Satellite communications facilitated by synchronized nodal regressions of low earth orbits

A satellite communication method enables a low earth orbit data collecting mission satellite to conveniently achieve continuous, real time, and cost effective global connectivity by "plugging in" to a constellation of low earth orbit communication satellites designed to provide mobile satellite service for terrestrial users. Orbit parameters for the mission satellite are selected to generate a nodal regression rate matched to that of the communication satellites so that initial alignment of orbit planes is sustained maintaining a favorable geometry between the communicating satellites. The resulting intra-satellite geometry is exploited through use of widebeam crosslink antennas, simplified pointing techniques, and variable transmission rates for optimum communication efficiency from the mission satellites to communication satellites and ground receivers.

SPECIFICATION 
FIELD OF INVENTION 
The invention relates to the field of satellite communications, and more 
particularly to methods for facilitating the transmission of information 
between low earth orbiting data collection satellites and low earth 
orbiting communication satellites. 
BACKGROUND OF THE INVENTION 
There are numerous operational satellite systems currently deployed in 
space, and several more in the planning stages. Satellites orbit the earth 
at prescribed altitudes and orbit inclinations, the inclination being the 
angle the plane of the orbit makes with the plane of the earth's equator. 
Satellite orbits can be categorized in three altitude ranges: Low earth 
orbit (LEO) from 300 to 1500 km; medium earth orbit (MEO) from 10,000 to 
20,000 km; and high earth orbit (HEO) from 35,000 km and above. Orbit 
altitudes and orbit inclinations are selected to achieve desired 
performance objectives (e.g., global access, ground resolution, orbit 
period, etc.), and also selected to avoid regions of high natural 
radiation which can damage satellite equipment. 
A satellite system can be deployed as a single satellite or as a plurality 
or constellation of similar satellites that occupy the same spherical 
altitudes, and cooperate together in performing a mission. For example, 
many communication satellite systems operate at HEO altitudes at near-zero 
inclination. Their resulting 24-hour orbital periods coincide with the 
earth's rotational rate causing the satellite to remain fixed above a 
specified equatorial ground point, a feature that facilitates ground 
antenna pointing. The Global Positioning System (GPS) is a multi-ring 
24-satellite MEO constellation that employs 20,000-km circular 12-hour 
orbits inclined at 55 degrees. U.S. Pat. Nos .5,551,624, 5,439,190, and 
5,415,368 disclose MEO constellation of communication satellites in 
10,000-km, six-hour orbits. IRIDIUM is a LEO constellation of 66 
communication satellites at 785 km altitude equally distributed in six 
planes inclined at 86.4.degree.. When specific mission satellites, such as 
earth imaging satellites, are deployed in the lower end of the LEO orbit 
range (for example between 300 to 600 km), the orbit is referred to herein 
as a very low earth orbit (VLEO). 
A satellite constellation may feature crosslinked communications enabling 
inter-constellation connectivity in a tangential or lateral direction from 
one satellite to another satellite around the orbital sphere. This 
inter-constellation connectivity allows a message to be uplinked radially 
from a ground site to any satellite in the constellation, then relayed 
tangentially from satellite to satellite around the globe and then 
radially downlinked to a remote receiving ground site. Communication 
satellite constellations are typically crosslinked to enable global 
connectivity. A signal from the ground may be received by a satellite 
through a radially directed communication uplink, retransmitted by that 
satellite to a second satellite through a tangentially or laterally 
directed crosslink, and then retransmitted by the second satellite to a 
ground receiver through a radially directed downlink. Some communication 
satellites operate primarily as relays, where messages are received from 
the ground and simply retransmitted back to the ground at another 
location. 
Earth-based or space-based microwave transmission can be directed through 
broad beams or narrow beams depending on the antenna design and 
transmission frequency, and are selected to be compatible with parameters 
and constraints of the particular link application in terms of range, 
satellite-to-satellite geometry dynamics, transmitter power, data rate, 
weather, or other operational factors. Broad beams or wide beams are 
created using small omni-directional antennas, and are best suited for 
coarse pointing, low signal power, low-data-rate applications. Narrow 
beams employ larger unidirectional antennas that require more precise 
pointing and generally deliver higher power to the receiver necessary for 
high-data-rate transmission. The beam width is selected to suit the 
particular application and can span the range from very broad to very 
narrow. The corresponding communications signals may include very 
low-data-rate command messages, or low-data-rate voice, or very 
high-data-rate multi-color images. There are always design trade-offs 
between transmitter power, beam widths, and data rates, but generally for 
high-data-rate communications the beam width is narrow and the power 
delivered to the receiver is high, and for low-data-rate communications 
and broadcasting the beam width is broad and the delivered power is lower. 
Satellite crosslinks generally employ very narrow microwave or laser 
transmission beams that focus the transmitted energy on the receiving 
antenna. Narrow beams allow efficient use of transmitter power over longer 
ranges, and maximize the transmitted data rate. A disadvantage of narrow 
beams is that they require very accurate pointing and tracking, and often 
require the use of a secondary wider beam to search for and acquire the 
receiver target and establish the link before narrow beam transmission can 
be initiated. This acquisition and hand-off procedure is repetitive as 
communicating satellites move in and out of view and the link must be 
rerouted. 
LEO mission satellites that monitor the earth collecting environmental 
records or ground images generate large quantities of data at high rates. 
If the collection satellite operates in the "store-and-dump" mode, the 
collected data is electronically stored onboard until the satellite 
overflies a ground read-out station, whereupon the data must be downlinked 
at an accelerated rate while the satellite is in view of the ground 
station. This method of operation requires the satellite to have ample 
onboard data storage and read-out capability, and suffers inherent time 
delays between data collection and read-out opportunities. Today, if the 
collection satellite operates in a "real time" relay mode, a wideband 
(high-data-rate) space link must be established and maintained between the 
data collecting mission satellites and a mission specific or partially 
dedicated HEO relay communication satellites. This method of operation 
requires precise tracking over long ranges (.about.25,000 km) using very 
narrow beams, and involves periodic reacquisition procedures as the 
satellites are eclipsed by the earth. The problems associated with 
long-range communications drive all aspects of the communication system 
design, and strongly influences the size, weight, power, and cost of the 
hosting satellite. 
Successful link closure between communicating satellites usually requires 
special hardware and processes to account for the time variation of the 
satellite-to-satellite geometry. The determination of precise antenna 
pointing angles and rates, range monitoring, Doppler shift correction, 
acquisition and hand-off procedures are typical processes in achieving and 
maintaining satisfactory link closure. For example, when a VLEO satellite 
communicates with a constellation of HEO communications satellites, the 
link must be reestablished with a second HEO satellite before the 
currently-linked HEO satellite passes out of view of the VLEO satellite. 
