Optimized land mobile satellite system for North American coverage

[0001] This application is a divisional application of U.S. patent application Ser. No. 11/030,282, entitled “Optimized Land Mobile Satellite System for North American Coverage,” filed Jan. 7, 2005, now U.S. Pat. No. 7,669,803 which claims the benefit of priority under 35 U.S.C. §119 from U.S. Provisional Patent Application No. 60/633,962, entitled “Enhanced Orbit Designs Utilizing Improved Attitude Steering Options,” filed Dec. 7, 2004. Furthermore, this application is related to U.S. patent application Ser. No. 10/176,936, filed Jun. 21, 2002. All of the aforementioned documents are incorporated herein by reference in their entirety for all purposes.

[0002] Not Applicable

[0003] The present invention relates generally to spacecraft constellations used for broadcast communication purposes and, more particularly, relates to spacecraft constellations which are optimized to provide North American mobile satellite services (“MSSs”).

[0004] Conventional land mobile satellite systems (“LMSSs”), such as the SIRIUS SATELLITE RADIO® mobile satellite system, use a constellation of three satellites in 24-hour repeating ground track, highly elliptical orbits (“HEO”) to provide high elevation angle coverage of the continental United States and Canada. The ground track of the orbit disclosed in U.S. Pat. No. 6,564,053 (“the Briskman patent”) is depicted as orbit 101 in FIG. 1. Orbit 101 has an orbit inclination of 63.4°, and a coverage of ±8 hours about the orbit apogee.

[0005] The conventional orbit described in the Briskman patent includes a pronounced figure-eight loop over northern Canada, which reduces the elevation angles to the coverage region (depicted as a cluster of asterisks). While the system and orbit described in the Briskman patent provides user-to-spacecraft elevation angles which are significantly higher than typical geostationary earth orbit (“GEO”) systems, the orbit described therein is suboptimal, based on a range of criteria related to system performance. Specifically, and as illustrated in FIG. 2, when the corresponding elevation angles for 125 North American cities are plotted, an elevation droop (denoted by reference 201) is evident when a spacecraft is near apogee, since high latitudes reduce elevation angles to the coverage region, resulting in a lower probability of link closures. The probability of link closure in an urban environment with at least one spacecraft and at least two spacecraft are depicted in the chart at FIGS. 3 and 4, respectively.

[0006] Due to their substantial build, launch and operational costs, it is considered highly desirable to overcome the deficiencies of conventional land mobile satellite systems. Specifically, it is desirable to provide an enhanced land mobile satellite system using an optimized orbit which reduces the system cost to achieve a given quality of service, by allowing the use of lower power and lighter satellites, and using fewer terrestrial repeaters or, conversely, allowing improved performance using the highest spacecraft power capability available.

[0007] The present invention relates generally to spacecraft constellations used for broadcast communication purposes and, more particularly, relates to spacecraft constellations which are optimized to provide North American MSS.

[0008] According to one arrangement, the present invention is a constellation, including a plurality of spacecraft, each of the plurality of spacecraft in its own approximately 24-hour orbit. Each of the orbits has a substantially teardrop-shaped or oval-shaped ground track, is optimized based upon performance criteria, and has an apogee longitude of approximately 90° west to approximately 100° west.

[0009] The present invention uses non-linear optimization to determine the improved orbit, where the improved orbits provide significantly higher elevation angles, and improved probability of signal availability. Higher elevation angles provide improved service quality by reducing signal blockages, resulting in higher signal strength to users in the coverage region. In contrast to conventional systems, the optimized orbit of the constellation of the present invention largely eliminates the figure-eight ground track, and instead exhibits a preferable teardrop or oval shape. The teardrop or oval shaped orbit results in the operational spacecraft remaining in the coverage region for a longer period of time.

[0010] Each of the orbits has a semi-major axis of approximately 42,164 kilometers, an argument of perigee of approximately 270°, an inclination of approximately 40° to approximately 60°, and an eccentricity of approximately 0.16 to approximately 0.4.

[0011] The plurality of spacecraft further includes a first, second and third spacecraft, and the orbits of each of the plurality of spacecraft are selected to bring each of the spacecraft to apogee at time increments of approximately eight hours. Each of the plurality of spacecraft includes a broadcast capability. The performance criteria include elevation angle from a coverage region to the plurality of spacecraft, and/or signal availability from at least one spacecraft, from at least two spacecraft, or from all operating spacecraft.

