- Buyers Guide
Tackling Satellite Interference
The satellite industry loses millions of dollars per year due to cases of interference and a great deal of manpower has to be given over to discovering its causes. Radio frequency interference (RFI) is caused by human error, bad installation, lack of training, poor equipment or system design and a lack of adherence to industry standards and guidelines. Occasionally, interference may be malicious, but this is rare and the main issues of interference lie largely within the heart of the satellite industry itself. The orbital spacing of satellites is being reduced and the fill rate is getting higher. It is getting crowded up there, leading to increased interference. The Satellite Interference Reduction Group (sIRG) is aiming to turn this around.
The effects of satellite interference are felt throughout the industry, yet it is we, the industry, that is causing the problem. Everything, from components and subsystem design, right through to the end user, affects satellite interference, so we need to work together for the resolution.
Types of Interference
There are several causes of interference and, as mentioned most of these are caused by the industry and can be avoided. There are of course some malicious attacks, but they only account for a very small percentage of the interference cases. By far the most significant cause of interference is human error. According to Ron Busch, VP Network Operations, Intelsat, as much as 90 percent can be attributed to human error. To follow is a quick rundown of the main types of interference. Satellite radio interference events may be generally categorized into eight main groups. These are:
This is usually accidental, due to user error (transmitting at the wrong time, wrong frequency or transmitting through the antenna instead of into a dummy load), equipment malfunction or due to poor cable shielding, causing retransmit of terrestrial signals (see Figure 1). Intelsat’s measurements show that 47 percent of satellite interference is due to unauthorized carriers, which is often caused by human error.
Adjacent Satellite Interference
This type of interference is also generally accidental, due to operator error or poor system design or installation practices. This type of interference is becoming more prevalent as a 2° spacing between satellites in the geostationary arc becomes more common. Smaller antennas mean larger beamwidths, so pointing the antenna correctly becomes more critical. Typical 3 dB beamwidths approach ± 0.75° or greater, so it does not take much to miss pointing the antenna such that it starts to illuminate an adjacent satellite at 2° spacing.
The use of RF spectrum is increasing and now traditional satellite frequency bands, such as the satellite downlink frequencies of 3.4 to 4.2 GHz, are being used terrestrially for the last mile broadband data networks. Tests carried out by sIRG, in conjunction with other groups, have demonstrated this to be a major issue. These frequencies are of paramount importance in the tropical regions as they are less prone to attenuation from weather effects. So the expansion of WiMAX and other BWA systems has limited the use of spectrum for satellite users.
Many satellite ground stations use intermediate frequency of either 70 MHz or L-Band (950 to 2150 MHz). These IF signals are upconverted to the satellite frequency bands, using block upconverters (BUC). Often, such IF frequencies are used for terrestrial services, such as FM broadcast radio or cell phone networks (GSM 3G and 4G). Badly planned or maintained ground stations retransmit these terrestrial signals to the satellite. Certainly, the increase in 3G and 4G services has seen a greater number of cell towers, which allied with an increasing number of small low cost satellite terminals, has led to increased retransmission of terrestrial signals.
This type of interference is usually caused by a state or large commercial user that objects to some content in the transmission that they are intentionally jamming. It is, generally, relatively easy to locate, but almost impossible to remove without political intervention, and even then this may prove difficult.
Cross Polarization Interference
Satellites increase the available spectrum by using polarity diversity. The RF signals are transmitted to the satellite in two polarities at the same frequency. A well aligned antenna should have greater than 30 dB of rejection of the opposite polarity (both transmit and receive), which is more than enough to ensure interference free communication. According to Intelsat, as much as 33 percent of interference is due to cross polarization leakage of signals from one pole into the other.
There are two main causes of cross polarization interference, the obvious one being that the antenna becomes misaligned. This may be due to several factors, ranging from being pushed around by high winds or other weather effects, to poor installation in the first place. Another is perhaps not so obvious and comes down to planning practices; that is incompatible modulation types (such as FM TV) being transmitted in the opposite polarization, analogue services such as FM or analogue video that generally, at some instance in time, collapse back to being un-modulated or a CW carrier.
