EAA Chapter 1241 Marathon, FL

January 2008 Flying with GPs ~ IV

This month we will continue our series on Flying with GPS by looking at the Wide Area Augmentation System (WAAS). I decided to move the discussion of Area Navigation (RNAV) and Required Navigation Performance (RNP) to next month and finish talking about the equipment (hardware). This article on WAAS will complete the airborne hardware and then we will talk more on usage.

In our WAAS discussion, we will look at the following:

• Why WAAS

• WAAS Capabilities

• WAAS Requirements

• WAAS Procedures

The answer to Why WAAS is that it improves the accuracy, integrity and availability of GPS signals while lowering cost. The FAA developed WAAS to allow GPS navigation to be used from takeoff through a Category 1 (200 feet ceiling and ˝ mile visibility) precision instrument approach. WAAS is very cost efficient for the FAA because, while a Category I ILS approach currently averages over 1 million dollars per runway, a WAAS approach can be obtained for a small fraction of that cost. Thus WAAS has become a critical component of the “Next Generation” Air Traffic System, which will provide improved safety and capacity.

WAAS will soon become an international system. ICAO (International Civil Aviation Organization) has already produced Standards and Recommended procedures (SARPs) for Satellite-based augmentation systems (SBAS). Europe and Japan are both developing WAAS type systems which will be compatible with WAAS. Europe’s system is EGNOS (European Geostationary Navigation Overlay System) while Japan’s is MTSAT (Multifunctional Transport Satellite-base Augmentation System). The merging of these systems will create a seamless worldwide navigation with WAAS benefits.

WAAS will cover a more extensive service area than traditional ground-based navigation aids. The U.S. WAAS network is currently comprised of 25 WRSs (wide-area ground reference stations), 2 WMSs (wide-area master station) and 4 GUSs (ground uplink station). The WRSs are each placed in a precisely surveyed location and linked to form the U.S. WAAS network. The WAAS network monitors signals from GPS satellites to determine satellite clock and ephemeris corrections as well as modeling the propagation effects of the ionosphere. Each WRS in the network relays data to a WMS which computes correction information and prepares a correction message. This correction message is then uplinked to a GEO (geostationary satellite) via a GUS and broadcast on the GPS frequency of 1575.42 MHZ (L1) to all WAAS receivers within the broadcast coverage area of the WAAS GEO.

Additional pseudo-range measurement to the aircraft WAAS receiver is provided by the WAAS GEO, in addition to providing a correction signal. This improves the availability of GPS by effectively providing another GPS satellite in view. GPS integrity is improved through real-time monitoring, while accuracy is improved by providing differential corrections to reduce errors. This improves performance enough to allow approach procedures with a GPS/WAAS glide path These are labeled LPV (localizer precision with vertical guidance) and effectively provide a GPS generated localizer and glide path with minimums as low as Category I ILS (200 ceiling and ˝ mile visibility). Future enhancements to the network will include additional ground reference stations, master stations, communications satellites and transmission frequencies as required.

GNSS (global navigation satellite system) navigation, which includes GPS and WAAS, is referenced to the World Geodetic System (WGS-84) and can only be used where the Aeronautical Information Publications – both electronic data and aeronautical charts – conform to WGS-84 or equivalent.

WAAS Capabilities provide approach procedures which include vertical guidance. Because these do not meet the ICAO Annex 10 requirements, a new class of approach procedures has been developed to support the worldwide use of satellite navigation for aviation applications. These new procedures are defined in ICAO Annex 6 and are called APV (approach with vertical guidance) approaches. They include LNAV/VNAV (Baro-VNAV) and LPV (GPS generated glide path).

Aircraft with a properly certified Flight Management Systems (FMS) can fly LNAV/VNAV approaches without use of WAAS. This is called Baro-VNAV. The FMS database for each LNAV/VNAV approach contains a charted descent angle which begins at or slightly beyond the FAF (final approach fix) and terminates at 50 feet over the runway threshold. These aircraft will display a glide path indicator to the pilot when the approach is active, which is flown by the pilot (or autopilot) in the same way a glide slope is flown. Because this is a “computed” pseudo glide path, its exact location is effected by altimeter errors. This is not the case with an electronic glide slope. (I.E. If the altimeter was set incorrectly and the aircraft subsequently crossed the FAF low, it would follow a glide path parallel and below the desired path and arrive at minimums short of the desired geographic spot. Conversely, if the error resulted in crossing the FAF high, it would follow a glide path parallel and above the desired glide path and arrive at minimums beyond the desired geographic spot.) Because altimeter errors are invisible (the altimeter reads the desired value even though the aircraft is not at the correct height), these approaches are not as accurate as those with a ground based electronic glide slope. The ground based electronic glide slope signal provides a descent path which is fixed in space and will provide the same descent path to all aircraft, irrespective of any altimeter errors. Because of the effect of altimeter errors on these approaches, Baro-VNAV approaches do not meet the more stringent standards of a precision approach. A properly certified WAAS receiver will use a WAAS electronic glide path to fly these approaches, thus eliminating the errors introduced by the use of Barometric altimetry.