[0012] According to a second arrangement, the present invention is a method of providing North American mobile satellite services, including the step of placing a plurality of broadcast spacecraft in similar approximately 24-hour orbits which may be rotated relative to each other, each of the plurality of spacecraft including a broadcast capability, each of the orbits having a substantially teardrop-shaped or oval-shaped ground track, being optimized based upon performance criteria, and having an apogee longitude of approximately 90° west to approximately 100° west. The method also includes the step of broadcasting from each of the plurality of spacecraft.

[0013] Each of the plurality of spacecrafts broadcasts in the northern hemisphere.

[0014] In the following description of the preferred embodiment, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and changes may be made without departing from the scope of the present invention.

[0035] The present invention provides a land mobile satellite system for North American coverage, which overcomes the deficiencies of conventional MSS systems. Specifically, the present invention provides an enhanced land mobile satellite system using an optimized orbit which reduces the system cost to achieve a given quality of service, by allowing the use of lower power and lighter satellites, and using fewer terrestrial repeaters.

[0036] The LMSS according to the present invention uses an improved orbit design that provides better user elevation angles and link availability performance. Depending upon the performance criteria to be optimized, a range of orbits is contemplated by the present invention, with specific example orbits described more fully below.

[0037] According to one arrangement, the present invention is a constellation, including a plurality of spacecraft, each of the plurality of spacecraft in its own approximately 24-hour orbit. Each of the orbits has a substantially teardrop-shaped or oval-shaped ground track, is optimized based upon performance criteria, and has an apogee longitude of approximately 90° west to approximately 100° west.

[0038] Each of the orbits has a semi-major axis of approximately 42,164 kilometers, an argument of perigee of approximately 270°, an inclination of approximately 40° to approximately 60°, and/or an eccentricity of approximately 0.16 to approximately 0.4.

[0039] The plurality of spacecraft includes a first, second and third spacecraft, and the orbits of each of the plurality of spacecraft are selected to bring each of the spacecraft to apogee at time increments of approximately eight hours. Each of the plurality of spacecraft includes a broadcast capability. The performance criteria include elevation angle from a coverage region to the plurality of spacecraft, and/or signal availability from at least one spacecraft, from at least two spacecraft, or from all operating spacecraft.

[0040] The present invention uses non-linear optimization to determine the improved orbit, where the improved orbits provide significantly higher elevation angles, and improved probability of signal availability. Higher elevation angles provide improved service quality by reducing signal blockages, resulting in higher signal strength to users in the coverage region. In contrast to conventional systems, the optimized orbit of the constellation of the present invention largely eliminates the figure-eight ground track, and instead exhibits a preferable teardrop or oval shape. The teardrop or oval shaped orbit results in the operational spacecraft remaining in the coverage region for a longer period of time.

[0041] Table 1, below, provides three example orbits which are contemplated by the present invention, and also provides a comparison to the conventional suboptimal orbit described in the Briskman patent.

[0042]

TABLE 1
Representative Orbits According to the Invention
Parameter	Conventional	Option 1	Option 2	Option 3
Inclination	63.4	48.2	45.4	58.9
Eccentricity	0.27	0.24	0.40	0.35
Apogee	96	95	95	95
longitude (° W)
Performance	Not	Minimum	Signal	Signal
Criteria	Optimized	composite	availability	availability
Optimized		elevation	from at least	from all
		angle	one spacecraft	operating
				spacecraft
Average	69.3	75.6	76.6	72.2
elevation angle
Minimum	60.4	70.4	67.5	65.1
elevation angle
Minimum	0.990	0.996	0.997	0.993
availability of
one spacecraft
(suburban)
Minimum	0.958	0.984	0.986	0.972
availability of
one spacecraft
(urban)
Minimum	0.746	0.765	0.770	0.775
availability of
two spacecraft
(suburban)
Minimum	0.489	0.516	0.523	0.545
availability of
two spacecraft
(urban)

[0043] According to one aspect (entitled “Option 1” in Table 1), the orbit is optimized based upon minimum composite elevation angle for each city in the coverage region to an available spacecraft, over a day of system operation. The optimization considers three spacecraft separated by 8 hours in repeating ground tracks that operate for ±8 hours about the orbit apogee, where each spacecraft is assumed operational in the northern hemisphere only. According to these parameters, the composite elevation for the i_th city is expressed in Equation (1), below:
θ_i(t)=max(θ_SC1i(t),θ_SC2i(t),θ_SC3i(t))
0≦t≦T_orbit  (1)

[0044] In Equation (1), θ_SC1i(t), θ_SC2i(t), and θ_SC3i(t)) represent the elevation angles to each spacecraft from the ith city or location in the coverage region, and T_orbit represents the orbit period, or one sidereal day, which is approximately 24 hours.

