16 October, 2009

To:	Hans-Peter Plag

From:	Pete Bender

Subject:  Roadmap and Resolutions

Hi Hans-Peter,

There are two main areas where I believe important changes need to be made,
both in the Roadmap and in the Workshop Resolutions.  The recommended changes
in both of these areas are consistent with the suggestions I made during the 
discussions of the Roadmap and the Resolutions at the Workshop.  

The most significant issue is what is said concerning Medium-Term 
Developments in the Roadmap and concerning the Medium-Term Perspective in the 
Resolutions.  Assuming two or more pairs of satellites with laser 
interferometry, it is now clear that only the approach of having one pair in 
a moderate inclination orbit can give a strong determination of the 
short-wavelength equipotential variations in the east-west direction over 
most of the globe.  These short-wavelength variations are key to improving 
the spatial resolution.  My talk at session B-2 emphasized the limitations of 
including only polar orbits, and I believe simulations by a number of other 
groups have also supported including a moderate inclination pair.  Thus,
although continuing simulations certainly are needed to pin down the best
choices of orbit parameters, I strongly recommend the following changes:

  1.  Roadmap, section 2.2, 2nd paragraph:  add at the end:  "At present, two
      or more pairs are preferred, with one pair probably in a moderate 
      inclination orbit."

  2.  Recommendations, 5), 2nd paragraph:  change to read as follows:
      "From today's point of view, laser satellite-to-satellite tracking
      (active laser interferometry) is the most probable candidate
      technology.  Two or more satellite pairs are preferred, with one pair
      probably in a moderate inclination orbit."  (Very few Workshop
      participants appeared to support the Alenia recommendation of using
      retro-reflectors, and the term active laser interferometry doesn't 
      exclude the use of retro-reflectors, as far as I know.)

The second main area concerns the Long-Term Developments in the Roadmap and
the Long-Term Perspective in the Resolutions.  Let me start with
Recommendations, 6).  The first paragraph discusses "a continuous series of
gravity field satellites" for monitoring mass redistribution.  However, I
have not heard any argument for "accelerometers and gradiometers based on
atom interferometry" or for "optical clocks" being plausible candidates for
this, even in the long-term future, as discussed below.

For atom interferometry, the only argument I have heard for accuracy and
resolution improvement involved many, many extremely simple satellites.
This was Candidate Concept 2 in the presentation given by Malte Schmidt at 
the Workshop.  However, I don't see how such satellites could be simpler than 
the drag-free pairs of satellites with laser interferometry that are expected 
to be used in the medium-term.  Thus the cost would be much higher.  Also, it 
seems clear that a complex system of at least 6 reference satellites in 
stable orbits would be needed in order to keep track of the locations of all 
the satellites to the required extremely high accuracy.  I've had some 
correspondence with Malte Schmidt after the Workshop, and I'd be glad to 
provide more information about it if you have time to look into this issue.

Concerning the optical clocks, the paper at the Workshop given by Axel 
Goerlitz discussed portable optical clocks for use on the ground, plus a
reference optical clock in a stable orbit for frequency comparisons.  No
"gravity field satellites" for monitoring mass redistribution were mentioned,
as far as I know.  And, I haven't understood why an optical clock in a stable
orbit is needed for frequency comparison, even at the highest possible
accuracy.  A very precise frequency transponder in geosynchronous orbit is
all that appears to be needed to compare transportable optical clocks with
reference optical clocks at stable locations.  Furthermore, it isn't clear
that transportable optical clocks with 1x10-18 accuracy would offer a
significant improvement in monitoring mass redistribution on the Earth.  I
have high regard for the co-authors on the talk at the Workshop, but I think
they have in mind mostly things like removing discrepancies between the 
geoids on different continents, rather than monitoring rapid changes in the 
Earth's mass distribution.  I'd be glad to go into this further, if you have 
time to discuss it during the workshop in Boulder next week.

Finally, my recommendations for the second area of changes are as follows:

  1.  Roadmap, section 2.3:  delete the second sentance:  "Examples of
      innovative technologies include accelerometers and gradiometers based
      on atom interferometry, gravity measurements through optical clocks 
      exploiting general relativity principles."

  2.  Recommendations, 6), 2nd paragraph:  delete the 2nd sentance:  "These
      include accelerometers and gradiometers based on atom interferometry,
      as well as gravity measurements through optical clocks exploiting
      general relativity principles."

I am strongly in favor of the development of 10-18 accuracy optical clocks
for use in space, and just finished working on a white paper arguing for this
that is being submitted in connection with a NAS Decadal Survey on Biological
and Physical Sciences in Space.  However, the argument for this is based on 
much improved fundamental physics tests in space, not geophysical 
applications.  I also believe that atom interferometers should be developed
for use in space for other fundamental physics applications, but I
unfortunately don't see how either of these can be justified for Earth 
gravity applications.

Sincerely,  Pete Bender