Satellite systems using HEOs

ITU 03006
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Henry J. Meyerhoff
Space Spectrum Coordination Consultants Ltd
At present, the ITU regulatory provisions for registering frequency
assignments on space stations depend on the category of their orbit: those in
the geostationary-satellite orbit (GSO), and those in other orbits (non-GSO).
This article advocates the addition of a third category: that of satellites in
highly elliptical orbits (HEO), and for which registration in the Master
International Frequency Register (MIFR) would follow the procedures applicable
to GSO systems.
The Radio Regulations define GSO as “the orbit of a geosynchronous
satellite whose circular and direct orbit lies in the plane of the Earth’s
equator”. But this definition is rather ambiguous in the sense that it gives
no bounds on either the degree of eccentricity of the circular orbit, or on the
inclination angle of the orbit within which the satellite may be registered as
being in the GSO.
Agenda
item 1.37 WRC-03: “To consider the regulatory and technical provisions
for satellite networks using highly elliptical orbits.” |
In the beginning, communication satellites were placed in low-earth orbits
(LEO), and 24 hour per day communication between associated earth stations
required a constellation of identical satellites that moved in and out of view.
There was also the Russian Molniya system based on HEOs in which the
satellite only operates around the apogee, and is switched off elsewhere. The
Molniya system, having a 12-hour orbit had two very narrow longitudinal
locations 180° apart in space with common latitude around 50° North, where
there appeared an operating satellite 24 hours per day.
Then came the realization of Arthur C. Clarke’s dream (expressed in 1945)
of a single satellite in the equatorial orbit that would revolve around the
planet once in 24 hours — a period of time that corresponded to the rotation
of the Earth about its axis. Major features of such a satellite were that:
- The satellite would appear stationary at a longitude above the equator,
thereby not requiring the associated earth station antennas to track it.
- Directional earth station antennas and satellite spot beams would allow
efficient frequency reuse for closely spaced satellites in the equatorial
plane.
- With a system of only three such satellites suitably separated in
longitude, it is possible to provide global coverage for all but the polar
regions.
The advent of the geostationary satellite for civilian telecommunication
services eclipsed the use of satellites in low-earth orbits, and also the
Molniya system. This led the fixed-satellite service (FSS) for the GSO to be
granted priority recording status in the Master International Frequency Register
over all other orbits categorized as non-GSO. That priority status requires
non-GSO not to cause unacceptable interference to the GSO in FSS bands.
On the other hand, the ITU procedures for registering space stations in the
GSO FSS were more complicated than for non-GSO systems, requiring a coordination
process before entry into the MIFR. This coordination process enables GSO FSS
systems to share the 360° of longitude orbit spectrum among themselves on the
basis of earth station antenna angular discrimination and satellite antenna spot
beam coverage. Sharing of the spectrum between GSO FSS space stations and
terrestrial services, based on the potential of steady state interference, is
assured by:
- requiring limits on the power-flux density (pfd) of the GSO FSS
satellites;
- restrictions on the pointing of terrestrial systems in the direction of
the GSO, assuming directive antennas.
The Molniya HEO system provides service to northern latitudes of Russia from
East to West with associated earth station limited tracking directive antennas.
ITU registration of this system as a non-GSO without coordination produced no
complaints of unacceptable interference, neither from the GSO nor from the
terrestrial community. The type of potential interference is of a steady state
nature as compared to that produced by other non-GSO, such as LEO systems.
Within the last decade, renewed interest was shown for LEO satellite systems
to provide FSS on a worldwide basis. Such systems, being classified as non-GSO,
could be registered in bands allocated to the FSS with the caveat that they do
not cause harmful interference to GSO FSS systems, but without qualifying what
that constitutes. It was recognized that networks using such satellites might
cause random intermittent interference to GSO FSS and terrestrial systems
compared to the steady state interference arising from GSO satellites. Study
Groups of the ITU Radiocommunication Sector (ITU–R) developed statistical
criteria to quantify, for coordination purposes, harmful interference levels
which such systems must not exceed in order to share the FSS (and later the
broadcasting-satellite service, or BSS) bands on an equal status with GSO
systems. (In order to meet the statistical criteria to protect the GSO systems,
mitigation techniques were suggested that would require the LEO satellites not
to operate within +/–10° of the equator, with the communication link replaced
intermittently with that of other satellites in the LEO system.) These
statistical criteria were then incorporated in the Radio Regulations for
application to all non-GSO systems.
