The positioning technique employing the ubiquitous signals of opportunity of non-cooperative satellites does not send special navigation signals, instead it passively receives satellite signals as noise, presenting advantages of concealment and difficulty for potential attackers. Thus, this study investigates the ranging principle and model using non-cooperative communication satellites and a time difference estimation algorithm. The technology of time difference measurement under non-cooperative observation mode was determined and simulated. A test platform for time difference measurement was built to receive the signal from an unknown geostationary Earth orbit communication satellite and verify the ranging feasibility and performance. The ranging accuracy was found to be smaller than 6 m, as demonstrated by experimental data, which shows the viability of the proposed positioning technique for ranging technology.

We are deploying a new station for sub-millimeter Very Long Baseline Interferometry (VLBI) to obtain shadow images of Supermassive Black Hole (SMBH). Sub-mm VLBI is thought to be the only way so far to get the direct image of SMBH by its shadow, thanks to the superb angular resolution and high transparency against dense plasma around SMBH. At the Summit Station on Greenland, we have started monitoring the opacity at sub-mm region. The Summit Station subtends long baselines with the Atacama Large Milimeter/submillimeter Array (ALMA) in Chile and Submillimeter Array (SMA) in Hawaii. In parallel, we started retrofitting the ALMA North America prototype telescope (renamed as Greenland Telescope: GLT) for the cold environment.