The acquisition and hand-off to the second HEO satellite must be 
accomplished over extremely long ranges and in rapid fashion to avoid gaps 
in communications. For another example, a constellation may include 
several satellites distributed in multiple orbit planes (sometimes 
referred to as rings of co-planar satellites), each ring defined by the 
orbit altitude and the orientation of its orbit plane. The constellation 
may exhibit inter-ring and intra-ring crosslink communications creating 
time varying Doppler shifts in the received carrier frequency especially 
during intra-ring communications. The onboard communication subsystem is 
usually required to retransmit received range tones which can then be used 
to appropriately adjust the transmission for Doppler correction. 
The rapidly changing relative geometry can be even more pronounced when 
communicating between two LEO satellites in different orbits and different 
orbit planes. At LEO, the severity of the relative geometry problem is 
largely dependent on the degree of alignment of the two orbit planes. When 
the planes of the orbits of two satellites are at a large angle to each 
other, the relative motion between the two satellites moving in their 
respective planes can be extremely large depending on the degree of planar 
misalignment. The two orbit planes are constantly rotating relative to the 
stars and relative to each other depending on the parameters of the two 
orbits. Even when the orbit planes are initially aligned, the orbit planes 
of unmatched orbits will slowly drift out of alignment due to the 
difference in their planar rotations. This well-known orbital perturbation 
effect that is caused by the earth's equatorial bulge is called nodal 
regression of the orbit plane, or the rotation of the orbit plane, 
relative to the stars, about the earth's polar axis. For LEO orbits, this 
rotational rate may be only a degree or two per day; however, in a 
relatively short time, it can result in severe misalignment of the planes 
of communicating LEO satellites with mismatched nodal regressions. The 
orbit plane rotational rate is a function of the orbit altitude and the 
inclination of the orbit plane. With proper selection of inclination, it 
is possible to match the nodal regression rates of two LEO orbits that are 
at different altitudes. The following equation would be used for 
calculating the correct inclination that results in matched nodal 
regression of circular orbits: 
##EQU1## 
where i.sub.1 =inclination of orbit #1 
i.sub.2 =inclination of orbit #2 
h.sub.1 =altitude of orbit #1 
h.sub.2 =altitude of orbit #2 
RE=earth radius 
If the satellite's orbit has an inclination angle of 90.degree., the 
resulting nodal regression rate is zero and the matching orbit would also 
have a 90.degree. inclination. 
There are a growing number of existing and planned LEO constellations of 
communication satellites designed to provide a variety of mobile 
communication services to terrestrial users. These constellations, 
referred to as Big LEO satellite communication systems, consist of dozens 
(sometimes hundreds) of crosslinked satellites distributed uniformly about 
the globe in multiple planes or rings at a common altitude. In some cases, 
each ring may include dozens of satellites equally spaced along its 
circumference. Big LEO systems use large constellations of satellites to 
provide telecommunications services at fixed and graduated data rates for 
users on the ground. For example, the IRIDIUM system currently features 66 
satellites distributed in six planes or rings and delivers low-data-rate 
mobile telephone service to subscribers in North America. For another 
example, Teledesic is a planned Big LEO system that will provide broadband 
(high-data-rate) global service suitable for rapid transmission of imagery 
and other very large data records. An early version of the planned 
Teledesic constellation called for 288 satellites uniformly distributed in 
12 rings about the globe. 
The need to provide uniform regional or global coverage with the fewest 
satellites drives the Big LEO orbits to the highest altitudes practical in 
the LEO range. (The earth's natural radiation environment constrains the 
upper bounds of the LEO altitudes.) Earth monitoring mission satellites, 
on the other hand, tend to fly at much lower (VLEO) altitudes in order to 
minimize sensor aperture (payload size and weight) needed to achieve the 
desired ground resolution. One or more VLEO mission satellites orbiting in 
the celestial presence of a Teledesic-like Big LEO system would suggest an 
architecture with multiple rings of Big LEO communication satellites 
criss-crossing aperiodically above the lower altitude mission satellites. 
The problems faced by the mission satellite in delivering its data 
retrievals to a ground site are formidable. Many mission satellites today 
store the collected data onboard until they overfly the ground station, 
and then read out the data at an accelerated rate while the station 
remains in view. Store and dump operations as described require adequate 
onboard data storage capacity, and a sophisticated data read-out 
capability. These operations are also disadvantaged by inherent time 
delays between data collection and delivery. Other mission satellites 
today transmit large data records to the ground site, in real time, using 
a dedicated HEO relay satellite system. This operation requires long-range 
communication technologies involving precise pointing and tracking with 
large narrow beam antennas and high power transmitters; in addition, the 
cost of a dedicated HEO relay may be as much or more than the mission 
satellite. Communications between a LEO mission satellite and a 
non-co-orbital LEO communication satellite requires overcoming a myriad of 
problems dealing with the rapidly varying relative geometry and a high 
nonperiodic acquisition and hand-off frequency. These and other 
disadvantages are solved or reduced using the invention. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a method for communicating data 
from a very low earth orbit (VLEO) mission satellite through a radially 
directed communication crosslink to a low earth orbit (LEO) communication 
satellite. 
Another object of the invention is to provide a method for communicating 
data between a LEO communication satellite and a VLEO mission satellite 
having an orbit with a nodal regression rate matched (equal) to the nodal 
regression rate of the orbit of the LEO communication satellite. 
Another object of the invention is to provide a method for communicating 
data between a LEO communication satellite and a VLEO mission satellite 
having an orbit inclination selected to result in a nodal regression rate 
matched to that of the LEO communication satellite. 
Yet another object of the invention is to provide a practical method for 
communicating data from a VLEO mission satellite to a planar ring of 
satellites which is part of a constellation of LEO communication 
satellites. 
Still another object of the invention is to provide a method for 
communicating data from a VLEO mission satellite to a ring of LEO 
communication satellites with the orbit of the mission satellite having a 
stable relative nodal alignment with the ring of communication satellites 
but at a different altitude and thus having a different orbital period 
which causes the ring of LEO communication satellites to clock past the 
VLEO mission satellite with the VLEO satellite successively communicating 
data and handing off in turns as each of the LEO communication satellites 
passes into proximity of the VLEO mission satellite. 
