They tell you how far you are from the runway threshold during an approach — Page 391, Lesson 399

This is the index of the book — the P section. I want to walk you through the key terms you'll meet, because this index is your map of the whole syllabus. Let's go through them in order, and I'll give you the professional definition of each one as we pass it. First, P and P1, P2, P3 — these are the three marker beacons on an Instrument Landing System, the ILS. P1 is the outer marker, P2 is the middle marker, and P3 is the inner marker. Each one transmits a different audio tone and a different light colour in the cockpit — P1 is a blue light, P2 is an amber light, and P3 is a white light. They tell you how far you are from the runway threshold during an approach. PAPIs — that's Precision Approach Path Indicators. These are the four lights beside the runway that show you whether you're too high, too low, or on the correct glide path. If you see two red and two white, you're on the correct approach path. PAR — Precision Approach Radar. This is a ground-based radar system where a controller gives you very precise guidance down to the runway, both in azimuth and in elevation, talking you down with heading and altitude instructions. Parabolic and Parabolic Dish — this is the shape of a reflector antenna. A parabolic dish focuses the energy from a feed horn into a narrow beam. The feed horn sits at the focus of the parabola, and the dish reflects the signal into a concentrated beam. That's how you get a very directional antenna. Parabolic Reflector — same idea, the reflecting surface itself. It's used in radar and in some communication antennas to concentrate the radiated energy. Parasitic Elements — these are the unpowered elements on a Yagi antenna. They're not connected to the transmitter; they just sit near the driven element and re-radiate the signal, which shapes the radiation pattern. The director is in front, the reflector is behind, and together they make the antenna directional. P Codes — this is the precision code in the GPS system. It's a long, encrypted code that gives you much higher accuracy than the coarse acquisition code, the C/A code. It's used by military and authorised users. PDOP — Position Dilution of Precision. This is a measure of how the geometry of the satellites you're using affects the accuracy of your position fix. If the satellites are spread out well, PDOP is low and your fix is accurate. If they're clustered together, PDOP is high and your fix is degraded. Pencil-shaped Beam — this is a very narrow, concentrated radar beam, shaped like a pencil. It's produced by a parabolic reflector and gives you very precise angular resolution. Period — this is the time taken for one complete cycle of a wave. It's the reciprocal of frequency. If the frequency is high, the period is short; if the frequency is low, the period is long. Phase Comparison — this is a method of measuring distance or direction by comparing the phase of two signals. In radio navigation, you compare the phase difference between a reference signal and a received signal to determine your position. Phased Array — this is an antenna made of many individual elements, each one fed with a signal at a slightly different phase. By adjusting the phase of each element, you can steer the beam electronically without physically moving the antenna. Phase Difference — this is the difference in phase between two signals at a given point. It's measured in degrees and is fundamental to phase comparison navigation. Phase Modulation — this is a method of encoding information on a carrier wave by varying its phase. The phase of the carrier is changed in accordance with the modulating signal. PLAN and Plan Mode — these are functions on a weather radar or a navigation display. Plan mode shows you a top-down view, like a map, with the aircraft at the centre and the weather or navigation information displayed around it. Polar Diagrams — these are graphical representations of an antenna's radiation pattern. They show you how the antenna radiates energy in different directions, usually in a polar plot with the antenna at the centre. Polarization — this describes the orientation of the electric field of a radio wave. It can be horizontal, vertical, or circular. It matters because the transmitting and receiving antennas must have matching polarization for efficient signal transfer. POS INIT — this is the position initialisation function on an FMS, the Flight Management System. You use it to enter your starting position before a flight, so the system knows where you are. Position Dilution of Precision — that's the same as PDOP, which we covered. It's the geometric effect on GPS accuracy. Position Reference System — this is a system that provides a reference position, often used in inertial navigation or in conjunction with GPS to give you a continuous, accurate position. Power — this is the rate at which energy is transferred. In radio, it's the strength of the transmitted signal, measured in watts. PPI — Plan Position Indicator. This is the display on a primary radar. It shows a top-down view of the area around the radar, with the radar at the centre and targets shown as blips at their bearing and range. pps — pulses per second. This is the repetition rate of a radar's transmitted pulses. It's the same as the PRF, the pulse repetition frequency. Practical Range — this is the maximum distance at which a radar can reliably detect a target, taking into account the actual conditions, not just the theoretical maximum. Precipitation Static — this is static noise caused by precipitation, like rain or snow, hitting the aircraft's antenna. It can interfere with radio reception. Precision Approach Radar (PAR) — we covered this. It's the ground radar that talks you down precisely. PRF — Pulse Repetition Frequency. This is the number of pulses transmitted per second by a radar. It determines the maximum unambiguous range and the Doppler resolution. Primary Radar — this is a radar that works by transmitting a pulse and listening for the echo reflected off a target. It doesn't require the target to have any equipment on board. PRN — Pseudo-Random Noise. This is the code used by GPS satellites to identify themselves and to allow the receiver to measure the time of arrival of the signal. P-RNAV — Precision Area Navigation. This is a more accurate form of RNAV that requires the aircraft to maintain a position accuracy of within 1 nautical mile for 95% of the flight time. Propagation — this is the way a radio wave travels from the transmitter to the receiver. It can be affected by the atmosphere, the ground, and other factors. Propagation Error — this is an error in a navigation measurement caused by the way the signal propagates, such as bending or delay in the atmosphere. Propagation Paths — these are the different routes a radio wave can take, such as the ground wave, the sky wave, and the space wave. Now, I want to draw your attention to two specific figures that relate to what we've just covered. The first is the parabolic reflector — — this shows you the horn feed to the parabolic reflector, which is exactly the parabolic dish and feed horn we talked about. The second is — this shows an aircraft maintaining the required track outbound from an NDB in zero wind, which is a practical application of track maintenance that you'll see later in the book. So that's the P section of your index. Every one of these terms will come up in the chapters ahead, and now you have a professional definition for each one.