Knowledge of the earth's gravity field continued to increase during the last four years. Altimetry data from the GEOS-3 satellite has provided the geoid over most of the ocean to an accuracy of about one meter. Increasing amounts of laser data has permitted the solution for 566 terms in the gravity field with which orbits of the GEOS-3 satellite have been computed to an accuracy of about one to two meters. The combination of satellite tracking data, altimetry and gravimetry has yielded a solution for 1360 terms in the earth's gravity field. A number of problems remain to be solved to increase the accuracy of the gravity field determination. New satellite systems would provide gravity data in unsurveyed areas and correction for topographic features of the ocean and improved computational procedures together with a more extensive laser network will considerably improve the accuracy of the results

Anderle, R. J.

English

NASA Technical Reports Server

N79 21487
Gravity Field, Geoid and Ocean Surface by Space Techniques
R. J. Anderle
Naval Surface Weapons Center
Dahlgren, Virginia 22448
Abstract. Knowledge of the earth's gravity field
continued to increase during the last four years.
Altimetry data from the GEOS-3 satellite has provided
the geoid over most of the ocean to an accuracy of
about one meter. Increasing amounts of laser data
has permitted the solution for 566 terms in the gra-
vity field with which orbits of the GEOS-3 satellite
have been computed to an accuracy of about one to two
meters. The combination of satellite tracking data,
altimetry and gravimetry has yielded a solution for
1360 terms in the earth's gravity field. A number of
problems remain to be solved to increase the accuracy
of the gravity field determination. New satellite
systems would provide gravity data in unsurveyed
areas and correction for topographic features of the
ocean and improved computational procedures together
with a more extensive laser network will considerably
improve the accuracy of the results.
Major improvements in our knowledge of the earth's
gravity field have been obtained since the GEOPS 4
Conference on The Geoid and Ocean Surface in August
1973 (Rapp, 1974). A dramatic improvement was ob-
tained by use of the GEOS-3 satellite altimetry da-
ta, which has defined the geoid over most of the
ocean areas to about a one meter accuracy (Brace,
1977, Hadgigeorge in press, Kahn et. al. 1977,
Marsh et. al. 1978, Marsh et. al. in press, Rapp
1978, Rapp in press, Yionoulis et. al., in press).
In each solution for the geoid based upon the al-
timetry data, errors in the satellite position were
larger than uncertainties in the altimeter measure-
ments. In order to reduce the effects of the orbit
errors, bias parameters for each satellite pass were
determined which minimized the discrepancy between
geoid determinations at intersections of north-west-
erly satellite subtracks with south-westerly subtracks
Some analysts further reduced the dependence of the
geoid determination on the accuracy of the satellite
orbit by fitting the altimetric data to a low order
reference geoid. Anderle (1977) raised the question
of biases in the computed geoid arising from syste-
matic errors in the computed satellite orbits, but
Rapp (private communication) found that the different
approaches to the computation of the geoid agreed
to about one meter. The computed ocean geoids are
also subject to effects of ocean tides and geostro-
phic effects. However, a new model of the princi-
pal ocean tide constituent for one degree areas
agrees with tide measurements to 5 cm (Schwiderski,
in press), and models for six additional consti-
tuents have been computed by Estes (1977).
Solutions for the earth's potential continued
to include an ever increasing number of coefficients
as additional satellite observations were acquired
at higher levels of precision and as computer pro-
Proc. of the 9th GEOP Conference, An International Symposium on the Applications of
Geodesy to Geodynamici. Oclobfr2-S, 1978. Depc. of Geodetic Science Rept. No. 280, The
Ohio State Univ.. Columbus, Ohio 43210.
grams were extended. High quality laser data played
an important role in the solutions by Gaposchkin
(in press) and Lerch (1978). Table 1 lists the more
recent solutions obtained at various agencies. Some
solutions were based solely on observations of satel-
lite motion while others, indicated by "general"
under the column headed optimization, included gravi-
metric, astro-geodetic, and/or altimetric data. The
solutions allowed the computation of the GEOS-3 satel-
lite altitude to an accuracy of 1.5 m (Douglas and
Anderle, 1977). The latest solution is believed to
represent the ocean geoid to an accuracy of one or
two meters (Lerch, et.al., 1978).
Another new source of data on the earth's gravity
