Information Note 97/04
BROADCASTING SERVICES TO NORFOLK ISLAND:
Effect of Transfer of SE HACBSS to National Beam
Introduction:
A programme is in place that will, eventually, see the conversion of existing HACBSS (ABC and SBS) and RCTS broadcasting services transferred from analogue (B-MAC) to digital transmission. This proposal has been active for some time but has been subject to delay and deferment as a consequence of continuing non-availability of appropriate digital hardware.
It now seems likely that a conversion will take place within about 6 months or so which we see at least some of these services transferred from zonal beams to a national beam on the OPTUS B-3 satellite. The main driving force behind conversion from B-MAC to digital is the ability to carry multiple programmes in a single transponder channel and thereby reduce costs of service delivery. At the same time, this will release transponders for other use and allow OPTUS to make more intensive use of the available transponders. A preliminary step along this path has been the adoption of "half-transponder" working whereby two B-MAC signals are combined so as to fully utilise a single transponder in place of the original implementation where each B-MAC signal occupied a major part of a single transponder. It is assumed that services currently delivered via NE, SE and Central zonal beams would be transferred to a National beam.
One disadvantage of any move from zonal to national beams is that the coverage patterns (foot-prints) of the beams are quite different with the result that signal levels outside mainland Australia will fall by significant amounts. A number of off-shore territories currently receive broadcasting services by means of "fringe" reception of zonal beam signals. In some cases, this "fringe" reception capability may not be retained in a move to National beam transmission.
Norfolk Island Situation:
Norfolk Island currently receives broadcasting services by means of terrestrial retransmission of ABC and SBS signals delivered via the SE Zonal beam. These SE beam signal levels are much higher than the signal levels expected if a National beam is used to distribute the services.
A theoretical study of the effect of transfer to National beam was undertaken by Murray Delahoy in 1995 and reported in Laboratory Report 95/24. The conclusion of the study is that transfer of HACBSS to National beam would result in loss of service to Norfolk Island. The basis for that outcome being that signal levels would fall to such an extent that they could not be recovered by use of any economically realistic larger antenna system at Norfolk Island.
The basis for the study was information supplied by OPTUS regarding the "General Design Level" (GDL) performance of the B-Series satellites. The GDL data is, in effect, the guaranteed end-of-life performance under all normal conditions of operation of the satellite. It is usual practice when designing a system to use the GDL data so as to ensure that the system has sufficient margins to ensure reliable operation during the life of the system. This is particularly so when the satellite signal is used as a feed for a terrestrial rebroadcast service because of the need to ensure full economic usage of the capital equipment required.
It is a normal (indeed an integral) aspect of this approach that, particularly early in the life of the satellite system, the actual performance (signal levels) will be significantly better than GDL limits. This is unavoidable as the satellite system suffers unavoidable gradual decline in performance due to factors such as ageing of batteries and solar panels and slow physical degradation of the entire system due to the harsh environment in space.
Within the main-beam of the satellite antennas this margin of performance is relatively small and can be calculated to a reasonable degree of accuracy. However, away from the main lobe of the antenna this is not the case. First, the antenna performance itself is not precisely defined - it is specified mainly as an "envelope" of permissible maximum signal levels rather than as actual nominal or minimum achievable levels, and the shape of the slope of the sides of the main lobe is not precisely defined. Secondly, as a consequence of the way in which the antenna patterns are synthesised, the performance of the beam away from the main axis is very sensitive to environmental influences at the satellite. Effects such as differential heating or cooling as the satellite passes into and out of sunlight can result in significant variations in detail to the off-axis antenna performance.
The SE zonal beam provides a relatively high level side-lobe which provides useable levels of signal on Norfolk Island that can be calculated with a good level of reliability, however, as far as the National beam is concerned, Norfolk Island falls a long way down the side of the beam where calculation of signal levels will be uncertain and actual signal levels might be expected to be highly variable and inconsistent. The expected National beam signal levels as calculated from GDL data are expected to be 24.4 dB lower than the SE beam signals that are currently in use. Such a margin is far greater than can be recovered by any economically realistic or practical upgrading of receiving installation at Norfolk Island.
Measurements that have been made within the "fringe" areas of the zonal beams have shown that actual signal levels can be significantly higher than those predicted from calculations based on GDL such that, in the absence of any better solution, an acceptable result can be obtained by using receiving installations with better than normal performance (eg bigger dishes and very low noise LNC units). The utility of this upgrade can only be assessed by means of on-ground measurement of the actual signal being considered for use. The ABC have recently undertaken some field measurements on Norfolk Island and have provided the results of those studies to the Department.
ABC Assessment of Norfolk Island Situation:
The ABC papers provide a number of useful pieces of information including an outline of present broadcasting arrangements, a comparison of relative signal levels from SE and National beams and an indication of variability in the National beam signal level.
In summary, the following information can be extracted from the ABC papers:
1. a 1.5 m dish provides just receivable pictures on SE HACBSS. This indicates that such dishes are operating at around the system threshold point of about 9 dB. (NOTE: It is usual to design a rebroadcast system to operate at about 5 or 6 dB above threshold (as a minimum) so as to obtain reliable and noise free signals for broadcast.)
2. 3 m dishes are currently used for rebroadcast, such antennas would have a performance advantage of about 6 dB compared to a 1.5 m dish so would seem to be operating at about the usual design point.
3. Tests were undertaken using a 4.5 m dish with low noise converter to examine a test signal in the National Beam in comparison with the SE HACBSS and receiving system noise floor.
4. Signals received on the test installation are such as to indicate that the results fairly reflect the receiving system performance and are not affected by the performance of the measuring instrument (spectrum analyser).
5. Results indicate:
SE HACBSS signal is about 20 dB above system noise floor. This is consistent with observations noted at (1) and (2) above. This signal level is appropriate for use as a calibrated reference point.
the national beam test signal was highly variable with signal levels ranging from 9.5 to 21.5 dB below the SE HACBSS Reference. Average value 15.9 dB.
the test signal level above system noise was seen to be in the range 2 to 11.9 dB with an average value of 7.6 dB.
(These two observations indicate significant fluctuation (about 8 dB) in the level of the SE HACBSS signal in addition to fluctuation in the test signal. Such an observation is not surprising given that both signals are outside the main beam of the respective foot-prints. One consequence of this is that greater performance margins need to be designed in to any system or else the reliability will be lower than usually expected.)
6. The ABC papers indicate that a replacement system would need performance of 10.4 dB better than that provided by the 4.5 m dish used for the tests.
Partial Report Ends….
References:
Laboratory Report 95/24 "Satellite Signal Coverage to Australian Island Territories"
Laboratory Report 96/1 "Coverage of Imparja Using the OPTUS B3 Satellite".