A brief discussion of the AFGL's (Air Force Geophysics Laboratory) program in absolute gravity is presented. Support of outside work and in-house studies relating to gravity instrumentation are discussed. A description of the current transportable system is included and the latest results are presented. These results show good agreement with measurements at the AFGL site by an Italian system. The accuracy obtained by the transportable apparatus is better than 0.1 microns sq sec 10 microgal and agreement with previous measurements is within the combined uncertainties of the measurements

Hammond, J. A.

Iliff, R. L.

English

NASA Technical Reports Server

The AFGL Absolute Gravity Program
James A. Hammond and Robert L. Iliff
Air Force Geophysics Laboratory, Terrestrial Sciences Division
Hanscom AFB, Massachusetts 01731
N79 21495
Abstract. A brief discussion of the AFGL's pro-
gram in absolute gravity is presented. Support of
outside work and in-house studies relating to grav-
ity instrumentation are discussed. A description
of the current transportable system is included and
the latest results are presented. These results
show good agreement with measurements at the AFGL
site by an Italian system and with previous meas-
urements by Hammond and Faller. The accuracy
obtained by the transportable apparatus is better
than 0.1 urn/sec^ (10 ugal) and agreement with pre-
vious measurements is within the combined uncer-
tainties of the measurements. The instrument will
be used extensively for field measurements in 1979.
The Air Force Geophysics Laboratory's program
in absolute gravity can be divided into three main
areas: support of outside research into measure-
ment techniques and of comparative measurements by
other absolute instruments; the study of the
physics of the measurement techniques and the
development of new instrumentation; and measure-
ments in the laboratory and at selected field sites
with the AFGL transportable system. The predomi-
nant focal point of this paper is the work on
current measurements, but in the interest of
completeness we will also briefly discuss the first
two areas.
The outside work supported by AFGL includes that
of Dr. James Faller and Mr. Robert Rinker of the
Joint Institute for Laboratory Astrophysics in
Boulder, Colorado (JILA/NBS). AFGL has supported
the development of a novel system for the isolation
of a reference reflector in an interferometer type
of absolute gravity instrument. This system, which
uses an electro-mechanical feedback system to syn-
thesize a very long period vertical mass-spring
support, is being designed and built into a package
which should be capable of directly supporting the
reference reflector on a gravity instrument.
AFGL supported the visit to the U.S.A. of the
transportable system developed by the 1st: tuto di
Metrologia "G. Colonetti" (IMGC) of Torino, Italy
with the cooperation of the Bureau International
de Poids et Mesures (BIPM). This work was sup-
ported by a grant to the IMGC administered through
the European Office of Aerospace Research and
Development as well as by in-house support from
AFGL and the Defense Mapping Agency Geodetic Survey
Squadron (DMA/GSS). The work involved transporting
the equipment and two people to six sites (Hanscom
AFB, MA; Denver, CO; Holloman AFB, NM; San Fran-
cisco, CA; Bismarck, ND; Miami, FL) . The system
had a mass of about 1500 kg when packaged for air
transport and the entire operation required six
weeks to complete (with a final remeasurement at
Hanscom AFB( seven measurements were made). The
uncertainty obtained was about .1 urn/sec^ (10
at most sites. [Marson and Alasia, 1978]
Proc. of (he 9th GEOP Conference. An Intermttfannl Symposium int the Applk-nt'nm* nf
<7<Wr.tv in Ciemlymimks. Ofl«her2-5.197S. Depl. of Geodetic Science Rept. No. 280, The
Ohio State Univ.. Columbus. Ohio 43210.
In the area of studying the physics of measure-
ment techniques we are planning to do accurate
measurements of the effect of air resistance on the
free fall type of measurement. The current data
analysis method allows very small effects to be
seen in the deviation of the fall of the reflector
from a purely uniform acceleration.
Another concern is that the laser wavelength
standard is reproducible and stable. Periodic
measurement of the laser used in our system will
be done to assure our wavelength standard is not
perturbed by time or the effects of transporting
the equipment.
AFGL is looking at new developments in electron-
ics and other areas to solve some of the current
problems with this kind of instrumentation. In
particular, several techniques for making the sys-
tem simple to operate are employed in the current