The power, which normally spreads across the bandwidth by the modulation, is now concentrated in the CW carrier. Even with 30 dB of cross pole signal rejection, there can still be sufficient power in the cross pole to disrupt digitally modulated (*PSK) signals (see Figure 2).
Intermodulation occurs when two or more RF signals meet and merge, the sums and the differences of their harmonic frequencies cause inter-modulation products. Most common is the third order harmonic for different products. This form of interference in generally caused by users “creeping” up their power for a perceived link budget advantage. The reality is that, when all users resort to power creep, it becomes self defeating, as the satellite TWTA moves toward a nonlinear state and intermodulation in the form of spurious signals and what appears to be an increased noise floor actually reduces the link margin. Power creep can also result in ground equipment becoming compressed and spectral re-growth is seen, which can result in adjacent carrier interference (ACI) as the carrier now expands beyond its allocated bandwidth (see Figure 3).
Sun Interference is due to the satellite, the Sun and the Earth station (E/S) antenna being aligned. Solar heavy noise will be received with the satellite signal, due to the satellite, the sun and the E/S being aligned, as shown in Figure 4, which occurs twice a year.
Scintillation (Level Variation)
Scintillation occurs due to the turbulent mixing of air masses in the ionosphere, as shown in Figure 5. The satellite signal fluctuates in level (by up to 12 dB) at affected Earth Stations, while neighboring Earth Stations may not experience the same effects. It often occurs between 19:00 and 23:00 Earth Station local time and mainly affects lower frequencies, such as C-Band. It is unpredictable, so affected E/Ss are recommended to disable normal auto tracking once noticed and engage in manual tracking using received traffic levels as the basis of keeping on track.
What is Causing These Cases of Interference?
There is a whole range of different causes of interference. A number of years ago, it was believed that the biggest cause was deliberate signal jamming. However, now it is known that it is not the case and in fact, the deliberate sabotage of satellite signals is a very small percentage of the problem and that is not currently the highest priority.
Indeed, our industry is the main cause. One factor is that crowded geostationary satellites are leading to closer spacing of satellite. The biggest reason right now is sub-standard equipment, lack of trained technicians, poor installations and consequently equipment failure, as well as human error. There is still a lot of unidentified carriers and insufficient incident coordination.
The Global VSAT Forum (GVF) comments that with VSAT terminal costs dropping well below $1000, the margin available for installation services is falling. Add to that the fact that installers are no longer necessarily experienced engineers spending days on site, but instead often junior technicians paid as little as $50 for a complete VSAT installation. There are a vast number of VSAT terminals being installed, over 100,000 per year, any of which can cause serious interference. GVF also comments that spot beams make satellites more sensitive to uplink signals and although this helps reduce VSAT size and cost, it makes transponders more sensitive to interference.
So What is the Industry Currently Doing?
sIRG is working together with other industry organizations and major players in the satellite industry to resolve interference and is supported by the key satellite operators. Indeed, Intelsat’s Ron Busch commented: “At Intelsat, we are very much aware that Radio Frequency Interference continues to be a problem. Indeed, our metrics continue to show a steady rate of interference events on our fleet.” Intelsat, like others, has its own Intelsat Interference Management Initiative, which concentrates on training, technology and processes, both internally and with the entire satellite community.
There are a number of ways to reduce interference and eventually it would be nice to eradicate it altogether. The only way this can be achieved is by working together. A lot of work has already been done with satellite operators, broadcasters and equipment manufacturers across the globe, establishing a number of initiatives and good working practices to tackle this growing problem.
