Geopotential model


In geophysics, a geopotential model is the theoretical analysis of measuring and calculating the effects of Earth's gravitational field.

Newton's law

states that the gravitational force F acting between two point masses m1 and m2 with centre of mass separation r is given by
where G is the gravitational constant and r̂ is the radial unit vector. For an object of continuous mass distribution, each mass element dm can be treated as a point mass, so the volume integral over the extent of the object gives:
with corresponding gravitational potential
where ρ = ρ is the mass density at the volume element and of the direction from the volume element to the point mass.

The case of a homogeneous sphere

In the special case of a sphere with a spherically symmetric mass density then ρ = ρ, i.e. density depends only on the radial distance
These integrals can be evaluated analytically. This is the shell theorem saying that in this case:
with corresponding potential
where M = ∫Vρdxdydz is the total mass of the sphere.

The deviations of Earth's gravitational field from that of a homogeneous sphere

In reality, Earth is not exactly spherical, mainly because of its rotation around the polar axis that makes its shape slightly oblate. If this shape were perfectly known together with the exact mass density ρ = ρ, the integrals and could be evaluated with numerical methods to find a more accurate model for Earth's gravitational field. However, the situation is in fact the opposite. By observing the orbits of spacecraft and the Moon, Earth's gravitational field can be determined quite accurately and the best estimate of Earth's mass is obtained by dividing the product GM as determined from the analysis of spacecraft orbit with a value for G determined to a lower relative accuracy using other physical methods.
From the defining equations and it is clear that outside the body in empty space the following differential equations are valid for the field caused by the body:
Functions of the form
where are the spherical coordinates which satisfy the partial differential equation are called spherical harmonic functions.
They take the forms:
where spherical coordinates are used, given here in terms of cartesian for reference:
also P0n are the Legendre polynomials and Pmn for 1 ≤ m ≤ n are the associated Legendre functions.
The first spherical harmonics with n = 0,1,2,3 are presented in the table below.
The model for Earth's gravitational potential is a sum
where and the coordinates are relative the standard geodetic reference system extended into space with origin in the center of the reference ellipsoid and with z-axis in the direction of the polar axis.
The zonal terms refer to terms of the form:
and the tesseral terms terms refer to terms of the form:
The zonal and tesseral terms for n = 1 are left out in. The coefficients for the n=1 with both m=0 and m=1 term correspond to an arbitrarily oriented dipole term in the multi-pole expansion. Gravity does not physically exhibit any dipole character and so the integral characterizing n = 1 must be zero.
The different coefficients Jn, Cnm, Snm, are then given the values for which the best possible agreement between the computed and the observed spacecraft orbits is obtained.
As P0n = −P0n non-zero coefficients Jn for odd n correspond to a lack of symmetry "north–south" relative the equatorial plane for the mass distribution of Earth. Non-zero coefficients Cnm, Snm correspond to a lack of rotational symmetry around the polar axis for the mass distribution of Earth, i.e. to a "tri-axiality" of Earth.
For large values of n the coefficients above in ) take very large values when for example kilometers and seconds are used as units. In the literature it is common to introduce some arbitrary "reference radius" R close to Earth's radius and to work with the dimensionless coefficients
and to write the potential as
The dominating term in is the "J2 term":
Relative the coordinate system
illustrated in figure 1 the components of the force caused by the "J2 term" are
In the rectangular coordinate system with unit vectors the force components are:
The components of the force corresponding to the "J3 term"
are
and
The exact numerical values for the coefficients deviate between different Earth models but for the lowest coefficients they all agree almost exactly.
For JGM-3 the values are:
For example, at a radius of 6600 km J3/ is about 0.002, i.e.
the correction to the "J2 force" from the "J3 term" is in the order of 2 permille. The negative value of J3 implies that for a point mass in Earth's equatorial plane the gravitational force is tilted slightly towards the south due to the lack of symmetry for the mass distribution of Earth's "north–south".

Recursive algorithms used for the numerical propagation of spacecraft orbits

Spacecraft orbits are computed by the numerical integration of the equation of motion. For this the gravitational force, i.e. the gradient of the potential, must be computed. Efficient recursive algorithms have been designed to compute the gravitational force for any and and such algorithms are used in standard orbit propagation software.

Available models

The earliest Earth models in general use by NASA and ESRO/ESA were the "Goddard Earth Models" developed by Goddard Space Flight Center denoted "GEM-1", "GEM-2", "GEM-3", and so on. Later the "Joint Earth Gravity Models" denoted "JGM-1", "JGM-2", "JGM-3" developed by Goddard Space Flight Center in cooperation with universities and private companies became available. The newer models generally provided higher order terms than their precursors. The EGM96 uses Nz = Nt = 360 resulting in 130317 coefficients. An EGM2008 model is available as well.
For a normal Earth satellite requiring an orbit determination/prediction accuracy of a few meters the "JGM-3" truncated to Nz = Nt = 36 is usually sufficient. Inaccuracies from the modeling of the air-drag and to a lesser extent the solar radiation pressure will exceed the inaccuracies caused by the gravitation modeling errors.
The dimensionless coefficients,,
for the first zonal and tesseral terms of the JGM-3 model are
n-
2-0.1082635854D-02
30.2532435346D-05
40.1619331205D-05
50.2277161016D-06
6-0.5396484906D-06
70.3513684422D-06
80.2025187152D-06

According to JGM-3 one therefore has that km5/s2 = km5/s2 and
km6/s2 = km6/s2

Spherical harmonics

The following is a compact account of the spherical harmonics used to model Earth's gravitational field. The spherical harmonics are derived from the approach of looking for harmonic functions of the form
where are the spherical coordinates defined by the equations. By straightforward calculations one gets that for any function f
Introducing the expression in one gets that
As the term
only depends on the variable and the sum
only depends on the variables θ and φ. One gets that φ is harmonic if and only if
and
for some constant
From then follows that
The first two terms only depend on the variable and the third only on the variable.
From the definition of φ as a spherical coordinate it is clear that Φ must be periodic with the period 2π and one must therefore have that
and
for some integer m as the family of solutions to then are
With the variable substitution
equation takes the form
From follows that in order to have a solution with
one must have that
If Pn is a solution to the differential equation
one therefore has that the potential corresponding to m = 0
which is rotational symmetric around the z-axis is a harmonic function
If is a solution to the differential equation
with m ≥ 1 one has the potential
where a and b are arbitrary constants is a harmonic function that depends on φ and therefore is not rotational symmetric around the z-axis
The differential equation is the Legendre differential equation for which the Legendre polynomials defined
are the solutions.
The arbitrary factor 1/ is selected to make Pn=−1 and Pn = 1 for odd n and Pn = Pn = 1 for even n.
The first six Legendre polynomials are:
The solutions to differential equation are the associated Legendre functions
One therefore has that