1. Introduction
In the past 10–15, years the problem of GPS (GPS means Global Positioning System) satellite-ground station communications has been replaced by the problem of autonomous navigation and inter-satellite communications (ISC) (links), which has been mentioned yet in 2005 in the monograph [
1]. Autonomous navigation means that satellites should have the capability to transmit data between them via inter-satellite cross-link ranging [
2] and, thus, to ensure navigation control and data processing without commands from Earth stations in the course of six months. It is important that next generation space missions will attain sub-millimeter precision of measuring distances beyond
m (1 micrometer = 1
m =
m) by means of ultrashort femtosecond pulse lasers. The theoretical description of such measurements is inevitably related to general relativity theory (GRT). This means that the relative ranging and relative velocity model should account also for the bending of the transmitting path of the signal, which is significant for such large distances between the satellites due to the action of the gravitational field. The bending of the light (laser) is related to the important and fundamental physical fact that due to the action of gravitational field, the signal travels a greater distance in comparison with the straight path distance in the absence of a gravitational field.
In the book [
3], autonomous navigation is defined as referring to “processes in which the spacecraft without the support from the ground-based TT&C (telemetry and tracking control) system for a long time, relying on its onboard devices, obtains all kinds of measurement data, determining the navigation parameters like orbit, time and attitude”. In this monograph, DORIS and PRARE navigation systems are determined as non-autonomous, because “DORIS system can determine the spacecraft’s orbit with high accuracy, but both need to exchange information with the ground stations”. Thus, the main aim of autonomous navigation is to reduce the dependance of spacecrafts on the ground TT&C network and, in this way, to enhance the capability of the system’s anti-jam and autonomous survivability. It is evident also that the determination of the satellite’s exact location is achieved through the reception, processing and transmitting of ranging signals between the different satellites.
Autonomous GPS navigation systems are necessary, in view of the proposal of researchers from the University of Texas, Aerospace Corporation, National Bureau of Standards, International Business Machines Corporation (IBM) and Rockwell Automation Inc., to monitor nuclear explosions, based on the GPS inter-satellite communication link.
The development of inter-satellite laser communication systems in space technologies that enable super-high-speed data transfers at rates greater than 1 Gbps is widely applied also in cube-/nano-satellite platforms such as CubeLCT, AeroCube-7B/C, CLICK, LINCS-A/B, SOCRATES and LaserCube [
4]. However, in order to establish laser communications, the high performance of the arc-second level pointing system is required. This is a difficulty for the nanosatellite platforms. It can be supposed why there is such a difficulty—since the data-transmission rate of 1 Gbps between the two nanosatellites is at an inter-satellite range of 1000 km, the effects of curving the trajectory of the laser signal may be considerable. In the publication [
5], concerning the optical links of small satellites, it has been admitted that the s.c. “pointing error” arises not only due to tracking sensors and mechanical vibrations, but also due to the base motion of the satellite. This fact, together with the ranging of signals and the pointing error, illustrates the assumption that it is not only the motion of two satellites, but the effect of curving of the signal trajectory, that is important. The paper [
5] also asserts that for a GPS constellation with 24 satellites, there are a total of 8–16 links—they can be forward and backward links in the same orbit, but as well as lateral links between adjacent orbits. Moreover, the distance of GPS inter-satellite cross-links can reach 49,465 km.
The establishment of inter-satellite links (ISL) and measurement communications is of primary importance for the relative ranging and relative velocity determination between the satellites from one constellation or from different constellations such as the GPS system, the Russian GLONASS, the European Galileo and the China Beidou second-generation system; all of them are considered to be inter-operable with each other.
The theory of inter-satellite communications (ISC) is developed in the series of papers by S. Turyshev, V. Toth, M. Sazhin [
6,
7] and S. Turyshev, N. Yu and V. Toth [
8]. The theory in these three paper concerns the space missions GRAIL (Gravity Recovery and Interior Laboratory) [
6], GRACE- FOLLOW-ON (GRACE-FO—Gravity Recovery and Climate Experiment—Follow On) mission [
7] and the Atomic Clock Ensemble in Space (ACES) experiment [
8,
9,
10] on the International Space Station (ISS).
9. Justification of the Theoretical Approach in This
Paper from the Viewpoint of General Relativity Theory and Differential
Geometry
As mentioned in the introduction, the basic approach of the calculation of the propagation time in this paper is based on finding the solution for the propagation time from the null cone equation and parametrizing the space coordinates for both cases, considered as plane elliptical orbits or space-oriented orbits by the orbital coordinates. So, in fact, the light-like geodesics are not used in this approach. In the previous chapter, it was demonstrated that the solution for the propagation time does not give any time, related to the motion of the satellite, but the calculated propagation time is of the order of microseconds (1 s) or nanoseconds ( s). This is the first fact in favor of the correctness of using the approach of the null cone equation. The second fact is related to the proper dimension of seconds of the expression for the propagation time for the case of a signal, emitted by a satellite on a plane elliptical orbit.
Now we shall present the third argument, related to a theorem, proved in the book on general relativity theory by Fock [
19]. This theorem is related to known ones from basic textbooks on differential geometry [
24,
26]. Let us first begin with the theorem in [
19]
Theorem 1. Let . is the null cone equation and the extremum of the integral is searched ; then, the Lagrange equation of motion will giveFrom the last equation, the geodesic equations can be obtained. Since the constant in (139) can be set up to zero, this would mean that the geodesic Equation (140) is compatible with the null cone equation . It is important to acquire the proper understanding of this statement: It means that every solution of the null equation (including in the proper parametrization of the differentials, related to the trajectory of motion of the satellite) is also a solution of the light-like geodesic Equation (
140). In the “backward direction”, however, the statement is not true, meaning that there might be other solutions of the geodesic equation, which are not solutions of the null cone equation.
The meaning of the geodesic as the shortest path, joining two given points (but only in the local sense, i.e a sufficiently short part of the geodesics), is confirmed also by the following theorem in the monograph by Prasolov [
26]. Let us present the exact formulation of this theorem:
Theorem 2. Any point p on a surface S has a neighborhood U such that the length of a geodesic γ going from the point p to some point q and lying entirely in the neighborhood U does not exceed the length of any curve α on S, joining the points p and q. Moreover, if the lengths of the curves γ and α are equal, then γ and α coincide as non-parametrizable curves.