WAAS certified receivers will be able to use a new type of APV approach procedure called LPV (localizer performance with vertical guidance). WAAS angular lateral precision, when combined with an electronic glide path, will allow the use of TERPS approach criteria very similar to that used with current Category I ILS approaches, adjusted for a larger vertical component limit. The resulting approach procedure minima (LPV) can have decisions altitudes as low as 200 feet HAT (height above touchdown) and visibility minimums as low as ˝ mile when the local terrain and airport infrastructure can support the lowest minima.

WAAS initial operating capability supports a level of service that provides LNAV, LNAV/VNAV and LPV approaches. GPS approach minima (LNAV, LNAV/VNAV and LPV) are published on the RNAV (GPS) approach charts.

WAAS requirements state that all WAAS avionics must be certified with TSO (Technical Standard Order) TS0-C145A, (Airborne Navigation Sensors Using the GPS Augmented by the Wide Area Augmentation System (WAAS) or TSO-146A, Stand-Alone Airborne Navigation Equipment Using the Global Positioning System (GPS) Augmented by the Wide Area Augmentation System (WAAS). These systems must be installed in accordance with Advisory Circular (AC) 20-130A, Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors, or AC 20-138A Airworthiness Approval of Global Positioning Systems (GPS) Navigation Equipment for use as a VFR and IFR Navigation System.

GPS/WAAS operation must be conducted in accordance with the FAA-approved aircraft flight manual (AFM) and flight manual supplements which will state the level of approach procedure that the receiver supports. IFR approved WAAS receivers support all GPS-only operations as long as lateral capability at the appropriate level is functional. WAAS provides integrity by monitoring both GPS and WAAS satellites. GPS/WAAS equipment which has a FDE (fault detection and exclusion) prediction program is capable of supporting oceanic and remote operations.

Prior to GPS/WAAS IFR operation, the pilot must review appropriate NOTAMs (Notices to Airmen) and aeronautical information. The FAA provides NOTAMs to advise pilots of the status of WAAS and the level of service available. Air carrier and commercial operators must meet the appropriate provisions of their approved operations specifications.

Unlike TSO-C129 avionics which were certified to supplement other means of navigation, WAAS avionics are evaluated without reliance on other navigation equipment appropriate to the route of flight being flown. Thus installation of WAAS avionics does not require the aircraft to have other avionics appropriate to the route of flight, as long as the aircraft is operated within the SBAS GEO coverage area. In the event of a WAAS failure, or when outside the SBAS coverage area, GPS/WAAS equipment reverts to a GPS-only operation and satisfies the requirements for basic GPS equipment.

WAAS allows pilots to use an airport which has only a GPS approach as an alternate airport provided it has weather reporting. When using WAAS for alternate airport planning, the weather required is based on the RNAV (GPS) LNAV minima, conventional GPS approach minima, or GPS “overlay” minima for a conventional approach with “or GPS” in the title. Upon arrival at the alternate, the approach may be flown to either LPV or LNAV/VNAV minima provided that the WAAS navigation system indicates that level of service is available. The FAA has begun the process of removing the alternate NA notation from affected airports, but this will take time.

WAAS Procedures have some differences from regular GPS. In addition to supporting all basic GPS functions, WAAS has the capability to generate an electronic glide path, independent of ground equipment or barometric aiding. This glide path is generated for both Baro-VNAV and LPV approaches and eliminates barometric altimetry errors. Altimetry errors such as cold temperature effect, incorrect altimeter setting, or lack of a local altimeter source are present in FMS Baro-VNAV operations without WAAS. WAAS receivers which can only support a glide path with performance similar to Baro-VNAV are restricted to the LNAV/VNAV minima line on GPS approach charts. WAAS receivers which can support the performance requirements for precision approaches (including update rates and integrity limits) are authorized to fly the LPV minima line. The lateral integrity changes dramatically from 0.3 NM (1824 feet) for GPS, LNAV and LNAV/VNAV approaches to 40 meters (131 feet) for LPV approaches. WAAS receivers also have vertical integrity monitoring for LNAV/VNAV and LPV approaches which contains the vertical error to 50 meters (164 feet).

The WAAS receiver notifies the pilot of the most accurate level of service supported by the combination of WAAS signal, receiver, and selected approach once the approach becomes active. It notifies the pilot by using the naming conventions on the minima line of the selected approach procedure. For example, if the receiver is not certified for LPV (or the WAAS signal does not support LPV approach minima) the receiver will notify the pilot with a message such as “LPV not available, use LNAV/VNAV minima” or “LPV not available, use LNAV minima.” Once this notification is given, the receiver is locked into this mode for the duration of the approach. It cannot change back to a more accurate level of service until the next time an approach is activated.

WAAS receivers have the ability to exclude a bad GPS signal and continue operating normally. Within the WAAS coverage area, this is accomplished by the WAAS correction information. When outside of the WAAS coverage area, it is accomplished by a receiver algorithm called FDE (fault detection and exclusion). This operation is automatic and invisible to the pilot.