Carrier ID has been the major initiative for sIRG thus far in the world of broadcasting. It is an industry-wide initiative and it is the most effective tool for tracking the source of interference quickly and efficiently. In fact, (at the time of this writing) we are working closely with satellite operators, broadcasters and equipment manufacturers to have the NIT Carrier ID in place in time for the 2012 Olympics.
The purpose of Carrier ID is to tag an RF carrier with a unique identity, in the form of an alphanumeric string. This unique identifier is tracked by the satellite operator after the carrier is commissioned. That way if that carrier were to be the cause of interference, the unique identifier could be read and the satellite operator contacted to rectify the problem.
There are currently two technologies available for Carrier ID. The first makes use of the Network Information Table (NIT) within DVB streams of satellite digital TV transmissions. A standard DVB stream analyzer can be used to extract the NIT from the DVB stream. The other method uses a Meta Carrier or sub-carrier developed by Comtech and is transmitted at the same frequency as the carrier. The Meta Carrier is transmitted at low power, approximately 22 dB below the actual carrier, so as not to impact the link budget. The Meta Carrier uses a spectrum spreading technique, such that a receiver can correlate and pull the Meta Carrier out of the noise and read the unique identifier. The industry has come together and suppliers of communication system monitoring (CSM) equipment now supply systems capable of extracting and displaying the unique identifier from both systems.
At the time of this writing, Intelsat plans to use Carrier ID for DVB and SCPC carriers during the 2012 Olympic Games. “We are prepared for the NIT version and are hopeful we can also include Comtech’s Meta Carrier ID technology. The idea is to get the community used to using Carrier ID and for satellite operators to test databases and processes to communicate with each other, when identifying an offending carrier” commented Ron Busch.
Bob Potter of SAT Corp. says, “Our goal is to support our customer base in meeting their target of using Carrier ID for the Olympics in 2012. To that end, we have CSM systems deployed in Europe and North America detecting and displaying Carrier ID information.”
Another piece of the puzzle for Carrier ID is the equipment manufacturer. A large number of the equipment manufacturers have integrated NIT Carrier ID capabilities into their products and we are now working with them to test those products in time for the Olympics and, of course, beyond. While at NAB, sIRG focused on making that happen and, as a result of our efforts, we now have encoders to test from Ericsson, Fujitsu, Harmonic, IDC, NTT and Vislink and, recently, a new modulator from Newtec that was launched at that show. SiS Live are assisting with test transmissions using the Vislink encoder, along with test carriers provided by Eutelsat, Intelsat and SES. In addition, Comtech is supplying various sub-sets of equipment and decoders for the new Meta Carrier ID technology. This testing is absolutely crucial for ensuring not only that there are encoders and modulators on the market able to handle NIT Carrier ID, but also to ensure interoperability between equipment. The tests we are currently carrying out will mean that carrier monitoring specialists and manufacturers alike can iron out any issues, as well as allowing time to develop the interfaces and decoding techniques for both NIT and the new ID technology. Once we have gathered the results from these tests, and after any necessary modifications, we will convey this information to all those involved with the Olympic transmissions. At that point, we can begin applying ID to all applicable carriers ready for the Olympics this summer.
The next stage will be to encourage those manufacturers to upgrade to the new Meta Carrier ID technology, which is currently with the DVB. In addition, users need to ensure they are purchasing only equipment with Carrier ID capability and replacing any existing equipment which does not allow Carrier ID. This is our single biggest challenge. Through open debate and educating our audiences, broadcasters and uplinkers are gradually getting on-board with Carrier ID. After all, they are arguably the biggest sufferers of satellite interference.
A major effort is coming from satellite operators. The various announcements and active support from key operators Eutelsat, Inmarsat, Intelsat and SES have ensured that Carrier ID continues to progress and the initial Carrier ID will be used for the Olympics. sIRG actively encourage all operators to commit to this program as they have the power and influence to ensure users can employ Carrier ID within their transmission systems.
Carrier ID essentially means that any carrier can be quickly identified, so when interference occurs, a satellite operator can identify instantly who is causing the problem. Figure 6 shows a synopsis of the Carrier ID and QA process.
