Satellite interference is a growing problem that affects all commercial satellites and satellite users. One underlying cause is the large number of transmit antennas now pointed to these satellites, including the ubiquitous VSAT. As the number of transmitters has increased, so has the number of interference events.
Damages caused by interferences range from disrupted traffic to lost revenue and unusable bandwidths. Among satellite users, one of the most vulnerable is video broadcasters. This is due to the sheer size of video signal bandwidths, which often span an entire satellite transponder. Video broadcasters may also find their signals share the same beam or polarization with hundreds or even thousands of other users. Such results in increased chances of their signals being adversely affected by interferences.
The trend of more frequent interference events is coming at a time when fixed satellite service (FSS) providers have been going through major consolidation to gain operational efficiency. This consolidation has resulted in larger fleet sizes for the surviving FSS providers, increased customer population, and higher service demand that, if improperly managed, can significantly add to or even overwhelm the work load of FSS customer service centers, resulting in lower quality of support.
Because of these trends, FSS providers, such as Intelsat, have been aggressively improving the efficiency and productivity of their service centers by adopting tools and best practices for three crucial network management functions.
- spectrum monitoring
- interference detection
In this article, we discuss the advances in technology that can now allow a single system to perform the functions of spectrum monitoring, interference detection and geolocation in a seamlessly integrated, cost-effective way. This level of integration and convergence of technologies has helped create tools that are highly automated, labor-saving, and surprisingly affordable .
Categories of Satellite Interference
The causes of satellite interference generally fall into one of three categories...
- human action
- equipment malfunction
- frequency spectrum
Interference caused by equipment malfunction has been on the rise over the last decade. As mentioned earlier, the root cause of this is the dramatic increases in the number of transmit capable antennas deployed worldwide, in particular, small VSAT terminals equipped with low-cost outdoor equipment. Many of the interferences seen on commercial satellites today are caused by some sort of ground equipment malfunction.
Interferences caused by frequency spectrum issues are also on the rise. As the demand for spectrum increases along with the number of new communications services being offered, the competition for available spectrum is intense. Satellite transmissions can be impacted by a variety of terrestrially based interference sources. Examples include terrestrial wireless services, line of sight microwave and various radar systems.
Dealing with Satellite Interference
The process of resolving satellite interference problems generally involves the following four steps: (1) Interference Detection; (2) Interference Classification; (3) Interference Isolation; and (4) Geolocation.
The first step in the process is to reliably detect interference when it occurs. This is accomplished by using a modern DSP based spectrum monitoring and interference detection system, such as the Glowlink Model 1000 (Figure 1, shown with touch-screen). The Model 1000 can be used to tightly manage transponder bandwidth and can be configured to detect even the weakest interferers.
Advanced signal processing techniques in the Model 1000 can detect both interferences that show up in unused bandwidth, as well as interferences that are hidden within the bandwidth of another traffic signal (Figure 2). This technology, known as Signal Under Carrier (SunCar) and pioneered by Glowlink, has been adopted by nearly all customers in their carrier monitoring systems.
A unique but interesting aspect of the SunCar technology is that it can also be used to separately measure two carriers with identical power, carrier frequency, and data rate. This means it can be used to monitor fully overlapping paired carriers, a new transmission technology used in various bandwidth-efficient modems on the market. This type of transmission conserves bandwidths, but amounts to self-interferences from a spectral perspective. Figure 3 shows two such signals that appear spectrally as one carrier, yet individually measurable by the Model 1000 with results shown in the report panel.
Tools such as the Model 1000 can also separate interference from other anomalies that may cause receive performance degradation. For example, phase jitter, uplink compression, adjacent channel interference, transponder compression or in-band interference can all cause a signals received Es/No performance to deteriorate. By isolating the problem to interference, the Model 1000 can dramatically improve the productivity of trouble-shooting.
In addition to being able to reliably detect interference, a versatile monitoring tool like the Model 1000 can also assist the operator in classifying and in turn isolating what causes the interference. Classification generally involves determining the characteristics of a particular interference signal: classifying the interference as stationary or non-stationary, such as misplaced communications carriers, spurious signals, and fixed noise mounds, or sweeping signals, bursting signals, hopping signals or various forms of intermittent signals. There are now available on the market various data fusion tools such as those built into the Model 1000, which can simplify this process.