WAAS receivers use different scaling (both laterally and vertically) than the linear scaling used by basic GPS receivers. WAAS receivers use +/- 1 NM until (2) NM from the FAF, where the sensitivity begins to increase similar to the angular scaling of an ILS approach. During long final approach segments, the initial scaling will be +/- 0.3 NM to provide equivalent performance to GPS (better than an ILS which is less sensitive far from the runway). Close to the runway threshold, WAAS scaling reverts to linear and stops becoming more sensitive. The total final approach course width is usually tailored to be 700 feet at the runway threshold. When the aircraft is vectored to final, instead of flying the complete published procedure, the VTF (Vector to Final) mode is used. Under this mode the scaling remains linear at +/- 1 NM until the point where the ILS angular course width reaches +/- 1 NM regardless of the distance from the FAWP (Final Approach Waypoint)

WAAS missed approach scaling is also initially different from basic GPS receivers. One difference is that the scaling does not ramp up as GPS receivers do, but abruptly changes from approach scaling to the missed approach scaling at approximately the departure end of the runway (or when the pilot selects missed approach guidance). The second difference is when the first leg of missed approach is a TF (track to fix) leg aligned within 3 degrees of the inbound course. The WAAS receiver then changes to +/- 0.3 NM sensitivity until the first waypoint in the missed approach procedure, where it abruptly reverts to +/- 1 NM sensitivity. This prevents close-in obstacles in the early part of a missed approach from raising the DA (decision altitude)

WAAS receivers add a new method for selecting the approach. In addition to the traditional GPS method using menu selection of airport, runway, procedure, and IAF (initial approach fix), WAAS receivers allow a channel selection method. This method is where the pilot enters a unique 5-digit number provided on the approach plate, and the receiver recalls the matching final approach segment from the receiver database. The receiver then lists the available IAFs and the pilot selects the desired IAF.

The ATD (along track distance) during the final approach segment of a LPV or LNAV/VNAV approach will be to the runway threshold as there is no MAWP (missed approach waypoint). The ATD for an LNAV final approach segment will be to the MAWP, although this is usually located on the runway centerline at the threshold and so is normally the same. These distances will vary slightly from ILS/DME distances as the ILS/DME transmitter is located further down the runway. Initiation of the missed approach on LNAV/VNAV and LPV is based solely upon reaching decision altitude (DA) with out seeing any of the required visual items of 14 CFR Section 91.175. It must not be delayed until the ATD reaches zero. WAAS receivers (unlike GPS receivers) will automatically sequence past the MAWP when the missed approach procedure has been designed for RNAV. The missed approach may also be selected by the pilot prior to the MAWP; however, navigation will continue to the MAWP prior to waypoint sequencing taking place.

That wraps us up for this month. Next month we will continue with RNAV and RNP. The thought for this month is “We make a living by what we get. We make a life by what we give.” – Winston Churchill. So until next month, be sure to Think Right to FliRite.

Garmin GNS 480 GPS/WAAS Receiver

Flight Advisor Corner by Tobie Tomlinson May 2006 

                                                                                    Annual Flight Safety Edition

In April 22^nd, I was in Nashua, New Hampshire giving a couple of presentations on “The Aging Airman” seminar at Daniel Webster College’s “New England Aviation Expo.”  (I don’t understand why a “youngster” like me keeps getting invited to present this!)  Anyway, just before the seminar, an AP release came across my desk about the “graying” of the pilot population in the U.S. and then the Scott Crossfield accident happened. 

September 2007                                                                                             Flying with Floats IX

This month we will wrap up the “Flying with Floats” series.  The final topics we will consider are the following:

• After Landing Procedures
• Anchoring
• Mooring
• Beaching
• Docking
• Ramping
• Postflight Procedures

After Landing Procedures are slightly different in seaplanes.  Most seaplane landings transition into a “Step Taxi” after landing.  This is to expeditiously move the seaplane from the landing area to the location where it will be secured.  Once the seaplane is firmly on the water, raise the flaps and add sufficient power to keep the seaplane in the planing position “on the step.”  The flaps are retracted to allow for a slightly higher step taxi speed, reduced wind effect and better visibility

When approaching the securing area reduce the power to idle, apply aft elevator pressure to raise the nose, and allow the seaplane to return to a plow taxi.  At this point the water rudders are lowered and the after landing checklist completed.  Next, remove your headset and seat belt so as to not become entangled with them when exiting the seaplane and then unlatch the door.  (The seat belt is secured by connecting the two ends across the seat underneath you.)  Lastly shut down all electrical equipment and turn off the Master Switch so that the battery will not be draining while you are out of the cockpit securing the seaplane.

Anchoring is an easy way to secure a seaplane on the water surface, but anchoring provides no way to reach shore unless a dinghy is available.  Because of this, the use of anchoring is limited mostly to recreation (i.e. fishing) or emergency use.  When selecting a site to anchor, the holding characteristics of the bottom are important to consider.  Other anchoring considerations are that the seaplane be out of the way of moving vessels, that it be in water deep enough to not “bottom out” as it moves with wind or tide changes, and that it has enough room to move around the anchor as the wind changes but not strike anything.  Use an anchor line that is approximately 7 times the water depth.  The anchor line should be tied off around the forward float attach fittings rather than the bow cleats.  This will prevent the anchor line from submerging the front of the floats if a stiff wind makes the seaplane “tug” on the anchor line.  The water rudders should be left retracted as they can interfere with the seaplane’s ability to respond to wind shifts.  If the seaplane is going to be left unattended at anchor, use a heavier anchor and insure that it is “set” into the bottom properly.  This setting is accomplished by anchoring the seaplane while it is facing into the wind and then letting it drift backwards and “set” the anchor.   Verify that the seaplane is not dragging the anchor by picking two points on the shore (one directly behind the other) and observing that they keep their alignment.  If they do not, the seaplane is drifting.  Secure the seaplane’s controls with the elevator down and the rudder centered.  This will cause the wind to keep the seaplane’s nose down, reducing the lift and wind resistance while at anchor.  Lastly do not forget to use some type of anchor light if the seaplane is to be left overnight.

