Open access peer-reviewed chapter

# On Six DOF Relative Orbital Motion of Satellites

By Daniel Condurache

Submitted: October 28th 2017Reviewed: January 8th 2018Published: February 14th 2018

DOI: 10.5772/intechopen.73563

## Abstract

In this chapter, we reveal a dual-tensor-based procedure to obtain exact expressions for the six degree of freedom (6-DOF) relative orbital law of motion in the specific case of two Keplerian confocal orbits. The result is achieved by pure analytical methods in the general case of any leader and deputy motion, without singularities or implying any secular terms. Orthogonal dual tensors play a very important role, with the representation of the solution being, to the authors’ knowledge, the shortest approach for describing the complete onboard solution of the 6-DOF orbital motion problem. The solution does not depend on the local-vertical–local-horizontal (LVLH) properties involves that is true in any reference frame of the leader with the origin in its mass center. A representation theorem is provided for the full-body initial value problem. Furthermore, the representation theorems for rotation part and translation part of the relative motion are obtained.

### Keywords

• relative orbital motion
• full body problem
• dual algebra
• Lie group
• Lie algebra
• closed form solution

## 1. Introduction

The relative motion between the leader and the deputy in the relative motion is a six-degrees-of-freedom (6-DOF) motion engendered by the joining of the relative translational motion with the rotational one. Recently, the modeling of the 6-DOF motion of spacecraft gained a special attention [1, 2, 3, 4, 5], similar to the controlling the relative pose of satellite formation that became a very important research subject [6, 7, 8, 9, 10]. The approach implies to consider the relative translational and rotational dynamics in the case of chief-deputy spacecraft formation to be modeled using vector and tensor formalism.

In this chapter we reveal a dual algebra tensor based procedure to obtain exact expressions for the six D.O.F relative orbital law of motion for the case of two Keplerian confocal orbits. Orthogonal dual tensors play a very important role, the representation of the solution being, to the authors’ knowledge, the shortest approach for describing the complete onboard solution of the six D.O.F relative orbital motion problem. Because the solution does not depend on the LVLH properties involves that is true in any reference frame of the Leader with the origin in its mass center. To obtain this solution, one has to know only the inertial motion of the Leader spacecraft and the initial conditions of the deputy satellite in the local-vertical-local-horizontal (LVLH) frame. For the full body initial value problem, a general representation theorem is given. More, the real and imaginary parts are split and representation theorems for the rotation and translation parts of the relative orbital motion are obtained. Regarding translation, we will prove that this problem is super-integrable by reducing it to the classic Kepler problem.

The chapter is structured as following. The second section is dedicated to the rigid body motion parameterization using orthogonal dual tensors, dual quaternions and other different vector parameterization. The Poisson-Darboux problem is extended in dual Lie algebra. In the third section, the state equations for a rigid body motion relative to an arbitrary non-inertial reference frame are determined. Using the obtained result, in the fourth section, the representation theorem and the complete solution for the case of onboard full-body relative orbital motion problem is given. The last section is designated to the conclusions and to the future works.

## 2. Rigid body motion parameterization using dual Lie algebra

The key notions that will be presented in this section are tensorial, vectorial and non-vectorial parameterizations that can be used to properly describe the rigid-body motion. We discuss the properties of proper orthogonal dual tensorial maps. The proper orthogonal tensorial maps are related with the skew-symmetric tensorial maps via the Darboux–Poisson equation. Orthogonal dual tensorial maps are a powerful instrument in the study of the rigid motion with respect to an inertial and noninertial reference frames. More on dual numbers, dual vectors and dual tensors can be found in [2, 16, 17, 18, 19, 20, 21, 22, 23].

### 2.1. Isomorphism between Lie group of the rigid displacements SE3and Lie group of the orthogonal dual tensors SO¯3

Let the orthogonal dual tensor set be denoted by.

SO¯3=R¯LV¯3V¯3RR¯T=I¯detR¯=1E1

where SO3¯is the set of special orthogonal dual tensors and I¯is the unit orthogonal dual tensor.

The internal structure of any orthogonal dual tensor R¯SO¯3is illustrated in a series of results which were detailed in our previous work [17, 18, 23].

Theorem 1. (Structure Theorem). For any R¯SO¯3a unique decomposition is viable

R¯=I+ερQE2

where QSO3and ρV3are called structural invariants, ε2=0, ε0.

Taking into account the Lie group structure of SO¯3and the result presented in previous theorem, it can be concluded that any orthogonal dual tensor R¯SO¯3can be used globally parameterize displacements of rigid bodies.