An additional object of the invention is to provide a method for improving 
the efficiency of communications between a VLEO mission satellite and an 
aligned ring of LEO communication satellites by incrementally varying the 
transmission data rate during the crosslinked communications to assure 
maximum data delivery compatible with the periodically varying 
transmission range and relative geometry. 
A further object of the invention is to provide a practical method for 
radially crosslinking data from a VLEO mission satellite to a tangentially 
crosslinked constellation of LEO communication satellites which can relay 
the data to another location and radially downlink the data to a specified 
ground receiver. 
The invention is a method for communicating collected data from a VLEO 
mission satellite to a LEO communication satellite that is a member of a 
constellation of interconnected communication satellites designed to 
service terrestrial users. The constellation of LEO communication 
satellites are deployed uniformly about the globe and distributed equally 
among one or more evenly spaced orbit rings or planes. The VLEO mission 
satellite is placed in orbit at a somewhat lower altitude, and has orbit 
parameters so selected to provide a nodal regression rate of its orbit 
plane matched to the nodal regression rate of the orbit planes of the LEO 
communication satellites. The matched nodal regression rates allows the 
orbit planes of the two satellite systems to maintain their relative 
initial alignment without further adjustment. If the mission satellite is 
inserted into an orbit plane that is nearly co-planar with one of the 
rings of the communication satellite constellation, that initial alignment 
of planes will naturally persist, keeping the VLEO mission satellite in 
time-varying proximity to one or more LEO communication satellites, and 
enabling convenient and continuous close-range communications through a 
radial satellite-to-satellite crosslink. The difference in altitudes of 
the two satellite systems results in a difference in orbit periods, with 
the lower mission satellite having a somewhat shorter period and thusly 
moving somewhat faster around its orbital ring. This difference in orbit 
periods causes the ring of LEO communication satellites to slowly clock 
past and overhead of the lower mission satellite. The short range radially 
crosslink from the VLEO mission satellite is sequentially closed with each 
succeeding LEO communication satellite in the aligned ring as its passes 
in proximity to the transmitting VLEO satellite. 
Data collected by a VLEO mission satellite, such as an earth monitoring 
imaging satellite, can be communicated in real time to each successive LEO 
communication satellite as it comes into proximity to the transmitting 
VLEO satellite. Once collected data is communicated from the VLEO 
satellite to one of the LEO communication satellites in the aligned ring, 
the collected data can be tangentially relayed through crosslinks and then 
radially downlinked to a ground receiver. The wideband global connectivity 
of the constellation of LEO communication satellites provides continuous 
real time communication of the collected data from the VLEO mission 
satellite to any ground receiver in view of any of the LEO communication 
satellites. 
The method matches nodal regression rates of the orbit planes of 
communicating satellites, and takes advantage of the difference in orbit 
altitudes of the VLEO mission satellite and the LEO constellation of 
communication satellites, and exploits the resulting satellite relative 
geometries to effect a number of significant implementation advantages 
that would improve communications performance, reduce mission satellite 
operations costs, and reduce design complexity and costs of mission 
satellite communications equipment. 
The enduring alignment of orbit planes of the communicating satellites that 
is enabled by matched nodal regression rates results in slowly varying, 
well behaved, periodic and predictable relative geometry between the VLEO 
mission satellite and the LEO communication satellite closing the link. 
The range between the communicating satellites, the range rate, and the 
angular rates of satellite-to-satellite antenna pointing angles are all 
relatively small when compared to satellite-to-satellite geometry dynamics 
in general. This benign relative motion and close range and predictable 
geometry allow the mission satellite to use a wide beam antenna for open 
loop tracking and data transmission. The method advantageously may not 
require precision closed loop tracking for link closure, but rather, 
coarse open loop pointing may be used because the relative positions of 
each of the LEO communication satellites in the link-up sequence is known 
apriori with sufficient accuracy. Because of the short range, antenna 
pointing requirements are less severe, and the VLEO satellite may 
advantageously use lower power transmitters with a smaller antenna 
aperture. The VLEO satellite need not be burdened with high-capacity 
onboard data recorders because the method enables continuous real time 
communication of the collected data from the VLEO satellite to the LEO 
communication satellites. The communication method advantageously 
unburdens the design of the wideband data collecting mission satellite of 
many costly mission-specific features including high capacity data 
recorders, sophisticated high-data-rate downlinks, high power 
transmitters, large aperture narrow beam crosslink antennas with precise 
pointing and tracking capability, and possibly a dedicated wideband 
satellite-borne data relay service. All of these elements drive up the 
size, weight, power, and design complexity of the mission satellite 
system. The communications method thusly provides significant potential 
for mission system cost reduction. 
The communication method provides a means for a space-based user to 
conveniently "plug-in" to a space-based wideband telecommunications system 
designed to service mobile ground-based users. One example of such a 
commercial wideband capability is the Gigalink Satellite Link and the 
Inter-satellite Link featured in a proposed design of the Teledesic 
Communications System. 
The communications method enables a VLEO earth monitoring mission 
satellites to conveniently crosslink wideband data in real time to 
broadband LEO commercial satellites designed to service terrestrial users. 
Orbits of the VLEO data collecting mission satellites are tailored to 
achieve and maintain short range and simple geometry relative to the LEO 
communication satellites eliminating the need for precision antenna 
pointing and tracking. The preferred LEO commercial communication 
satellite constellation high-data-rate connectivity would be used to 
radially crosslink receive, tangentially crosslink relay and downlink 
mission data and telemetry to mission ground stations and other 
terrestrial receivers for processing and dissemination. Processed 
information may then be routed to users worldwide via standard commercial 
or special application broadband space and terrestrial assets. 
Two factors influence the resulting satellite-to-satellite geometry 
exploited by the communications method are the number of communication 
satellites in each ring which determines their angular spacing, and the 
inclination angle of the Big LEO orbits which affects the degree of 
co-planarity of the aligned orbit planes of the mission satellite and the 
selected ring of communication satellites. The method is more compatible 
with larger number of communication satellites per ring and with higher 
inclined orbits. An early version of the proposed Teledesic design called 
for a constellation of 288 satellites in highly inclined orbits with 24 
satellites in each of 12 rings. The proposed Teledesic system may be a 
preferred constellation that can readily take advantage of the 
communications method. 