Mooring eliminates the problem of anchor dragging and its use is more common.  The problem of reaching the shore still remains, but the issue can be resolved by leaving a dinghy attached to the mooring while using the seaplane.  A mooring buoy is approached straight into the wind at minimum speed.  Always approach a mooring buoy with the outside of a float - never between the floats.  This is to avoid damage to the propeller and/or underside of the fuselage.  To prevent overrunning the mooring buoy, shut down the engine as you approach the buoy and let the seaplane coast up to the buoy.  After the engine is shut down, approach the buoy, turn the mags “off”, exit the seaplane, and stand on the float deck.  The mooring buoy can then be grasped either by hand or with a boat hook.  The mooring lines are tied off around the forward float attach fittings, just like an anchor line.

Beaching is a desirable alternative if the shoreline is suitable.  Rocky shorelines will damage the floats, and mud bottoms are not suitable due to the probability of getting the seaplane mired in the mud.  Suitable beaching areas should be of sand and free from obstructions.  Inspect the proposed site carefully from both the air and water before using it.  Approach the beaching area at a 45 degree angle so that the seaplane can be quickly turned away if the area proves to be unsatisfactory.  This also allows the shoreward float to get closer to the beach before bottoming out which makes for less wading in shallower water.  Water rudders should be retracted before entering the shallow water next to shore to protect them from damage.  Once contact with the beach is made, exit the seaplane, push it around, and then pull it backwards onto the beach.  If the wind is suitable, sailing backwards onto the beach with the water rudders up is the most desirable method of beaching a seaplane.  The aft bodies of the floats do not dig into the beach as hard as the forward float bottoms, so it is easier on the floats.  Also, the seaplane can get closer to the beach this way and is in the proper position to “power off’ the beach when departure time comes.  Do not leave a beached seaplane unattended without securing a tail tie down line to a solid object on the shore.  This is because wave action rapidly washes the sand out from under the floats and may re-float the seaplane.  Do not forget the effect of tides if operating in salt water estuaries.  It is important to get the seaplane firmly on the beach so that the waves do not keep pounding the floats against the bottom and damage them.  If the seaplane is to be left overnight, tie-down stakes and ropes should be used, just as in a landplane.  In a pinch, with high winds expected, the floats can be filled with water.  This makes the seaplane very secure, but filling and subsequently pumping out the floats involves a lot of work.  Flying boat pilots who beach “gear down” should clear the main gear wheel wells of any sand or debris prior to departure.

Docking is the most popular way of securing float equipped seaplanes.  Docking does not work well for hulled seaplanes (flying boats) because their wing tip floats will not clear the dock and they prevent bringing the hull alongside the dock.  This procedure is essentially the same as mooring, except that approaching directly into the wind may not be an option.  Proper planning of the dock approach, compensating for the wind and current, and skill in handling the seaplane in congested areas are the keys to success.  Remember that bumping into things with a seaplane’s extremities can result in extensive damage and be very expensive!  Plan the approach to the dock as much into the wind as possible and verify the responsiveness of the water rudders to ensure that they will maneuver the seaplane with the existing wind and current.  If control is marginal, turn away and plan an alternate method of approaching the dock. When within coasting range of the dock, shut down the engine, turn the mags “off”, and steer the side of the floats against the dock as gently as possible.  Next, exit the seaplane, stand on the float deck, step onto the dock, and secure the line from the rear float strut to a mooring cleat on the dock. Then proceed to secure the front mooring line to a mooring cleat on the dock.  Be very careful in letting inexperienced people assist in docking or mooring a seaplane.  It is quite possible to walk far enough forward on the float deck to be struck by the propeller!             

Ramping is the most popular way of securing amphibious seaplanes.  Ramping is also used quite a bit with straight floats.  Again, ramping may not be an option for a hulled seaplane because their wing tip floats will not clear the obstructions along the sides of narrow ramps.  These aircraft can only be ramped in locations which provide a wide enough ramp, or clear area, to allow adequate clearance for the wing tip floats.  A high-wing, float-equipped aircraft usually has sufficient wing height to clear all but the highest obstructions.  When using boat ramps to take amphibious seaplanes out of the water, be very careful about light poles and other obstructions, as these ramps are not designed with wing clearances in mind!

Wooden ramps are sometimes used at seaplane bases and - when kept wet - will allow a seaplane on straight floats to be taken completely in and out of the water by sliding up or down the ramp on the keels of the floats.  Concrete (or paved) boat ramps are not suitable for straight floats and should only be used by amphibians.  With a stone or a widely spaced plank ramp, check to insure that the smaller bow tires of the amphibian will not become lodged in the gaps before attempting their use.