[0045] Orbit parameters are determined which minimize the cost function, expressed in Equation (2), below:

[0046] $\begin{array}{cc}J={\left[\sum _{i=1}^N{w_i\left(90°-\min \left[{\theta }_i\left(t,e,i,L,\omega \right)\right]\right)}^2\right]}^{1/2}& \left(2\right)\end{array}$

[0047] In Equation (2), 90°−min[θ_i(t, e, i, L, ω)] represents the maximum co-elevation angle for the i_th city over a single sidereal day, e represents the orbit eccentricity, i represents the orbit inclination, L represents the longitude of the ascending node, ω represents the argument of perigee, N represents the number of cities (125), and w_i represents normalized city weighting factors, expressed below in Equation (3), for preliminary analysis:

[0048] $\begin{array}{cc}1-\sum _{i=1}^N{w}_i{w}_i=\frac{1}{N}=\frac{1}{125}& \left(3\right)\end{array}$

[0049] The cost function (Equation (2)) is based on a composite elevation angle computed for 125 cities in the coverage region, which includes the continental United States and Canada. Alaska and Hawaii are not considered in this optimization, and equal weighting is applied to each of the 125 North American cities.

[0050] As indicated in Table 1, optimization based upon minimum composite elevation angle results in an orbit which provides elevation angles approximately 10 degrees higher than the conventional orbit. Furthermore, the optimized orbit provides a higher minimum probability of link availability in both suburban and urban environments. The optimized orbit has reduced inclination and eccentricity, compared to the conventional orbit.

[0051] FIG. 5 depicts the ground track of the above-described improved orbit, which is optimized based upon minimum composite elevation angle for each city in the coverage region to an available spacecraft, over a day of system operation. A distinguishing feature of the improved orbit, denoted by reference 501, is that it exhibits a distinctive teardrop shape, compared to the figure-eight ground track of the conventional system's orbit. The three small circles in FIG. 5 indicate the position of spacecraft at the start and end of the 16 hour service region and at apogee, which occurs at a longitude of roughly 95° west.

[0052] In contrast to the conventional orbit 101, the optimized orbit substantially eliminates the pronounced figure-eight loop over northern Canada. In particular, the optimized orbit exhibits a teardrop shape, resulting in the operational spacecraft remaining longer over the coverage region. To illustrate the improvement, FIG. 6 depicts the corresponding composite angles for the improved orbit to 125 North American cities. Comparing FIG. 6 to FIG. 2, it is clear that optimization of orbits eliminates the “elevation droop” associated with conventional orbits.

[0053] According to a second and a third aspect of the present invention (respectively entitled “Option 2” and “Option 3” in Table 1), the orbit is optimized to maximize the signal availability to users in the coverage region. According to the second aspect, orbit parameters maximize the probability that at least one spacecraft's signal will be available at all locations at all times. According to the third aspect, orbit parameters maximize the probability that all operating spacecraft's signals will be available at all times. In all cases, the orbit parameters are determined based upon results for 125 representative North American cities.

[0054] The probability of signal availability is computed based upon known three-state statistical LMSS channel models, such as the model described in Y. Karasawa et al., Analysis Of Availability Improvement In LMSS by Means Of Satellite Diversity Based On Three State Propagation Channel Model, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, Vol. 46, No. 4 (November 1997) (“the Karasawa article”), which is incorporated by reference herein for all purposes. Using this model, the probability that a signal reaching the user will exceed a minimum needed for the receiver to operate x_o is expressed by Equation (4), below:

[0055] $\begin{array}{cc}p\left(x\ge x_0\right)={\int }_{X_0}^{\infty }\left\{P_A{f}_a\left(x\right)+P_B{f}_b\left(x\right)+P_C{f}_c\left(x\right)\right\}dx& \left(4\right)\end{array}$

[0056] In Equation (4), P_A is the probability that the line-of-sight (“LOS”) from the user to the spacecraft is unobstructed, P_B is the probability that the LOS is partially obstructed, and P_C is the probability that the LOS is entirely obstructed. Also, ƒ_a, ƒ_b, and ƒ_c are probability density functions (“PDFs”) that provide a statistical description of multi-path and signal attenuation effects for each of the obstruction cases. Further detail of this model, including appropriate parameters for urban and suburban environments, is omitted for the sake of brevity, and may be found by referencing the Karasawa article.

