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En route navigation using the GNSS
Rev. 17a — page content was last changed 2 January 2013
|Flight Planning and Navigation|
Global Navigation Satellite System [GNSS] technology freely provides precise 3D position-fixing capability — latitude, longitude, altitude — and velocity data and has excellent navigation capability when combined with a stored flight plan. Planned improvements in the integrity and availability of GNSS, and implementation of ground-based and satellite-based augmentation systems in Australia, will ultimately make ground-based radio navigation systems (ADF, VOR/DME) redundant. However, as with the ground-based radio navigation systems, pilots operating under the day VFR may use GNSS only to supplement map reading and other visual reference navigation techniques.
For more information concerning the current regulatory use of GNSS in VFR navigation see AIP ENR 1.1 paragraphs 19.2 and 19.5 and AIP GEN 1.5 paragraph 8.5.4.
Massive growth is expected in the application of the satellite navigation systems, with aviation being a very small part of the total market. The European GNSS Agency 2012 world market estimate is $48 billion and the 2020 market forecast is $128 billion. The split-up by market sector for cumulated revenues from GNSS devices for the period 2010 to 2020 is:
The European Space Agency's new  Navipedia website contains a large store of information regarding GNSS in general and Galileo in particular.
For a more generalised GNSS overview download the Navipedia Galileo booklet.
NAVSTAR satellites continuously broadcast information on very low power at two UHF L band frequencies. The civilian standard positioning service [SPS] 'coarse/acquisition' ranging code at 1575.42 MHz (the L1-C/A code) and an encrypted precise positioning service code (the PPS code) at 1227.60 MHz. The L1-C/A code is freely available to all while the PPS code is only available to authorised users — chiefly military. The basic L1-C/A code is designed to provide a latitude/longitude position-fixing accuracy within 300 metres 99% of the time and within 100 metres laterally and 140 metres vertically 95% of the time; but probably better than 30 metre lateral accuracy is achievable most of the time without augmentation. Some manufacturers quote accuracies of 3 metres, or less. At present it is far more accurate than necessary for flight under the VFR though perhaps the higher accuracies might be valuable if using georeferenced taxiway diagrams at a major airport.
All civilian GPS receivers are SPS units receiving the L1 code, however a new, easier to track civilian signal [L2C i.e. Civilian] has been added at 1227.60 MHz to provide some redundancy and improve accuracy for dual-frequency receivers. The accuracy obtained is said to be as good as that obtained with the military PPS code.
Each satellite continually transmits three sets of information contained in the L1 code navigation message:
*Note: in astronomy the ephemeris term describes a table of predicted positions and is derived from the Greek word for diary.
In essence the aviation GPS receivers use the information, 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. Although the receiver would normally calculate an altitude, external input of an aircraft's altitude — baro-aiding — can provide a further range measurement — that from the centre of the Earth, thus simulating an additional satellite; but see 'The WGS84 ellipsoid and the geoid-ellipsoid separation'.
Aircraft positions are calculated by the receiver in terms of latitude, longitude and GPS altitude. The receiver chips contain mathematical models of the Earth. The most accurate, and commonly used for aviation purposes, is the World Geodetic System 1984 [WGS84] which is the lateral datum for WACs and — in Australia — VNCs, VTCs, aerodrome reference points and VOR sites. The Australian Height Datum is the vertical datum for Australian charts. See 'Defining the shape of the Earth – ellipsoids and geoids'.
Note: GLONASS uses a different coordinate system to WGS84 which might result in a 20 metre or so difference in position relative to the GPS calculation.
For an outline of the Global Positioning System visit Navigation Programs - Global Positioning System in the Federal Aviation Administration's website.
For more information concerning the use of GNSS in VFR navigation see AIP ENR 1.1 paragraphs 19.2 and 19.5. Note the wording of sections 19.2.1e and 19.5.1d together with the latter's link to AIP GEN 1.5 section 8. Also read CAAP 179-1(1) section 5 'Navigation using Global Navigation Satellite Systems'.
The simplest handheld satnav receivers always contain a re-writeable user's database to store a reasonable number of planned flight routes and perhaps a thousand user-defined waypoints (icon, name, latitude and longitude). The actual track date from previous flights can also be stored. Aviation portables will also provide a recognised standard aviation navigation database, possibly compiled by Jeppesen (the Pacific Basin) and 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. Those simplest 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. Display diagonal dimensions are typically 4.3 or 7 inches for the portable units.
Purchase cost for the simpler aviation 4.3 inch receivers may be $500 - $750 while the cost of those in the middle of the portable range may be $1200 to $1750.
The illustrated AvMap EKP IV receiver is in the middle of the portable range. It first appeared around 2006 and is representative of a 7 inch colour 800 × 480 pixel LCD receiver with split screen capability. The image shows two of the navigation screens — moving map on the left and horizontal situation indicator [HSI] on the right. A third screen is a navigation and location screen with a course deviation indicator [CDI] display. A compact flash memory card contains Jeppesen NavData plus land cartography. The amount of cartographic detail displayed can be varied. Internal RAM can store up to 1000 user-defined waypoints and up to 15 flight plans.