The water rudders are left down for directional control when approaching a ramp, and they are subsequently raised as the bows of the floats contact the ramp.  The elevator control is held full aft throughout the ramping maneuver.     

When approaching the ramp at the correct speed, the impact with the ramp is cushioned by the bow wave of the float.  When the seaplane is too slow or decelerating, the bow wave moves farther aft on the float and causes the ramp impact to be harder.  Apply additional power just prior to contacting the ramp.  The higher power raises the float bows and creates more of a bow wave cushion, lowering the ramp impact force on the floats

When the wind is toward shore and the ramp must be approached downwind, it is necessary to approach the ramp with enough speed to maintain control.  In this instance it is important to not cut the power until the seaplane has contacted the ramp and sides up on it.  Cutting the power before reaching the ramp will cause the seaplane to weathervane and hit the ramp sideways or backwards.

The most difficult ramping condition is when the wind is blowing parallel to the shore.  This is especially true when the wind is strong enough to make control marginal.  In this instance, if the ramp has been approached upwind, it may not be possible to turn the seaplane crosswind without excessive speed.  This may be overcome by taxiing directly downwind toward the ramp until very near the ramp.  When the seaplane is directly abeam the ramp, momentarily close the throttle and let the seaplane weathervane to the correct ramping position.  Then apply enough power to pull the seaplane up the ramp and out of the water.  Do not attempt this when the winds are high and the ramp is slippery, as the seaplane may be blown off the ramp sideways.

When the seaplane stops moving, shut down the engine and complete the secure checklist.  The seaplane should have been powered far enough up the ramp so that the waves will not hit the seaplane and work it back into the water.  On the other hand, if the seaplane is too far up the ramp, shoving off will be difficult.  Ramps are usually quite slippery, so remind passengers to be very cautions with their footing when walking on the ramp.

In strong winds, experience and proficiency are necessary for safe ramping.  If the conditions are questionable, the safest procedure is to taxi upwind to the ramp and have a helper attach a line to the floats.  The seaplane can then either be left floating, or pushed into a position where a vehicle can haul it up the ramp.

Postflight Procedures also have some items unique to seaplanes.  Any time the seaplane has been operated in salt water, the entire seaplane must be flushed with plenty of fresh water to minimize corrosion.  When the seaplane is secured, move the fuel selector to a position which blocks the fuel tanks from being interconnected.  This is especially important when the seaplane is left in the water.  It prevents fuel from transferring into the low wing (if one float becomes lower in the water) and possibly capsizing the seaplane.  Lastly, insure that the seaplane is properly securely (tied) before leaving.  The rest of the Postflight items are similar to a landplane.

This completes the seaplane series.  Next month we will start a new topic.  The thought for this month is a quote from John A. Shedd.  “A ship in harbor is safe, but that is not what ships are for.”  So until next month, be sure to: Think Right to FliRite!

October 2007                                                                                                   Flying with GPS ~ I    

This month we are starting a new series on “Flying with GPS.  A considerable amount of information has been published about operating GPS receivers.  This ranges from “GPS for Dummies” to “The GPS in Quantum Physics for Techies.”   Hopefully we can generate some useful information that is somewhere in between.

GPS (Global Positioning System) was developed by the U.S. Military and is a relatively new technology (in the grand scheme of things), having only become operational on the 8^th of December, 1993.  It consists of a constellation of 24 satellites, each in a geostationary orbit above a designated spot on the earth’s surface.  These satellites are strategically positioned so that there is no place on the earth’s surface which cannot view a minimum of 5 satellites for navigation.  Each satellite broadcasts its identity and a precise satellite time signal on the L1 GPS frequency of 1575.42 MHz. The aircraft GPS receiver tracks multiple satellites and determines a pseudo-range measurement which it uses to triangulate a 3 dimensional position.  The typical GPS receiver is capable of tracking 9 or more satellites with a minimum of 4 satellites required to establish an accurate three-dimensional position.  The orbital parameters for each satellite (ephemeris data) are sent to that satellite and embedded in the GPS signal which it transmits.  The DOD (Department of Defense) is responsible for operating the GPS system and monitoring each satellite for proper operation.   The GPS system utilizes Cartesian earth-centered, earth-fixed coordinates specified in the 1984 World Geodetic System (WGS-84).  Thus any charts used for GPS navigation must be annotated as WGS-84 compliant 

For those of us fortunate enough to live in America during the early part of the 21^st century, (vs. being born on the Mongolian Plain in the 15^th century), the progress in electronics is truly astounding!  The transistor has to rival the gasoline engine (or penicillin) for the discovery which has had the greatest effect on modern society.  I used to wonder at my grandparents who went from the horse and buggy to the interstate highway system, or my parents who went from the grossly underpowered, early biplanes of the post WW1 era to the Boeing 747.  My generation has now gone from the “Air Boy Senior” low frequency radio (powered by a pack of 9 volt telephone batteries) and the ultra modern VHF Narco “Superhomer” tube radio to Garmin 1000 cockpits.  When I took my Private Pilot flight test (1961), a new requirement for radio navigation had just been added to the test.  Because the Cessna 140’s that Northern Airway’s flight school was using did not have the modern VOR radios, we were taught to use the then still existing Burlington Low Frequency Range to satisfy this requirement.  (In those days, an actual FAA inspector still gave the Private Pilot Flight Test!)