In fact, what does it mean that “the length of the geodesics from point
p to point
q does not exceed the length of any other curve” (but only locally, i.e., in the vicinity of the given neighborhood)? This means that this geodesic line is minimal. So, this confirms the theorem in another, third well-known book on differential geometry by Mishtenko and Fomenko [
24]:
Theorem 3. The geodesic line is minimal if it is not longer than any smooth path, joining the endpoints and .
So, if we have, for example, the functional for the null cone equation, and we are searching for the extremals, these extremals will be derived from the geodesics—the multitude of the solutions for the extremals from the geodesics turns out to be larger in comparison with the one for the other local curves in the vicinity, in particular, for the functional for the null cone equation.
On the basis of combining the results in the previously mentioned monographs by Fock [
19,
26], one easily establishes the validity of the following theorem in [
24]:
Theorem 4. The extremals of the length functional are smooth functions, derived from the geodesics (by means of smooth change of the parameter along the curve). Then, each extremal of the functional (called also action functional of the trajectory) is also an extremal of the functional . However, each extremal of the functional is not necessarily an extremal of the functional .
There is a simple inequality
[
24] in confirmation of the above theorem, which can be immediately proved by means of the Schwartz inequality
for the following functions [
24]:
It should be noted that the parameter
s in the book [
19] is defined simply as a “space parameter along a curved line”. In the above formulaes,
s is also a space parameter. But as noted in [
24], when
, i.e.,
and the parameter
s is proportional to the length of the arc of the curved line, an equality can be derived
. Thus, every solution of the null cone equation
(i.e., a zero-extremal of
) will also be a zero extremal of
. If this zero extremal corresponds to the propagation of the signal along the geodesics of the minimal length, the propagation time will satisfy Fermat’s principle regarding the least time of propagation of light between two space points. Consequently, even if there are other extremals for the length functional, of interest are only those, related to the minimal geodesics, obtained from the action functional
, which ensures the fulfillment of Fermat’s principle.
10. New Approach for Analytical Calculation of Elliptic Integrals—Analytic Relation Between Elliptic Integrals in the Weierstrass and in the Legendre Form
In this section, some new analytical algorithms for the calculation of elliptic integrals will be presented, which have been published in the paper [
33]. The main advantage of the theoretical method is that (1) it will be applicable for various values of the modulus
q of the elliptic integral, no matter whether
q is a small or a big number, and (2) the integrals, encountered in the calculation of the propagation time in the preceding sections, were in the Legendre form, which contains the specific fourth-order polynomial
under the square root in the denominator.
However, in the previous
Section 4, the definitions for elliptic integrals with arbitrary polynomials of the third and of the fourth degree were given. Such integrals are very often encountered in cosmology, nonlinear evolution equations, black hole physics, etc. For example, in the definition for the luminosity in cosmology, the more general polynomial of the fourth degree under the square root in the denominator was used. Also, there is an interesting interplay between the methods of the standard three-dimensional uniformization and of the four-dimensional uniformization, described in the monograph by Whittaker and Watson [
35]. This method will be exposed in detail further and will allow not only the analytical computation of the Weierstrass invariants
and
for four-dimensional elliptic integrals of the type
but also the comparison of elliptic integrals in the Weierstrass form
, both of them defined by formulaes (
48)–(
51) in
Section 4.1, concerning the definitions of higher-order elliptic integrals.
Some physical applications of elliptic integrals were presented in the paper [
33].
10.1. Transforming an Elliptic Integral in the Legendre Form into an
Elliptic Integral in the Weierstrass Form
The purpose of this section will be to propose a transformation, which will transform the elliptic integral
in the Legendre form into an elliptic integral in the Weierstrass form
For the purpose, the transformation
shall be applied. It is possible to calculate the parameters
a and
b, so that the integral (
144) is satisfied. These parameters represent complicated functions of the modulus parameter
q of the integral
In (
147),
is a rational function, depending on the modulus parameter
q of the elliptic integral in the Legendre form
However,
and
remain undetermined. This will turn out to be an advantage of the applied formalism, since, further, another representation of the integral in the Weierstrass form shall be found with different Weierstrass invariants
and
, which will depend explicitly on the modulus parameter
q. Then, by comparing both representations in the Weierstrass form and proving a new theorem,
and
will be expressed through
and
in a complicated way.
Further, the following important property will be very important.
Corollary 1. The parameter b satisfies the cubic equation, i.e., The proof is straightforward, if the expressions for
and
are expressed from (
146) and (
147) and are substituted into the above cubic equation.
10.2. Equivalence Between a Formulae from Integral Calculus and the Recurrent System of Equations for the Elliptic Integral in Terms of the y-Variable
Due to the property (
149), the elliptic integral (
144) can be represented in the equivalent way:
where
. Such integrals in the monograph by Timofeev [
46] are analytically computed according to the formulae
Since this is a formulae from integral calculus, it is interesting to prove the following theorem, which most surprisingly establishes the equivalence between the above relation from integral calculus with some of the recurrent system of equations for the case of a fourth-degree polynomial in the denominator of the elliptic integral [
34]. The recurrent system for the case of a cubic polynomial under the square root has been investigated in the monographs of Fichtenholz [
38] and Smirnov [
45]. Below, the formulation of the newly proved theorem is given.
Theorem 5. The integral (151) is equivalent to the second equationfrom the recurrent system of equationswhere the fourth-degree polynomial is defined asThe coefficients in the above polynomial have the valuesThe corresponding elliptic integrals of the first kind and of the third kind and of the n-th order are Due to the established equivalence, it can be concluded that the integral (
151) cannot be used for the calculation of the zero-order elliptic integral in the Legendre form (
143), because the recurrent relation (
152) contains also the higher-order elliptic integrals
and
.