Theorem 2 (Representation Theorem). For any orthogonal dual tensor R¯defined as in (Eq. (2)), a dual number α¯=α+εdand a dual unit vector u¯=u+εu0can be computed to have the following Eq. [17, 18]:

R¯α¯,u¯=I+sinα¯u¯+1cosα¯u¯2=expα¯u¯E3

The parameters α¯and u¯are called the natural invariants of R¯. The unit dual vector u¯gives the Plücker representation of the Mozzi-Chalses axis [16, 24] while the dual angle α¯=α+εdcontains the rotation angle αand the translated distance d.

The Lie algebra of the Lie group SO¯3is the skew-symmetric dual tensor set denoted by so¯3=α¯LV¯3V¯3α¯=α¯T, where the internal mapping is α¯1α¯2=α¯1α¯2.

The link between the Lie algebra so¯3, the Lie group SO¯3, and the exponential map is given by the following.

Theorem 3. The mapping is well defined and surjective.

exp:so¯3SO¯3,expα¯=eα¯=k=0α¯kk!E4

Any screw axis that embeds a rigid displacement is parameterized by a unit dual vector, whereas the screw parameters (angle of rotation around the screw and the translation along the screw axis) is structured as a dual angle. The computation of the screw axis is bound to the problem of finding the logarithm of an orthogonal dual tensor R¯, that is a multifunction defined by the following equation:

log:SO¯3so¯3,logR¯=ψ¯so¯3expψ¯=R¯E5

and is the inverse of (Eq. (4)).

From Theorem 2 and Theorem 3, for any orthogonal dual tensor R¯, a dual vector ψ¯=α¯u¯=ψ+εψ0is computed, represents the screw dual vector or Euler dual vector (that includes the screw axis and screw parameters) and the form of ψ¯implies that ψ¯logR¯. The types of rigid displacements that is parameterized by the Euler dual vector ψ¯as below:

1. roto-translation if ψ0,ψ00andψψ00ψ¯R¯andψεR;

2. pure translation if ifψ=0andψ00ψ¯εR¯;

3. pure rotation if ψ0andψψ0=0ψ¯R.

Also, ψ<2π, Theorem 2 and Theorem 3 can be used to uniquely recover the screw dual vector ψ¯, which is equivalent with computing logR¯.

Theorem 4. The natural invariants α¯=α+εd,u¯=u+εu0can be used to directly recover the structural invariants Qand ρfrom (Eq. (2)):

Q=I+sinαu+1cosαu2ρ=du+sinαu0+1cosαu×u0E6

To prove (Eq. (6)), we need to use (Eq. (2)) and (Eq. (3)). If these equations are equal, then the structure of their dual parts leads to the result presented in (Eq. (6)).

Theorem 5. (Isomorphism Theorem): The special Euclidean group SE3and SO¯3are connected via the isomorphism of the Lie groups

Φ:SE3SO¯3,Φg=I+ερQE7

where g=Qρ01, ΦSO3, ρV3.

Proof. For any g1,g2SE3, the map defined in (Eq. (7)) yields

Φg1g2=Φg1Φg2E8

Let R¯SO¯3. Based on Theorem 1, which ensures a unique decomposition, we can conclude that the only choice for g, such that Φg=R¯is g=Qρ01. This underlines that Φis a bijection and keeps all the internal operations.

Remark 1: The inverse of Φis

Φ1:SO¯3SE3;Φ1R¯=Qρ01E9

where Q=ReR¯,ρ=vectDuR¯QT.

### 2.2. Dual tensor-based parameterizations of rigid-body motion

The Lie group SO¯3admits multiple parameterization and few of them will be discussed in this section.

#### 2.2.1. The exponential parameterization (the Euler dual vector parameterization)

If R¯=R¯α¯u¯, then we can construct the Euler dual vector (screw dual vector) ψ¯=α¯u¯,ψ¯V¯3which combined with Theorem 2 and Theorem 3 lead to

R¯=expψ¯=I¯+sincψ¯ψ¯+12sinc2ψ¯2ψ¯2E10

where

sincx¯=sinx¯x¯,x¯εR1,x¯εRE11

#### 2.2.2. Dual quaternion parameterization

One of the most important non-vectorial parameterizations for the orthogonal dual tensor SO¯3is given by the dual quaternions [20, 21]. A dual quaternion can be defined as an associated pair of a dual scalar quantity and a free dual vector:

q¯̂=q¯q¯,q¯R¯,q¯V¯3E12

The set of dual quaternions will be denoted Q¯and is organized as a R¯-module of rank 4, if dual quaternion addition and multiplication with dual numbers are considered.