“In
order to use the orbit spectrum efficiently, it is considered that
registration of proposed HEO systems follow procedures more akin to those
that apply to GSO systems (i.e. sharing based on steady state
interference).” |
However, just as the renewed interest was appearing for large numbers of
low-earth orbiting satellites, various HEO satellite systems (like the Molniya)
were being proposed for FSS and BSS. The type of interference they might cause
to other HEO and GSO systems sharing the same frequency bands is of a steady
state rather than the random intermittent nature coming from large numbers of
LEO satellites. Furthermore, these proposed HEO systems are inherently
compatible with, and complementary to, GSO systems where HEOs maintain a wide
angular separation from the GSO.
Consequently, in order to use the orbit spectrum efficiently, it is
considered that registration of proposed HEO systems follow procedures more akin
to those that apply to GSO systems (i.e. sharing based on steady state
interference).
The paragraphs below highlight a number of determining features of the
category of HEO satellite systems that would follow the procedures applicable to
GSO systems, with coordination based on potential steady state interference for
their registration in the MIFR.
- The satellites in HEO constellation have repeating ground tracks, operate
only in portions of their orbit, the active arcs, and are switched off
elsewhere. At all times there is only one satellite in the active arc. For
highly inclined orbits (25<i°<155), this ensures that there is a
minimum angular separation of at least 20° between an operating satellite
and the GSO. Most HEO constellation earth station antennas will track the
satellite in its active arc, but some may have a broad beam that covers a
small active arc but is of sufficient directivity to protect the FSS GSO.
For BSS, GSO and HEO systems rely on satellite spot beam discrimination for
frequency reuse. To better protect terrestrial networks, the service area
for HEO systems is generally restricted to earth stations having high
elevation angles to the operating satellite. Of practical interest for
inclusion in the third category are the HEOs with periods of 24, 16, 12 and
8 hours.
- A distinction has been drawn between FSS and BSS systems, in that a common
feature of BSS systems is cheap omnidirectional non-tracking receiver
antennas. For sharing the same frequencies, such systems, be they GSO or
HEO, rely entirely on the satellite spot beam, rather than their angular
separation.
HEO constellations may share with LEO non-GSO systems. The latter systems,
in order to stay within the statistical sharing criteria developed for the
protection of GSO systems, may necessitate the application of a similar
mitigation technique to protect HEO systems that is used to protect the GSO.
It has already been found that not more than four such LEO systems can share
the orbit spectrum, and certainly to require them to provide similar
protection to HEO constellations as given to GSO networks would add to their
complexity.
- In order to share the FSS/BSS orbit spectrum, HEO constellations must
coordinate with each other as well as with GSO and terrestrial networks. HEO
constellations sharing criteria with terrestrial systems are based on pfd
masks as with GSO systems. Whereas the spacing of GSO satellites for
effective orbit spectrum utilization over common service areas has been
reduced to a few orbital degrees, the HEO constellations will require
broader angular separation among themselves since the active arcs occupy a
greater portion of the sphere than the dot corresponding to a GSO orbit
location. At this stage, it is impossible to predict the future number of
HEO constellations. However, for the efficient use of orbit spectrum, a
relatively small number (less than 10) should be taken to estimate the
aggregate interference to terrestrial systems with a given pfd mask. As far
as sharing with GSO systems, the minimum angular separation from the GSO
should provide sufficient protection, given that GSO networks among
themselves can share when separated by only a few orbital degrees. Hence the
HEO constellations with high inclination angles complement the GSO systems.
- In the azimuth elevation angle diagram, terrestrial systems make use only
of the circumference belt corresponding to elevation angles normally less
than 3°. Although terrestrial networks will be subjected to additional
interference from other azimuths than the two ranges corresponding to the
position of the GSO, HEO constellations make effective use of the total
sphere surrounding any terrestrial station.
- As far as sharing criteria for the HEO constellations described above are
concerned, the type of interference they may cause to other HEO, GSO and
terrestrial systems is of a steady state nature, and bears no relationship
to the intermittent statistical interference associated with LEO non-GSO
systems.
- Conclusion. It would be desirable to include a new definition in the Radio
Regulations that would state that: “A satellite in a highly elliptical
orbit has a repeating ground track, only operates in a portion of its orbit,
the active arc, and is switched off elsewhere. At all times there is only
one satellite in the active arc. The inclination angle of the orbit lies
between 25° and 155°. The period of the orbit is a multiple of that of the
geostationary-satellite orbit. Eccentricity of the orbit should be greater
than 0.05.
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