Receiving International Satellite Signals
by Mark Long
from The World of Satellite TV, Asian Edition
copyright 1995, 1996, 1997 MLE INC. All Rights Reserved.
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The reception of INTELSAT and STATSIONAR satellites
often requires the use of large antennas. These satellites are delivering satellite TV
programming to areas such as Australia, New Zealand and the South Pacific where a limited
range of services from domestic satellites are available, as is the case in Australia, or
no service at all. The appropriate dish size for INTELSAT or STATSIONAR satellite
reception at your location will depend on which beam pattern is being used by the
particular services you wish to view. INTELSAT and STATSIONAR reception often requires the
use of special satellite feedhorns, receivers, and antenna tracking devices in order to
obtain a high-quality picture.
Circular
Polarization
For
the best possible reception of INTELSAT and STATSIONAR satellites, you will need to change
your feed polarization from horizontal/vertical (which is used by the Palapa, AsiaSat,
Apstar and PAS satellites) to circular polarization. Instead of positioning the microwave
energy in a plane, whether vertical or horizontal, circular polarization is transmitted in
a circular pattern, that rotates in either a clockwise or a counter-clockwise direction.
Although your regular feedhorn can still pick up circularly polarized signals, you end up
losing half of the available signal. This can make the difference between a noise-free
picture and one that is not tolerably viewable.
Special international
feedhorns have been created which can receive both linearly and circularly polarized
satellite signals, so that you can receive programming from the international satellites
as well as regional satellites such as Palapa or PAS-2. These products are available from
manufacturers such as Chaparral Communications,
National ADL, and Seavey Engineering
When your feedhorn skew is adjusted to optimally receive right hand circularly polarized (RHCP) signals, you will be unable to see any left hand circularly polarized (LHCP) signals, and vice versa. This cross-polarization isolation is identical to the isolation between the horizontal and vertical polarizations used by domestic and regional satellites. To maximize your reception of both circular and linear satellite transmissions be sure to follow the installation instructions provided by the feedhorn manufacturer when installing the feed on your system.
International
Frequency Schemes
The
INTELSAT V and VII series satellites, as well as PAS-2 and PAS-4, transmit to Earth using
C-band frequencies in the 3.7 to 4.2 GHz range, while INTELSAT VI and the STATSIONAR
satellites transmit to Earth using the 3.65 to 4.2 GHz frequency range. The transponder
numbering and center frequencies for INTELSAT, PAS, and STATSIONAR channels differ
somewhat from what regional satellites such as AsiaSat, Palapa, and Apstar use. Satellite
TV receivers and IRDs which incorporate programmable tuning will give your system added
flexibility when pulling in international signals.
Video Bandwidth
INTELSAT
and PanAmSat sometimes transmit TV signals using only half of a transponder's available
bandwidth. This is typically the case when INTELSAT transmits short duration news feeds on
one of its global beam transponders. When receiving half-transponder signals, the
receiver bandwidth must be narrowed from its normal wide setting to less than 20 MHz
(INTELSAT) or 27 MHz (PAS) in order to obtain optimum system performance.
Even when a satellite signal occupies the entire transponder, your system may not receive enough signal to produce a good picture. This is often the case when the antenna isn't large enough to gather a sufficient amount of signal. Many of todays receivers come with adjustable bandwidth filters built into them. These filters can be adjusted on a channel-by-channel basis to provide optimum reception of each available satellite TV service. Once you have determined the best filter setting, you can store that setting in the receiver's memory circuit for automatic recall any time that you select that channel for viewing. If you have a satellite receiver which does not have a built-in bandpass filter, you can purchase an outboard filter and connect it to the second IF loopthrough ports (usually labeled IF IN and IF OUT ) on the receiver's back panel. The frequency of the filter you buy also must match the IF loopthrough frequency scheme of the receiver. This information is either provided on the receiver's back panel or in the receiver's instruction manual.
While a bandpass filter can be used to remove impulse
noise or sparklies from your reception of the weaker international satellite TV
channels, you will need to readjust the filter setting each time you change channels to
get the best possible system performance.
There are limits to the effectiveness of using filters to improve satellite reception. Exact reproduction of video color and detail requires that the satellite receiver process an incoming signal's total transmitted bandwidth. By narrowing the filter's bandwidth, you remove the portion of the signal that contains the greatest amount of noise. The image will be noticeably improved, but some color fidelity and picture detail will be lost.
Filtering techniques cannot be used to improve the reception of digitally compressed satellite TV services. Since all available TV services in a digital DTH bouquet are multiplexed into a unified data stream, reducing receiver bandwidth will result in a loss of essential data that the digital IRD needs to capture the signal.
Audio Companding
The audio portion of some of the Russian STATSIONAR satellite TV services using 7.0 or 7.5
MHz audio subcarriers has been compressed or companded and consequently sounds
distorted on receivers not equipped to re-expand it at the receive end. A special add-on
audio filter is needed to restore the audio to its original format.
Energy Dispersal
(ED)
Energy dispersal systems insert a constantly changing waveform into the video, preventing
the carrier from ever resting on its main frequency. ED also spreads the power of any
transponder across its total bandwidth; this prevents the satellite transmissions from
causing interference to ground-based services using the same frequencies. The ED waveform
is filtered out of the video at the TVRO receiver terminal. The dispersion removal
circuits used by most satellite receivers may not entirely take out the ED waveform
sometimes used by STATSIONAR satellites. In some cases, the receiver's automatic frequency
control circuit can be modified to do the job.
Coping With Video
Standards
The international community uses several techniques to encode the picture and color
components of video. On Asiasat alone, satellite TV viewers will encounter all three of
the basic color encoding systems being used around the world today: NTSC, PAL, and SECAM.
