Query: How many satellite constellations does the BeiDou Navigation Satellite System have?
Context: The BeiDou Navigation Satellite System (BDS, simplified Chinese: 北斗卫星导航系统; traditional Chinese: 北斗衛星導航系統; pinyin: Běidǒu wèixīng dǎoháng xìtǒng) is a Chinese satellite navigation system. It consists of two separate satellite constellations – a limited test system that has been operating since 2000, and a full-scale global navigation system that is currently under construction.