AFGL system. The system is completely automated
and data are analyzed and corrected for gravity
tides in real time. Optical and mechanical align-
ment are simplified over previous systems and self-
checks on timing accuracy can be performed inde-
pendent of a gravity measurement. A new timing
and data analysis system is being obtained that
should have increased reliability over the current
system.
Measurements are currently being made with a
system that incorporates the mechanical parts from
the first generation instrument [Hammond, 1970],
and uses a control system and support base (with
optics) built at AFGL. The timing and data
analysis equipment were integrated by JILA.
Figure 1 shows schematically the absolute gravity
system. The laser length standard is a Lamb-dip
stabilized He-Ne laser which is periodically
compared with an Iodine stabilized laser in our
laboratory. The oscillator is a Rubidium fre-
quency standard and the timing electronics allow
very precise (i 125 psec) measurement of a large
number of time values during the fall of the
reflector. The reference system in the current
arrangement is simply a retroreflector mounted to
the base with no seismic isolation.
Figure 2 is a photo of the system as it looked
at the time of this symposium. The vacuum chamber
has been reduced in height by 45 cm so that the
free fall path is now about 60 cm. A smaller
vacuum pump is used (30 I/sec pumping speed) and
the pump magnetic field is reduced considerably
from the earlier system. An "old fashioned",
simple free fall technique is used because several
apparently inherent problems obtained with a
"chamber-in-a-chamber" system, resulting in our
setting that chamber aside, at least temporarily.
This system has a total mass of about 700 kg when
packed for air transport and it is contained in
nine or ten boxes which can be handled by one or
two people.
The first field measurements were made in June
of 1978, approximately six months after the deci-
sion to convert the old vacuum chamber for use with
the new system. At the time of that field trip we
were using a computation technique that used 150
245
https://ntrs.nasa.gov/search.jsp?R=19790013324 2020-03-21T23:59:42+00:00Z
time measurements from three different positions in
the free fall path. This technique produced good
statistics and fairly .good repeatability at the
AFGL site, but a systematic effect' was known to
exist prior to the June 1978 field trip.
In spite of this systematic effect, (which has
since been eliminated) several important things
were demonstrated by these field measurements at
Denver, CO, Holloman AFB, MM, and San Francisco,
CA:
1. Portability of the system, short operating
time (three sites in ten days)
2. 10 ugal accuracy is possible at most sites
3. Sites must be chosen carefully.
The third result expresses a difficulty that
proved to be very serious in San Francisco. The
site chosen was the actual IGSN-71 site in a museum
in Golden Gate Park. Preliminary measurements with
a short period seismometer did not indicate such a
serious noise problem, but it was so bad that we
were only able to get a standard error of .15 urn/
sec2 (15 with about 2400 drops. The IMGC
system didn't experience as much trouble because
they use a seismometer for an inertial support of
the reference retroref lector .
In August of 1978 a least-squares program was
adopted for doing the data analysis . this is a
program in which the positions (Xi) and the times
(tj) are fit to a constant acceleration formula:
i = 1/2 g
The results are: g, S0 (the last two are of no
interest, except to fix the positions of the meas-
urement), and a table of residuals for each drop.
The residuals for each position in the path can be
averaged and then plotted as in Figure 3. These
residuals represent the deviation of the relative
path difference between the reference reflector ar.
the freely falling reflector from what it would be
if the reference reflector were not accelerating .at
all, and the free falling reflector were acceler-
ating uniformly at g. Thus, a vibration of the
reference reflector at a constant frequency.appears
as a vibration whose frequency in space decreases
as the falling reflector moves to the bottom. In
Figure 3, then, the ordinate gives the magnitude of
these average residuals in Angstroms and the
abscissa gives the position of the object in the
vacuum chamber measured from the start of the meas-
urement (approximately 8 cm from the zero velocity
position).
Even with the vibration shown in Figure 3 the g
value obtained showed a much reduced bias when
compared with the IMGC measurement and with the
older measurement [Hammond and Faller, 1970] . The
repeatability and the standard error of the g value