However, the communications method is also a viable concept when applied to 
much smaller LEO constellations and to orbits with lower inclinations, 
and, with minor operational constraints on link closure duty cycles, may 
well outperform available alternatives for these applications. The 
advantages of short range link closure, and the savings gleaned from 
unburdening the mission satellite system of several mission specific, high 
cost communications features will become more apparent from the following 
detailed description of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The communication method provides a means of favorably constraining the 
variation in the relative geometry of satellites in low earth orbit (LEO) 
in order to facilitate their cooperative interaction, for example, 
satellite-to-satellite communications. The method applies preferably to 
low altitude data collecting mission satellites and their interaction with 
proposed constellations of LEO communication satellites designed 
specifically to provide mobile satellite communication services to 
terrestrial users. These large multi-satellite systems are often referred 
to as Big LEO systems. The method enables collected data from a mission 
satellite to be transmitted conveniently to the Big LEO satellites in a 
manner similar to that of a user on the ground. The transmitted data is 
then relayed to one or more ground sites for processing and dissemination 
using the wideband connectivity of the Big LEO constellation as if the 
transmitted data was a typical commercial message. The communication 
method is useful for servicing users of the collected data. By enabling 
the mission satellite to plug in to the Big LEO system, the method 
eliminates the need for a dedicated data communication capability and 
provides a low-cost means for the mission satellite system to continuously 
deliver real time data to the ground. 
An embodiment of the invention is described with reference to the Figures. 
A Big LEO constellation typically has a plurality of circular rings of 
satellites uniformly spaced around the equator, and at equal orbit 
altitudes and inclinations, with each plane or ring containing several 
satellites with uniform angular spacing around the ring. For illustrative 
purposes, FIG. 1 shows a polar view of only one ring of satellites from a 
Big LEO constellation. The ring is shown at time T1 and the same ring is 
shown again some time later at T2, during which time the orbit plane has 
experienced appreciable nodal regression. Nodal regression refers to the 
rotation of the orbit plane about the earth's polar axis measured relative 
to the stars. The figure also shows a VLEO mission orbit at time T1 with 
its orbit plane initially aligned with the plane of a Big LEO ring. The 
orbit parameters of the mission satellite have been selected to result in 
a nodal regression rate equal to that of the Big LEO ring, and the initial 
alignment of the two orbit planes that was established at T1 is 
continually maintained with no further adjustment as indicated at time T2. 
For illustration, the communication satellites are in highly inclined 
circular orbits at a given altitude, for example, 1350 km. The altitude of 
the mission satellite is somewhat lower, for example, 560 km. 
Referring to FIGS. 1 through 3, FIG. 2 shows an in-plane view of the 
exemplary ring of 24 equally-spaced LEO communication satellites, and the 
aligned orbit of the VLEO mission satellite. The difference in altitudes 
of the two orbits results in a difference in orbit periods which causes 
the mission satellite to move around its orbit at a slightly faster rate. 
A hypothetical cosmonaut onboard the mission satellite would observe a 
continuous parade of LEO communication satellites slowly passing by 
overhead as the mission satellite overtakes them one by one. The exemplary 
altitudes previously stated would result in a 26 minute spacing between 
communication satellite passes. If the mission satellite was linking data 
up to the ring of communication satellites, the link would be handed off 
to each succeeding communication satellite at 26.5 minute intervals. Fewer 
satellites in the ring of communication satellites would result in longer 
intervals between hand-offs with less favorable geometry during the 
hand-off operation. 
FIG. 3 is an edge view of the orbit plane of the ring of LEO communication 
satellites and the orbit plane of the mission satellite. FIG. 3 
illustrates the resulting difference in inclination angles that is 
required to match the nodal regression rates of the two orbits having 
different altitudes. This difference in inclination angle results in an 
out-of-plane component in the satellite relative geometry that must be 
accounted for in the calculation of antenna pointing angles. The 
out-of-plane component is maximum at extreme latitudes, and zero at the 
equator. 
Referring to FIGS. 1 through 6, every satellite orbit is defined by a 
six-element parameter set which establishes the path of the satellite, and 
the state of the satellite along the path at a particular instant. Once 
these parameters are known at any given point in time, the position of the 
satellite can be computed at any other time thereafter. For circular 
orbits, which is the preferred form of the invention, the important 
non-zero orbital elements include "r" the radius of the orbit measured 
from the earth's center, "i" the inclination angle of the orbit plane, 
".OMEGA." the right ascension of the ascending node which establishes the 
instantaneous angular position of the orbit plane with respect to the 
stars, and "M" the mean anomaly angle that defines the instantaneous 
angular position of the satellite around its orbit measured from the 
equator. These elements are illustrated graphically in FIG. 4 which 
depicts the circular orbits of the LEO communication satellites and the 
VLEO mission using a typical earth centered triad reference system. The 
orbit plane of the VLEO mission satellite is depicted as being initially 
aligned with the orbit plane of the preferred ring of communication 
satellites. Those skilled in the art know how to deploy a mission 
satellite in a predetermined orbit with specified parameters and orbit 
plane orientation. Selection of the appropriate radius and inclination 
parameters for the VLEO mission satellite orbit will result in a nodal 
regression rate matched to that of LEO communication satellites, and will 
assure continued alignment of the two orbit planes with no need for 
further adjustment. 
Referring again to FIG. 4, the mission satellite mean anomaly Mm defines 
the position of the mission satellite along its orbital ring, and the 
communication satellite mean anomaly Mc defines the similar orbital 
position of the communication satellite to which the mission satellite is 
transmitting. .DELTA.M is the difference in mean anomalies, expressed as 
Mc-Mm, and defines how far ahead or behind the communication satellite is 
relative to the mission satellite. .DELTA.i is the difference between im 
and ic. The satellite positions in their respective orbits define the 
azimuth angle AZ of the range vector from the mission satellite to the 
communication satellite. The satellite positions also define their 
respective earth centered radius vectors and the earth central angle 
.theta. between the radius vectors rc and rm. 
FIG. 5 is a view of the communicating satellites in the plane formed by the 
radius vectors, rm and rc. This geometry defines the depression angles 
.alpha. of the range vector emanating from the mission satellite to the 
communication satellite, as well as the communication satellite off nadir 
angle A. Together AZ and .alpha. determine the required angular direction 
of the boresight of the transmitting antenna onboard the mission 
satellite. The values of AZ and .alpha. are readily calculated using 
conventional algorithms. 
