It remains doubtful whether the indirect ocean tide effect can be seen with an
orbiting gradiometer since the signal level seems at the noise floor of a cryogenic
instrument. Compare also (Paik,1986) who claims 10- 4 E/VHz for the proposed
SGGM although it should be mentioned that Paik (pers.comm.) recently suggested
that a further increase in accuracy down to 10- 5 E / V Hz may be feasible. A fundamental question seems to be whether the goal of a GRM mission is to observe the
static field or whether it should be used for measuring temporal gravitational effects
as a result of mass variations being the result of geophysical processes.
How to deal with the problem
To reduce the indirect ocean tide effect in the orbit of a GRM one should follow
an approach to integrate the variational equations and to estimate the alma and f3ima
coefficients in eq.(14). In fact, this procedure has been applied in the computation of
ocean tide models from satellite orbit perturbations, cf. (Lerch et al,1992) and (Cheng
et al,1990). Similar procedures have been applied for Topex/Poseidon, see also the
discussion under the heading "Ocean tide modeling" on page 24423 in (Nerem et
aI, 1994). Nevertheless these satellite solutions for the indirect ocean tide potential
are only successful in providing the low degree harmonics coefficients alma and f3ima.
However these models miss the resolution of a global ocean tidal model but are successful in for instance estimating a global rate of energy dissipation by the ocean tide,
cf. (Schrama and Ray,1994).
In the case of a GRM there will be other obstacles. In our opinion the key issue
should be to separate the Earth's gravity field from the indirect ocean tide coefficients
aim and 131m. Whether this is possible remains to be investigated because of the
relatively short mission duration of approximately six months or so typically chosen
for a GRM combined with the fact a sun-synchronous orbit is likely to be adopted.
The consequence of the latter is that S2 will map in the same way as a stationary
gravity field. Independent computations whose results are not shown in this paper
indicate smooth S2 maps of VrrUO(p) without any satellite tracks, which is a logical
consequence of the Sun always being in the same position with respect to the orbital
plane. Because of sampling, similar M2 maps of VrrUO(p) underlying table 1 don't
show this smooth response and satellite tracks are clearly visible.
In our opinion future work should go into the direction of conducting more realistic
error analysis of orbiting gradiometers. In (Schrama, 1991) we discussed the necessity
of satellite tracking techniques for estimating the low degree and order potential
coefficients. In this paper we conclude that other gravitational effects may play a
role and that our earlier results in (Schrama,1991) should be reviewed in light of the
indirect ocean tide potential.
REFERENCES
Bertiger W.1. et al (1994), GPS precise tracking of Topex/Poseidon: Results and
implications, JGR oceans, Vol 99, 24449-24464.
Bettadpur and Eanes (1994), Geographical representation of radial orbit perturbations due to ocean tides: Implications for Satellite Altimetry, JGR oceans, Vol 99,
139
orbiting gradiometer since the signal level seems at the noise floor of a cryogenic
instrument. Compare also (Paik,1986) who claims 10- 4 E/VHz for the proposed
SGGM although it should be mentioned that Paik (pers.comm.) recently suggested
that a further increase in accuracy down to 10- 5 E / V Hz may be feasible. A fundamental question seems to be whether the goal of a GRM mission is to observe the
static field or whether it should be used for measuring temporal gravitational effects
as a result of mass variations being the result of geophysical processes.
How to deal with the problem
To reduce the indirect ocean tide effect in the orbit of a GRM one should follow
an approach to integrate the variational equations and to estimate the alma and f3ima
coefficients in eq.(14). In fact, this procedure has been applied in the computation of
ocean tide models from satellite orbit perturbations, cf. (Lerch et al,1992) and (Cheng
et al,1990). Similar procedures have been applied for Topex/Poseidon, see also the
discussion under the heading "Ocean tide modeling" on page 24423 in (Nerem et
aI, 1994). Nevertheless these satellite solutions for the indirect ocean tide potential
are only successful in providing the low degree harmonics coefficients alma and f3ima.
However these models miss the resolution of a global ocean tidal model but are successful in for instance estimating a global rate of energy dissipation by the ocean tide,
cf. (Schrama and Ray,1994).
In the case of a GRM there will be other obstacles. In our opinion the key issue
should be to separate the Earth's gravity field from the indirect ocean tide coefficients
aim and 131m. Whether this is possible remains to be investigated because of the
relatively short mission duration of approximately six months or so typically chosen
for a GRM combined with the fact a sun-synchronous orbit is likely to be adopted.
The consequence of the latter is that S2 will map in the same way as a stationary
gravity field. Independent computations whose results are not shown in this paper
indicate smooth S2 maps of VrrUO(p) without any satellite tracks, which is a logical
consequence of the Sun always being in the same position with respect to the orbital
plane. Because of sampling, similar M2 maps of VrrUO(p) underlying table 1 don't
show this smooth response and satellite tracks are clearly visible.
In our opinion future work should go into the direction of conducting more realistic
error analysis of orbiting gradiometers. In (Schrama, 1991) we discussed the necessity
of satellite tracking techniques for estimating the low degree and order potential
coefficients. In this paper we conclude that other gravitational effects may play a
role and that our earlier results in (Schrama,1991) should be reviewed in light of the
indirect ocean tide potential.
REFERENCES
Bertiger W.1. et al (1994), GPS precise tracking of Topex/Poseidon: Results and
implications, JGR oceans, Vol 99, 24449-24464.
Bettadpur and Eanes (1994), Geographical representation of radial orbit perturbations due to ocean tides: Implications for Satellite Altimetry, JGR oceans, Vol 99,
139