[0057] At any given time in the service region of the orbit, the probability P_A is computed using Equation (5), below:

[0058] $\begin{array}{cc}{P}_A=1-\frac{{\left(90-\theta \right)}^2}{a}& \left(5\right)\end{array}$

[0059] In Equation (5), θ is the elevation angle from the ground to the spacecraft, and a is a parameter whose value depends on the obstruction environments. Based upon empirically-derived expressions that relate the ratio of P_B and P_C, and given that P_A+P_B+P_C=1, the probability P_B is determined by Equation (6), and P_C is determined by Equation (7), below:

[0060] $\begin{array}{cc}{P}_B=\langle \begin{array}{c}\frac{4\left(1-P_A\right)}{5}\mathrm{for}\mathrm{suburban}\mathrm{area}\\ \frac{\left(1-P_A\right)}{5}\mathrm{for}\mathrm{urban}\mathrm{area}\end{array}& \left(6\right)\\ P_C=\langle \begin{array}{c}\frac{\left(1-P_A\right)}{5}\mathrm{for}\mathrm{suburban}\mathrm{area}\\ \frac{4\left(1-P_A\right)}{5}\mathrm{for}\mathrm{urban}\mathrm{area}\end{array}& \left(7\right)\end{array}$

[0061] The probability that an insufficient signal is available at any given time for link closure from all j=1, K operational spacecraft is computed in Equation (8), below:

[0062] $\begin{array}{cc}{P}_i^{\mathrm{none}}\left(t\right)=\prod _{j=l}^K\left(1-p_i^j\left(t\right)\right)& \left(8\right)\end{array}$

[0063] In Equation (8), p_i^j(t) is the probability of signal availability computed from Equation (4) for the i_th ground location, based on the elevation angle to the j_th spacecraft. Similarly, the probability that sufficient signal is available for link closure simultaneously from all operational spacecraft is given by Equation (9), below:

[0064] $\begin{array}{cc}{P}_i^{\mathrm{all}}\left(t\right)=\prod _{j=l}^K{p}_i^j\left(t\right)& \left(9\right)\end{array}$

[0065] Using the results of these equations, cost functions are defined which, when minimized, result in optimized orbits for the second and third aspects. The cost function for the second aspect expressed in Equation (10), and the cost function for the third aspect is expressed in Equation (11), below:

[0066] $\begin{array}{cc}J={\left[\frac{1}{N}\sum _{i=1}^N{\left({\stackrel{\_}{P}}_i^{\mathrm{none}}\right)}^2\right]}^{1/2}& \left(10\right)\\ J={\left[\frac{1}{N}\sum _{i=1}^N{\left(1-{\stackrel{\_}{P}}_i^{\mathrm{all}}\right)}^2\right]}^{1/2}& \left(11\right)\end{array}$

[0067] In Equation (10), P_i^none is the average value of the probability given by Equation (8) over an orbit period, which is nominally one sidereal day. In Equation (11), P_i^all is the average value of the probability given by Equation (9).

[0068] Although the results assume a nominal link margin (in the absence of fade) of 8 dB, the optimized orbit parameters are insensitive to the link margin and whether the environment is suburban or urban.

[0069] FIG. 7 depicts the ground track of an example improved orbit according to present invention, where the orbit is optimized based upon signal availability from at least one spacecraft, over a day of system operation, superimposed on the conventional, FIG. 1 ground track. The optimized ground track, denoted by reference 701, has an oval shape, and the spacecraft moves across the coverage region in a predominantly east-to-west direction. In contrast, the conventional orbit moves across the coverage region in a predominantly north-to-south direction.

[0070] FIG. 8 depicts the corresponding elevation angles of the FIG. 7 improved orbit. Comparing FIG. 8 to FIG. 2, an increase in corresponding elevation angles is apparent.

[0071] FIG. 9 depicts the probability of link closure in an urban environment with at least one spacecraft, for the FIG. 7 orbit. Similarly, comparing FIG. 9 to FIG. 3, it is evident that there is a significant increase in link closure probability using the optimized orbit according to the present invention.

[0072] FIG. 10 depicts the ground track of an example improved orbit according to present invention, where the orbit is optimized based upon signal availability from all operating spacecraft, over a day of system operation, superimposed on the conventional, FIG. 1 ground track. The optimized ground track, denoted by reference 1001, has a teardrop shape, and the spacecraft spends increased time over the coverage region compared to the FIG. 1 ground track.

[0073] FIG. 11 depicts the corresponding elevation angles of the FIG. 10 improved orbit. Comparing FIG. 11 to FIG. 2, an increase in corresponding elevation angles is also apparent.

[0074] FIG. 12 depicts the probability of link closure in an urban environment with two spacecraft, for the FIG. 10 orbit. Comparing FIG. 12 to FIG. 4, a significant increase in link closure probability is evident using the optimized orbit according to the present invention.