Being at a lower altitude, the VLEO satellite will have an orbit period 
shorter than that of the Big LEO communication satellites. This will cause 
the outer ring of the LEO communication satellites to clock, that is, 
rotate in the plane relative to the aligned mission satellite, so that the 
communication satellites overfly the mission satellite in continuous 
succession, one after another. The difference in orbit altitudes in the 
exemplar form results in an in-plane rotation of the outer ring relative 
to the inner ring of approximately 34 degrees per hour. Thus, a LEO 
communication satellite will pass overhead the mission satellite every 26 
minutes. By hand-off switching from one communication satellite to the 
next at predetermined time periods corresponding to specified .DELTA.M 
intervals, the mission satellite maintains a periodic and predictable 
crosslink geometry. The pointing of the crosslink antenna of the mission 
satellite could be controlled open-loop by a simple clock which times the 
successive overhead passages of LEO communication satellites. There are 
several options for closing the link between the mission satellite and the 
Big LEO communication satellite. The simplest approach would be to 
position a body-fixed antenna at the optimum off-zenith angle on the 
mission satellite, and close the link each time a communication satellite 
passes through the beam. However, limitations on beamwidths compatible 
with wideband communication would probably result in periodic outages. 
Multiple fixed antennas with tailored beams would improve the closure duty 
cycle. 
The method prefers an architecture featuring VLEO mission satellites such 
as one or more earth monitoring satellites, orbiting below a constellation 
of Big LEO communication satellites designed to service terrestrial users. 
The orbit plane of the mission satellite is aligned with a ring of the Big 
LEO constellation, and the orbit parameters of the mission satellite are 
selected to achieve a matched nodal regression rate so the original 
alignment is maintained. The method exploits the predictable and benign 
nature of the resulting relative geometries of the VLEO and LEO 
satellites. The favorable close range of the communicating satellites, in 
the order of 1000 km, reduces requirements for high pointing accuracy. The 
mission satellite may use a clock-driven phased array antenna or a 
clock-driven gimballed parabolic antenna to keep the crosslink beam 
focused on the receiving communication satellite. The geometry clock would 
simply step the antenna boresight through prescribed increments of the 
excursion arc of the receiving communication satellite as it passed 
overhead. When the receiving communication satellite reaches a pre-set 
angular excursion limit .DELTA.Mh relative to the VLEO mission satellite, 
the antenna beam is repositioned to link up with the next receiving 
communication satellite in line. Using this approach, continuous closure 
of the wideband link is maintained without the requirement for more 
precise tracking and antenna pointing capabilities. 
If the parameters of the mission orbit are selected so that the orbit plane 
has the same nodal regression rate as a plane of communication LEO 
satellites, the two planes can be initially aligned, and that alignment 
can be continually maintained with little or no further adjustment. The 
resulting relative geometry is described by a ring of equally-spaced LEO 
communication satellites passing at precise intervals, one after another, 
overhead the transmitting mission satellite. The relative geometry is 
predictable and well behaved, that is, the relative geometry changes 
slowly, which is convenient for antenna pointing. The periodic and slowly 
varying dynamics of the relative geometry are significant features of the 
communication method. The mission satellite interfaces only with the ring 
of Big LEO satellites in the aligned plane. Once the mission data has been 
received, it can be relayed through the crosslinks of any of the 
communication satellites in the Big LEO constellation, in any plane, like 
any other communication message. 
The communication method enables link closure using predetermined open loop 
antenna pointing sequences and provides continuous real-time data 
transmission. However, in some applications, it may prove advantageous to 
have the LEO communication satellites in the aligned plane emit a 
low-power pilot tone at a selected frequency to enable the mission 
satellite to execute an efficient acquisition. If the communication LEO 
satellites should add such a feature to accommodate the mission 
satellites, only the LEO communication satellites in the aligned ring are 
affected. For some orbits, it may be advantageous to transmit data only at 
discrete orbital positions or at certain relative positions of the 
communicating satellites when geometries are most favorable. In such a 
situation, interim data could be recorded onboard the mission satellite 
for later transmission. 
The receiving antenna on a Big LEO communication satellite is typically a 
phased array antenna. The beam that emanates from a phased array antenna 
is electronically steered by changing the relative phase of adjacent 
elements forming the array. Such an antenna features high agility with no 
moving parts. However, as the beam is electronically pointed away from its 
normal direction, the efficiency decreases. There is a practical limit on 
how far from the normal direction the beam can be pointed. As applied to 
the communication method, the communication geometry and range are most 
favorable when the mission satellite is directly below the LEO 
communication satellite. As this relative geometry changes, communications 
gradually deteriorate, and the ability to transmit at maximum data rates 
becomes more marginal. The worst case geometry occurs when the mission 
satellite is midway between adjacent LEO communication satellites at the 
end and beginning of the overhead excursion arc. At this instant, the 
difference in mean anomalies of the communicating satellite's (.DELTA.M) 
equals .DELTA.Mh triggering an immediate hand-off to the next 
communication satellite in line. As the number of Big LEO satellites in 
each ring increases, their relative spacing decreases, and the worst case 
geometry for continuous mission satellite communications improves. 
The communication method can be described with reference to FIG. 6. The 
diagram applies to a cooperating ring of communication satellites C in a 
Big LEO constellation and a crosslinking VLEO mission satellite. All 
satellites are in known circular orbits. The VLEO mission satellite is at 
a lower altitude with a correspondingly different inclination preselected 
to result in a nodal regression rate matched to that of the Big LEO ring. 
The orbit of the mission satellite is established so that its orbit plane 
is initially aligned with the plane of the ring of Big LEO satellites. The 
matched nodal regression rates will assure continued alignment of the 
orbit planes. The difference in orbit periods will cause the ring of Big 
LEO satellites to rotate in the plane and slowly parade by overhead 
relative to the mission satellite. Satellite orbit parameters and initial 
conditions are used to calculate the relative positions of the satellites 
and generates pointing angles for the mission satellite antenna to 
initiate transmission to an overhead communication satellite. The mean 
anomalies of the communicating satellites are continually updated and 
compared, and the antenna boresight is continually repositioned to track 
the predictable overhead path of the communication satellite. When the 
difference in mean anomalies (.DELTA.M) reaches a prescribed maximum 
(.DELTA.Mh), the antenna pointing processor is signaled that the 
communication satellite has reached the extreme of its excursion arc above 
the mission satellite, and the antenna is repositioned to close the link 
with the next communication satellite in line. As the hand-off is 
initiated, the mean anomaly value MC is appropriately reset to reflect the 
position of the next communication satellite in the sequence, and the 
mission satellite antenna begins tracking that satellite through its 
excursion arc. The tracking and hand-off sequence continues, one after the 
other, enabling continuous close-range communications between the VLEO 
mission satellite and the LEO communication satellites. The relatively 
close range enables open loop tracking using a preferred wide beam antenna 
of small aperture. As the antenna aperture is decreased, the antenna beam 
becomes wider and more compatible with less precise open loop pointing and 
tracking. As the antenna aperture is increased, the antenna becomes 
narrower and the transmitted power becomes more focused and may require 
closed loop precision pointing and tracking. The communication method 
takes advantage of the resulting short range and placid geometry dynamics, 
and may preferably use small antenna apertures, of the order of six 
inches, with open loop pointing and tracking. 