The product of two dual quaternions q̂¯1=q¯1q¯1and q̂¯2=q¯2q¯2is defined by

q̂¯1q̂¯2=q¯1q¯2q¯1q¯2q¯1q¯2+q¯2q¯1+q¯1×q¯2E13

From the above properties, results that the R¯-module Q¯becomes an associative, non-commutative linear dual algebra of rank 4 over the ring of dual numbers. For any dual quaternion defined by (Eq. (12)), the conjugate denoted by q̂¯=q¯q¯and the norm denoted by q̂¯2=q̂¯q̂¯can be computed. For q̂¯=1, any dual quaternion is called unit dual quaternion. Regarded solely as a free R¯-module, Q¯contains two remarkable sub-modules: Q¯R¯and Q¯V¯3. The first one composed from pairs q¯0¯,q¯R¯, isomorphic with R¯, and the second one, containing the pairs 0¯q¯,q¯V¯3, isomorphic with V¯3. Also, any dual quaternion can be written as q̂¯=q¯+q¯, where q¯=q¯0¯and q¯=0¯q¯, or q̂¯=q̂+εq̂0, where q̂,q̂0are real quaternions. The scalar and the vector parts of a dual unit quaternion are also known as dual Euler parameters .

Let denote with Uthe set of unit quaternions and with U¯the set of unit dual quaternions. For any q̂¯U¯, the following equation is valid [17, 20]:

q̂¯=1+ε12ρ̂q̂E14

where ρV3and q̂U. This representation is the quaternionic counterpart to (Eq. (2)). Also a dual number α¯and a unit dual vector u¯exist so that:

q̂¯=cosα¯2+u¯sinα¯2=exp12α¯u¯E15

Remark 2: The mapping exp:V¯3U¯,q̂¯=exp12Ψ¯, is well defined and surjective.

Remark 3: The dual unit quaternions set U¯, by the multiplication of dual quaternions, is a Lie group with V¯3being it’s associated Lie algebra (with the cross product between dual vectors as the internal operation).

Using the internal structure of any element from SO¯3the following theorem is valid:

Theorem 6. The Lie groups U¯and SO¯3are linked by a surjective homomorphism

:U¯SO¯3,q¯+q¯=I¯+2q¯q¯+2q¯2E16

Proof. Taking into account that any q̂¯U¯can be decomposed as in (Eq. (15)), results that q̂¯=expα¯u¯SO¯3. This shows that relation (Eq. (16)) is well defined and surjective. Using direct calculus, we can also acknowledge that q̂¯2q̂¯1=q̂¯2q̂¯1.

An important property of the previous homomorphism is that for q̂¯and q̂¯we can associate the same orthogonal dual tensor, which shows that (Eq. (16)) is not injective and U¯is a double cover of SO¯3.

#### 2.2.3. N-order modified fractional Cayley transform for dual vectors

Next, we present a series of results that are the core of our research. These results are obtained after using a set of Cayley transforms that are different than the ones already reported in literature [17, 25, 26, 27].

Theorem 7. The fractional order Cayley map f:V¯3U¯

cayn2v¯=fv¯=1+v¯n21v¯n2,nE17

is well defined and surjective.

Proof. Using direct calculus results that fv¯fv¯=1and fv¯=1. The surjectivity is proved by the following theorem.

Theorem 8. The inverse of the previous fractional order Cayley map, is a multifunction with n branches f1:U¯V¯3given by

v¯=q̂¯2n1q̂¯2n+1E18

Remark 4: If v¯Rthen cayn2v¯is the parameterization of a pure rotation about an axis which does not necessarily pass through the origin of reference system. Meanwhile, if v¯εRthe mapping cayn2v¯is the parameterization of a pure translation. Otherwise, cayn2v¯is the parameterization of roto-translation.

Taking into account that a dual number α¯and a dual vector u¯exist in order to have

q̂¯=cosα¯2+u¯sinα¯2,E19

from (Eq. (18)), results that:

v¯=tanα¯+22nu¯,k=01n1E20

The previous equation contains both the principal parameterization v¯0=tanα¯2nu¯, which is the higher order Rodrigues dual vector, while for k=1n1the dual vectors v¯k=tanα¯+22nu¯are the shadow parameterization  that can be used to describe the same pose. Based on v¯0=tanα¯2nand v¯k=tanα¯+22n, results that v¯k=v¯0+tann1v¯0tann.

