Satellite Downlink Interference Information
Sheet
Courtesy of Spacecom
Systems
Preface
Throughout its history, satellite communication has always
been challenged by external noise sources. From natural occurrences, such
as background radiation from the movement of particles in space or manmade
sources like automobile ignition coils, RFI (Radio Frequency Interference)
is everywhere. In addition, although universal, it is not equally distributed.
RFI can be more pronounced in certain geographic locations or in certain
frequency bands. Additionally, man-made RFI is becoming more prevalent
as electronics products become a bigger part of our everyday lives. Because
of the potential for RFI to cause degradation to vital satellite delivered
information, our engineers have prepared this information sheet designed
to inform you about RFI and how to overcome it.
What is RFI?
In its basic form, RFI is reception of any signal other than
the one that is intended, and as described above, is both natural and man-made.
This "unintended reception" can result from any one or any combination
of many different sources.
Examples of
natural RFI experienced at downlink sites are:
-
Background Radiation - Random electron
motion throughout the universe is measurable and contributes to thermal
noise. Thermal noise in space is generated by the movement of charged particles
through magnetic fields. Thermal noise in the atmosphere is compounded
by re-radiation of any existing noise present. Both of these can contribute
from 3 to 13 degrees Kelvin of "noise" to a communication link depending
on the frequency of the link. This effect of thermal noise against a relatively
low-level satellite signal can be significant.
To minimize effects of this type of noise, satellites
have higher power transponders and amplifier manufacturers design cooler
noise temperature LNB's. A good communication link design can overcome
the effects of these noise sources.
-
Solar flares - Excessive solar radiation
can damage spacecraft.
-
Faraday rotation - This phenomenon is
linked to the 11-year rotation of the sun. It affects linear, but not circularly
polarized signals. The effects are more severe at lower frequency regions
such as C-Band and are usually not noticeable at Ku-Band frequencies. US
Domestic C- and Ku-Band satellites are linearly polarized, and transmit,
as well as receive, horizontal and vertical polarities. The two polarities
on a satellite are isolated from each other by 30dB (average), and enable
the spacecraft to utilize a sharing scheme called frequency reuse. During
periods of Faraday rotation, as the satellite signal passes through the
atmosphere, it becomes de-polarized causing undesirable reception of the
opposing polarity.
To minimize or eliminate these effects, satellite
users, like SpaceCom, attempt to ensure there are no other reused frequency
signals on the opposing polarity, as well as choose a satellite location
that offers a shorter path through the atmosphere.
-
Rain Fade - Rain fade is a single term
for many conditions regarding degradation due to water droplets along the
reception path. The effect of a high frequency signal passing through rain
can cause signal attenuation and accounts for the majority of the problems
associated with rain fade. However, rain is not the sole source of the
problem. Moisture laden clouds (as those associated with rain) extend up
through the atmosphere sometimes reaching altitudes of 75,000 feet. By
the time a signal passes through a cloud system, it can be attenuated by
as much as 1 dB. In addition to this phenomenon, rain can absorb and attenuate
signals even further as well as depolarize them and add unwanted thermal
noise. Moreover, water droplets on a feed horn window cause detrimental
effects too.
It is important to know that when it comes to rain
fade effects, higher frequency signals like Ku-Band degrade faster,
harder, and longer than their lower frequency C-Band counterparts.
There are several ways to overcome the effects of degradation
due to water. For constant rate transmission, ample power from the
satellite is required to maintain adequate fade margin, i.e., FM Cubed.
If the power must remain constant, and bandwidth is available, then FEC
or other error correction techniques can be employed. Designers of the
satellite link play an important role in determining the applicable transmission
scheme for the desired result. As for the user, larger antennas generally
do not offer much protection.
-
Sun Fade - Twice a year, during the
vernal and autumnal equinox's, the sun lines up directly behind the satellite.
Thermal noise created by the sun rivals, and often overpowers, a satellite's
output.