The link closure processor uses the calculated range and relative geometry 
of the communicating satellites, along with transmitter and receiver 
design characteristics, to compute the available link margin and the 
corresponding maximum allowable data rate, that is, the highest 
transmission rate in terms of bits per second (bps) consistent with a 
pre-established minimum link margin. Hence, the link closure processor 
continually monitors those parameters affecting the available link margin 
and incrementally adjusts transmission rates in a somewhat cyclic fashion 
as the communication satellites pass overhead the transmitting mission 
satellite. As a result, data rates are maintained at near maximum and 
transmission efficiency is enhanced. 
An exemplary satellite communication method is described in reference to a 
computer simulation readout presented in the following LINK CLOSURE TABLE. 
The example is for a Big LEO communication system comprising a 
constellation of 288 satellites uniformly distributed in 12 orbit planes, 
with 288/12 equaling 24 satellites in each planar ring, with 
360.degree./24 equaling a 15.degree. satellite angular spacing around each 
ring, at a circular orbit altitude of 1370 km, with an orbit inclination 
equaling 85.degree., with an orbit period equaling 112.97 minutes, and 
with a mean anomaly rate, that is mean motion, equaling 360.degree. per 
112.97 minutes that is equal to 3.19.degree. per minute. The VLEO data 
collection mission satellite is in a circular orbit at an altitude 
equaling 560 km, with an orbit period equaling to 95.71 minutes, with a 
mean anomaly rate equaling 360.degree./95.71 min or 3.76.degree./min. The 
inclination angle of the mission satellite orbit is set at 86.600.degree., 
which results in a nodal regression rate of the orbit plane equal to that 
of the orbit planes of the Big LEO constellation. The mission satellite is 
inserted into orbit so that its orbit plane is aligned with one of the Big 
LEO rings. The relative mean anomaly rate between the mission satellite 
and the Big LEO satellite is equal to 3.76.degree./min minus 
3.19.degree./min or 0.57.degree./min. Thus the time spent linked to each 
communication satellite as it passes overhead the mission satellite is 
equal to 15.degree./0.57 deg per min or 26.3 minutes. The link closure 
specifications include a crosslink antenna aperture on the mission 
satellite equaling 0.5 ft, a pointing accuracy equaling 1.25.degree., with 
transmitter power equaling 10 watts, and a carrier frequency equaling 30 
GHz, resulting in a crosslink beamwidth of 5.degree.. Communication 
satellite receiver characteristics include a Gain/Temperature (G/T) 
performance parameter equaling 6.0 dB, an energy per bit/noise density 
(Eb/No) equaling 9.6 dB, with combined circuit, polarity, and 
implementation losses equaling 5.5 dB, and with a minimum required link 
margin of 1 dB. At time zero, the mean anomalies of the mission satellite 
(Mm) and the first communication satellite (Mc, c=1) are equal to zero, 
that is, both satellites simultaneously cross the equator, arbitrarily at 
zero longitude, at time equal to zero. 
The link closure history is calculated and printed out by computer 
simulation, as a well known practice in the art. Definition of printed 
columns are that: TIME is the time in minutes; SAT is the identification 
number of the communication satellite currently closing the link; LAT is 
the latitude of the mission satellite in degrees; LON is the longitude of 
the mission satellite in degrees; DELMA is the mean anomaly of the active 
communication satellite minus mean anomaly of mission satellite in 
degrees; AZ is the azimuth of mission satellite crosslink antenna 
boresight in degrees; ALFA is the off-zenith angle of the mission 
satellite antenna boresight in degrees; RANGE is the distance between the 
mission satellite and the link closing communication satellite in 
kilometers; BETA is the off-nadir angle of the phased array receiving 
antenna of the active communication satellite in degrees; LM is the link 
margin in dB assuming a constant data rate of 50 megabits per second 
(Mbps) (LM varies with range and geometry); and MAXDR is the maximum 
allowable data rate in Mbps at the minimum required link margin. The Link 
Closure Table shows results in one minute increments. 
__________________________________________________________________________ 
LINK CLOSURE TABLE 
TIME SAT 
LAT LON DELMA 
AZ ALFA 
RANGE 
BETA 
LM MAXDR 
(Min) (Deg) 
(Deg) 
(Deg) 
(Deg) 
(Deg) 
(Km) (Deg) 
(dB) 
(Mbps) 
__________________________________________________________________________ 
0. 1 0.00 
0.00 
0.00 0.00 
0.00 
810. 0.00 
6.88 
194. 
1. 1 3.75 
359.97 
-0.57 
171.17 
5.56 
813. 4.97 
6.82 
191. 
2. 1 7.51 
359.95 
-1.15 
171.23 
11.02 
824. 9.86 
6.64 
183. 
3. 1 11.26 
359.93 
-1.72 
171.27 
16.32 
840. 14.57 
6.35 
171. 
4. 1 15.02 
359.91 
-2.30 
171.30 
21.39 
863. 19.06 
5.96 
157. 
5. 1 18.77 
359.90 
-2.87 
171.33 
26.18 
891. 23.27 
5.49 
141. 
6. 1 22.53 
359.91 
-3.45 
171.37 
30.67 
925. 27.17 
4.96 
125. 
7. 1 26.28 
359.92 
-4.02 
171.43 
34.84 
963. 30.76 
4.39 
109. 
8. 1 30.03 
359.96 
-4.60 
171.49 
38.70 
1005. 
34.04 
3.78 
95. 
9. 1 33.78 
0.02 
-5.17 
171.56 
42.26 
1051. 
37.02 
3.15 
82. 
10. 1 37.54 
0.11 
-5.75 
171.64 
45.54 
1099. 
39.72 
2.51 
71. 
11. 1 41.29 
0.23 
-6.32 
171.73 
48.55 
1151. 
42.15 
1.88 
61. 
12. 1 45.04 
0.40 
-6.90 
171.82 
51.32 
1204. 
44.35 
1.25 
53. 
13. 1 48.78 
0.63 
-7.47 
171.92 
53.88 
1260. 
46.32 
0.63 
46. 
14. 2 52.53 
0.93 
6.95 11.27 
51.79 
1214. 
44.71 
1.14 
52. 
