a sine signal, we can look at different parameters. If this summary increased your curiosity for more details, just follow on, but for a good understanding, we'll need to first clarify a few notions: Phase, modulation and sidebands. Missing the phase role in the ILS (and conventional VOR) is actually missing the cleverness of these technologies. Phase is used in many sophisticated radio applications. By taking appropriate protection measures in the Fresnel zone, the phase-based ILS is therefore very reliable. On the other hand, the phase of a signal remains unaltered until the signal is reflected, diffracted or enters a material with a different refractive index. in cellular networks) because the cost is marginal, but RSS depends on many factors which cannot be controlled. RSS-based positioning improved by averaging algorithms is commonly found for rough positioning ( e.g. They miss the point the ILS is not based on amplitude comparison, but on depth of modulation comparison.Ī precise location cannot be determined by simply using received signal strength (RSS) of two or more signals. The sidebands amplitude has been altered.Īs the sidebands are the modulation, in effect there is a change in the DM (the ratio between modulation and carrier amplitude), and a DDM appears magically, increasing as we move away from CSB, that is as we move away from the centerline.Ĭountless oversimplified descriptions put the focus on 90 and 150 Hz, each one allegedly transmitted on a specific side of the runway (cf.The carrier amplitude hasn't changed in the process.When the sidebands interfere, the way they are constructed (by tweaking phases) makes 150 Hz more prominent on the left of the runway, and less prominent on the right side (details below, drawings will clarify). Phase of the 150 Hz tone is inverted in SBO (90 Hz is kept unchanged).Īs SBO contains only the sidebands when CSB and SBO interfere in space, only the sidebands can actually interfere, the carrier found in CSB is unaltered.SBO phase depends on the runway side (+/-90°, CSB is kept unchanged).The heart of the system creating the DDM is two signals: CSB (carrier and sidebands, see below what is a sideband) and SBO (sidebands only) and surprise! they are containing the same quantity of 90 and 150 Hz tones and are transmitted in equal quantity on both sides of the runway. The 90 and 150 Hz tones are not used directly. Changing the relative strength of a signal changes both amplitudes in the ratio, but keeps the ratio unchanged. The depth of modulation is the ratio between the modulation amplitude and the carrier amplitude. When de-modulated is effectively a difference in amplitude modulation depth? So is it actually that the entire modulated signal strength decreases which The beam strength decreases as you move away from it's own centreline,
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