If Rev¯0then Rev¯kcotn, which allows the avoidance of any singularity of type Reα¯2n=π2+π.

Theorem 9. If v¯V¯3is the parameterization of a displacement obtained from (Eq. (20)), then

±q̂¯=11+v¯2npnv¯+qnv¯v¯E21

where

pnX=k=0n/21k2knX2kE22
qnX=k=0n1/21k2k+1nX2kE23

In (Eq. (22)) and (Eq. (23)),.represents the floor of a number and knare binomial coefficients.

Remark 5. The structure of the polynomials pnXand qnX, given by (Eq. (22)) and (Eq. (23)), can be used to obtain the following iterative expressions:

pn+1X=pnXX2qnXqn+1X=qnX+qnXp1X=1,q1X=1E24

In order to evaluate the usefulness of the iterative expressions, we provide the second to third order polynomials and the resulting dual quaternions and dual orthogonal tensors:

p1X=1;q1X=1;v¯=tanα¯2u¯;±q̂¯=11¯+v¯21+v¯;R¯=I¯+21+v¯2v¯+v¯2;E25
p2X=1X2;q2=2;v¯=tanα¯+24u¯;k=0,1¯;±q̂¯=11¯+v¯21v¯2+2v¯;R¯=I¯+41+v¯221v¯2v¯+2v¯2;E26
p3X=13X2;q3=3X2;v¯=tanα¯+26u¯;k=0,2;¯±q̂¯=11¯+v¯2313v¯2+3v¯2v¯;R¯=I¯+23v¯21+v¯2313v¯2v¯+3v¯2v¯2.E27

### 2.3. Poisson-Darboux problems in dual Lie algebra and vector parameterization

Consider the functions Q=QtSO3Rand ρ=ρtV3Rto be the parametric equations of any rigid motion. Thus, any rigid motion can be parameterized by a curve in SO¯3where R¯t=I+ερtQt, where t is time variable. Let h¯0embed the Plücker coordinates of a line feature at t=t0. At a time stamp tthe line is transformed into:

h¯t=R¯th¯0E28

Theorem 10. In a general rigid motion, described by an orthogonal dual tensor function R¯, the velocity dual tensor function Φ¯defined as

ḣ¯=Φ¯h¯,h¯V¯3E29

is expressed by

Φ=Ṙ¯R¯TE30

Let Φ¯=Ṙ¯R¯T, then Ṙ¯R¯T+R¯Ṙ¯T=0¯, equivalent with Φ¯=Φ¯T, which shows that Φ¯so¯3R.

The dual vector ω¯=vectṘ¯R¯Tis called dual angular velocity of the rigid body and has the form:

ω¯=ω+εvE31

where ωis the instantaneous angular velocity of the rigid body and v=ρ̇ω×ρrepresents the linear velocity of the point of the body that coincides instantaneously with the origin of the reference frame. The pair (ω,v) is usually refereed as the twist of the rigid body.

#### 2.3.1. Poisson-Darboux equation in dual Lie algebra

The next Theorem permits the reconstruction of the rigid body motion knowing in any moment the twist of the rigid body that is equivalent with knowing the dual angular velocity [5, 18].

Theorem 11. For any continuous function ω¯V¯3Ra unique dual tensor R¯SO¯3Rexists so that

Ṙ¯=ω¯R¯R¯t0=R¯0,R¯0SO¯3E32

Due to the fact that orthogonal dual tensor R¯completely models the six degree of freedom motion, we can conclude that the Theorem 11 is the dual form of the Poisson-Darboux problem  for the case when the rotation tensor is computed from the instantaneous angular velocity. So, in order to recover R¯, it is necessary to find out how the dual angular velocity vector ω¯behaves in time and also the value of R¯at time t=t0.

The dual tensor R¯can be derived from ω¯, when is positioned in space, or from ω¯B, which denotes the dual angular velocity vector to be positioned in the rigid body.

Remark 6. The dual angular velocity vector positioned in the rigid body can be recovered from ω¯B=R¯Tω¯, thus transforming (Eq. (32)) into:

Ṙ¯=R¯ω¯BR¯t0=R¯0,R¯0SO¯3E33

(Eq. (32)) and (Eq. (33)) represent the dual replica of the classical orientation Poisson-Darboux problem [17, 28, 29].