Isolation is the third step in the process whereby the source of the interference is determined. Once interference has been detected and classified, tools such as the Model 1000 can help the satellite operator further identify the interference in terms of sweep rate, burst rate, burst interval, or modulation related parameters. In many cases, such signal DNA is sufficient to quickly identify and remove the source of the interference. The Model 1000 also has various built-in tools to help operators resolve common interference problems, including cross-pol bleed over, adjacent channel interference, and misplaced but otherwise legitimate carriers.
When a satellite interference cannot be resolved using the above process, it is necessary to perform geolocation, defined as the process whereby the interference emitter is located so appropriate action can be taken.
How Geolocation Works
When an interfering emitter transmits to a satellite, a small fraction of its energy illuminates adjacent satellites via the sidelobes of the transmitting antenna (Figure 4). Geolocation exploits this tiny bit of spilt-over energy to make time and frequency difference measurements of the interference signal as it passed through the primary or affected satellite and one or more adjacent satellites. Making these measurements can be extremely challenging, however.
A system that can perform this type of measurements where the signals differ by a factor of 10,000commonly referred to as 40 dB--is generally considered marginal. Systems where measurements can be achieved for a factor of 100,000 or better are more dependable.
Geolocation works by making relative measurements between satellite pairs and combining these measurements with the ephemeris of the satellites to locate the unknown emitter on earth. The relative measurements include time difference of arrival (TDOA) and frequency difference of arrival (FDOA). These measurements translate to lines of position on earth where the emitter could be located. The intersection of two such lines then forms an estimate of the interferers location.
Geolocation solutions may be formed by intersecting two TDOA lines, two FDOA lines or one TDOA and one FDOA line. Intersections of two TDOA or two FDOA lines requires measurements from two pairs of satellites whereas an intersection of one TDOA line with one FDOA line can be accomplished from a single pair of satellites (Figure 5).
There are differences between these techniques that can affect the performance of a geolocation system. The next few paragraphs examine these differences.
Properties of TDOA Measurements
TDOA measurements between a pair of satellites hinges on determining the difference in path length from the emitters location on earth to the primary satellite and the adjacent satellite. A key advantage of this approach is that it is independent of signal frequency, and therefore equally effective from UHF to Ka. However, measuring these time differences is extremely challenging and only a geolocation system that is properly designed will work correctly.
Another important factor of an effective system is the ability to perform fast signal acquisition in order to deal with hoppers or fast sweepers. The TDOA approach lends itself naturally to these types of interferers.
FDOA measures the differences between frequency Doppler shifts between a pair of satellites relative to the interference emitter. Two things are immediately obvious: First, unlike TDOA, FDOA measurements are frequency dependent. For geolocation, this means that the observed Doppler shift shrinks as the frequency band goes from Ku to C, making it harder to measure. Second, geostationary satellites in general do not move much, making Doppler even smaller. For a significant number of geostationary satellite pairs, there is little Doppler shift even at Ku band.
As the Doppler shift on geostationary satellites can be as small as a hundredth of 1 Hz, a geolocation system using FDOA technique must acquire the signal for a relatively long duration (> one minute) and over a relatively narrow bandwidth to yield sufficient samples for processing. Because of these constraints, FDOA, while suitable when there is large satellite motion and stationary signals, can often be rendered ineffective, especially against increasingly common interferences such as frequency sweepers or bursting signals.
Properties Of TDOA-TDOA Solutions Versus TDOA-FDOA Solutions
As mentioned earlier, intersections of TDOA lines, FDOA lines or a combination of both TDOA and FDOA lines can yield an emitter location. One intrinsic property of the TDOA lines approach is that it does not change shape or orientation over time, thereby allowing geolocation to be performed any time of the day. In contrast, FDOA lines are constantly changing orientation during a day. This has the effect of producing highly inconsistent results, and therefore is effective only during certain times of the day when the intersecting lines are nearly orthogonal. Unfortunately, the interferer cannot always be expected to be cooperative enough to appear in these desirable time windows!
Available Tools on the Market
One geolocation system on the market that is optimized for performance under various constraints is Glowlinks Model 8000. The Model 8000 is the worlds first fully integrated spectrum monitoring, interference detection and geolocation system in a compact 4U chassis (Figure 6). The system is also priced to be affordable for almost everyone in the satellite business, not just large customers with large capital equipment budgets.
Due Diligence = A Viable Solution
Because of the growing problem of interference and the increasing complexity of interfering signals, the satellite community needs effective and affordable tools to help them detect, identify, and remove these interferences. There are many products on the market with various degrees of effectiveness and attributes, Potential buyers of these systems must do their due diligence and insist on equipment demonstration, performance evaluation, and customer references, as well as considerations such as pricing, system attributes, and equipment reliability.