15. 2 56.27 
1.34 
6.38 12.62 
49.18 
1162. 
42.66 
1.74 
59. 
16. 2 60.01 
1.89 
5.80 14.18 
46.38 
1113. 
40.41 
2.34 
68. 
17. 2 63.74 
2.65 
5.23 16.00 
43.38 
1067. 
37.95 
2.94 
78. 
18. 2 67.46 
3.71 
4.66 18.18 
40.17 
1023. 
35.28 
3.53 
89. 
19. 2 71.17 
5.26 
4.08 20.84 
36.76 
983. 32.40 
4.09 
102. 
20. 2 74.85 
7.66 
3.51 24.16 
33.16 
947. 29.32 
4.63 
115. 
21. 2 78.48 
11.68 
2.93 28.43 
29.42 
915. 26.09 
5.12 
129. 
22. 2 82.00 
19.47 
2.36 34.12 
25.62 
888. 22.78 
5.55 
143. 
23. 2 85.13 
38.44 
1.78 41.94 
21.92 
866. 19.53 
5.91 
155. 
24. 2 86.59 
88.57 
1.21 52.96 
18.58 
850. 16.57 
6.19 
165. 
25. 2 84.73 
133.67 
0.63 68.33 
16.03 
839. 14.32 
6.37 
172. 
26. 2 81.50 
150.05 
0.06 87.82 
14.87 
835. 13.28 
6.44 
175. 
27. 2 77.96 
157.05 
-0.52 
108.10 
15.46 
837. 13.81 
6.40 
173. 
28. 2 74.32 
160.76 
-1.09 
125.02 
17.61 
845. 15.72 
6.26 
168. 
29. 2 70.63 
162.99 
-1.67 
137.40 
20.75 
860. 18.49 
6.01 
159. 
30. 2 66.92 
164.46 
-2.24 
146.15 
24.38 
880. 21.69 
5.68 
147. 
31. 2 63.20 
165.47 
-2.82 
152.45 
28.18 
905. 25.01 
5.27 
134. 
32. 2 59.46 
166.19 
-3.39 
157.12 
31.96 
936. 28.29 
4.79 
120. 
33. 2 55.73 
166.72 
-3.97 
160.71 
35.62 
971. 31.43 
4.27 
106. 
34. 2 51.98 
167.11 
-4.54 
163.54 
39.12 
1010. 
34.39 
3.71 
93. 
35. 2 48.24 
167.40 
-5.12 
165.84 
42.41 
1053. 
37.15 
3.12 
82. 
36. 2 44.49 
167.62 
-5.69 
167.75 
45.49 
1099. 
39.68 
2.52 
71. 
37. 2 40.74 
167.78 
-6.26 
169.36 
48.37 
1147. 
42.01 
1.92 
62. 
38. 2 36.99 
167.90 
-6.84 
170.75 
51.05 
1199. 
44.13 
1.31 
54. 
39. 2 33.24 
167.98 
-7.41 
171.95 
53.54 
1252. 
46.06 
0.71 
47. 
40. 3 29.49 
168.04 
7.01 5.00 
51.63 
1211. 
44.59 
1.17 
52. 
41. 3 25.73 
168.07 
6.44 4.71 
48.88 
1157. 
42.42 
1.81 
60. 
42. 3 21.98 
168.08 
5.86 4.35 
45.89 
1105. 
40.01 
2.44 
70. 
43. 3 18.23 
168.09 
5.29 3.89 
42.65 
1056. 
37.34 
3.08 
81. 
44. 3 14.47 
168.08 
4.71 3.30 
39.12 
1010. 
34.40 
3.71 
93. 
45. 3 10.72 
168.06 
4.14 2.55 
35.31 
968. 31.16 
4.32 
107. 
46. 3 6.96 
168.04 
3.56 1.54 
31.19 
929. 27.63 
4.90 
123. 
47. 3 3.21 
168.01 
2.99 0.07 
26.78 
896. 23.79 
5.43 
139. 
48. 3 -0.55 
167.99 
2.41 2.18 
22.09 
867. 19.68 
5.90 
154. 
49. 3 -4.30 
167.96 
1.84 5.34 
17.17 
844. 15.33 
6.29 
169. 
50. 3 -8.05 
167.93 
1.26 11.63 
12.14 
827. 10.85 
6.59 
181. 
51. 3 -11.81 
167.91 
0.69 26.85 
7.30 
816. 6.53 
6.77 
189. 
52. 3 -15.56 
167.90 
0.11 75.91 
4.26 
812. 3.81 
6.84 
192. 
53. 3 -19.32 
167.89 
-0.46 
132.05 
6.64 
815. 5.94 
6.79 
190. 
54. 3 -23.07 
167.90 
-1.03 
150.02 
11.37 
824. 10.17 
6.62 
182. 
55. 3 -26.82 
167.92 
-1.61 
157.12 
16.39 
841. 14.64 
6.34 
171. 
56. 3 -30.58 
167.96 
-2.18 
160.83 
21.32 
863. 19.00 
5.97 
157. 
57. 3 -34.33 
168.02 
-2.76 
163.13 
26.03 
890. 23.13 
5.51 
141. 
58. 3 -38.08 
168.11 
-3.33 
164.71 
30.46 
923. 26.99 
4.99 
125. 
59. 3 -41.83 
168.24 
-3.91 
165.88 
34.60 
961. 30.56 
4.42 
110. 
60. 3 -45.58 
168.42 
-4.48 
166.79 
38.44 
1002. 
33.82 
3.82 
96. 
61. 3 -49.33 
168.65 
-5.06 
167.53 
41.98 
1047. 
36.79 
3.20 
83. 
62. 3 -53.07 
168.97 
-5.63 
168.16 
45.25 
1095. 
39.48 
2.57 
72. 
63. 3 -56.81 
169.40 
-6.21 
168.71 
48.26 
1146. 
41.92 
1.94 
62. 
64. 3 -60.55 
169.98 
-6.78 
169.19 
51.03 
1198. 
44.12 
1.32 
54. 
65. 3 -64.28 
170.77 
-7.36 
169.63 
53.59 
1253. 
46.10 
0.70 
47. 
66. 4 -68.00 
171.89 
7.07 12.39 
52.46 
1228. 
45.23 
0.98 
50. 
67. 4 -71.70 
173.54 
6.49 13.61 
49.90 
1176. 
43.23 
1.58 
57. 
68. 4 -75.38 
176.10 
5.92 15.01 
47.15 
1126. 
41.03 
2.18 
66. 
69. 4 -79.00 
180.48 
5.34 16.64 
44.19 
1079. 