The unit uses an external antenna and needs external power supply — battery life is limited and may be regarded as an emergency supply. An NMEA 0183 serial input/output data port is included.
There are a limited number of portable/panel mountable aviation GPS receiver manufacturers: Bendix King, Garmin and AvMap are probably the best known in Australia.
In Australia the geoid-ellipsoid separation is quite significant, varying between minus 110 feet and plus 220 feet. At Cooktown, north Queensland, the airfield elevation is 26 feet (7.9m) and the AHD is 65.2m above the WGS84 ellipsoid (the N value) so the ellipsoidal height at the airfield would be 73.1 metres and a GPS receiver that doesn't adjust for the geoid-ellipsoid separation would indicate an airfield elevation of 240 feet; i.e. altitude is overstated by 214 feet. See 'Altitude and Q-code definitions'.
The GLONASS system uses a different ellipsoid so the geoid-ellipsoid separation would differ from the WGS84 separation.
Some handhelds might provide just a basic cartographic which may be a monochromatic or colour representation of a few significant line or point features (highways, railroads, coastlines) on which aviation-related detail (airfields, PRD areas and controlled airspace boundaries) is overlaid. This is generally sufficient for VFR navigational use, but there are more expensive handhelds on the market which provide a topographic, colour moving map display — possibly with terrain clearance indication — but note these may not be WACs, VNCs or VTCs and such map displays for Australia are likely to have some detail deficiencies. Also the screen area is too small to show normal map detail. GNSS terrain clearance warning is not particularly useful for VFR navigation; Mark 1 eyeball, which also takes in the cloud base, is superior.
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'. 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 GNSS CDI are not spaced at two-degree intervals, but generally 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.
Note: there are no certification standards for the hand-held GPS devices used in sport and recreational aviation; i.e. they are non-TSO'd. There is little to gain by installing a more expensive TSO'd device in a RA-Aus aircraft. The TSO'd receivers are not necessarily more accurate than a non-TSO'd receiver.
Panel-mounted GNSS receivers are certified to comply with TSO C129 which is a 'supplemental means' standard for IFR en route navigation that includes Receiver Autonomous Integrity Monitoring (RAIM). RAIM is an aircraft-based GNSS augmentation system (ABAS) that identifies any satellite that is not meeting specified standards and alerts the pilot. Portables only have the ability to inform the user when navigation has ceased entirely; they don't warn when a significant degradation in accuracy (the precision of the position solution) is occurring.
TSO C145 and C145a add a fault detection and satellite exclusion system (FDE) plus capability to use an American satellite-based augmentation system (SBAS) known as the Wide Area Augmentation System (WAAS); though WAAS is not available in Australia. TSO C146 and C146a includes SBAS and is a standalone system with the same status as ADF and VOR, i.e. a primary means of navigation. No satellite-based augmentation system is yet planned for Australia though the Japanese SBAS, known as MSAS, has one of its six ground reference stations in Canberra.
Ground Based Augmentation Systems (GBAS) are local systems installed at airports that transmit data to aircraft during precision approach and landings (GNSS Landing Systems or GLS) and will eventually replace Instrument Landing Systems (ILS). One GLS system is fully functioning at Sydney airport (2012) but handling only GLS receiver equipped Qantas Airbus A380 and Boeing 737-800 approach and autoland operations. See CASA 156/12.
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 GPS) 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.
*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.
For further information see CAAP 179A-1 Navigation using GNSS , also see the CASA booklet Overview of GNSS  and take particular note of the human factors section.
Note: a robotic antenna calibration facility installed at Geoscience Australia and used to calibrate the 200 GNSS antennae forming part of the Australian Geophysical Observing System (AGOS) is expected to increase their satellite positioning precision to less than one millimetre.
When used in the moving map navigation mode, the GNSS display exactly complements the en route navigation techniques expounded in section 8.3. The following is a basic illustration of GPS practice, for example let's take our planned flight from Oxford to Tottenham:
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).
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 map reading and visual reference to the ground, not the GPS display. That display should only be a fractional portion of your continuing scanning pattern. The VFR rules (ERSA ENR 1.1 para 19.2) state 'the pilot must positively fix the aircraft's position by visual reference to features shown on topographical charts at intervals not exceeding 30 minutes.'
The 'GO TO' function is for emergency use; you must not use it as a substitute for proper route planning.
Interference. Ensure that (1) 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.
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 readingAirservices Australia's document 'Safety_Net: using GPS as a VFR navigation tool' concerning avoiding airspace infringements while using GNSS.
An excellent online book written by John Bell of Orlando, Florida titled 'A practical guide to using GPS in the cockpit'. The book is in pdf format (4 MB), and also contains links to additional material. It was published in 2006 (developed from an earlier publication) and updated November 2007.
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.Pre-flight safety and legality check | 7. Airmanship & flight discipline | 8. En route adjustments |
| 9. Supplementary navigation techniques | [10. En route navigation using the GNSS] | 11. Using the ADF |
| 12. Electronic planning & the EFB | 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 © 2001–2013 John Brandon [contact information]