10.3. A New Theorem, Concerning the Equivalence Between Some of the
Equations in the Recurrent System
Theorem 6. From (153), the corresponding equation for can be transformed into the equation for by means of the transformation and the simple relations (for the concrete case for )In such a way, only one independent equation can be obtained from the recurrent system of equations 10.4. Relations Between the Recurrent System of Equations in Terms of the y and x Variables
10.4.1. The Recurrent System of Equations in Terms of the x Variable
Now, we shall write the recurrent system of equations for the elliptic integrals
as this has been performed in the monographs [
38,
45]. Generally, the polynomial of a fourth-order under the square root is denoted as
but for the concrete case of the integrals (
162), the coefficients will be given by
For this choice of the coefficients, due to
, the polynomial becomes a cubic one, so we change the variables in (
163) from
to
, i.e.,
, where the Weierstrass invariants (coefficients)
and
will be defined in the next sections.
The recurrent system of equations is easily found to be (omitting the upper indice “
” in
)
It should be noted also that the variable transformation
in (
162) contains other parameters
and
, which are different from the parameters
and
in the transformation (
145). Now, we shall transform the three-degree polynomials under the square root in (
162) into the known four-degree polynomials for the elliptic integral in the Legendre form. The transformation
(as well as the previously applied transformation (
145)
) are generalizations of the transformation
in the paper [
54], where
are constant roots of a cubic polynomial. The transformation transforms the integral
into the integral
, where
. Under another transformation,
, the
x-variable will transform as
where a new variable
has been introduced. The two recurrent systems of equations, (
153) (written in terms of the
y-variable) and the recurrent system (
165) for the
x-variable, are not independent because
where
is determined so that
is transformed into the expression
. Consequently, the last expression is satisfied if
is defined as
Two central problems remain to be solved:
How to combine the derived equations in terms of the
x variable with the found relation (
161);
How to estimate the parameter
in Equation (
168). In fact, further, in the next sections, it will be proved that
and
will depend in a complicated way on the modulus parameter
q, but this dependence will be found from another Weierstrass representation of the integral
(
143). This new representation will not contain the conformal factor
in Equation (
144), but instead will be characterized by different Weierstrass invariants
and
, which will be explicitly determined by the method of the s.c. “fourth-degree polynomial uniformization”. This algebraic method will be considered in detail in the following sections.
10.4.2. Combining the Two Recurrent Systems of Equations
From the two recurrent system of equations in terms of the
y-variable and the
x-variable, the following equation can be obtained:
where
In (
169), the expression for
is defined as
. The notations
and
mean that these Weierstrass invariants are expressed as complicated functions of the parameter
.
Now, it can be noted that if it is possible to make the replacement
, then another equation will be obtained:
and in this way, the Equations (
170) and (
174) will give the following solution for the zero-order elliptic integral in the Legendre form:
As a consequence of the replacement
, one also has the following equality, which follows from the defining relations (
168):
If this is a quite a general condition (not imposing any restrictions), then the replacement
will be justified. Evidently, this will depend on the determination of
, which will be given in the next sections.
10.5. Another Integral in the Weierstrass Form After Applying the
Four-Dimensional Uniformization
Further, by “four-dimensional uniformization”, it shall be meant that the polynomial of the fourth degree under the square root in the denominator of the elliptic integral shall be “uniformized” by introducing complicated functions, depending on the Weierstrass function and its derivative, which represent functions of the complex variable z. The term “uniformization” shall be clarified in the next section.
10.6. The Standard Method for Uniformization in Elliptic
Functions Theory—Reminder of the Basic Facts
In accord with the standard notions in elliptic functions theory [
34,
55], the s.c. “fundamental parallelogram” is defined on the two-periodic lattice (see also the monograph [
56])
The two periods
and
constitute the basis on the complex plane and, thus, each complex number can be represented as
. Then, the Weierstrass function
is an example of an elliptic function. By definition,
is an elliptic function if it is meromorphic and doubly periodic. The double periodicity of the elliptic function means that the equality
is fulfilled, where
n and
m are integer numbers. A complex function is meromorphic, if it does not have singular points other than poles [
34]. The singular points can be of three kinds [
57,
58]:
The first derivative of the Weierstrass function
(
178) is
and it satisfies the parametrizable form of the cubic algebraic equation
where the functions
and
are called also uniformization functions for the above cubic algebraic curve
.
The summation in the defining formulaes (
178) for the Weierstrass function
is over all points on the two-dimensional lattice on the complex plane, i.e., on all possible numbers
m and
n. The coefficient functions
and
are the s.c. “invariants of the Weierstrass function”
defined as infinite convergent sums over the period
of the two-periodic lattice. The prime “′” above the two sums means that the period
is excluded from the summation, so that the expressions would not tend to infinity.
10.7. Application of the Weierstrass Integral and of the Weierstrass
Elliptic Curve in the Parametrizable Form
This section has the purpose of clarifying the importance of finding the Weierstrass invariants
and
for the calculation of an elliptic integral in the Weierstrass form
. In the next sections on the basis of the methods for four-dimensional uniformization, a primary goal will be to find the Weierstrass invariants
and
not only for integrals in the Weierstrass form, but also for integrals in the Legendre form and also for more general integrals (
50) (for
)
with an arbitrary polynomial of the fourth-degree under the square in the denominator. Here, it shall be clarified that finding the Weierstrass invariants will be possible after transforming the integral initially given in the Legendre form into an integral in the Weierstrass form, and in such a way, the Weierstrass invariants
and
will depend in a complicated manner on the modulus
q of the integral in the Legendre form. Thus, as it will be demonstrated by a theorem in the monograph by [
59], for known
and
, it will be possible to determine the solution (and the roots) of the cubic equation
. Then, the periods
and
of the two-dimensional lattice of the fundamental parallelogram (
177) can be calculated.
Let us first note that finding a solution
of the integral
in the Weierstrass form is equivalent to knowing the roots of the cubic polynomial
. This “equivalency” can be written as [
57]
In fact, the equality
is a solution also of the elliptic integral in (
185) and this is the known problem of “inversion” for elliptic integrals. In (
183),
is the Weierstrass function [
57]:
From the second representation (
186) for the Weierstrass function
, it becomes clear why, in the literature, it is denoted by
. The following theorem, proved in Ch. 6 of the monograph by Knapp [
59], allows us to understand how the result of integration for the elliptic integral (
143)
can be represented in an analytical form by means of the Weierstrass integral, after calculating the Weierstrass invariants
and
by means of the method of four-dimensional uniformization. This method will be presented in the next sections.
