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The Global Positioning System
Rev. 14 — page content was last changed November 18, 2009
consequent to editing by RA-Aus member Dave Gardiner
|Flight Planning and Navigation|
Global positioning system [GPS] technology provides precise position-fixing capability, and excellent DR capability when combined with a stored flight plan. Planned improvements in the integrity and availability of GPS, and increases in signal strength, will ultimately make ground-based radio navigation systems redundant. GPS navigation will then be accepted as a sole means of en route navigation — provided the aircraft is fitted with an approved GPS system and the pilot appropriately trained.
GPSGPS or the NAVigation Satellite Timing And Ranging [NAVSTAR] system consists of a minimum 24 satellites (of which usually three are operating spares) orbiting Earth at an altitude of 20 000 km, with each unit taking about 12 hours to complete one orbit. The NAVSTAR orbits are arranged in six planes with three or four satellites in each plane. This configuration ensures that a minimum of four satellites would be in view from most locations on Earth at any time. Each NAVSTAR unit is solar powered and equipped with atomic clocks for extremely accurate time measurement. The satellites have an operational life of around 10 years, so there is a continuing replacement program, which also allows phased introduction of new technology and expanded facilities. Early launch of replacements often provides more than 24 units in orbit.
NAVSTARs continuously transmit information on very low power at two UHF L band frequencies, a coarse acquisition ranging code (the C/A code) on 1575.42 MHz and an encrypted precise positioning service code (the P code) on 1227.6 MHz. The C/A code is freely available to all while the additional P code is only available to authorised users. The C/A code is designed to provide a latitude/longitude position-fixing accuracy within 300 metres 99% of the time and within 100 metres 95% of the time; but probably better than 30-metre accuracy is achievable most of the time. Similar accuracy is achieved in reporting an aircraft's elevation above a particular reference level. At present it is far more accurate than NDB/ADF or VOR and certainly more accurate than necessary for VFR flight.
Future developmentMassive growth is expected in the application of the satellite navigation systems. The 2003 forecasts for European usage in 2015 were:
In aviation a 'sole means' precision IFR navigation system has to meet certain standards with respect to accuracy, integrity, availability and continuity of service. GPS by itself cannot meet those standards. However, the ICAO nations are developing a sole means global navigation satellite system based on GPS and Galileo augmented with ground- and space-based correction (or differential) systems, airborne avionics plus digital data link communications and surveillance between aircraft and ground stations. (See ADS-B surveillance technology.) GNSS will eventually make obsolete all VORs, NDBs and other ground-based systems* and — as manufacturers are prepared to develop low-cost light aircraft avionics — it may have considerable spin-off benefits to recreational fliers. I suggest you read the article 'GPS Revolution' in the Flight Safety Australia January – February 2000 issue.
*But not a self-contained onboard system like an Inertial Navigation System. GPS is used to supply position-fixing data to INS which, while its electronic DR is very accurate, still has a gyro drift inaccuracy up to one nautical mile per hour.
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 issue.
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 and clock error on the Trimble Web site.
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. 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.
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.
When used in the moving map navigation mode, the GPS display exactly complements the en route navigation techniques expounded in section 8.3. For example let's take our planned flight from Oxford to Tottenham:
Entering routesLet's assume our GPS receiver contains a standard aviation database, in which case the only waypoint already existing would be Tottenham (YTOT) and the others would have to be entered into the users database thus:
Oxford S33° 02.5' E144° 35.1'
Warraway S33° 06' E146° 02.5'
Junction S32° 40.6' E146° 57'
The route would be given an identification and simply entered into the route database as:
Route ID: Oxford – Warraway – Junction – YTOT
The processor will calculate and provide an alphanumeric display of the required track and the distance for each leg, which must then be verified with the flight plan data. If they don't agree the cause must be found and corrected. The database contains the isogonals for the region so the required tracks can be displayed as magnetic or true. Note that the map screen will also include details from the standard aviation database; in this case the airfields at Ivanhoe (YIVO), Lake Cargelligo (YLCG) and Condobolin (YCDO).
Monitoring progressOnce airborne, and the receiver has locked on to the required number of satellites, the planned route is pilot-activated. The receiver recalculates the aircraft's position at set intervals of one or two seconds, or less, keeps a history log of the track made good, and continually recalculates groundspeed and distance off track.
The moving map display at left shows the situation at our position fix at Trida enroute from Oxford. The indication is that we have drifted left of track, the bearing to the first waypoint at Warraway is 085° magnetic and the distance to run is 52.4 nm. The track made good was 077° magnetic and the ground speed since activating the route is 55 knots.
In this screen the system acts exactly as if there is an NDB at Warraway and the GPS receiver homes to it by indicating the bearing. However, if you just fly that bearing, without any heading adjustment for the crosswind component, the bearing will keep changing due to the drift and, like the ADF, you will eventually arrive at Warraway — but the track followed will be curved and the magnetic heading flown will be changing consistently. Thus to maintain a constant heading, and the direct track, you still have to calculate and apply the wind correction angle.