38.62 
2.78 
75. 
70. 4 -82.49 
189.21 
4.77 18.56 
41.01 
1034. 
35.98 
3.38 
86. 
71. 4 -85.51 
211.27 
4.19 20.89 
37.62 
993. 33.13 
3.96 
99. 
72. 4 -86.50 
265.49 
3.62 23.79 
34.01 
955. 30.05 
4.51 
112. 
73. 4 -84.30 
305.17 
3.05 27.50 
30.23 
921. 26.79 
5.02 
126. 
74. 4 -81.00 
319.41 
2.47 32.43 
26.33 
892. 23.40 
5.48 
140. 
75. 4 -77.43 
325.72 
1.90 39.25 
22.43 
869. 19.98 
5.87 
153. 
76. 4 -73.78 
329.14 
1.32 49.04 
18.76 
850. 16.73 
6.17 
165. 
77. 4 -70.09 
331.23 
0.75 63.33 
15.70 
838. 14.02 
6.39 
173. 
78. 4 -66.38 
332.61 
0.17 83.01 
13.88 
832. 12.41 
6.49 
177. 
79. 4 -62.65 
333.58 
-0.40 
105.41 
13.93 
832. 12.44 
6.49 
177. 
80. 4 -58.92 
334.26 
-0.98 
124.91 
15.81 
838. 14.12 
6.38 
173. 
81. 4 -55.18 
334.77 
-1.55 
139.01 
18.92 
851. 16.87 
6.16 
164. 
82. 4 -51.44 
335.15 
-2.13 
148.64 
22.63 
870. 20.15 
5.85 
153. 
83. 4 -47.69 
335.43 
-2.70 
155.34 
26.56 
894. 23.60 
5.45 
139. 
84. 4 -43.95 
335.63 
-3.28 
160.18 
30.49 
923. 27.02 
4.99 
125. 
85. 4 -40.20 
335.79 
-3.85 
163.81 
34.30 
958. 30.30 
4.47 
111. 
86. 4 -36.45 
335.90 
-4.43 
166.63 
37.93 
996. 33.39 
3.91 
98. 
87. 4 -32.69 
335.98 
-5.00 
168.88 
41.35 
1038. 
36.26 
3.32 
85. 
88. 4 -28.94 
336.03 
-5.58 
170.72 
44.55 
1084. 
38.91 
2.71 
74. 
89. 4 -25.19 
336.06 
-6.15 
172.26 
47.53 
1133. 
41.33 
2.10 
64. 
90. 4 -21.44 
336.07 
-6.72 
173.56 
50.30 
1184. 
43.54 
1.49 
56. 
91. 4 -17.68 
336.08 
-7.30 
174.69 
52.87 
1237. 
45.55 
0.88 
49. 
92. 5 -13.93 
336.07 
7.13 1.53 
52.02 
1219. 
44.89 
1.08 
51. 
93. 5 -10.17 
336.05 
6.55 0.90 
49.31 
1165. 
42.76 
1.71 
59. 
94. 5 -6.42 
336.02 
5.98 0.16 
46.38 
1113. 
40.40 
2.34 
68. 
95. 5 -2.66 
336.00 
5.40 0.76 
43.21 
1064. 
37.80 
2.97 
79. 
96. 5 1.09 
335.97 
4.83 1.92 
39.77 
1018. 
34.94 
3.60 
91. 
97. 5 4.85 
335.94 
4.25 3.43 
36.07 
976. 31.81 
4.20 
105. 
98. 5 8.60 
335.92 
3.68 5.30 
32.09 
937. 28.40 
4.78 
119. 
99. 5 12.35 
335.90 
3.10 7.85 
27.86 
903. 24.74 
5.31 
135. 
100. 
5 16.11 
335.88 
2.53 11.49 
23.42 
874. 20.84 
5.77 
150. 
__________________________________________________________________________ 
The Link Closure Table indicates that the maximum allowable data rate 
varies with the cyclic variation in range and geometry between the 
communicating satellites. As shown, at the lowest range of 810 km, the 
link margin is the highest enabling he highest transmission data rate. 
Later in the cycle, the range increases and the geometry between the 
communicating satellites (described by the angle beta) degrades, causing a 
drop in link margin and a corresponding decrease in allowable data rate. 
Later again, a new cycle begins and link closure conditions improve and 
the allowable data rate again is high. Hence, the crosslink efficiency can 
be optimized by a cyclic variation in the transmission data rate as the 
communication satellite overflies the mission satellite. The communication 
method can therefore be enhanced to have a variable data transmission rate 
during the linkup interval depending on the cyclic variation in range and 
geometry between the mission satellite and the overhead communication 
satellite, having the highest data rate, for example 200 Mbps, at the 
overhead position at the midposition in the linkup excursion arc, and 
lowest data rate, for example 50 Mbps at the beginning and end of the 
excursion arc. The data rate could be stepped up and down by a clock in 
increments, such as, in 10 Mbps increments from the low 50 Mbps 
transmission rate up to the high 200 Mbps rate and then back down to low 
50 Mbps, as the communication satellite travels through the linkup 
excursion arc over the mission satellite. 
Further still, because of the difference in inclination angles of the 
orbits of the mission satellite and the communication satellites, the 
minimum and maximum range through the excursion arc varies with latitude. 
At the equator, the communication satellites pass directly overhead the 
mission satellite, whereas at the northern and southern most latitudes, 
the communication satellites pass at a maximum angular offset (.DELTA.i) 
from the plane of the mission satellite orbit. Hence, the range is minimum 
when a communication satellite is at the mid-point of the excursion arc 
and the mission satellite is near the equator, and the range is maximum 
when a communication satellite is at the beginning or end of the excursion 
arc and the mission satellite is near the poles. Thus, the maximum data 
transmission rate is determined by the available link margin which varies 
with the readily predicable range and geometry and can be varied through 
the linkup excursion arc and varied somewhat with satellite latitude. 
Those skilled in the art are capable of deploying satellites in prescribed 
orbits and establishing communication links and hand-off procedures. The 
present invention is characterized by a mission satellite having matched 
nodal regression with at least one communication satellite. When matched 
to a ring of communication satellites, the method enables continuous 
real-time communication between the mission satellites and the ring of 
communication satellites that can then crosslink the communicated data 
around the globe and then downlink the communicated data to a desired 
ground station. Those skilled in the art can make enhancements, 
improvements, and modifications to enhance the invention. However, those 
enhancements, improvements, and modifications may nonetheless fall within 
the spirit and scope of the following claims.