Detecting and Locating Interference
For those instances, which still exist when there is no Carrier ID, operators can call upon geolocation to find the source of the problem. Integral Systems Europe (ISE) is one of the companies providing geolocation tools. The company uses a monitoring tool to spot when interference occurs on a satellite. In this case, two signals with the same characteristics can be separated and characterized (see Figure 7).
These signals can then be geolocated by satID, the companies integrated geolocation product, to identify where the offending carrier is located. This can be further backed up by more accurate geolocation using its plane based system, Moscito. Before take-off, the mission is prepared from a laptop. Once in the air, the pilot engages autopilot and records GPS, video and spectrum power. If communication is possible, the pilot will also receive live data, which is then downloaded to the laptop after landing and processed using ISE’s software (see Figure 8).
The interference detected by the Monics CSM system is shown in Figure 9. The CSM system was used to measure cross polarization and found no matching carrier. Monics continued to monitor and characterize the interference. The signal that was causing ACI interference put the transponder close to a nonlinear mode and blocked a fee paying service from accessing the satellite. The carrier was not employing Carrier ID technology.
While the CSM continued to monitor the signal, it was decided the quickest form of action was to use a geolocation tool to locate the source of the transmission and then further determine the reason for interference (see Figure 10). Generally, ground based satellite signal geolocation systems need to monitor the target signal (interferer) through two satellites. The primary satellite being the satellite that is experiencing interference and the secondary satellite is close by and has a similar frequency plan.
The signal is monitored through the two satellites and analyzed to produce two lines of position, TDOA, a Time Difference of Arrival, which is generally a north south line and FDOA, Frequency Difference of Arrival, which generally is an east west line. The TDOA is generated based on the different distances to the two satellites and FDOA is generated based on the different frequency shift through the two satellites based upon their relative motion to each other. Through geolocation, the offending ground station is found relatively easily (see Figure 11) and, after a few phones calls, found that the reason for the interference was simply human error. The problem was quickly rectified and the carrier was removed from the satellite.
TX Spectrum Monitoring
As discussed, Carrier ID is by far the most effective solution for monitoring and resolving interference quickly and efficiently. However, this is not yet common practice and in those instances where it is not in place, operators are therefore forced to use other methods to combat this costly and challenging problem. Geolocation is one solution and Satellite Operator Arabsat urges that regularly monitoring the uplink can be a crucial “tool” for spotting interference. Arabsat has observed that any undesired signal above the noise threshold but low enough (typically more than 26 dBc below the desired carrier level) will affect adjacent users.
Talal Mahfouz, Arabsat Operations Center (AOC) expert, commented, “More crucially, it can be hard to determine the source by a satellite operator, even if it is clearly visible on the downlink spectrum.” He added, “HPA re-growth, HPA inter-modulation, retransmission MRN and converter harmonics are part of the undesired signals RFI types. However these could easily be detected in the early stages by the Uplinker, once the output spectrum has been checked. Indeed, L-Band analyzers, which are the most commonly available, can monitor MRN and converter harmonics if connected to the upconverter input test point.” Arabsat would like to see every Earth Station uplink operators apply transmit spectrum monitoring. The graphs shown in Figures 12 to 14, supplied by Arabsat, show TX spectrum monitoring at work.
The RF environment is becoming ever more crowded with ever increasing competition for bandwidth, and satellite communication is very much a part of that conundrum. Satellites in the geostationary arc are moving closer together (2°); more people are accessing satellites than ever before and thus interference is very much a fact of life. However the industry, through innovation, technology and a willingness to cooperate, is fighting back against interference. Carrier ID is a proven technology and another tool to use in reducing interference.
The author thanks the support from Bob Potter, President, Sat Corp., Ron Busch, Vice President, Network Operations, Intelsat and Talal Mahfouz, Operations Center (AOC) expert, Arabsat.