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.
- 10.1 Global navigation satellite systems [GNSS]
- 10.2 GPS receivers and augmentation systems
- 10.3 Performance standards for installed receivers in IFR aircraft
- 10.4 GPS VFR applications
- 10.5 GPS constraints
GNSS is the generic term covering satellite-based, three-dimensional position fixing, timing and navigation systems. The first such — and currently dominant — system is NAVSTAR or GPS, initially developed by the United States Department of Defense for position fix coordination of the inertial navigation systems [INS] on board military aircraft and cruise missiles. GPS has since become freely available — as a valuable primary and supplemental navigation aid — to civilian aircraft of all types and all nations, with the compliments of the U.S. government. The Russian GLONASS GNSS system now has 24 satellites in orbit and providing world coverage. Some GPS receivers can utilise both GLONASS and GPS satellites singly or jointly. The European Union’s Galileo GNSS system is undergoing trials with several satellites in orbit and is scheduled for full operation capability by 1920. China’s national satellite system is being expanded into ‘Compass‘, a 35 satellite GNSS system.
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:
- Road: personal navigation devices and in-vehicle devices – 54%
- Location based services: smart phones etc – 43.7%
- Aviation – 1%
- Agriculture – 1%
- Land and hydrographic surveying – 0.6%
- Commercial maritime – 0.1%
It is estimated that 80% of all applications would be satisfied by a one standard deviation accuracy of one metre. (One standard deviation refers to a statistically normal distribution where 68% of all measurements will be within that accuracy.)
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.
GPS 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 around 19 000 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 plus satellite trials often provides more than 30 units in orbit. Initial Operating Capability was established in 1993 with 24 operational NAVSTAR units in orbit.
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:
- its own ID, current date and time
- ephemeris* data that the receiver uses to calculate the precise position of that satellite at the start of the navigation message and which includes current orbital position data and predicted orbital positions for the next few hours plus its ‘health’ status
- almanac data providing future orbital position and time information for all the currently operational GPS satellites in the constellation.
The almanac data helps the receiver determine which other satellite ephemeris data to use. The almanac data is valid for 30 days or more.
*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.
There are four parameters for assessing the performance of a navigation system: availability/vulnerability, accuracy, integrity (i.e. trustworthiness) and continuity of service.
- Availability/vulnerability: the basic GPS civilian service is available nearly 100% of the time.
- Integrity refers to the trustworthiness of the device, i.e. the information provided is of the required standards for the particular application and the user is alerted when it is not.
- If a particular system is demonstrated to satisfy all four parameters for a flight phase then it may be classified as a sole-means navigation system — for that phase. When operating under the day visual flight rules, only navigation by visual reference to the ground satisfies all four parameters.
- A supplemental-means navigation system may be used in conjunction with a sole-means navigation system. Pilots operating under the VFR may use GNSS to supplement map reading and other visual reference navigation techniques. Any GPS receiver may be used but if it is an installed (i.e. not readily demountable) receiver it must be fitted in accordance with CAAP 35-1 or AC21-36; see AIP GEN 1.5 section 8.5.4. GNSS is only officially regarded as a primary-means night VFR navigation if the GPS receiver system accords with the FAA’s Technical Standard Order [TSO] C129 or TSO C145/6 series, or has other CASA approval.
- If a system navigation meets the integrity and accuracy requirements all the time, but falls short on availability/vulnerability or continuity of service, it may be approved as a primary-means navigation system for a flight phase, if specified procedures are employed. Day VFR navigation does not use primary-means systems, only the visual reference system plus supplemental-means systems as required.
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‘.
Manufacture of GPS receivers for all applications is a multi-billion dollar industry supplying, just in the avionics field, a wide range from inexpensive handhelds — not much more advanced than the 1995 Magellan SkyBlazer first-generation receiver illustrated — to very expensive aviation panel mounts with integrity monitoring, ground-based position correction capability and colour moving-map position and terrain awareness warning systems [TAWS] displays. Inbuilt GPS reception capabilility has been added to smartphones and other portable consumer electronic devices, — perhaps in the form of a Trimble chip set — but such GPS capability may not be suitable for aviation use. Some dual receivers have GPS and GLONASS reception capability.
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.
‘GPS altitude‘ is calculated as the elevation above the WGS84 ellipsoid, which differs from the Australian height datum geoid [AHD]. Aviation GPS receivers navigation data should include tables/algorithms (based on latitude/longitude grids of varying cell sizes) of the geoid-ellipsoid separation values to allow the altitude above the AHD (rather than the WGS84 ellipsoid) to be displayed. Non-aviation receivers generally display only GPS altitude.
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.
Aviation GNSS receivers offer a variety of screen displays with user-configurable content that varies between models. The most useful displays for supplementary-means purposes are a moving topographical map screen, a horizontal situation indicator screen and an alphanumeric navigation screen.
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.
There are packaged GNSS engines available which output the navdata, via a Bluetooth connection, to an iPad, iPhone, Android or other display device. The cost for aviation types is $75 to $150. For example the Garmin GLO for aviation costs about $150 and receives position date from GLONASS and GPS satellites (thus 48 satellite potential) with an update rate of 10× per second. Weight is 60 grams, USB connection also available.
Unlike aircraft operating under the day Visual Flight Rules, aircraft operating under the Instrument Flight Rules must use a GNSS system that meets the minimum performance standards of a Technical Standard Order (TSO) issued by the United States Federal Aviation Administration (FAA). The manufacturer of the system must hold a TSO authorisation issued by the FAA aircraft certification office; such receivers are then generally acknowledged as being ‘TSO compliant’ or “TSO’d”.
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.
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.
The primary use of GNSS in sport and recreational aviation is in en route navigation — monitoring flight progress against the established flight plan and providing the heading corrections necessary to maintain the required track. This requires entering the planned route into the GNSS database, activating that GNSS route on take-off and making the necessary adjustments, as indicated by the GNSS, to maintain track.
When used in the moving map navigation mode, the GNSS display exactly complements the en route navigation techniques expounded insection 8.3. The following is a basic illustration of GPS practice, for example let’s take our planned flight from Oxford to Tottenham:
|Segment||Altitude||Distance||Track [mag]||Heading [mag]||Ground speed||ETI||Comms|
|Oxford – Warraway Mountain||3500||74||083°||079°||67||66||ML 124.9|
|Warraway – junction||3500||52||050°||050°||65||48||ML 124.9|
|Junction – Tottenham||3500||33||029°||031°||65||30||ML 123.9|
|QNH: 1027||Last light: 1755 hrs AEST||Fuel margin: 40 mins|
Let’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).
Once airborne, and the receiver has locked on to the required number of satellites, the planned route is pilot-activated. The navigation computer within 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 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 ontopographical charts at intervals not exceeding 30 minutes.’
Aviation 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 nor the route that avoids controlled airspace or restricted areas — though the device should warn when the aircraft is approaching such areas. 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.
Antenna placement. The capability of a handheld receiver is greatly reduced if the receiver antenna is sited where it is shadowed by the airframe or is within one metre of a VHF antenna. An externally mounted antenna usually provides the best reception.
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.
Airservices 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.