Reception of all three video standards by your satellite TV system will require the use of
a multistandard monitor or a special VCR that transcodes the TV transmissions from their
original standard to one that can be displayed by your regular TV set.
NTSC. Named after the U.S. National Television Standards Committee which selected this method of color coding back in 1953, NTSC utilizes a separate 3.58 MHz chroma subcarrier to contain the color information. NTSC is used for all of STAR TV's Northern Beam broadcasts on AsiaSat 1. The countries within the region using NTSC include Japan, South Korea, Taiwan, and the Philippines.
SECAM. The French system is called SECAM (for Sequence a Memoire). This system FM modulates the color information onto two subcarriers which are then recombined at the television set. Within the region, SECAM is currently used for satellite broadcasting by member countries of the Commonwealth of Independent States, Vietnam, and Mongolia.
PAL. A third system, developed by the Germans, is called PAL (for Phase Alteration by Line). This system is used to transmit all of STAR TV's Southern Beam broadcasts on AsiaSat 1. The countries within the region using PAL for satellite broadcasting include Indonesia, Malaysia, New Zealand, Thailand, and the People's Republic of China.
Multi-standard monitors and receivers are available so that you can view overseas satellite TV signals in full color. Some models will automatically detect and display the correct standard. Both Instant Replay and Panasonic offer special VCRs that will convert PAL, SECAM, and NTSC signals so that you can record them for later viewing, or view them live on any TV set in your home. We would recommend one of these for any first-class international earth station installation.
You also can receive a black and white picture from any standard on any television. When receiving 625-line video on an NTSC monitor, first readjust the set's vertical hold control to stop the picture from rolling. Then readjust the vertical linearity control to reduce the picture's height so that the 625-line picture will fit on the screen. The linearity control may be on the back of your set. If this adjustment requires the use of a screwdriver, use an insulated tool to prevent the possibility of shock.
Before making the linearity adjustment, the top and bottom of the picture appears cut off and everything looks skinnier than it should be. For someone with a 625-line set, the vertical linearity would be adjusted so that the video fills the entire screen. Unadjusted, 525-line video on a 625-line set looks compressed, with no picture present at the very top and bottom of the screen.
Switchable Video
Deemphasis
Every satellite TV receiver has a deemphasis filter which follows the demodulator stage;
slightly different components values are necessary for 525-line video than for 625-line
video. If this circuit is not modified, there will be a barely perceptible degradation of
the video, more noticeable on marginal signals. Use of the incorrect deemphasis network
also can pose problems when recording satellite TV programs with your VCR or interfere
with the remodulation schemes used in SMATV or CATV RF signal distribution systems. (See
WSTV, Chapter 10, for more information on SMATV and CATV distribution systems.)
Manufacturers of receivers for the Asian marketplace provide switchable deemphasis
circuits within their products.
Tracking Inclined
Orbit Satellites
The Sun, the Moon, and the Earth's gravitational field all affect the movement of
satellites orbiting the Earth. These natural effects impel telecommunications satellites
to wander from their assigned orbital positions over the Earth's equator. The amount of
movement can be predicted, with the direction of movement primarily in the north/south
direction over 24-hour intervals.
All geostationary satellites are launched with enough stationkeeping fuel to counteract the effects of these natural forces for a period of several years, and in the case of the very newest satellites, for up to fourteen years. It is the precise expenditure of fuel at periodic intervals over the lifetime of each spacecraft which permits each satellite to remain continuously on station at its assigned geostationary orbital position.
The controlling engineers for older INTELSAT V satellites, as well as for all of the Russian STATSIONAR satellites, typically cease expending stationkeeping fuel once the satellites have expended most of their remaining fuel. Once the north/south stationkeeping maneuvers have ceased, the natural forces described above will cause the spacecraft to begin drifting in the north/south direction. This is called an inclined orbit.
During the first few months of inclined orbit operations, the amount of drift in the north/south direction is minimal and is transparent to all but the largest receiving antennas on Earth. Eventually, however, the amount of drift will grow and the satellite's signal will fall outside of each receiving antenna's main beam for portions of each 24-hour orbital inclination period. The amount of inclination will increase at a rate of approximately ±0.85 degrees per year, and as time goes on, the length of the diurnal (twice daily) outages at all receiving stations also will increase.
The only way for a satellite TV system to maintain continuous contact with an inclined orbit satellite is to add a second motor drive to permit the operator to make periodic adjustments to the antenna's declination setting. An adjustment of the antenna's elevation angle alone will not move the antenna along the satellite's north/south path unless the satellite receiving system is located at a longitude that is less than 10 degrees from the longitudinal location of an inclined orbit satellite.
The least expensive way to track the movements of an inclined orbit satellite is to add a second actuator, which replaces the antenna's declination bracket, and an actuator controller. The operator can then manually make adjustments periodically to keep the inclined orbit satellite in view throughout each 24-hour period. This solution is not practical for home satellite TV applications where multiple satellites, including the inclined orbit satellite, will regularly be accessed by the receiving system. Any movement of the polar mount antenna outside of the normal operational boundaries will affect the antenna's ability to properly track geostationary satellites.
Automated receivers and control systems are available from U.S. companies such as Chaparral Communications which receive data from the satellite receivers AGC (Automatic Gain Control) signal level in order to obtain information on the relative signal strength of the inclined orbit satellite. If the signal level falls below an established threshold level, the system's microprocessor initiates a search routine that commands the movement of the antenna's tracking system until optimum reception has been restored. These systems also have memory circuits which will automatically return to the correct antenna declination setting for normal satellite reception.