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12.540 Principles of the Global Positioning System Lecture 03 Prof. Thomas Herring Room 54-820A; 253-5941 tah@mit.edu http://geoweb.mit.edu/~tah/12.540

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02/10/1012.540 Lec 032 Review In last lecture we looked at conventional methods of measuring coordinates Triangulation, trilateration, and leveling Astronomic measurements using external bodies Gravity field enters in these determinations

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02/10/1012.540 Lec 033 Gravitational potential In spherical coordinates: need to solve This is Laplace’s equation in spherical coordinates

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02/10/1012.540 Lec 034 Solution to gravity potential The homogeneous form of this equation is a “classic” partial differential equation. In spherical coordinates solved by separation of variables, r=radius, =longitude and =co-latitude

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02/10/1012.540 Lec 035 Solution in spherical coordinates The radial dependence of form r n or r -n depending on whether inside or outside body. N is an integer Longitude dependence is sin(m ) and cos(m ) where m is an integer The colatitude dependence is more difficult to solve

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02/10/1012.540 Lec 036 Colatitude dependence Solution for colatitude function generates Legendre polynomials and associated functions. The polynomials occur when m=0 in dependence. t=cos( )

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02/10/1012.540 Lec 037 Legendre Functions Low order functions. Arbitrary n values are generated by recursive algorithms

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02/10/1012.540 Lec 038 Associated Legendre Functions The associated functions satisfy the following equation The formula for the polynomials, Rodriques’ formula, can be substituted

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02/10/1012.540 Lec 039 Associated functions Pnm(t): n is called degree; m is order m 0 http://mathworld.wolfram.com/LegendrePolynomial.html

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02/10/1012.540 Lec 0310 Orthogonality conditions The Legendre polynomials and functions are orthogonal:

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02/10/1012.540 Lec 0311 Examples from Matlab Matlab/Harmonics.m is a small matlab program to plots the associated functions and polynomialsMatlab/Harmonics.m Uses Matlab function: Legendre

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02/10/1002/17/0912.540 Lec 0312 Polynomials

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02/10/1002/17/0912.540 Lec 0313 “Sectoral Harmonics” m=n

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02/10/1002/17/0912.540 Lec 0314 Normalized

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02/10/1012.540 Lec 0315 Surface harmonics To represent field on surface of sphere; surface harmonics are often used Be cautious of normalization. This is only one of many normalizations Complex notation simple way of writing cos(m ) and sin(m )

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02/10/1002/17/0912.540 Lec 0316 Surface harmonics Zonal ---- Tesserals ------------------------Sectorials

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02/10/1012.540 Lec 0317 Gravitational potential The gravitational potential is given by: Where is density, G is Gravitational constant 6.6732x10 -11 m 3 kg -1 s -2 (N m 2 kg -2 ) r is distance The gradient of the potential is the gravitational acceleration

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02/10/1012.540 Lec 0318 Spherical Harmonic Expansion The Gravitational potential can be written as a series expansion Cnm and Snm are called Stokes coefficients

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02/10/1012.540 Lec 0319 Stokes coefficients The Cnm and Snm for the Earth’s potential field can be obtained in a variety of ways. One fundamental way is that 1/r expands as: Where d’ is the distance to dM and d is the distance to the external point, is the angle between the two vectors (figure next slide)

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02/10/1012.540 Lec 0320 1/r expansion Pn(cos ) can be expanded in associated functions as function of ,

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02/10/10 Computing Stoke coefficients Substituting the expression for 1/r and converting to co-latitude and longitude dependence yields: 12.540 Lec 0321 The integral and summation can be reversed yielding integrals for the Cnm and Snm Stokes coefficients.

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02/10/10 Low degree Stokes coefficients By substituting into the previous equation we obtain: 2212.540 Lec 03

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02/10/10 Moments of Inertia Equation for moments of inertia are: The diagonal elements in increasing magnitude are often labeled A B and C with A and B very close in value (sometimes simply A and C are used) 2312.540 Lec 03

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02/10/10 Relationship between moments of inertia and Stokes coefficients With a little bit of algebra it is easy to show that: 2412.540 Lec 03

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02/10/1012.540 Lec 0325 Spherical harmonics The Stokes coefficients can be written as volume integrals C 00 = 1 if mass is correct C 10, C 11, S 11 = 0 if origin at center of mass C 21 and S 21 = 0 if Z-axis along maximum moment of inertia

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02/10/1012.540 Lec 0326 Global coordinate systems If the gravity field is expanded in spherical harmonics then the coordinate system can be realized by adopting a frame in which certain Stokes coefficients are zero. What about before gravity field was well known?

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02/10/1012.540 Lec 0327 Summary Examined the spherical harmonic expansion of the Earth’s potential field. Low order harmonic coefficients set the coordinate. –Degree 1 = 0, Center of mass system; –Degree 2 give moments of inertia and the orientation can be set from the directions of the maximum (and minimum) moments of inertia. (Again these coefficients are computed in one frame and the coefficients tell us how to transform into frame with specific definition.) Not actually done in practice. Next we look in more detail into how coordinate systems are actually realized.

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