field acquired recently was from satellite-to-satel-
lite tracking. Data were acquired between two low
orbiting satellites in the Apollo-Soyuz experiment
and also between high and low satellites, ATS-F and
GEOS-3. Agreement of gravity anomalies for two de-
gree and five degree squares computed from high-low
satellite to satellite tracking data with anomalies
computed from this data was around five to ten milli-
gals (Hajela, 1977, Vonbun 1977, Vonbun 1978).
Highly precise values for effects of gravity co-
efficients of specific order have been obtained by
analysis of resonance effects on satellite motion
(King-Hele, et. al., 1978, Klokocnik, 1978, Reigber
in press) and evaluated by Wagner (in press). Such
results are useful in evaluation of general solutions
based on less sensitive data.
A number of outstanding problems need to be ad-1
dressed in the course of further refinement of our
knowledge of the gravity field:
1. drag and solar radiation effects on satel-
lite motion,
2. other small effects,
3. correlation of coefficients,
4. combination of heterogeneous data,
5. computational problems, and
6. instrument limitations.
The following comments expand briefly on these
problems:
1. Changes in the atmospheric drag due to solar
radiation and changes in magnetic flux affect the
motion of satellites in ways which can be misinter-
preted as effects of the earth's gravity field. The
effects of direct solar radiation and earth's albedo
on the more complicated satellite satellite configu-
ration can similarly be misinterpreted.
2. Other smaller effects which must be accurately
modeled or determined include
a. crustal motion,
193
https://ntrs.nasa.gov/search.jsp?R=19790013316 2020-03-22T00:00:42+00:00Z
inents,
b. polar motion and earth's rotation,
c. solid earth, ocean and atmospheric tides,
d. ionospheric effects on electronic measure-
e. tropospheric effects on measurements, and
f. ocean topography on geoids computed from
altimetric data.
3. The separation of gravity coefficients computed
from observed satellite motion is made difficult by
the limited number of satellites with different or-
bital motion for which accurate observations are
available because satellites with different orbital
inclinations or orbital periods are sensitive to
different orders and degrees of gravity coefficients.
Yet, apart from resonance effects, satellite motion
is insensitive to high order terms in the earth's -
gravity field. Separation of the coefficients is
possible through the use of altimetric data over the
oceans and gravimetric data. But such data are not
available in many regions, and the precision of such
data is insufficient to compute accurate satellite
orbits.
A. Neglect of systematic instrument and envi-
ronmental effects, truncation of the gravity field
representation, and other model errors have generally
yielded standard errors of solutions for gravity
coefficients which are overly optimistic. As a re-
sult, solutions based on combinations of various types
of data have usually required the use of arbitrary
weights for the different data classes employed in
order to obtain a reasonable contribution to the so-
lution from each set of data.
5. The computation of the gravity field from
observed satellite motion is a costly undertaking.
The high cost is one factor which limits the number
of coefficients used to define the gravity field in
such computations, and the frequent use of an incon-
sistent number of gravity parameters for different
data which are then combined in a single solution.
6. Instrument limitations for lasers include
weather and the cost of installations. S-band ra-
dars require a transponder on the satellite, have
limited availability due to their heavy workload, and
are subject to ionospheric refraction errors. Dop-
pler receivers require a transmitter on the satellite
and are of lower precision than lasers on S-band
radars.
Solutions to many of these problems will be
addressed by the panel members. Gaposchkin and
Smith will discuss primarily advanced computational
techniques; Fishell and Reigber will discuss ad-
vanced measurement techniques, and Whitehead will
address the topographic effects on the ocean geoid.
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Anderle, R. J. , L. K. Beuglass and M. G. Tanenbaum,
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Naval Surface Weapons Center Technical Report,
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AGENCY
DMA
NSWC
NSWC
NOAA
GRGS/SFB
SAO
GRAVITY FIELD DETERMINATIONS
NUMBER OF
DESIGNATION COEFFICIENTS
DOD WGS-72 472
NWL 10-E 401
NWL 1G 396
P = 4 252*
GRIM 2 950
SE VI. 3 604
OPTIMIZATION
General
NAVSAT
GEOS-3
General
General
General
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196


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