10.8. A Theorem about the Unique Correspondence Between the Periods and on the Two-Dimensional
Complex Lattice and the Weierstrass Invariants for Elliptic
Integrals in the Weierstrass Form
Theorem 7 ([
59])
. There exists an unique correspondence between the lattices Λ on the complex plane and the pair of the complex numbers, the Weierstrass invariants, such that the discriminant of the cubic polynomial is different from zero. If are the roots of the above polynomial, i.e.,then the periods on the complex lattice Λ (177) can be found from the following integrals:In the first integral, the unique branch of the square root is chosen with cuts from to and from to ∞, and in the second integral, the branch is with cuts from to and from to ∞. Moreover, for every lattice
on the complex plane, defined according to (
177), a biholomorphic mapping is determined as
, where
denotes the elliptic curve
. The mapping
is defined by means of the Weierstrass elliptic function
and its derivative
and the inverse mapping—by the corresponding elliptic integral.
Since
and
are uniformization functions, they will satisfy the cubic polynomial [
34]
from where the following relations are easily derived [
34], following from the algebraic properties of the roots of a cubic equation
Now, it may easily be calculated that
If the discriminant
(a well known notion from higher algebra) is different from zero, then the cubic equation
will not have coinciding roots, i.e.,
This property is very important also for the theory of elliptic curves (see also the monograph [
60]) since if the property
is fulfilled, then a lattice
on the complex plane exists, so that the Weierstrass invariants
and
can be defined as (
184)
At this point, it becomes clear why the correspondence
in the cited monograph [
59] is in both directions—previously, we proved that if the lattice
is given, then the Weierstrass invariants
and
can be defined, i.e.,
.
10.8.1. The Consistency Problem for a Non-Zero Discriminant for the Case, When the Invariants in the Weierstrass Integrals are Expressed Through the Modulus Parameter q of the Integral in the Legendre Form
Now, the statement is proved in the opposite directions—if the Weierstrass invariants are given, then a complex lattice can be defined
. Note that this statement is not trivial and it can be reformulated in the following way: If
and
are given (real or complex) numbers or even functions of some parameter (such examples will be shown further), then a two-dimensional lattice
on the complex plane with periods
and
can be defined, so that the sums in the defining equalities for
and
in (
184) will be convergent. This is also a problem, which needs further investigation.
Further, it shall be understood why this is important in view of the fact that an elliptic integral in the Legendre form (with a modulus parameter
q) shall be transformed to an elliptic integral in the Weierstrass form
. The Weierstrass integral is characterized by invariants
and
, which shall be expressed by complicated polynomials, depending on the modulus parameter
q, i.e.,
and
. So, a natural question arises: Will the integral in the Weierstrass form with the calculated values for the Weierstrass invariants
and
be correctly defined? The answer will be affirmative, because from the above correspondence, it will follow that the lattice
can be defined. However, if
and
are defined as
and
, then the condition
for the discriminant should also be fulfilled, which, in fact, will mean that the higher-order
polynomial of
q should have no zeroes! Further, in the section for four-dimensional uniformization, we shall obtain the parametrization for
and
(
212) in the form of the following higher-order polynomials of the modulus parameter
q of the elliptic integral (
143) in the Legendre form
and (
213) (the notation in both equalities will be
and
)
Then, will the higher-order polynomial
have the property to be without any roots? The same question appears with respect to the found second pair of Weierstrass invariants (
226) (the same as (
212)),
and also (
227)
In view of the theorems, which have been proved, it follows that in both cases, the higher-order polynomial
should have no roots! Since both the polynomials
depend on higher degrees of
q, which is defined for
, it is known from higher algebra that there are many theorems [
61] investigating the case when an arbitrary polynomial of the
n-th order will have (or will not have) roots in the circle
. One such theorem, formulated in the monograph by Obreshkoff [
61], is the theorem by Schur [
62], published in 1918. The concrete application of this theorem requires cumbersome analytic calculations, because it should be applied to a chain of algebraic equations of diminishing degrees. For example, for the case of the polynomials (
226) and (
227), the polynomial
will be of the 12th degree! For polynomials of the fourth degree, the Schur theorem has been applied several times in the papers [
20,
40] and the results have shown excellent consistency with the physical meaning, related to these polynomials. Moreover, in [
20], it has been demonstrated that the Schur theorem can be used both for the case of proving that a certain polynomial has roots in the circle
and for the case of proving that it does not have such roots. In the next sections, we shall give only the formulation of this theorem.
On the other hand, if in one of the cases for the polynomial , it is proved that there are roots, then this will mean that either the corresponding parametrization should be rejected, or that the elliptic integral in the Legendre form is not defined for the values of q, satisfying the above higher-order polynomial . In standard monographs on elliptic functions and integrals, this interesting problem is not investigated.
Now, it remains to prove that the three roots
are different and they will correspond to three different values for the Weierstrass function
,
,
, corresponding to the values
The proof is standard and can be found in the known monographs by [
34,
57,
60].
A detailed study on whether the periods
are real and imaginary depending on the roots of the polynomial
and on the properties of the integrals (
188) and (
189) is given in Ch.
of the known monograph [
63].
10.8.2. A Differential Equation for the Weierstrass Function as a
Consequence of the Group-Theoretical Law for Summing up Points on the Cubic
Curve
Since the group law for summing up points on the cubic curve and the resulting differential equation will be applied further for finding other formulaes for the Weierstrass invariants
and
in (
228), we shall review briefly the derivation of this equation, given in [
34].