[0075] To further illustrate the performance improvement using the FIG. 10 orbit, FIGS. 13A to 13C show the elevation angle time history over a day for New York, Los Angeles, and Seattle, respectively, for the improved orbit and conventional orbit. The solid lines (denoted by reference 1301) show the optimized orbit elevation angles, and the dashed lines (denoted by reference 1302) show the elevation angles for spacecraft in the conventional orbit. The significant increase in the composite elevation angle, which is the envelope formed by the maximum of the elevation angles for all operating spacecraft, is readily seen. Furthermore, in addition to the increase in the minimum composite angle which occurs at 8-hour intervals, including 0, 8, 16 and 24 hours in FIGS. 13A to 13C, the elevation angles for the other two spacecraft at these times are generally higher than for conventional orbits. These attributes account for the improvement in the two spacecraft link availability achieved by the FIG. 10 orbit.

[0076] Regarding the optimization of orbits based upon performance criteria, different optimization criteria will result in different optimized orbits, and other spacecraft and system design constraints may favor the use of one orbit over another. Extensive analysis has shown that optimal orbits for North American LMSS have inclinations between 40° and 60°, eccentricities between 0.16 and 0.4. The ground tracks of the optimal orbits have an oval or teardrop shape, and lack a significant figure-eight loop. The longitude of the orbit apogee, and correspondingly the longitude of the highest latitude of the ground track, is between 90° and 100° west.

[0077] The optimal orbits contemplated by the present invention are included within the inclination and eccentricity region depicted in FIG. 14. The inclination and eccentricity region is depicted in FIG. 14 for inclinations of 40° to 60° and on the right-hand side of curve 1401 in FIG. 14, with the lower-bound eccentricities of 0.16 at 40° inclination and 0.34 at 60° inclination.

[0078] FIGS. 15 to 18 illustrate specific example orbits from the family of optimal orbits defined within the inclination and eccentricity region of FIG. 14, which are contemplated by the present invention. In particular, FIG. 15A depicts the ground tracks for a family of orbits with an inclination of 45°, and with eccentricities from 0.18 to 0.40, in increments of 0.02. FIG. 15B depicts a corresponding plot of average and minimum elevation, which shows that the optimal elevation angle orbit is included within the specified parameter region.

[0079] FIG. 16A depicts the ground tracks for a family of orbits with an inclination of 50°, and with eccentricities from 0.22 to 0.40, in increments of 0.02. FIG. 16B depicts a corresponding plot of average and minimum elevation, which shows that the optimal elevation angle orbit is included within the specified parameter region.

[0080] FIG. 17A depicts the ground tracks for a family of orbits with an inclination of 55°, and with eccentricities from 0.28 to 0.40, in increments of 0.02. FIG. 17B depicts a corresponding plot of average and minimum elevation, which shows that the optimal elevation angle orbit is included within the specified parameter region.

[0081] FIG. 18A depicts the ground tracks for a family of orbits with an inclination of 60°, and with eccentricities from 0.34 to 0.40, in increments of 0.02. FIG. 18B depicts a corresponding plot of average and minimum elevation, which shows that the optimal elevation angle orbit is included within the specified parameter region.

[0082] According to a second arrangement, depicted in FIG. 19, the present invention is a method of providing North American mobile satellite services. Briefly, the method includes the step of placing a plurality of broadcast spacecraft in similar approximately 24-hour orbits which may be rotated relative to each other, each of the plurality of spacecraft including a broadcast capability, each of the orbits having a substantially teardrop-shaped or oval-shaped ground track, being optimized based upon performance criteria, and having an apogee longitude of approximately 90° west to approximately 100° west. The method also includes the step of broadcasting from each of the plurality of spacecraft.

[0083] In more detail, the process begins (step S1900), and a plurality of broadcast spacecraft is placed in similar approximately 24-hour orbits which may be rotated relative to each other (step S1901). Each of the plurality of spacecraft include a broadcast capability. Each of the orbits has a substantially teardrop-shaped or oval-shaped ground track, and is optimized based upon performance criteria. Furthermore, each of the orbits has an apogee longitude of approximately 90° west to approximately 100° west.

[0084] Each of the plurality of spacecraft broadcasts (step S1902), providing North American mobile satellite services, and the process ends (step S1904). Each of the plurality of spacecrafts broadcasts when the spacecraft is in the northern hemisphere.

[0085] The invention has been described with particular illustrative embodiments. It is to be understood that the invention is not limited to the above-described embodiments and that various changes and modifications may be made by those of ordinary skill in the art without departing from the spirit and scope of the invention.