One of the main objectives of the Cosmic Lens All-Sky Survey ( CLASS ) collaboration has been to find gravitational lens ( GL ) systems at radio wavelengths that are suitable for the determination of time delays between image pairs .
The survey is now near completion and at least 18 GL systems have been found .
Here , I will discuss our efforts to measure time delays from several of these systems with the ultimate aim of constraining the Hubble Constant ( H _ { 0 } ) .
Thus far three CLASS GL systems ( i.e .
B0218+357 , B1600+434 and B1608+656 ) have yielded measurements of time delays , from which values of H _ { 0 } \approx 60–70 km s ^ { -1 } Mpc ^ { -1 } have been estimated .
Although most GL systems give similar values of H _ { 0 } , statistical and systematic uncertainties are still considerable .
To reduce these uncertainties , I will shortly mention two monitoring programs that we are undertaking to ( re ) measure time delays in 15 CLASS GL systems and address several important issues for the near future .