The tensorial (Eq. (32)) and (Eq. (33)) are equivalent with 18 scalar differential equations. The previous parameterizations of the orthogonal dual tensors allow us to determine some solutions of smaller dimension in order to solve the dual Poisson- Darboux problem.

#### 2.3.2. Kinematic equation for Euler dual vector parameterization

Consider Ψ¯V3Rsuch that R¯=expΨ¯. According to the (Eq. (10)), the Poisson-Darboux problem (32) is equivalent to

Ψ̇¯=T¯ω¯Ψ¯t0=Ψ¯0E34

whereexpΨ¯0=R¯0, and T¯is the following dual tensor:

T¯=Ψ¯2cotΨ¯2I¯12Ψ¯12Ψ¯cotΨ¯2Ψ¯2E35

The representation of the Poisson-Darboux problem from (Eq. (33)) is equivalent to

Ψ̇¯=T¯Tω¯BΨ¯t0=Ψ¯0E36

#### 2.3.3. Kinematic equation for high order Rodrigues dual vector parameterization

Let v¯V¯3Rsuch that R¯=caynv¯. The problems (32) and (33) are equivalent to:

v̇¯=S¯ω¯v¯t0=v0¯,E37
v̇¯=S¯Tω¯Bv¯t0=v¯0E38

where caynv¯0=R¯0, and S¯is the following dual tensor :

S¯=pnv¯2qnv¯I¯12v¯+1+v2qnv¯npnv¯2nv¯2qnv¯v¯2E39

and the polynomials pn,qnare given by the (Eq. (22)), (Eq. (23)) and (Eq. (24)).

(Eq. (34)), (Eq. (36)), (Eq. (37)) and (Eq. (38)) are equivalent with six scalar differential equations. This is a minimal parameterization of the Poisson-Darboux problem in dual algebra.

#### 2.3.4. Kinematic equation for dual quaternion parameterization

Let q̂¯U¯Rsuch that q̂¯=R¯. According to (Eq. (16)), the Poisson–Darboux problems (32) and (33) are equivalent to:

q̂̇¯=12ω¯q̂¯q̂¯t0=q¯̂0E40

and

q̂̇¯=12q̂¯ω¯Bq̂¯t0=q¯̂0E41

where q¯̂0=R¯0

The (Eq. (40)) and (Eq. (41)) are equivalent to eight scalar differential equations.

## 3. Rigid body motion in arbitrary non-inertial frame revised

To the author’s knowledge, in the field of astrodynamics there aren’t many reports on how the motion of rigid body can be studied in arbitrary non-inertial frames. Next, we proposed a dual tensors based model for the motion of the rigid body in arbitrary non-inertial frame. The proposed method eludes the calculus of inertia forces that contributes to the rigid body relative state. So, the free of coordinate state equation of the rigid body motion in arbitrary non-inertial frame will be obtained.

Let R¯Dand R¯Cbe the dual orthogonal tensors which describe the motion of two rigid bodies relative to the inertial frame.

If R¯is the orthogonal dual tensor which embeds the six degree of freedom relative motion of rigid body C relative to rigid body D, then:

R¯=R¯CTR¯DE42

Let ω¯Cdenote the dual angular velocity of the rigid body C and ω¯Dthe dual angular velocity of the rigid body D, both being related to inertial reference frame. In the followings, the inertial motion of the rigid body C is considered to be known. If ω¯is the dual angular velocity of the rigid body D relative to the rigid body C, then, conforming with (Eq. (42)):

ω¯=ω¯Dω¯CE43

Considering ω¯DBbeing the dual angular velocity vector of the rigid body D in the body frame, the dual form of the Euler equation given in  results that:

M¯ω¯DḂ+ω¯DB×M¯ω¯DB=τ¯BE44

In (Eq. (44)) τ¯B=FB+ετB, where FBthe force applied in the mass center and τBis the torque. Also in (Eq. (44)), M¯represents the inertia dual operator, which is given by M¯=mDdI+εJ, where Jis the inertia tensor of the rigid body D related to its mass center and mDis the mass of the rigid body D. Combining M¯1=J1d+ε1mDIwith (Eq. (44)) results:

ω̇¯DB+M¯1ω¯DB×M¯ω¯DB=M¯1τ¯BE45

Taking into account that ω¯D=R¯ω¯DB, the dual angular velocity vector can be computed from

ω¯=R¯ω¯DBω¯CE46

this through differentiation gives:

ω̇¯+ω̇¯C=Ṙ¯ω¯DB+R¯ω̇¯DBE47

If the previous equation is multiplied by R¯T, then

R¯Tω̇¯+ω̇¯C=R¯TṘ¯ω¯DB+ω̇¯DBE48

which combined with Ṙ¯=ω¯R¯generates:

R¯Tω̇¯+ω̇¯C=R¯Tω¯R¯ω¯DB+ω̇¯DBE49

After a few steps, (Eq. (49)) is transformed into

ω̇¯+ω̇¯C=R¯ω̇¯DB+ω¯×ω¯CE50

which combined with (Eq. (45)) gives:

ω̇¯+ω̇¯C=RM¯1τ¯BRM¯1ω¯DB×M¯ω¯DB+ω¯×ω¯CE51

Because ω¯DB=R¯Tω¯×ω¯C, the final equation is:

ω̇¯+ω̇¯C=RM¯1τ¯BR¯Tω¯+ω¯C×MR¯Tω¯+ω¯C+ω¯×ω¯CE52

The system:

Ṙ¯=ω¯R¯ω̇¯+ω̇¯C=R¯M¯1[R¯Tτ¯¯R¯Tω¯+ω¯C××M¯R¯Tω¯+ω¯C]+ω¯×ω¯Cω¯t0=ω¯0,ω¯0V¯3R¯t0=R¯0,R¯0SO¯3E53

is a compact form which can be used to model the six D.O.F relative motion problem. In the previous equation the state of the rigid body D in relation with the rigid body C is modeled by the dual tensor R¯and the dual angular velocities field ω¯. This initial value problem can be used to study the behavior of the rigid body D in relation with the frame attached to the rigid body C. In (Eq. (53)), all the vectors are represented in the body frame of C, which shows that the proposed solution is onboard and has the property of being coupled in R¯and ω¯.

Next, we present a procedure that allows the decoupling of the proposed solution.

In order to describe the solution to (Eq. (53)), we consider the following change of variable:

ω¯=R¯Tω¯+ω¯CE54

This change of variable leads to ω̇¯=Ṙ¯Tω¯+ω¯C+R¯Tω̇¯+ω̇¯C=R¯Tω¯ω¯+ω¯C+R¯Tω̇¯+ω̇¯C. The result is equivalent with ω̇¯=R¯Tω¯C×ω¯+ω̇¯+ω̇¯Cor

ω¯C×ω¯+ω̇¯+ω̇¯C=R¯ω̇¯E55

After some steps of algebraic calculus, from (Eq. (54)), (Eq. (55)) and (Eq. (52)), results that:

M¯ω̇¯+ω¯×M¯ω¯=τ¯ω¯t0=ω¯0E56

Where τ¯=R¯Tτ¯is the dual torque related to the mass center in the body frame of the rigid body D and ω¯0=R¯0Tω¯0+ω¯Ct0. (Eq. (56)) is a dual Euler fixed point classic problem.

For any R¯SO¯3R, the solution of (Eq. (53)) emerges from

Ṙ¯=ω¯R¯R¯t0=R¯0E57

Making use of (Eq. (54)), results that R¯ω¯=ω¯+ω¯C. If ~operator used, the previous calculus is transformed into R¯ω¯=ω¯+ω¯CR¯ω¯R¯T=Ṙ¯R¯T+ω¯C. After multiplying the last expression by R¯, we obtain the initial value problem:

Ṙ¯=R¯ω¯ω¯CR¯R¯t0=R¯0E58

Using the variable change (Eq. (54)), the initial value problem (53) has been decoupled into two distinct initial value problems (56) and (58).

Let R¯ω¯CSO¯3Rbe the unique solution of the following Poisson-Darboux problem:

Ṙ¯+ω¯CR¯=0R¯t0=IεrCt0E59

Considering R¯=R¯ω¯CR¯, a representation theorem of the solution of (Eq. (53)) can be formulated.

Theorem 12. (Representation Theorem). The solution of (Eq. (53)) results from the application of the tensor R¯ω¯Cfrom (Eq. (59)) to the solution of the classical dual Euler fixed point problem:

Ṙ¯=R¯ω¯M¯ω̇¯+ω¯×M¯ω¯=τ¯ω¯t0=ω¯0R¯t0=R¯0E60

where ω¯0=R¯0Tω¯0+ω¯Ct0,R¯0=I+εrCt0R¯0, τ¯=R¯Tτ¯.

Different representations can be considered for the problem (60).

