=b : Grounding electrode of HVDC Italy-Greece
Not all HVDC schemes have grounding systems visible on satellite pictures. Some use grounding systems consisting of single or multiple rods running deeply
in the ground or grounding electrodes in the sea. Other schemes have no ground electrodes at all, as they use a metallic conductor instead of the earth as return conductor.
HVDC back-to-back schemes have no ground electrodes.
Transmitters
http://wikimapia.org/#lang=de&lat=48...70292&z=17&m=b : Olgij Longwave Transmitter
http://wikimapia.org/#lang=de&lat=43...58502&z=17&m=b : RMC Transmitter
http://wikimapia.org/#lang=de&lat=-2...88541&z=18&m=b : Former Le Chabrier OMEGA-transmitter
http://wikimapia.org/#lang=de&lat=-4...90983&z=16&m=b ; Former Trelew OMEGA-transmitter
http://wikimapia.org/#lang=de&lat=23...55011&z=18&m=b : Afif LORAN-C transmitter
http://wikimapia.org/#lang=de&lat=68...7779&z=17&m=bs : ZEVS
Although nearly all transmitter for frequencies below a few megahertz requires a grounding system, such devices are usually not visible when the transmitter is situated on fertile land.
Linear Mode
