MOST PEOPLE IN/OUT OF UNIFORM DO NOT UNDERSTAND SPACE RELATED ISSUES.
AFTER ALL IT IS 'ROCKET SCIENCE'.
HOWEVER TO BE IGNORANT OF THE COMING TIDE IS SACRILEGE FOR A MAN IN UNIFORM.
A satellite can be used for
many military purposes. Primarily, military satellites are used for Communication,
Intelligence, Surveillance & Reconnaissance, Navigation, Meteorological
Observations and Electronic Warfare. Theoretically any satellite, can carry out
these functions whether it is in High, Geostationary, Mid or Low Earth orbits. However,
from a practical point of view it is the Low Earth Orbit (LEO) which is most
commonly used for military satellites. It is highlighted at the outset that
understanding LEO is more important for exploiting space rather than knocking a
satellite down. The LEO is the simplest, cheapest and safest
location for the deployment of satellites, space stations, and crewed space
missions. Most military personnel do
not understand the fundamentals of a LEO and hence are at a loss to appreciate the
essentials of ASAT tests and their implications. In this context this article
is an attempt to give readers a basic understanding of LEOs and their utility
as military platforms.
Height
A Low Earth Orbit Satellite is one that is
in orbit 160 -2000 km above the earths surface. Any object below 160 Km suffers
from a phenomenon called orbital delay and descends into the atmosphere to burn
or crash land. In practice, LEO satellites orbit between 300 -1000km. The
reason being that below 300km the drag is high, and it is difficult to maintain
a satellite at that altitude. Above 1000km there are something called Van Allen radiation
belts which are zones of charged particles. They cause a lot of ionospheric
interference and endanger satellites which invariably have sensitive equipment.
A major factor in deciding the height of the satellite is the bandwidth
available for operations and the latency (communications time lag). A LEO
satellite gives the best results for this.
LEO satellites can also be on an equatorial
orbit, inclined orbit or a polar orbit. In an equatorial orbit, the area
covered is a latitudinal band only. Normally LEO satellites are kept in Polar
or Inclined orbits which enables max coverage of the Earth. A polar orbit is
essentially a north south orbit around the poles. The earth revolves west to east.
The satellite orbit and the earth’s rotation allow it to cover most of the earth
surface after it has covered a complete cycle of orbits in a certain period. Orbits can be circular or elliptical. Circular orbits are popular, because then the satellite
is at a constant altitude requiring a constant strength of signal to communicate.
Elliptical orbits are useful to increase the stay period over an area.
LEO satellites orbit the
earth at an average speed of 7.8 Km/ sec when their altitude is 300 km. As the
altitude increases to about 1500 km, the speed reduces to 7.12 km/sec. They circumvent
the Earth in 90 minutes to 120 minutes (approx.). It means that a LEO satellite
circumvents the earth 16-12 times a day depending upon the height at which it
is placed. Lower the orbit height, higher is the number of orbits and vice
versa. As a thumb line guide, a LEO satellite will traverse the entire length
of Pakistan (north to south) in about 7-8 minutes. By the time the satellite finishes
one orbit of 90 minutes, Pakistan would have passed by underneath. Some part of
Iran would be directly below the satellite. Twelve hours later, Pakistan will be
once again under the satellite for about 7-8 minutes. Hence in a 24-hour period,
Pakistan would have been seen for 14-16 minutes.
Every satellite has a
footprint or the area it covers on the surface of the Earth which is directly
proportional to its orbital height. As the satellite passes over the Earth, the
location of the footprint changes. The continuous path of the footprint is
called a Swath. Satellite A has a larger
foot print than Satellite B as shown in the diagram. Hence higher the
satellite, greater the foot print. A corollary to this is that, with increase
in height and decrease in speed, the satellite stays over the target area for a
longer time. Its ability to stay and stare over a larger area increases. As
compared to a Geostationary orbit, which has a large footprint which is
stationary, a LEO satellite has a small and momentary footprint and can observe / communicate
with only a fraction of the Earth at a time. In order to overcome this issue of
interrupted coverage LEO satellites are used in a constellation to provide
continuous coverage.
Satellites could be in constellations
varying from 2-840. However, in a practical sense any constellation of 12-24
satellites give a fair coverage of an area for surveillance. Which implies that
if Pakistan is under surveillance by a constellation consisting o f 12-24 satellites the
coverage would be between 3-6 hours in a 24-hour period with a look period of
15 minutes at a time. The intervening periods will have to be correlated or change
detection techniques must be used. LEO constellations are more costly and
complex to launch and operate.
Launching
Satellites are placed in
space by launching a rocket. The higher the height at which a rocket is placed
the greater the effort required. Which means
a heavy rocket and extensive launching facilities. A LEO satellite requires a
very small rocket in comparison to a Geostationary rocket. Also, LEO satellites
and space stations are more accessible for crew and servicing.
Segments
Any Satellite system has a
space segment and an earth segment. The space segment consists of the sensor,
satellite, power, transmission and control sub systems. Control sub system will
be controls of the satellite as well as the sensor. Sensors can be steered, directed
or focused as required. The Earth segment consists of the tracking antenna,
transmission, downlink/ data receivers, sensor control and orbit control sub
systems. The overall mission control also forms part of the Earth Segment.
Sensors
A satellite is essentially a
platform in space which hosts a payload. Theoretically, a LEO satellite can
carry any number of sensors as part of its payload. However due to
considerations of launch capabilities and on-board power requirement, the
sensors are restricted. More sensors imply greater weight. Greater weight
implies a more powerful launch. Similarly, power is required to operate the
sensors and the satellite controls. A complex sensor needs greater power. As the height increases, the amplifier power
requirements increase for signal transmission. This in turn demands more on-board
power. Satellites draw solar power and operate with it. To that extent, a LEO in polar orbit is well
suited since it is in sun for fifty percent of its orbit and gets recharged repeatedly.
However overall power available is a severe limitation. Another aspect is that
for a given sensor while the footprint increases with height, the power requirement
also increases, and resolution detail decreases. So, this contradictory
phenomenon must be reconciled. A major
factor is the technology adopted on the sensors. Advanced sensor technologies
reduce sensor size, power requirements and weight. This enhances the capability
of the satellite. A sensor could be active or passive. A passive sensor uses an
external source of energy or the target emissions for detection. An active
sensor generates its own energy to either illuminate the target or act upon it.
Any satellite will have dedicated and separate communication channels for controlling
the satellite, controlling each of the sensors and downloading data from the
sensors.
Passive Sensors
Most military sensors fall
in the passive category. They use the illumination by the sun or the emissions by
the target to detect them. Battle field sensors such as day/night cameras, Thermal
Imaging systems, electronic warfare receivers, communication transponders, and meteorological
are all passive systems. Communication transponders can be used for Command and
Control, Air Space Management, UAV/ Air craft control and enhancing range of
communications.
Active Sensors
Active systems are more
complex since they must generate some energy to act on the target. Active sensors
include any Radars to include Synthetic Aperture Radars, LIDARs, laser-based
devices, electronic emissions for jamming, and Directed Energy Weapons. The biggest challenge in an active system is power generation / management and
weight management. It must be remembered that energy generation consumes weight
and power at a phenomenal rate. In this context, though satellites are spoken
of as platforms for Directed Energy Weapons, technologically we are not yet there.
USA started the Star Wars Program in the Ronald Reagan era and had to close it
down since it was prohibitively costly.
Deployment of
Satellites
Space Debris
Extremely informative and complete article on LEO satellites and their deployment. A must read by one and all.
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ReplyDeleteIt’s very informative.Thanks for sharing these wonderful ideas. You can check more about LEOs
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