The GPS doesn't directly calculate the heading to fly, either to regain the planned track or fly direct to Warraway. You don't need to estimate the track error from your chart the GPS shows the track made good as 077°, the track required was 083° thus the track error is 6°. You can then apply the double track error technique to regain and hold the original track. You will know you have regained track when the GPS indicates that the bearing to the waypoint is 083° so you then make the necessary heading adjustment to maintain track.
If you continue to adjust your heading so that track made good — not your heading — matches the bearing you will theoretically continue tracking directly to the waypoint.
Alternatively if you prefer to fly directly to the waypoint, rather than first regaining the original track, a quick mental calculation of track error/closing angle will provide the wind correction angle to accomplish that.
e.g. track required = 083° track made good = 077° thus opening angle = 6°.
Track required = 083°, bearing to waypoint = 085° thus closing angle = 2°
Opening + closing angles = 8° WCA to track direct to the waypoint.
In fact that calculation is simplified by using the difference between the track made good and the bearing; i.e. bearing = 085°, track made good = 077° thus WCA = 8°.
Some handhelds may do the calculation for you in providing a TURN display, which is just that difference between the track made good and the bearing.
The alphanumeric display at left, for the same flight situation, shows the same bearing, distance, track made good and ground speed information. In addition the CDI indicates that the required track is about 3 nm to the right of the present position, the estimated time en route to reach Warraway is another 55 minutes and the actual distance off track or the cross-track error [XTE] is estimated at 3.15 nm. The type of navigation using this display is very much the same as tracking in on a selected VOR radial and obviously to maintain a constant heading you still have to calculate and apply the wind correction angle.
Handheld aviation GPS receivers normally provide an E6-B page where, if you enter the heading being flown and the true airspeed, the receiver will estimate the 'winds aloft' from the TMG and ground speed, and then calculate the heading to fly. Due to the limited key board it is not so easy to input data during flight in a light aircraft , so it is much easier to just use the variant of the track error/closing angle calculation outlined above.
Remember that during flight under the Visual Flight Rules you are required to navigate by visual reference to the ground, not the GPS display. That display should only be a fractional portion of your continuing scanning pattern.
Emergency search featureAviation GPS receivers all provide an emergency search key, possibly labelled 'GOTO/NRST'. Pressing this key once will bring up a screen displaying the 10 nearest airfields extracted from the database together with the distance and bearing to each. Highlighting one of these airfields and then pressing the 'GOTO/NRST' key again will bring up the alphanumeric navigation screen to 'go to' that airfield. However, and this is a big however, the GPS indicates the direct route to the selected airfield not the safe route. The GPS does not take into account the type of terrain or the height of terrain — the GPS indicated route might be over 'tiger country' or straight through a mountain.
The 'GO TO' function is for emergency use; you must not use it as a substitute for proper route planning.
Interference. Ensure that (10 the mounting/placement of the GPS unit, and associated cables, within the cockpit can cause no interference with the magnetic compass; and (2) other equipment within the cockpit (including mobile phones) can't interfere with the GPS receiver.
Integrity. Approved panel-mounted GPS receivers have an inbuilt integrity monitoring and warning system called Receiver Autonomous Integrity Monitoring [RAIM]. Handhelds only have the ability to inform the user when navigation has ceased entirely; they don't warn when a significant degradation in accuracy is occurring. Read the article 'GPS failure' contained in the Flight Safety Australia January – February 2002 issue.
Satellite signal quality [SQ]. The SQ number is an indication of signal to noise ratio for each satellite in view: 0–1 is useless, 2–3 is undesirable and 7–9 is good. The SQ may be indicated as an unnumbered bar chart but the scale usually reflects the 0–9 range.
Horizontal dilution of precision [HDOP]. Some handhelds may show the HDOP value reflecting the relative geometric positioning of the satellites in view. Low HDOP (less than 02) is best, high HDOP (greater than 06) is not so good for accuracy.
Ease of use. The keypads of aviation handhelds are not designed for entering data whilst flying a light aircraft, thus it is very difficult to change route details whilst airborne. Some GPS receivers now on the market, purporting to be aviation receivers, seem to have been designed for the much larger road vehicle market.
Improper use of the 'go to' function. There is always the temptation to use the 'go to' function as a replacement for proper flight planning.
Further readingI suggest an excellent online book written by John Bell of Orlando, Florida and titled 'A practical guide to using handheld GPS in the cockpit'. The book is in both html and pdf format, and also contains links to additional material.
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Groundschool – Flight Planning & Navigation Guide
| Guide content | 1. Australian airspace regulations | 2. Charts & compass | 3. Route planning | 4. Effect of wind |
| 5. Flight plan completion | 6. Safety audit | 7. Airmanship & flight discipline | 8. Enroute adjustments |
| 9. Supplementary navigation techniques | [10. Global Positioning System] | 11. Using the ADF |
| 12. Electronic planning & navigation | 13. ADS-B surveillance technology |
| Operations at non-controlled airfields | Safety during take-off & landing |
|The next section of the Flight Planning & Navigation Guide discusses how to use NDB/ADF for VFR navigation|
Copyright © 2001–2009 John Brandon [contact information]