were as good as before. A rough calculation showed
that even this small vibration, initiated by the
release of the falling body, could produce a
systematic effect of the order of .60 urn/sec2. In
fact, the g value then obtained agreed with the
IMGC to within the uncertainties of the measure-
ments .
We decided to get rid of the vibration even
though it didn't cause a large bias with the least
squares analysis technique. To do this the chamber
was isolated from the reference reflector and the
rest of the optics by placing it on a separate
vibration isolation system. The results of aver-
aging 150 drops worth of data are shown in Figure 4.
The solid line is the result of subjecting syn-
thetic data, with a 3 p.m/sec /m gradient included,
to the same least squares analysis. Thus most of
the systematic appearance, if not all, is caused by
the vertical gradient. If one attempts to fit the
MECK
SYSTEM
/V
REF.
SYSTEM
/A
OPTICAL INTERFEROMETER
^
LASER LENGTH STANDARD
OPTICAL
FRINGE
SIGNAL
(MODULATED
SINE WAVE)
COMPUTATION A
READOUT
A
\ID
TIMING ELECTRONICS
OSCILLATOR TIM
STANDARD
E
ABSOLUTE GRAVITY INSTRUMENT
SCHEMATIC
Fig. 1. Schematic diagram of a system for meas-
uring absolute gravity.
246
Fig. 2. The AFGL Transportable Absolute Gravity
Instrument.
t 0.03 urn/sec
. ±0.03 un/sec2
.25 + 0.01 mn/sec2
+ 0.03 mn/sec2
+ 0.05 nm/sec2
gradient as an additional parameter, the least
squares fit becomes poorly determined and the g
values have a higher scatter.
The most recent value obtained at AFGL on pier 1,
Haskell Observatory is:
Measured Value 9803783.21
Gradient Correction + 3.77
Velocity of Light Correction
Wavelength Uncertainty
Estimated Uncertainty for
Atmospheric Pressure and
Other Possible Systematic
Effects
Value at Floor Level 9803786.73 + 0.07 urn/sec2
This can be compared with the IMGC 1978 measure-
ments and the 1970 Hammond-Faller value at this
site.
Marson and Alasia (1978) 9803786.59 +0.10 urn/sec2
Hammond and Faller (1971)9803786.71 ± 0.42 urn/sec
There is a discrepancy at close to a significant
level between the AFGL and IMGC values. Effects
such as water table level and air pressure could
contribute to the difference. Pressure changes
could result in as much as 0.10 urn/sec2 full swing.
Those who use these data will have to do the best
they can (weighted means) to incorporate all
possible measurements at any site. Future opera-
tors of these types of instruments must make it a
rule to make some notation about the barometric
pressure. Discrepancies in gravity tide effects
could also contribute to these differences. It is
possible to perform corrections for tides which
fully include the effects of ocean loading.
In summary, the AFGL program in absolute gravity,
which contributed so significantly to the IGSN-71
with absolute gravity measurements at eight sites,
will continue to produce significant results and
advances in the future. There are several impor-
tant ways for absolute measurements of gravity to
help in solving problems in geodynamics. The
necessity for accurate calibration lines for rela-
tive instruments, as well as for the absolute
values for networks and at sites of special signif-
icance, should be obvious enough that looking at
instrument acquisition as a long term goal would
appear more important than making patchwork meas-
urements with prototype instruments. The agencies
247
-30
GRAVITY RESIDUALS (ANGSTROMS)
NEW BASE
50 DROP AVERAGES
•20
-10
. . 10 CM 20 CM 30 CM 40 CM 50 CM
-10
-20
-30 Fig. 3. Average residuals from least squares fit
of gravity data to quadratic equation showing
systematic vibration of reference reflector.
- 10
GRAVITY RESIDUALS (ANGSTROMS)
CHAMBER SUSPENDED
R:AL DATA aso DROP AVERAGES)
THEORETICAL DATA - . 3 MGAL/M GRADIENT
Fig. 4. Average residuals showing effect of
isolating vibration from retroreflector and theo-
retical curve for gravity data with a gradient
included.
50 CM
248
interested should be looking at more than just the
numbers describing the results. They should look
at the systems with an eye to seeing them become
operational instruments rather than specialized
systems which only a Ph.D. physicist can operate.
References
Hammond, J., A laser-interferometer system for the
absolute determination of the acceleration of
gravity, Report, AFCRL-70-0163, Air Force
Cambridge Research Laboratories, Bedford, Mass.,
March 3, 1970.
Hammond, J., and J. Faller, Results of absolute
gravity determinations at a number of different
sites, J. Geophys. Res., 76, 7850, 1971.
Marson, I., and F. Alasia, Absolute gravity meas-
urements in the United States of America,
Report, AFGL-TR-78-0126, May 1978.
249


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