Let
denote the three roots of the cubic curve
. If a straight line
(
are coefficients) intersects the cubic curve, then, from the cubic equation
also from the Wiets formulae for the roots and from the equalities (
195) for the three values of the Weierstrass function at the points
,
and
, it follows [
34] that
Since
and
are uniformization functions for the cubic curve and also for the straight line
, let us write it in terms of the Weierstrass function
Expressing, from here, the coefficient
f as
and substituting into the cubic Equation (
195), the following differential equation is obtained, which is a consequence from the group-theoretic law for summing up points on a cubic curve:
Closely related with the group theoretical law on elliptic curves is the problem of rational points on elliptic curves. Both topics are extensively discussed in the monographs [
34,
64,
65].
12. Comparison of Elliptic Integrals in Different Variables and with
Different Weierstrass Invariants
In this section, the method for calculating the Weierstrass invariants
and
will be presented, which is based on the combination of two approaches. In the first approach (see
Section 10.1), the initial integral
was brought to the Weierstrass form (
144) (multiplied by the conformal factor
) by means of the transformation (
145)
, as a result of which, the integral acquires the form (
144). This will be Equation (
228) below.
The second approach is based on the method of “four-dimensional uniformization”, as a result of which, the integral is brought to the second representation (
203)
. This will be Equation (
229) below.
As a result of the two representations, the following two equalities can be written:
Now, we shall use the following theorem, proved in the monograph by Hancock [
67], in order to transform the integral (
228).
Theorem 11. The elliptic integral in the Weierstrass representation (144) (equation (A) in (228) in terms of the “x”-variable) can be written in terms of the variable σ asafter performing the variable transformationwhere and are the roots of the cubic polynomial .
In the original formulation of the theorem in [
67] (page 29), the transformation
has been applied to the integral
. Note, however, the minus sign in front of the right term in Equation (
230), which is important and can easily be confirmed by a direct calculation. Applying, again, the above theorem with respect to Equations (
229) and also (
203), the equality (
230) can be written as
where the variable transformation
has been performed and the notations
are introduced. From Equations (
230) and (
232) (combined with (
203)), we obtain
Note the important peculiarity of Equation (
234): The Weierstrass invariants on the left-hand side of (
234) are
,
, while on the right-hand side, they are
.
12.1. Algebraic Roots of the Cubic Equations and Comparison of the
Weierstrass Invariants—The Case of Positive Discriminant
From the Cardano formulae, we have, for the roots of the cubic polynomial
(
), the following formulae (see the monographs [
61,
68]):
This is only the first root of the cubic equation
. Further, we shall investigate the case of a positive discriminant
for the cubic equation
[
61]. For the given cubic equation
with the first root (
235), the discriminant is
Let us assume that the roots of this polynomial are
and the roots of the polynomial
are
. Then, the other two roots
can be found if the formulae for the s.c.
n-th root of the number 1 is taken into account. This simple formulae is
For
, and by means of the formulae
the roots
can be expressed as
From the denominators of the two integrals in (
234), we should have the equality of two cubic polynomials under a square root, the first expression for the polynomial obtained from the square of the second cubic polynomial, multiplied by
This expression can be written in the form
The first root
of the cubic equation on the right-hand side of (
242), analogously to the Cardano formulae (
235) for the previous equation, can be expressed as
From here, after multiplication of the right-hand side of (
243) with
, only the root
can be expressed as
Now, it is important to note that
is not another root of the cubic Equation (
242)
If one assumes that
is another root (different from the roots
), then this would imply that the polynomial on the left-hand side of (
242) should be of the fourth degree, which evidently will be a contradiction. Consequently, the root
should coincide with one of the roots
. Let us first assume that
. From the comparison of the corresponding expressions under the cubic and square roots of (
235) and (
244), it follows that the equality is fulfilled when the following relations between the Weierstrass invariants
and
are fulfilled:
The last relation can be rewritten, expressing the dependance of
on
:
It can be noted that these relations are different from the ones obtained in a previous paper [
33]:
In fact, the correct relations are (
245) and (
246) (and not those in (
247), which are wrong), because they are obtained from the relation for the roots (
242). This relation takes into account the multiplication both of the roots and the variables in (
242) with the conformal factor
and not with
, as this is applied in the paper [
33].
12.2. Finding the Relation Between the Other Two Roots and for the Case When —The Case
of Positive Discriminant
The other two roots (
of the second Equation (
241)
can be written similarly to the expressions (
239) and (
240) for
and
, but with indices “2”:
Taking into account that
, and also the equality (
244), it can be obtained that
If, substituted in the formulaes (
248) and (
249) for
and
, the following relations are found between the roots (
and (
:
where
are the complex roots, given by the formulaes (
239) and (
240).
12.3. Expressing the Ratio of the Weierstrass Invariants and as a Rational Function of Higher-Order Polynomials, Depending on the Modulus Parameter q of the Elliptic Integral in the Legendre Form—The Case of Positive Discriminant
Finding the relation between the Weierstrass invariants
and
in (
245) and (
246) is rather insufficient and does not give any valuable information because of the following two reasons:
A. The variable
a is not just a parameter, but represents a complicated function, depending on the ratio
and on the rational function
, and is determined in (
146) as
, where (
148)
.
B. The invariants
do not contain any useful information—they appeared after the variable change (
233)
when the equality of the two integrals (
234)
was obtained. Although we found the relation between the roots of the polynomials in both sides of (
234), it is more important to find the dependance of the invariants
on the invariants
, expressed through polynomials of the modulus
q and calculated in the process of the four-dimensional uniformization, presented in the previous sections.
That is why, now, we shall combine the transformations (
245) and (
233). We obtain
Taking into account (
146) for
with (
148) for
and substituting in the above equalities, it follows that
Dividing the above two equalities, it can be obtained that
After some transformations, the ratio
can be expressed as a function of powers of the ratio
(which is a rational function of polynomials of the modulus parameter
q-see (
143) and (
213)) and powers of
, which is also a rational function of higher-order polynomials in the nominator and in the denominator according to (
148):
This is an important formulae, enabling to express
as a linear function of
and a complicated function
:
In such a way, substituting the above expression in formulae (
235) for the (real-valued) first root
of the cubic polynomial
, one can obtain
where
and
(obtained from the discriminant formulae (
236)
) denote the following functions:
In (
259), the following new notations were introduced:
Since the “modified” discriminant
(
261) is linearly dependant on the Weierstrass invariant
, it is evident that
can be found if the ratio
is known. This will be used in the next sections.
