Amplitude Calibration System
                       Prototype Development Plan v2.0


Prototype 1: Semi-Transparent Vane System

             The tests of the first prototype of a semi-transparent
             vane (STV) calibration system have been made at the IRAM
             30m telescope, with encouraging results.

             Further tests with this system are required to confirm
             that the STV concept can work at the 3% level.  The
             test that needs to be made is to check that the losses in
             the vane are due to absorption to within 3%.  This test
             will require a measurement of the transmission of the
             vane on an astronomical source with very good weather
             conditions.  Unfortunately, the 30m telescope is unavailable for
             further tests; the feasibility of some astronomical tests at
             the ATF with evaluation front ends is being investigated.  
             Tests are needed at frequencies for which both
             ALMA and the ATF operate, with 1.3mm and 3mm most critical.

Prototype 2: Wire Grid Amplitude Calibration System

             Design and construction of a test prototype for a more
             advanced device containing a polarization grid has been
             stalled.  This second prototype will allow us to study the
             anticipated advantages of using a grid, which are
             anticipated to allow calibration accuracy of 1%.  In
             fact, in the previous observing tests made with Prototype
             1 at the IRAM 30m telescope, some preliminary
             measurements with the grids of the 30m receivers have
             been made.  These tests were quite encourageing, and we
             are confident that tests could be made which would allow
             for a precision of about 1%.  These tests will be pursued at
             (insert name of lab at Madrid) at a maximum frequency of
             60 GHz.  Tests are needed at frequencies
             for which ALMA operates, with 1.3mm and 3mm most critical.
             These tests might be pursued at the ATF.

Prototype 3: Multi-Load Amplitude Calibration System

             This more advanced design would incorporate the concepts
             detailed in ALMA Memo 461.  Three types of couplers will be
             tested and compared: semi-transparent vane; wire grid;
             dielectric film.  An ALMA design will be developed, and
             testing will be performed at the ATF telescopes with ALMA
             prototype front ends, currently planned to be available
             for Q3 2005. Tests are needed at frequencies
             for which both ALMA and the ATF operate, with 1.3mm and 3mm
             most critical.