Fast forward to today and even some of the new light sport aircraft are coming equipped with multiple TV sets which provide more navigation and information capability then I ever saw in any of the airliners that I flew!  What used to be deluxe, full panel instruments are now buried somewhere out of the way in case they are ever needed for “emergency standby” use!

So the question becomes, why GPS.  The first answer is obviously navigation flexibility.  Because the GPS signals are satellite based rather than ground based, local terrain does not cause “signal shadows” which are so common with the VOR navigation system.  Also because all the satellites operate on the same frequency, and at least 5 satellites are always in view, you can never go “out of range” as you eventually do with all ground based navigation systems. This navigation flexibility allows a direct routing to any point within the non-stop range of an aircraft, thus reducing trip time and subsequently trip cost.  The second reason becomes economic.  Not only does GPS navigation provide the potential for lower trip time and cost, it also lowers the cost for the U.S. DOT (Department of Transportation).  This is because the GPS system encompasses the entire earth’s surface; thus the need for multiple ground stations with their attendant purchase, site preparation and installation, and maintenance costs is eliminated.  Third is the reliability of the system which has always been above 95% usability.  The fourth is the reason the DOD developed the system -­­­­- accuracy!  VOR (Very High Frequency Omni Directional Range) was developed in the 1950s to solve the inherent inaccuracies in the then prevalent Low Frequency Range and Non Directional Beacon navigation systems.  However; the VOR navigation system   uses 1 degree angular courses which cause the accuracy to degrade over distance.  The VOR navigation error is 475 feet per mile, so the potential error at 10 miles is 4,750 feet and at 100 miles it is 47,500 feet.   The GPS navigation system provides a constant width course lane (instead of angular) meaning that the potential navigation error remains constant. This is a big advantage in and of itself, but the GPS accuracy is also much better.  The typical lateral navigation error with GPS is only 110 feet.  With WAAS (Wide Area Augmentation System) the typical navigation error is lowered to 10 feet!

GPS receivers come in five basic types.  They are as follows:

1.      Hand Held Portable Units.  These range from simple battery powered (stick it inside your pocket & go hiking) units, through portable car navigations systems, and up to the fancy Garmin GPSMAP 496 color aviation navigator with uplink XM weather, terrain warning, and  a pseudo instrument panel.  These are limited to VFR use only for reasons that we will cover later.  They do not require any approval to use in an aircraft, but if attached to the aircraft structure with a permanent mounting bracket,  the mounting bracket and any permanently installed external antenna must  be FAA approved under 14 CFR Part 43.

2.      Panel Mounted VFR Units.  These are sometimes less expensive, “dumbed down” versions of the earlier monochrome IFR units or just earlier units.  A popular panel mounted VFR unit is the Garmin 250XL which also includes a 720 Channel com radio.  These units do not drive external CDIs (Course Deviation Indicators) nor are they required to provide any warning for loss of navigational accuracy.  Sometimes VFR installations are simply an older IFR unit which the owner did not bother to IFR certify.  Typically they do not have a current data base.  Any time a panel mounted unit is limited to VFR use, the aircraft must display a prominent placard in the pilot’s direct field of view which states: “GPS navigation system limited to VFR use only.”

3.      Panel Mounted IFR Stand Alone Units.   These can range from some older basic monochrome navigation units such as the KLN90B or the Garmin 300 to some pretty fancy color boxes such as the Garmin 530 with integrated Nav/Com radios, Traffic Situation Display, Terrain Warning and XM weather.  These units drive a separate CDI and provide a warning to the pilot when navigational accuracy is degraded.  This is done by an internal monitoring system called RAIM (Receiver Autonomous Integrity Monitoring) which we will go into later.  These units provide certified lateral navigational accuracy for IFR enroute, terminal and (when loaded with a current data base) LNAV approach capability. Appropriate backup conventional navigational systems must be installed and operational when using these units under IFR.  Aircraft so equipped file a “G” suffix on their flight plans.

4.      Panel Mounted IFR ~ WAAS Certified ~ Stand Alone Units.  These are the newest of the “Top End” IFR boxes such as the Garmin (Global Navigation System) GNS-480 which is WAAS certified and contains integrated Nav/Com radios.  These units are considered “stand alone” navigators and do not require appropriate backup (conventional navigation systems) when using them under IFR.  They are capable of displaying the Traffic Situation, Terrain Warning and XM weather when hooked to the necessary input radios.  They drive an external CDI and Vertical Path (or Glide Slope) Indicator and have an internal FDE (Fault Detections and Exclusion) monitoring system.  This is a higher level internal monitoring system than the RAIM system used on Non-WAAS certified units. Because of this ability these units are allowed to use alternate airports which have only GPS approaches when they otherwise meet the alternate airport criteria.  These units can be used for LNAV, LNAV/VNAV (Baro-VNAV), and LPV (Localizer Precision with Vertical Guidance) approaches.  Aircraft so equipped file a “G” suffix on their flight plans.

