A role of resonances for the earth's gravity field determination: 
                     past, present, and future

Jaroslav Klokocnik, Astronomical Institute, Czech Academy of Sciences,
          CZ - 251 65 Ondrejov Observatory, the Czech Republic



Orbital resonances of the Earth's artificial satellites were and are
used to evaluate  linear combinations of  the  harmonic geopotential
coefficients (lumped coefficients). This data source, if independent
of comprehensive gravity field models (based on satellite as well as
terrestrial data),  has often been used  to test the accuracy of the 
harmonics of the relevant (resonant) order(s).  The resonant results
(primarily the lumped coefficients  as  "directly"  observable quan-
tities) are -- from physical viewpoint  --  potentialy more accurate
than any others from satellite data and would be still more accurate
if only there were suitable "resonant" satellites precisely tracked.
(Many satellites cannot be tracked too accurately  because they have
not laser retroreclectors or other (active) advanced tools for orbit
determination).

During the last decade, the accuracy of the gravity field models has
increased substantially; the role of the resonant (lumped) coeffici-
ents is limited  excluding the lowest degree and order coefficients.
These coefficients and their time derivatives can be determined from
high-orbiting satellites (geostationary near 1:1 resonance, and ETA-
LONs or even from GPS orbits at 2:1 resonance),  see project COGEOS.

But  the concept of the "lumped values" is a natural one, alive, and
has been recently used to define  "latitudinal lumped coefficients",
derivable from satellite altimetry cross-over data.  The method will
presented and the accuracy assessments of recent gravity models like
JGM 2, 3 or GRIM4S4  with satellite altimetry from  ERS-1 and TOPEX/
Poseidon will be shortly reviewed.



resonances
Earth artif. satellites
Earth gravity field
astronomical standards - accuracy assessment
lumped geopotential coefficients
latitude lumped coefficients
satellite cross-over altimetry


*******************************
Jaroslav Klokocnik,

Astronomical Institute, Academy of Sciences,
the Czech Republic
CS - 251 65  Ondrejov Observatory

Phone:  (0042)
office       204 857 158 (direct)
             204 85 201, ext. 158
Fax:    (0042) 2 88 16 11 [via Praha]
Telex:    12 15 79     astr c
E-mail:   jklokocn@asu.cas.cz
          or
          ASTFOZ@CSEARN.BITNET
*******************************