12.4. A Simple Proof for the Positivity of the Weierstrass Invariants for the One Pair of Weierstrass Invariants (226) and (227)
From (
261), the condition for a non-zero discriminant can be written as
Now, we shall prove that this expression is different from zero and also is positive. The nominator of the expression for (
148)
and also the expression for
(
227),
are different from zero. For the nominator of
, the positivity is easily established, because it can be represented in the form
which, evidently, is greater than zero.
Consequently, whether
will be positive depends on the ratio
. The invariant
(
226)
is evidently positive.
Let us check the sign of expression (
227) for
, which can be rewritten as
This expression will be positive when the inequalities
and also
are fulfilled. The first case (
267) should be disregarded, because it means that
, which is impossible because
q should be in the range
.
The second expression (
268) is positive when
, which is also impossible. Consequently, the expression in the square brackets in (
266) cannot be positive when
and
.
However, when
and
(always fulfilled since
), the expression
is positive, because both expressions in the round brackets are negative and (
269) is with a plus sign.
Therefore, we proved that for all values of
q, the invariant
(
263) is positive and from the assumption for the positivity of the discriminant (
236)
it follows also that
.
12.5. The Possibility for a Positive and Real Invariant ,
Imaginary Invariant and a Negative Discriminant—A Theorem in
[60]
Formally, it may seem that if
is negative but the condition for the moduli is fulfilled, then the discriminant
will be positive. In fact, here, we may have the case when
is positive but
can be imaginary. This problem is not investigated in the mathematical literature, but it is important that the possibility for
to be imaginary does not contradict a theorem in problem 12, paragraph 6 of chapter 1 in the monograph by Koblitz [
60]:
Theorem 12. The roots of the polynomial are all real ones if and only if the invariants and are all real ones and the discriminant is positive.
In this paper, we are not investigating the case when all the roots of the polynomial
are real; we deal with the case when only the first root
is a real one and the other two roots
and
are imaginary. Consequently, it is not obligatory that the invariants
and
should be real ones. We obtained that the invariant
is positive, but let us remember that for the invariant
, we found the formulae (
258)
We already proved that the ratio
is positive and real; also, we proved that the nominator of the function (
148)
is positive. The denominator can be decomposed as
Now, if one assumes that this expression is positive, then this is possible only when the following inequalities are simultaneously fulfilled:
The other two inequalities with the positive sign should be discarded because
. But if the inequalities (
271) are fulfilled and
,
and
are positive, then from the real-valuedness of
and
and also the positivity and real-valuedness of the discriminant
from the theorem in [
60], it should follow that the roots
of the polynomial
should all be real. But this will be in contradiction to the case of one real root and two imaginary roots, investigated in this paper.
Consequently, an important conclusion can be made that there can be two cases:
1st case: positive invariant , imaginary (or both imaginary invariants and ) and positive discriminant . Then, not all the roots should be real ones (the presently investigated case).
2nd case: real roots (the case not investigated in this paper). Then, the invariants and should be real and the discriminant should be negative. From the point of view of practical applications for the inter-satellite communications, it is evident that the case of a negative discriminant and modulus parameter (remember that for a GPS orbit and ), such a case seems to be realistic and should be investigated.
Note that in the strict mathematical sense, the theorem in the monograph by Koblitz [
60] should be reformulated and proved.
The case of a negative discriminant for a general cubic equation
is investigated in details in the monograph by Obreshkoff [
61] and the roots have been expressed by the concise formulae
and the angular variable
is given by the formulae
We have not investigated the case of the other pair of invariants (
212)
(identical with the invariant (
226)) and (
214)
; when the second invariant is given by a polynomial of the sixth degree and depending on its roots, it can be either positive or negative.
12.6. Finding the Weierstrass Function and the
Weierstrass Invariant From the Known Roots of the Cubic Equation and
the Group-Theoretical Law for Summing up of Points on the Cubic Curve
The main goal of every investigation, related to the cubic algebraic equation in its parametrizable form, is to find the expression for the Weierstrass function
by known Weierstrass invariants
and
. Since we started from a Weierstrass integral, derived from an elliptic integral in the Legendre form, previously, we found how the invariant
can be expressed through the other invariant
(
258)
Now, it remains to find the other Weierstrass invariant
and the Weierstrass function itself. This shall be performed by using the found roots
for the case of a positive discriminant (
236)
, the relation (
274) between these roots with the values of the Weierstrass function at the points on the complex plane
(for convenience we omit the factor
in the argument of the Weierstrass function
),
and also the group-theoretical law for summation of the points
on the elliptic curve, expressed analytically by the formulae (
199)
Taking into account (
274), let us take the ratio
of the Weierstrass function at the two different values
and
and also the ratio
at the two values
and
:
where the root
is given by formulae (
235) and the complex roots
and
are given by formulaes (
239) and (
240), respectively. Since
is a complex root, the Weierstrass function
is also a complex function and can be represented as
, where
is the real part of
and
is the imaginary part.
12.6.1. The First Equation
The first equation in (
275) can be written as
Denoting
the above equation can be represented as
Setting up equal to zero the real and imaginary part, one can obtain
12.6.2. The Second Equation
Let us now make use of the second equation in (
275), which can be written as
Using the group-theoretical law for summing up of points, represented by formulae (
199) for expressing
, the above equation can be written as
Using the two equalities in (
279), Equation (
281) can be represented as
, where the real part
and the complex part
are
Since the equation
is fulfilled when the real and imaginary part are equal to zero, we have to set up
and
. Also, we have assumed that the modulus parameter
q of the elliptic integral in the Legendre form might depend on the complex coordinate
z, on which, by definition, depends the Weierstrass function
. However, in view of the complexity of the equations, for now, we shall consider the simplified case
.