5.      Remote Mounted Integrated Units.   These units are typically found on larger aircraft and provide GPS signals to the FMS (Flight Management System) which is an “on board” computer.  This computer accepts inputs from multiple navigation systems such as IRS (Inertial Reference System), VOR/DME and GPS, and then provides navigation output information to the instruments, Flight Director and Autopilot. These are always IFR units although they may or may not be WAAS certified.  Aircraft so equipped use an E (Single FMS) or F (Dual FMS) suffix when filing their flight Plans.

RAIM (Receiver Autonomous Integrity Monitoring) is also referred to as fault detection.  It is required on all IFR certified GPS units to alert the pilot to the fact that navigational accuracy has degraded.  This can occur either because there are not enough satellites being received to provide integrity monitoring or because the system has detected a potential error which exceeds the tolerance allowed for that phase of flight.  This may be due to either an insufficient number of satellites in view or because poor satellite geometry has caused the error in the position solution to become too large.  Although 4 satellites are sufficient to provide an accurate navigational fix, 5 satellites are required to detect an integrity anomaly and are thus the minimum number of satellites required for GPS IFR navigation.  The receiver needs the 5 satellites (or 4 satellites and a barometric altimeter setting ~ known as baro-aiding) to determine if a satellite is providing corrupted information.  RAIM is necessary for IFR operations because up to 2 hours can elapse before an erroneous satellite transmission can be detected and corrected by the satellite control segment.  Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position.  

FDE (Fault Detection and Exclusion) is an advanced RAIM system used on all WAAS certified receives and some advance non-WAAS units.  This system isolates a corrupt satellite signal and removes it from the navigation solution.  To do this the receiver needs to see 6 satellites (5 with baro-aiding).

Baro-aiding is a method of augmenting the GPS integrity solution by using a non-satellite input source.  To insure baro-aiding is available, a current altimeter setting must be entered into the GPS receiver.  GPS derived altitude information should not be relied upon to determine aircraft altitude as the vertical error can be quite large and no integrity monitoring is provided.  

Selective Availability was the intentional degradation of the GPS accuracy by the military to deny precise GPS positioning data to a potential enemy for hostile use.  This feature of the GPS was discontinued on May 1, 2000.

This looks like a convenient place to break for this month.  Next month we will continue with some navigations concepts and terms, as well as to start discussing the use of the various type GPS receivers.

The thought for this month is: “If at first you don’t succeed, try reading the instructions!”  So until next month, Think Right to FliRite!

November 2007                                                                                              Flying with GPS ~ II

This month we will continue our series on Flying with GPS by looking at the VFR use of GPS. 

As discussed last month, GPS navigation makes possible a direct routing to any point within the range of an aircraft.  It does not; however, relieve the pilot of responsibility to maintain adequate terrain separation as well as to be knowledgeable about the types of airspace through which the proposed direct routing will pass, especially TFRs.

VFR Flights may be undertaken using a fully IFR certified panel mounted receiver, a VFR-only panel mounted receiver (which may exist in an IFR certified aircraft), or a hand-held receiver.  VFR-only and hand-held GPS receivers are not authorized for IFR navigation, instrument approaches, or as a primary instrument flight reference, even if they are mounted in an IFR capable aircraft.  Their use during IFR operations is restricted to aiding “situational awareness” and all navigation during such IFR operations must be done by conventional (non-GPS) means!

The Limitations of the various types of GPS receivers should be clearly understood by the pilot to avoid navigational errors.  It is never wise to depend on any single, “sole source” for information in an aircraft, including navigation information.  VFR GPS navigation should be part of a navigation solution that includes VOR, pilotage and even dead (deduced) reckoning.

The Critical Concerns in the VFR use of GPS navigation are as follows:

• RAIM Capability (Receiver Autonomous Integrity Monitoring)
• Antenna Location
• Database Currency

RAIM Capability is not provided in hand-held GPS receivers, or in many panel mounted VFR- only GPS receivers.  Thus no warning would be provided to the pilot for the loss of the required number of satellites (5) in view or the development of a position error.  Because no alert would be provided to the pilot in these critical instances where the navigation solution has deteriorated, an undetected navigation error could occur. When using these types of GPS receivers, a systematic cross-check with other navigation types is required to identify this failure and prevent a serious navigation error from occurring!

Antenna Location is the next consideration.  While antenna location is carefully planned in IFR installations to insure a clear view of available satellites, this is rarely the case with panel mounted VFR-only installations.  With these antennas, location is usually more a matter of convenience than performance.  Because of this, some portion of the aircraft structure may block the antenna’s view of available satellites, which will cause a greater opportunity for the loss of navigation signal.

Antenna Blockage by aircraft structure is particularly a problem for hand-held receivers.  Typically hand-held receivers use a suction cup to fasten the GPS antenna on the inside of the aircraft’s windows.  Hand-held receivers provide great utility and are especially favored by rental pilots.  They also find frequent use in older, light aircraft because of the ease with which they may be installed.  Because antenna location is limited to only the cockpit (or cabin) windows, it is rarely optimized for providing a clear view of the available satellites.  Thus certain combinations of aircraft-satellite geometry can cause a loss of navigation signal.  Because no RAIM capability exists in these receivers, no warning will be provided to the pilot when erroneous position information causes a navigation error!

