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GPS receivers

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10.2 GPS receivers







Manufacture of GPS receivers for all applications is a multi-billion dollar industry supplying, just in the avionics field, a wide range from comparatively inexpensive handhelds, such as the old Magellan SkyBlazer illustrated, to very expensive panel mounts with integrity monitoring, ground-based position correction capability and colour moving map position displays.


In essence the aviation GPS receivers use the information contained in the C/A code, emanating from each satellite in view, to measure the time lapse of a received radio signal, calculate the distance to each satellite's position and then establish the receiver's three-dimensional position by trilateration — a form of triangulation — of the distances from a minimum of three satellites.


But simultaneous range calculations from four satellites are necessary to correct for the clock error in the GPS receiver — see below


. Although GPS would normally calculate it, external input of an aircraft's altitude can provide a further range measurement — that from the centre of the Earth, thus simulating an additional satellite.


For more detail download the


Garmin GPS Guide for Beginners


; it's in PDF format and about 524k. Also read the article


'How GPS works'


contained in the online version of CASA's magazine


Flight Safety Australia


April 1999




The distance calculation is derived from the time taken for a satellite radio signal to reach the receiver. As electromagnetic waves in space propagate at a speed close to 300 000 km/sec, the time taken for the signal to reach the surface from a satellite overhead is 20 000 / 300 000 seconds — about 0.067 seconds. It is also evident that if a position accuracy of, say, one metre is desired then the clock in the GPS receiver must be able to measure transmission times in nanoseconds (billionths of one second).


For further information on the timing techniques read


distance calculation




clock error


on the Trimble Web site.


Handheld GPS receiver databases


Aircraft positions are calculated by the receiver in terms of latitude, longitude and elevation. The receiver chips contain mathematical models of the Earth. The most accurate, and commonly used for aviation purposes, is the World Geodesic System 1984 [WGS84] which is the datum for WACs and — in Australia — VNCs, VTCs, aerodrome reference points and VOR sites. Check Geoscience Australia for more information about


geodesic datums


. There is also a useful


distance measuring facility


(between Australian locations) on the Geoscience Australia site.


Handheld receivers always contain a re-writeable user's database to store a number (maybe 500 or more) of user-defined waypoints (name, latitude and longitude) and maybe 50 flight plan routes, each typically allowing a maximum of 30 waypoints. Aviation handhelds will also provide a recognised standard aviation navigation database, invariably compiled by Jeppesen, containing: location/elevation coordinates and other information for all aerodromes referenced in ERSA; plus VORs, NDBs and intersections shown on ERCs; plus controlled and 'special use' (PRD areas) airspace. Those location references may also be used as waypoints when defining routes. The receivers provide elementary 'moving map' graphics that display the aircraft's position and the relative position of all the waypoints and aviation-related detail within a user-defined range. The diagram or 'map' can be configured to remove unnecessary items from the display and thus present a less cluttered image. Screen sizes are typically 40 × 55 mm for the less expensive units.


For more information see


navigation databases


in the electronic flight planning and navigation module.


Calculating height


Altitude is calculated as the height above the WGS84 ellipsoid, which differs from the geoid. In Australia the


geoid-ellipsoid separation


varies between minus 100 feet and plus 200 feet. Aviation GPS receivers should include tables (based on latitude/longitude grids of varying block sizes) of the geoid-ellipsoid separation values which allow the altitude above the geoid to be displayed.


Configuring displays


Aviation GPS receivers offer a variety of screen displays with user-configurable content that varies between models. The most useful displays for navigation purposes are: a moving map screen, and an alphanumeric navigation page that includes a course deviation indicator.


Some handhelds also provide a very basic ground map which may be a monochromatic or colour representation of a few significant line features (highways, railroads, coastlines) on which aviation-related detail is overlaid. This is generally sufficient for VFR non-primary navigational use, but there are some expensive handhelds on the market which provide a topographic, colour moving map display — but note these are not WACs, VNCs or VTCs and such map displays for Australia are likely to have some detail deficiencies.


The basic moving map, which is the preferred navigation mode, is usually configured to show an aeroplane image at the lower centre of the screen representing the aircraft's position in relation to the flight planned track between current waypoints, airfields and controlled airspace, etc. The display can be configured as 'north up' or 'track up'; most people seem to prefer the latter. The track made good will also be displayed, together with bearing and distance to the next waypoint. The display can normally be zoomed in or out and thus represent an area ranging from a few square miles to thousands of square miles.


The alphanumeric display might show track made good, ground speed, distance and bearing to the next waypoint, ETE to the next waypoint plus a course deviation indication similar to that of a VOR omni bearing indicator. The division dots on a GPS CDI are not spaced at two-degree intervals, but indicate distance off track — with the interval between dots being user scalable from maybe 0.25 nm up to 5 nm. Some devices may change scale automatically as the waypoint is neared. The bar indicates where the required track is in relation to the aircraft; e.g. if the interval is set at one nm and the bar is located three divisions to the right of centre then the required track is 3 nm to the right.







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