12.6.3. First Way for Representation of and of
Further, from the second Equation (
283) for
, one may express the ratio (
277)
as
Substituting
from (
284) in the first Equation (
282) for
and denoting
, one obtains the following quadratic equation with respect to the variable
X:
It is known that the roots of the simple quadratic equation
are found from the formulae [
61]
Consequently, the roots of the quadratic Equation (
285) are
This relation can be substituted in formulae (
284) for
and for the moment investigating only the case of a positive sign in front of the square root,
can be written as
Respectively, the solution (
287) for
can be represented as
The last two relations are the first way for representing
and
. Now, it shall be proved that there is a second way for representing these expressions.
12.6.4. Second Way for Representation of and of
The second way is based on the standard formulae for the parametrizable form of a cubic curve:
Substituting into (
284) for
, we obtain the second representation:
Next we shall use also expression (
258) for
through
:
Taking this equality into account and also that according to (
260),
, the second representation (
291) for
can be written also in the form
Setting up equal the two representations (
288) and (
291) for
, one can obtain
12.6.5. Another Interpretation of the Equation (293)
for the Equality of the Two Representations of —A Theorem from
Higher Algebra for Investigating Whether There are Roots of an Algebraic
Polynomial
After some transformations, the above equation can be represented in the form of a sixth-order algebraic equation with respect to
:
This equation of the 6th degree is interesting, because it does not always have roots. It will, however, have roots for certain values of the invariant
. Thus, the values of
are interrelated to the values of
. It is interesting to investigate the following problem: According to (
293), for a given function
, there should be only one value of
. However, the algebraic Equation (
294), if solved as a quadratic equation with respect to
and for a given function
, should have two roots with respect to
, which will contradict the unique expression of
from (
293).
Now, let us give the formulation of the theorem of Schur from higher algebra, which gives the possibility to establish whether a polynomial has roots. The formulation of the theorem, originally published by Schur [
62] in 1918, is taken from the monograph [
61], where many other theorems concerning roots of polynomials can be found. The theorem of Schur has been applied in the paper [
20] for establishing whether two different algebraic equations of the fourth degree have roots.
The first of these equations was related to the introduced in [
20] physical notion of a “space-time interval”, which, for a particular case, was proved to be positive, negative or even equal to zero. By means of the Schur theorem, it was proved that the corresponding algebraic equation has roots.
The second equation was related to the notion of “geodesic distance”, which is related to the propagation of a light or radio signal in a gravitational field and, therefore, should be positive. Respectively, again, by applying the Schur theorem, it was proved that the corresponding algebraic equation does not have roots. The confirmation of the physical definitions has been commented on in the paper [
40] and also the review paper [
22]. This consistency between the mathematical results and the corresponding physical definitions give some grounds to believe that the Schur theorem is a reliable mathematical tool for the investigation, also, of Equation (
294).
Theorem 13 ([
62])
. The necessary and sufficient conditions for the polynomial of n-th degreeto have roots only in the circle are the following ones:- 1.
The fulfillment of the inequality - 2.
The roots of the polynomial of degreeshould be contained in the circle , where is the s.c. “inverse polynomial”, defined asIn the case of fulfillment of the inverse inequalitythe degree polynomial (again with the requirement the roots to remain within the circle ) is given by the expression
In the paper [
20], the Schur theorem has been generalized for a “chain” of algebraic equations, when it is applied to a chain of algebraic equations with diminishing degrees. Many other theorems for investigating when a polynomial has roots or does not have can be found also in the contemporary monograph [
69].
12.7. Finding the Weierstrass Function and the
Invariant
Equation (
293), further, will play an important role, since it allows expressing the invariant
as a function of the Weierstrass function
. More importantly, it shows that for a given function
, there is only one value of
.
Yet, Equation (
293) is not sufficient for finding both the Weierstrass invariant
and the Weierstrass function
. We need one more equation, and for the purpose, we will use the defining equality for
m (
277)
, where
and
were previously defined by the formulaes (
262)
Thus, it can easily be obtained that
. Taking into account (
262), it can be derived that
where
has been defined by the equality (
261)
Denoting the first term in the above equality as
and remembering the defining equality (
260)
one can obtain the simple expression
Consequently, the discriminant
(
261) can be represented as
Making use of (
301), one can find an expression for
:
Transforming this equality and using (
303), one obtains the final formulae for
, from where
can be expressed and
can be expressed in two different ways:
In this formulae, we substitute
, expressed in (
291) through the Weierstrass invariant
and higher powers of the Weierstrass function
. After some lengthy calculations, the following complicated expression is obtained:
where
and
are the expressions (note that, everywhere, the dot “.” means multiplication, for example,
)
This equation and also Equation (
293)
can be considered as a system of two equations with respect to
and
. If, from (
293), the Weierstrass invariant
is expressed and then substituted into (
306), then a complicated equation is obtained only with respect to
. It is not clear how to find its roots (and also how many there are), but if
is found, then
is uniquely expressed from (
293).
There is one more possibility, namely, to use the second representation for
by formulae (
292), which is to be substituted in Equation (
305) for
. We shall not present here this method, which is more complicated and will lead to the derivation of a cubic equation with respect to
, which, after solving, has to be substituted in the earlier derived algebraic Equation (
294) of the sixth order with respect to
. However, again, a higher-order algebraic equation has to be solved, so this method is not effective.
13. Conclusions
In this paper, the propagation time of a signal was analytically calculated by making use of the null cone equation from the metric element, defined for the near-Earth space. Two different cases were considered, corresponding to two different parametrizations of the space coordinates of the satellite orbit: The first parametrization is in terms of the eccentric anomaly angle E, which is the dynamical parameter for the case of a plane elliptic orbit, and the second parametrization is in terms of all the six Kepler parameters for the case of space-oriented orbit with only the true anomaly angle f considered to be the dynamical parameter for the motion of the satellite.
The propagation time for the first case is proved to contain the three known elliptic integrals of the first, second and the third kind; all of them are zero-order elliptic integrals. The definitions of the different kinds of elliptic integrals are given in
Section 4.3.
The propagation time for the second case is investigated in detail in
Section 6. It is expressed in more complicated elliptic integrals of the second and of the fourth order. These are higher-order elliptic integrals, the mathematical definitions of which are given in
Section 4.1. It should be mentioned these integrals have been known in the mathematical literature for a long time, but the known physical applications are not so many compared to the elliptic integrals of the first, second and third kind.