Using dual quaternion representation R¯=q̂¯, (Eq. (60)) is equivalent with the following one:

q̂¯̇=12q̂¯ω¯M¯ω̇¯+ω¯×M¯ω¯=τ¯ω¯t0=ω¯0q̂¯t0=q̂¯0E61

For the n-th order of Cayley transform based representation R¯=caynξ¯,ξ¯=tanα¯2nu¯, the (Eq. (60)) becomes:

ξ¯̇=S¯ξ¯ω¯M¯ω̇¯+ω¯×M¯ω¯=τ¯ω¯t0=ω¯0ξ¯t0=ξ¯0E62

where the tensor S¯is:

S¯==pnξ¯2qnξ¯I¯+12ξ¯+1+ξ¯2qnξ¯npnξ¯2nξ¯2qnξ¯ξ¯ξ¯E63

when pnXand qnXare defined by (Eq. (22)) and (Eq. (23)).

Different particular cases can be analyzed for the (Eq. (62)):

1. Let ξ¯=tanα¯2u¯be the Rodrigues dual vector for n = 1:

S¯=12I¯+12ξ¯+12ξ¯ξ¯

• Let ξ¯=tanα¯4u¯be the modified Rodrigues dual vector (Wiener-Milenkovic dual vector) for n = 2:

S¯=1ξ¯24I¯+12ξ¯+12ξ¯ξ¯.

• The initial value problem (62) is a minimum parameterization of the six degrees of freedom motion problem. The singularity cases can be avoided using the shadow parameters of the n-th order Modified Rodrigues Parameter dual vector.

## 4. A dual tensor formulation of the six degree of freedom relative orbital motion problem

The results from the previous paragraphs will be used to study the six degrees of freedom relative orbital motion problem.

The relative orbital motion problem may now be considered classical one considering the many scientific papers written on this subject in the last decades. Also, the problem is quite important knowing its numerous applications: rendezvous operations, spacecraft formation flying, distributed spacecraft missions [3, 4, 6, 7, 8, 9, 10].

The model of the relative motion consists in two spacecraft flying in Keplerian orbits due to the influence of the same gravitational attraction center. The main problem is to determine the pose of the Deputy satellite relative to a reference frame originated in the Leader satellite center of mass. This non-inertial reference frame, known as “LVLH (Local-Vertical-Local- Horizontal)” is chosen as following: the Cxaxis has the same orientation as the position vector of the Leader with respect to an inertial reference frame with the origin in the attraction center; the orientation of the Czis the same as the Leader orbit angular momentum; the Cyaxis completes a right-handed frame. The angular velocity of the LVLH is given by vector ωC, which has the expression:

ωC=ḟChChC=1rC2hC=1+eCcosfCtpC2hCE64

where vector rCis

rC=pC1+eCcosfCtrC0rC0E65

where pCis the conic parameter, hCis the angular momentum of the Leader, fCtbeing the true anomaly and eCis the eccentricity of the Leader.

We propose dual tensors based model for the motion and the pose for the mass center of the Deputy in relation with LVLH. Both, the Leader satellite and the Deputy satellite can be considered rigid bodies.

Furthermore, the time variation of rCis:

ṙC=eChCsinfCtpCrC0rC0E66

In order to a more easy to read list of notations, for t=t0there will be used the followings:

ωC0=1+eCcosfCt0pC2hCE67
ṙC0=eChCsinfCt0pCrC0rC0E68

where rC0rC0is the unity vector of the X-axis from LVLH.

The full-body relative orbital motion is described by the (Eq. (53)) where the dual angular velocity of the Chief satellite is:

ω¯C=ωC+εṙC+ωC×rCE69

and the dual torque related to the mass center of Deputy satellite is:

τ¯=μrc+r3rc+r+ετ.E70

The representation theorem (Theorem 12) is applied in this case using the conditions (66)(69), the solution of the Poisson-Darboux problem (59) is:

R¯ω¯C=(IεrCt)(Isinfc0hChc+1cosfc0hC2hc2).E71

In (71), we’ve noted hc=hcand fc0=fctfct0.

Theorem 13. (Representation Theorem of the full body relative orbital motion). The solution of (Eq. (53)) results from the application of the tensor R¯ω¯Cfrom (Eq. (71)) to the solution of the classical dual Euler fixed point problem (60).

### 4.1. The rotational and translational parts of the relative orbital motion

The complete solution of (Eq. (53)) can be recovered in two steps.

