  In the next decade or two, extremely accurate tests of general relativity
under extreme conditions are expected from gravitational wave observations of
binary black hole mergers with a wide range of mass ratios. In addition, major
improvements are planned in both strong and weak equivalence principle tests;
clock measurements based on the ACES program on the ISS; more accurate
light-bending measurements; and other new types of tests. However, whether
these tests are all consistent with general relativity or not, it still appears
desirable to proceed with a much improved measurement of the Shapiro time
delay. A suggested approach is based on using a high-quality optical clock in a
drag-free spacecraft near the sun-earth L1 point and a smaller drag-free
transponder spacecraft in a two-year period solar orbit. Laser phase
travel-time measurements would be made between the two spacecraft over a period
of 10 or 20 days around the time when the line of sight passes through the Sun.
The requirements on the optical clock stability and on the drag-free systems
will be discussed. The accuracy achievable for the time-delay appears to be
better than 1 part in 100 million.