For both cases, the sum of the
and
propagation time corrections are real-valued expressions. For the case of plane elliptical orbits, this was proved in
Section 5.1, and for the second case of space-oriented orbits, this was proved in
Section 7. This proves the correctness of the theoretical approach of parametrizing the space coordinates
or
in the null cone equation with the orbital coordinates, known from celestial mechanics.
Numerical calculations for the propagation time for the first case of plane elliptical orbit are performed in
Section 8 and they confirm that during the propagation time, the signal travels a much greater distance than the distance travelled by the satellite. It turned out also that theorems from general relativity theory [
19] and from differential geometry [
24,
26] confirm the conclusion that various parametrizations of the space coordinates, related to the orbital motion of the satellite, can be used. This proof comes in three stages: The real-valuedness of all the expressions for the propagation time for all the two cases, the theorems from general relativity theory and differential geometry and, lastly, the numerical calculations confirm the substantial fact that no matter whether a parametrization of the space coordinates (related to the satellite orbits) is used, the propagation time for a signal, expressed from the null cone equation, gives the typical values for signal propagation and not for the moving satellite.
It is important to clarify that the calculated propagation time gives the correspondence between the initial and final values of the dynamical parameter (the eccentric anomaly angle E or the true anomaly angle f) and the initial and final moments of propagation of the signal. However, in such a theoretical setup, and are not related to any real processes of transmission of signals between satellites on one and the same orbit or on different space-oriented orbits. In such a modified setting, the initial position of the signal-emitting satellite and the final position of the signal-receiving satellite have to be given and they will correspond to the initial and final moments of propagation of the signal. They have to be calculated in view of the fact that the trajectory of the signal is curved due to the action of the gravitational field. Due to this, the propagation time will not be the (presumed to be known at each moment of time) Euclidean distance between the satellites, divided by the velocity of light. The nontrivial moment in such a problem is that both satellites are moving (at the moments of emission and receiving of the signal and also during the time of propagation of the signal) so that the moments of emission and reception of the signal (they are denoted by and ) should incorporate these important peculiarities. In other words, the curvature of the signal trajectory will have to be such that at a certain space point of the second satellite, the signal will be intercepted, provided also that the signal is emitted at the proper position by the first (signal emitting) satellite.
The last three sections of this paper (
Section 10.1,
Section 11.1 and
Section 12) are dedicated to the problem of finding new solutions of elliptic integrals in the Legendre form by means of transforming them to elliptic integrals in the Weierstrass form, which, after inversion, have as a solution the Weierstrass elliptic function. The necessity of studying such integrals (at first, in the pure mathematical aspect, but subsequently, applications in problems related to physics might be searched out) is dictated not only by their important application in GPS inter-satellite communications with account also of the general relativity effects, but also by the wide application of elliptic integrals in various problems of mechanics, cosmology, black hole physics, integrable systems, etc. A short review of these different applications has been given in the paper [
33].
In this paper (as well in [
33]), the representation of the integral is exposed in two different ways; one of them is based on the transformation
(
are proved to be functions of the modulus parameter
q of the integral in the Legendre form), and the second method is related to the s.c. “four-dimensional uniformization”, exposed in the monograph by Whittaker and Watson [
35]. After comparison of the Weierstrass invariants in the two representations, the main result is the representation (
257) for the ratio of the two invariants
where
is the function (
148),
,
and
are the Weierstrass invariants, obtained after the four-dimensional uniformization [
35]. This result is not trivial and previously not investigated in the monographs on elliptic functions. Further, it was proved also that the Weierstrass invariant
and the Weierstrass function
can be found from a complicated system of two nonlinear equations.
Another interesting result was obtained in this paper, suggested by a theorem in the monograph by Koblitz [
60]. If
is a real number (or a real and positive function) and there is one real root and two imaginary ones for the cubic polynomial
(the case investigated in this paper), then the other invariant
should be imaginary and the discriminant
should be negative (a case not investigated in this paper). It should be kept in mind also that the case “negative discriminant and imaginary
” does not follow directly from the formulation of the theorem, but rather than from the “non-fulfillment” of the theorem, which is an unusual moment. No doubt, more strict mathematical treatment is necessary.
The case of a negative discriminant would correspond to three real roots of the cubic polynomial (see [
61,
68]) and this case needs a separate investigation. However, the theorem in [
60] has a very specific and intricate formulation: From it, for example, follows that for a positive discriminant, one may have also a case with three real roots of the polynomial (contrary to the standard case in the algebraic books [
61,
68]), but then, the invariants
and
should also be real! So, the interesting fact is that the Weierstrass invariants
and
have a purely “algebraic” meaning whether the roots are only real or one real and two complex. Consequently, the theorem in [
60] should be reformulated for all the possible cases and investigated again.
Transformations of the elliptic integral with a general fourth-order polynomial under the root in the denominator into an integral in the Legendre form have been studied also in the monograph [
70] (also in [
42]), but in [
70], transformation to an integral in the Weierstrass form has not been performed. However, in [
42], the inverse transformation from an integral in the Weierstrass form with the polynomial
to the integral in the Legendre form [
42]
was made, where the transformation
has been applied and the modulus of the elliptic integral
in the Legendre form was determined by the formulae
An interesting problem for future research is the following: The ratio (
257) of the invariants
and
can be used for calculating the roots
. The resulting expressions will depend on the invariant
and can be substituted in the formulae (
312) for
. Then, it is interesting to see whether the invariant
can be determined in such a way that the calculated
modulus according to (
312) will coincide with the initially given
q in the integral
.
There are also another problems, related to the implementation of the Schur theorem from higher algebra. Besides the ratio (
257), another ratio
can be obtained, if it is assumed that the roots
and
do not coincide (
), but the roots
and
coincide, i.e.,
This case also is not considered in this paper, but the investigation should be performed in the same manner.
Also, an important problem for further investigation is the algebraic treatment of the higher-order algebraic equation for the discriminant (
236)
of the cubic polynomial in the Weierstrass elliptic integral, which, in this investigation, was assumed to be positive.