Consider first the real part of (Eq. (53)). This leads to an initial value problem:

Q̇=ωQω̇+ω̇c=QJ1[QTτQTω+ωc××JQTω+ωc]+ω×ωcωt0=ω0,ω0V3Qt0=Q0,Q0SO3E72

which has the solution Q=Qt, the real tensor Qbeing the attitude of Deputy in relation with LVLH. In (Eq. (72)), ωis the angular velocity of the Deputy in relation with LVLH, ωcis the angular velocity of LVLH, τis the resulting torque of the forces applied on the Deputy in relation with is mass center, Jis the inertia tensor of the Deputy in relation with its mass center. The angular velocity of Deputy in respect to LVLH at time t0is denoted with ω0and Q0is the orientation of Deputy in respect to LVLH at time t0.

Consider now the dual part of (Eq. (53)). Taking into account the internal structure of R¯, which is given by (Eq. (2)), after some basic algebraic calculus we obtain a second initial value problem that models the translation of the Deputy satellite mass center with respect to the LVLH reference frame:

r¨+2ωc×ṙ+ωc×ωc×r+ω̇c×r++μrc+r3rc+rμrc3rc=0rt0=r0,ṙt0=v0E73

where μ>0is the gravitational parameter of the attraction center and r0,v0represent the relative position and relative velocity vectors of the mass center of the Deputy spacecraft with respect to LVLH at the initial moment of time t00.

Based on the representation theorem 12, the following theorem results.

Theorem 14. The solutions of problems (Eq. (72)) and (Eq. (73)) are given by

Q=RωCQr=RωCrrcE74

where Qand rare the solutions of the the classical Euler fixed point problem and, respectively, Kepler’s problem:

Q̇=QωJω̇+ω×Jω=τωt0=Q0Tω0+ωct0Qt0=Q0E75

and

r¨+μr3r=0;rt0=rc0+r0;ṙt0=ṙC0+v0+ωC0×rC0+r0E76

where

RωC=Isinfc0hChc+1cosfc0hC2hc2)E77

and rcis given by (Eq. (65)).

Remark 7: The problems (72) and (73) are coupled because, in general case, the torque τdepends of the position vector r.

The relative velocity of the translation motion may be computed as:

v=RωCṙωcRωCrechcsinfctpcrc0rc0E78

This result shows a very interesting property of the translational part of the relative orbital motion problem (73). We have proven that this problem is super-integrable by reducing it to the classic Kepler problem [11, 12, 31, 32]. The solution of the translational part of the relative orbital motion problem is expressed thus:

r=rtt0r0v0;v=vtt0r0v0E79

The exact closed form, free of coordinate, solution of the translational motion can be found in [11, 12, 31, 32, 34].

## 5. Conclusions

The chapter proposes a new method for the determination of the onboard complete solution to the full-body relative orbital motion problem.

Therefore, the isomorphism between the Lie group of the rigid displacements SE3and the Lie group of the orthogonal dual tensors SO¯3is used. It is obtained a Poisson-Darboux like problem written in the Lie algebra of the group SO¯3, an algebra that is isomorphic with the Lie algebra of the dual vectors. Different vectorial and non-vectorial parameterizations (obtained with n-th order Cayley-like transforms) permit the reduction of the Poisson-Darboux problem in dual Lie algebra to the simpler problems in the space of the dual vectors or dual quaternions.

Using the above results, the free of coordinate state equation of the rigid body motion in arbitrary non-inertial frame is obtained.

The results are applied in order to offer a coupled (rotational and translational motion) state equation and a representation theorem for the onboard complete solution of full body relative orbital motion problem. The obtained results interest the domains of the spacecraft formation flying, rendezvous operation, autonomous mission and control theory.

## Nomenclature

 a real number a¯ dual number a real vector a¯ dual vector A real tensor A¯ dual tensor V3 real vectors set V¯3 dual vectors set V3R time depending real vectorial functions V¯3R time depending dual vectorial functions a∼¯ skew-symmetric dual tensor corresponding to the dual vector a¯ fc true anomaly pc conic parameter hc specific angular momentum of the leader satellite LV¯3V¯3 dual tensor set q̂ real quaternion q¯̂ dual quaternion R real numbers set R¯ dual numbers set SO3 orthogonal real tensors set SO¯3 orthogonal dual tensor set SO3R time depending real tensorial functions SO¯3R time depending dual tensorial functions U unit quaternions set U¯ unit dual quaternions set

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© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Daniel Condurache (February 14th 2018). On Six DOF Relative Orbital Motion of Satellites, Space Flight, George Dekoulis, IntechOpen, DOI: 10.5772/intechopen.73563. Available from:

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