As with rain fade, sun fade can be overcome with
link design which makes use of adequate power and/or modulation techniques
like those employed in FM Cubed and HyperCubed.
Examples of man-made RFI experienced at downlink
sites are:
-
Unintentional radiators - electric shavers,
light switches, automobile ignition coils and spark plugs, fluorescent
lights, computers, and power convening units. These devices as well as
many others not classified as transmitters can prove to emit destructive
RFI.
It is recommended that a receive system is shielded and
any unintentional radiators are removed from the general area.
-
Intentional radiators - walkie talkies,
amateur radio, cellular or PCS phones, terrestrial microwave, other satellites,
same satellite, and paging transmitters. Many of these radiators either
share or abut frequencies dedicated to satellite communication. Without
careful coordination RFI could be a continuous problem.
Walkie talkies, amateur radio, cellular or PCS phones
operate near, but usually never in the bands associated with satellite
C and Ku-Band downlinks including L-Band signals from an LNB. However,
close physical proximity to and relative high power of these radiators
can cause amplifiers in LNB’s and receivers to become overloaded, effectively
pushing the desired signal out of the way.
Keeping these emitters away from the downlink
is an effective means of eliminating the RFI.
Terrestrial microwave has no direct frequency related
threats to domestic Ku-Band spectrum, but C Band is another matter. C-Band
satellite receive systems must co-exist and share frequencies with terrestrial
microwave radio paths. Both the satellite and microwave systems share a
portion of the same C-Band frequency spectrum. When a signal is received
from both a satellite and a microwave source at the same frequency, the
receiver has no way to determine which signal is to be ignored. This can
cause problems ranging from reduced quality of reception to total loss
of signal depending on the severity of the interference.
In order to minimize the possibility of interference
from microwave paths, C-Band Fm Squared was engineered to fit into a mostly
interference-free portion of the spectrum. This has proven to be an effective
method of dealing with terrestrial microwave interference. Should the interference
be so strong that the technology alone is not sufficient, in-line filters
have proved successful in reducing the interference to acceptable levels.
Other satellites can be picked-up along with the desired
satellite. Since satellites are spaced 2 degrees from each other, it is
possible for a single antenna to receive two or more satellites simultaneously.
To alleviate this problem, the space segment provider tries to ensure the
adjacent satellites do not use the frequency spectrum occupied by its signal.
Currently, Hughes and AT&T strive to maintain this
arrangement for our FM Cubed, HyperCubed, and FM Squared signals. Individual
downlinks can prevent or reduce adjacent interference by selecting
the largest 2-degree compliant antenna possible and ensuring it is assembled
and mounted per the manufacturer's instruction.
The same satellite can interfere with the desired
signal due to cross-polarization problems. These problems could be in either
the uplink and or downlink chain.
To minimize or prevent same satellite RFI, satellite
providers try to ensure cross-polarized signals are eliminated or are not
objectionably unbalanced. Therefore, the feed horns of both uplink and
downlink dishes are aligned to provide 30dB of isolation or better.
Paging transmitters operating near the L-Band region
affect signals traveling from the LNB to the receiver. This problem is
most commonly seen at shared paging transmitter sites that have multiple
transmitters active at the same time. If the right combination of carrier
frequencies occurs, harmonics that are at the L-band frequency range can
be generated.
To reduce this type of interference, shielding
the device receiving the interference should be investigated. Typically
either the LNB, cabling, or the receiver should be examined.
Summary:
RFI is prevalent in satellite systems; nonetheless, with
proper planning and implementation satellites are a proven technology for
distributing vast amounts of information. The policies and practices by
the FCC, satellite operators, satellite vendors, and end-users help ensure
RFI issues remain at a minimum.
Please contact Salem Music Network at 615-367-2210
if you have any questions regarding the interference information.
Also, please check out Spacecom's Technical
Bulletins section on their site for updated information.