Database Currency is the third consideration.  Most GPS receivers use an up-dateable database to store navigation fixes, airports and instrument procedures.  IFR GPS navigation requires that GPS receiver database be kept current with the latest effective database revisions.  No such requirement exists for VFR GPS navigation!

The Database drives the moving map display which typically displays various types of airspace.  When the data base is not kept current, the moving map display may be providing erroneous information about the location of critical Special Use Airspace areas, such as Class B airspace or Restricted Areas.  Having bad information is worse than having none!  Many pilots have gotten into trouble for wandering unintentionally into class B airspace while flying too close to the edge with an outdated database.

Some GPS Navigation terms are as follows:

• RNP ~ Required Navigation Performance is the navigation accuracy required for that phase of flight expressed in distance from “on course.”
• ANP ~ Actual Navigation Position is the navigation accuracy being currently provided by the GPS receiver expressed in distance from “on course.”
• DTK ~ Desired Track is the magnetic course to the active waypoint.
• TRK ~ Track is the magnetic course currently being flown.
• BRG ~ Bearing is the compass direction from the aircraft present position to the active waypoint.
• XTK ~ Cross Track is the distance the aircraft is off the Desired Track to the right or left.
• DIS ~ Distance is the nautical miles from the aircraft present position (PP) to the active waypoint.

Always Check for RAIM Capability before using any GPS receiver for aviation navigation.  When you are using a receiver which does not have RAIM capability, be suspicious of the GPS position if it disagrees with the position derived from other navigation sources.

Always Check for Database Currency before using any GPS receiver for aviation navigation.  If the database is expired and cannot be updated, disregard the moving map display of airspace for critical navigation decisions.  The navigation waypoints which you are planning to use should be verified with current navigation charts to assure their accuracy. 

When using Hand-Held Receivers, be prepared for intermittent navigation signal loss.  As these sets typically do not possess RAIM capability, no warning will be provided.  Be sure to comply with 14 CFR Part 43 when mounting a hand-held receiver in the aircraft.  While hand-held receivers themselves require no approval, any modification to support the hand-held receiver (such as the installation of an external antenna or the installation of a permanent mounting bracket) does require approval. 

Enter the Desired Navigation Points before starting to taxi, not while moving!  Verify your planned route against a current sectional chart to insure that proper terrain clearance is maintained and that you are aware of any Special Use Airspace (SUA) which impacts your route of flight. 

Become Very Familiar with your Receiver’s Operation in order to minimize your “head-down” time while enroute.  As most receivers are not intuitive, you must take the time to learn the various displays, knob functions, and keystrokes used to operate the receiver.  Be sure to keep a sharp lookout for other aircraft and to minimize the time that you keep your eyes inside the cockpit while in-flight!  Use the computer based tutorials provided by some of the manufacturers to learn about your receivers operation.

Remember, No Standard of Accuracy or Integrity is Provided unless an IFR certified receiver is installed in accordance with IFR requirements.  It is still the pilot’s responsibility to navigate the aircraft, and GPS is just one of the tools available for that job. 

Lastly, Be aware of VFR Waypoints which are starting to appear on sectional charts.  They are established to provide navigation data for pilots unfamiliar with the area.  They also provide a waypoint definition of the existing reporting points, enhanced navigation in and around Class B and Class C airspace, and enhanced navigation around other SUA (Special Use Airspace) areas.  When operating under VFR, it is very important to depend on current aeronautical charts specifically published for visual navigation.  These include World Aeronautical Charts (WACs), Sectional Charts and - where appropriate - Terminal Area Charts.  The GPS use of VFR waypoints does not relieve the pilot of the responsibility to comply with all the airspace requirements of 14 CFR Part 91!  They also do not provide any protection against inadvertently penetrating a TFR (Temporary Flight Restriction) area.   

VFR Waypoint Names consist of five letters with the first two being “VP” and are retrievable from navigation databases.  Stand-alone VFR waypoint names are not intended to be pronounceable nor are they to be used in ATC communications.  They are depicted using the same four-point star symbol used for IFR waypoints.  When VFR waypoints are collocated with visual check points on the chart, they will be identified by the small magenta flag symbol.  When this is the case, their names will be pronounceable (based on the name of the visual check point) and they may be used for ATC communication.  The Latitude/longitude of all established VFR checkpoints is listed in the appropriate regional Airport/Facility guide.

When Filing VFR Flight Plans you may use the five letter identifier as a waypoint in the route section of the flight plan when there is a course change at that point or to describe the route of flight.  You must only use VFR waypoints when operating under visual conditions.  VFR waypoints shall not be used for IFR flights as they will not be recognized by the IFR computer system and will be rejected for IFR routing purposes.

Be Especially Vigilant for other aircraft when operating near VFR waypoints as was always the case with VOR (Very High Frequency Omni Range) stations.  The increased accuracy of GPS receivers and modern altimeters make “random misses” less likely than in the past.  In these instances it is good practice to turn on landing lights for visibility and to utilize ATC flight following services (or any other ATC services which may be available) for increased traffic awareness.

This looks like a good place to break for this month.  Next month we will continue with the IFR use of GPS navigation.   The thought for this month is Artificial Intelligence is no match for Natural Stupidity!  So until next month, remember to Think Right to FliRite!