A major task when setting up an SNG unit is ‘acquiring the bird’ – that is locking onto the booked satellite. The satellite occupies a fixed position relative to earth and its azimuth and elevation will be relative to the position of the uplink. To establish a link it is essential that the precise position of the SNG unit is fixed. Vehicle mounted SNG equipment and some flyaway units are equipped with automatic acquisition facilities. The position of the SNG unit can be calculated by one or a combination of three methods.
Global positioning system (GPS): This is a military system developed by the US using 24 satellites. They individually transmit a constant stream of timing information that the GPS receiver can process to calculate position. A commercial version of the system with less precision allows a position to be calculated to within 10 m.
Flux-gate compass: This is a piece of equipment that establishes location by comparing electromagnetic fields with the magnetic force from the North Pole.
Beacon receivers: Communication satellites used in SNG work continuously transmit a unique identifying beacon signal. A receiver on the SNG unit can sweep a narrow frequency range until it picks up the signal of the beacon identifier and aligns the dish to maximize the signal.
An essential part of operating an SNG unit is obtaining various authorizations and permission. These include permission from the appropriate authorities to operate in their country or area, registration with a satellite system operator, a space segment booked for the required time slot. Satellite systems operators will require evidence before registration and accepting bookings that appropriate designated equipment is being used and operated by competent, trained personnel. Considerable damage can be done to their satellite and to other users of the satellite by incompetent operational procedure.
Elevation angle
The elevation of the path to the satellite above the horizontal (the look angle) is critical to establishing an SNG link. A low elevation angle of the satellite just above the horizon can cause difficulty in clearing trees, buildings, pylons and other terrestrial objects, resulting in the attenuation of the signal by absorption or multipath reflection distortion; a low elevation path through the atmosphere before it emerges into space is much longer and this increases rain attenuation; electrical noise is generated by heat near the earth’s surface, which can be picked up by the side lobes of the receiving antenna. Careful site selection is therefore important to ensure a clear line-of-sight to the designated satellite. The antenna requires protection against strong winds, which could shift the antenna’s precise alignment to the satellite. Ku-band signal transmissions are subject to degradation from heavy rainfall, particularly in the tropics where cloud-bursts occur frequently; C-band transmissions, however, suffer negligible attenuation when passing through belts of high rainfall.
A typical operational procedure for an SNG transmission would be for the uplink to call the satellite control centre and identify uplink registration and booking details including frequency and polarization. Five minutes before transmission the uplink ‘brings up’ a ‘clean’ carrier (unmodulated) to about 5–10% of full power. The control centre checks correct frequency and polarization. At this reduced power the control centre request modulation to be switched on, which is often a test signal with identifying caption. When the control centre has checked this signal it will agree that transmission can commence. It is from this point that the chargeable period commences. Test signal is removed and programme signal is now switched in. At the end of transmission the uplink ‘brings down’ the signal and end of transmission is agreed with control centre.
SNG safety
Because flyaway units and satphone antennas are often placed on the ground, there are essential safety procedures that must be complied with. Microwave transmitting equipment emits non-ionizing radiation at high power. This is a health risk that can burn the skin or internal organs. This must be avoided by rigging the antenna in a restricted location not accessible by the public or unauthorized broadcasting personnel; marking an exclusion zone in front of the antenna; checking the radiation levels around the perimeter of the safety zone and supervising and checking the danger area especially during transmission.
Operational points
Microwave radiation should not be operated near aircraft flight paths or military installations unless prior permission has been obtained. The antenna must be directed away from any vantage point to which the public have access by at least 5° and directed away from any mast on which radio equipment is installed by at least 10°. The SNG unit must not be positioned under power lines. No person must be able to get within 20 feet of the front of the antenna at the same height as the antenna. Avoid siting the SNG close to helicopter operating zones or radio transmitters. Make an assessment of wind strength. Strong winds can wobble the dish off satellite alignment and that is contrary to access regulations. Eutelsat specify no more than 0.3° of movement of beam angle. Check, with a radiation detector, the flexible waveguide at the bottom of the feed arm with power on for radiation leakage. Check that electrical connections and waveguide are protected from water ingress. There are also electrical hazards associated with SNG equipment due to the very high voltages required. One other important safety hazard is the weight of manual handling of flyaway SNG flight cases in transit and when setting up and de-rigging. Standard advice on lifting heavy weights should be observed.