echo free space无: not have; there is not; be w ...回波: echo; return waves区: H region; H回波区: echo area; echoing area雷达回波区: rea radar echoing area下载手机词典可随时随地查词查翻译关注微信公众号：chachacidian，回复:无回波区，即可在微信中查询翻译
