
Interview to Dr. E. Myles
Standish from JPL/NASA ( Jet Propulsion Laboratory )
By Rui Miguel Fernandes
Summary: The JPL ephemeris files, are the most
accurate in the world. The man behind its calculation is Dr E. Myles Standish,
that worked for years in the JPL's headquarters in research  still he hasn't
stopped.
He's the reason why Cosmos hasn't stopped trying to improve itself after 8
years...
This interview was conducted before his retirement.
Let's find out a little more about him.
Cosmos  This is not exactly a question, but talk a little bit
about yourself  where did you graduate, etc.
Dr.
E. Myles Standish  BA and MA: Wesleyan University in Connecticut; PhD:
1967 Yale University, also in Connecticut. After I received my degree at Yale, I
stayed there and taught astronomy for four years. Then I came to JPL where I
have worked ever since. At JPL, I have always concentrated on the planetary
ephemerides (knowledge of where the planets are and how they are moving). So,
I'm like a mapmaker; but, the map is in motion. The navigation team at JPL
needs to know this information when they send a spacecraft to one of the
planets. As the years have gone by, they have wanted the information to be more
and more accurate.
Most of my papers are related to the ephemerides and are published in the major
astronomical journals. Once in a while, I have published the results of some
historical research, but even then, these have been subjects somehow related to
planetary positions. The JPL planetary and lunar ephemerides are used throughout
the world; the planetary positions in all of the major national almanacs are
based upon the JPL ephemerides: previously, "DE200";now "DE405".
Cosmos  Are you a personal observer of the stars and planets,
that is, are you a pratical astronomer, or do you prefer the math related to it
 the pencil and the computer?
Dr. E. Myles Standish  I never use a telescope, but I
certainly do use measurements made by others using optical telescopes (now with
CCD's attached), radio antennas (planetary radar and VLBI), lunar laser ranging,
and spacecraft tracking data (ranging and doppler). On the other hand, I do
spend most of my time on a computer, processing the measurements and the motions
and positions of the planets and moon.
Cosmos  Is celestial mechanics your favourite issue in the
astronomy field?
Dr. E. Myles Standish  My job requires celestial mechanics,
astronomical measurements, physics, numerical analysis, and computing. I think
that my favorite is the computing. I have seen the amazing improvements made in
computers over the past 4 decades, so I am fully aware of how powerful (and
inexpensive) today's computers are. It's a joy to have them and use them.
Cosmos  When and how was created the idea of generating the
JPL ephemeris files? When we're they computed?
Dr. E. Myles Standish  In the 1960's, JPL realized that the
existing ephemerides were not good enough for spacecraft navigation, so they had
no choice: JPL had to improve the ephemerides themselves: there is a big saving
of fuel if the ephemeris is good: then one does not need to make big midcourse
corrections to compensate for a bad target position. We have been improving the
ephemerides ever since. When a significant improvement becomes possible (more
observations, more accurate observations, better handling of the observations,
etc.), then we create a new ephemeris.
Cosmos  What principles did you use in Physics to elaborate
them and what computational techniques we're required ?
Dr. E. Myles Standish  Three things are needed in order to
create an ephemeris:
a) the equations of motion which are the forces acting upon the planets and
moon  these come from our knowledge of gravitational physics,
b) a computer program to numerically integrate the equations of motion
(analytical formulae are not accurate enough for modernday ephemerides), and
c) the initial conditions (positions and velocities at some given date) and
related constants (masses, etc.)  these are determined by a least squares
adjustment of them in order to make the ephemerides fit the observational
measurements as well as possible.
Cosmos  How accurate are them?
Dr. E. Myles Standish  The measurements of Sun, the four
innermost planets (Mercury, Venus, Earth, Mars), and the Moon are accurate to a
few tens of meters, due to the highly accurate measurements of them. However,
since the asteroids affect the motions, and since the masses of the asteroids
are not well known, it is not possible to compute the forces from the asteroids
with high accuracy. Therefore, the positions and motions of the inner planets
are known to an accuracy of about 1 kilometer.
The accuracy of the outer planets is much worse: 100 km at Jupiter on out to a
few thousand kilometers at Pluto. This is because the only measurements of the
outer planets are optical  much less accurate than the radar and spacecraft
measurements of the inner planets.
Cosmos  What we're the primary and secondary difficulties in
their calculation in the physics and computational areas? How did you manage to
program the initial conditions and physical constants?
Dr. E. Myles Standish  The key for the planets is the variety,
accuracy, and timecoverage of the observations. These allow the accurate
adjustment of the initial conditions and constants: in effect, one uses the
existing ephemeris, produces what that ephemeris predicts for the value of each
measurement, and then compares these "computed" values with the "observed"
values. A mathematical computation ("least squares", invented by Gauss) then
gives what corrections should be applied to the initial conditions which were
used for the existing ephemeris. Finally, one adds these corrections to the
existing initial conditions and uses the new initial conditions to produce a new
ephemeris.
Cosmos  Someone  a portuguese mathematician  asked me: why
Chebychev polynomials in the interpolation ?
Dr. E. Myles Standish  Chebychev polynomials seem to be
accurate, economical, and efficient. Of course, they are not the only choice,
but certainly they are a good one.
Cosmos  Can we make a brief comparison chart in terms of
accuracy of the JPL ephemeris and, let's say the VSOP87 or the ELP2000/82?
Dr. E. Myles Standish  VSOP87 and ELP2000/82 were fit to JPL's
DE200 (an ephemeris that we made 24 years ago). Therefore, it doesn't make much
sense to compare them with DE200. If the comparison were to be made and if it
showed any significant differences, it would come from the weakness of the fit
of those ephemerides to DE200. On the other hand, EPM2000 from my colleagues in
St. Petersburg, Russia, was made in a way very similar to the ephemerides made
at JPL. Comparisons with that ephemeris are quite useful, though one must keep
in mind that both EPM2000 and the JPL ephemerides have used mostly the same
observational measurements.
Cosmos  Are there any plans for improve the ephemeris or, more
important, to expand the time span?
Dr. E. Myles Standish  The ephemerides are improved every few
years or so  when more and better measurements become available. Also, we have
extended the coverage of DE405 from 10,000 BC to 10,000 AD. It's fairly easy
now, with the fast modern computers that are available.
Cosmos  What are you currently doing in the JPL headquarters 
what kind of research?
Dr. E. Myles Standish  Currently, we are processing spacecraft
measurements from the MGS and Odyssey spacecraft, both of which are orbiting
Mars. The measurements are both ranges (distances) and VLBI (angular
directions). The combination is giving us excellent positions of Mars at the
present time  useful for the upcoming arrivals of spacecraft at Mars this month
and next month.
Cosmos  What are your future plans of research?
Dr. E. Myles Standish  I am in the process of writing
descriptions of the whole ephemeris process so that others here can assume the
job of refining the ephemerides.
Cosmos  Would you like to leave a message to all that follow
your work?
Dr. E. Myles Standish  To anyone who follows this work, I
would like to say that I hope you enjoy this field as much as I have; it has
been interesting for me and has given me great satisfaction.
Cosmos  Thank you very much for this time.
Dr. E. Myles Standish  You're very welcome.
