GPS-Global Positioning System basics
GPS avionic system is a long range navigational aids and it stands for Global Positioning System. It provides aircraft position fix (3D). This technology is without any dought the most precise system used in navigation by civilian and military aircrafts. To understand why, we first need to realize that generally there exist a tradeoff between position fix accuracy and the coverage area in systems seen thus far. In other words, we cannot have the best of both worlds. However, we know for a fact that satellites operate at the UHF band and hence accurate position fix reading. As for ensuring a large and excellent coverage area we must place the GPS Transponder far away in outer space at roughly 20,000 km form the earth surface. As a result, GPS becomes an exceptional navigation tool ever invented due primarily to its accurate position fix and large coverage area.
To obtain a 2D position fix we need at least three satellites. Whereas for a 3D fix a minimum
of four satellites are needed. Logically speaking, this does not make any sense because it
goes without saying that for a least position fix calculation, the size of the dimension
and satellite must be equal. Technically speaking, that is right; however, from a practical or
cost effective approach we will always require an extra satellite.
Number of satellites = Number of dimensions +1.
GPS ground facilities consists of monitor stations, master control station and ground stations used both as Tx and Rx. Number of monitor stations worldwide are six. It receives navigation signals from satellites and then transmits it to the master control station for processing. Number of master control station is one and Once or sometimes twice a day, master control determines any navigation adjustments or updates needed and then forwards it to the ground antennas. There updates are orbital information, clock synchronization and status of the ionosphere layer. The number of ground antennas are about four. They receives updates from the master control station and emits signals to the satellites.
In the space, GPS has following features:
• It operates in various frequency ranges in UHF (L band).
• The orbital planes-6, Inclination-55 Degree, number of active satellites per plane are 4
• position or altitude of satellites is about 10988 nm (20350 Km). Minimum of five satellites are in view at all the times.
• Speed of GPS satellite is about 11265 Km/hr.
• Sun radiation forms source of energy used by solar arrays of satellite. Onboard batteries are used during darkness period and solar eclipse time.
• Each satellite contains either 3 or 4 atomic clocks. Only one of the clocks is actively used; the others remain on standby in case of an emergency or during maintenance.
• At all the times, total of 24 satellites must remain active to ensure a worldwide coverage. The remaining 5 satellites are there for backup in case an unexpected malfunction occurs or a deliberate shutdown is forced for maintenance.
• Total coverage angle is about twice of 13.84 degrees.
• Satellites need to be re-positioned to their original predefined location using small onboard rocket boosters once a year. Hence there will be downtime of about 12 hours.
• Each GPS satellite transmit three signals viz. Pseudo random code(PRC), Ephemeris data and Almanac data. PRC contains satellite ID since each SAT has a unique PRC. Also, the PRC is used to calculate the time it takes a signal to go from SAT to GPS-Rx; and therefore, it is often referred to as time signal. Ephemeris data contains satellite position w.r.t the time. Almanac data contains information about satellite status.
In the Aircraft GPS Rx system has following features:
• Operates in the UHF band
• GPS-Rx will observe data form at least 4 SATs in order to solve for the unknowns using the 4 equations
• The storage part of the Rx, which is updatable, contains a database with information about Airspace, Airports, navigation facilities etc.
• The main purpose of GPS is to calculate Aircraft position fix; however other output parameters are also derived such as TAS and Heading, GS and Track, WS and WA, distance to next WPT and/or to destination, ETA to next WPT and/or to destination, moving map display, Intensity of each signal received, Condition of each satellite tracked.
• GPS is the most accurate NAV system ever invented.
• GPS provides continuous real-time navigation information.
• GPS is an all-weather system.
• GPS is available 24/7 to the international community.
• GPS is available free of charge without any subscription or license.
• Unlimited users could take advantage of GPS without degradation of the signals quality.
• For military activities, GPS could hit the target without causing major collateral damages.
• SATs use atomic clocks, and they are very precise; however sometimes
discrepancies do happen and hence time measurement errors.
• The satellites should in general maintain their predefined orbital positions, however drifts do occur by gravitational pulls (earth, moon, sun).
Ionosphere and troposphere leads to slow down of signal propagation speed. This rate is variant and non constant. This leads to complications in the correction/compensation.
• The GPS-Rx will detect the desired NAV signals; however, the signal might be slightly distorted with noise due to the wireless nature of the system.
• Ideally we want signals to go straight from satellite to GPS receiver; however, on occasions the signals get bounced, and the Rx will detect these bounced signals.
• It is very costly to maintain GPS constellation annually.
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