GPS Procedures

GPS Simulation -

GPS is not particularly well simulated in MS Flight Simulator. This seems odd since the sim knows precisely where you are at all times much as a GPS receiver would, but the emphasis to-date has been on ground-based Navaids, and GPS has some catching up to do. We are just starting to see some GPS gauges and products that are close to realistic. One example of this is a payware product from Reality-XP which simulates the Apollo GX-50. This company also has an excellent rendition of the Garmin 530, the premier IFR GPS/NavCom instrument of the day.
Reality-XP GL-50 GPS
Although even this product falls short (shown here as version 1.2) as it requires flight plans created as the standard FS2002 plans (using FS2002 or other planner such as FSNavigator rather than the GL50 gauge itself) and it lacks the ability to load SIDs, STARS and Approach Procedures. The FSAvionics GNS530 will include a full Jeppesen database with instrument procedures.

Prior to Reality-XP, all GPS gauges use the FS2002 GPS. Limitations here include requiring use of the default flight planner, which is cumbersome and unrealistic.

FS2002 GPS Showing Route Info Mode

You can't modify the flight plan while in flight (at least not without pausing the sim in mid-air!) and if you choose to fly direct to a waypoint, maybe one that exists later in your plan, it abandons the entire plan and replaces it with the single "direct-to" waypoint. No terminal procedures available here either. It does however couple to your CDI which is a requirement for IFR certification of the avionics.

Still the all-around best navigation tool is FSNavigator. You can consider it to be a GPS with a moving map. It doesn't simulate real-world avionics in looks, but functionally includes most of the things a GPS will do and it's sort of a cross between a GPS and a Flight Management Computer. Add the custom HSI that goes with FSNavigator and you have everything you need (this gauge is free, it also comes installed on the Altair Fokker 100). Features include a world map with VORs, NDBs, intersections, airways and airports. Build your flight plan by dragging fixes from the map. Fly your flight plan by coupling it to your auto-pilot. It is also helpful (and accurate) for planning when to begin descent from cruise to meet crossing restrictions. Check these screenshots.

GPS Common Functions -

Under VFR, GPS is often used as an aid to situational awareness. Much as a VOR receiver is not required for VFR flight, yet it is often used to aid in navigation nonetheless, so it is with GPS. Not required for VFR, it is very useful. As for IFR, its usage varies country-by-country. In the US, some GPS receivers are certified for IFR enroute and terminal operations, and others are certified for IFR approach procedures as well. The certifications relate to items that MS Flight Simulator has covered. This includes things like internal updateable database of fixes, fixes can't be manually altered, RAIM integrity alarms, turn anticipation, autosequencing of approach fixes. Other items Flight Simulator doesn't do so well on include approach arming within 30 miles of airport, approach annunciators, CDI scaling from 1 mile to .3 mile sensitivity, and single pilot action to go from present position to any waypoint. So GPS approaches remain seldom used and even less well understood and the ILS approach remains the favorite instrument approach. As GPS simulation improves, look for this to change. Some common terms that apply to GPS:

  • Track - Aircraft track over the ground. This is the direction the aircraft is going versus heading, which is the direction the nose of the aircraft is pointing.
  • Desired Track or DTK - Not to be confused with bearing, this is the course line to your next waypoint from previous waypoint.
  • Bearing - The direction to the next waypoint from present position.
  • Distance - Unlike DME which measures slant distance from the aircraft to the DME station, GPS measures ground distance. Crossing a VOR-DME station at 6,000 feet AGL, your DME would ready 1 NM. GPS would read zero.
  • OBS or Non-Sequential Mode - A GPS following a flight plan assumes you wish to sequence from one waypoint to the next one in the plan. This is always true except when flying a hold or procedure turn. In this case, you want to be able to turn this auto-sequencing off. In other words, don't remove the waypoint from the plan after fix passage as would normally be the case.

The Scale Problem
GPS is fundamentally different from ground-based navaids when it comes to crosstrack error as shown by the Course Deviation Indicator. With navaids such as a VOR, radials emanate from the station in 360 degrees. The distance represented by one dot of deflection, or full scale deflection for that matter, is a function of distance from the station. One dot of deflection is about 2.5 degrees of error. A little math gives us a one dot deflection being one-half mile of cross-track error at 15 miles from the station, but only about 1,000 feet 5 miles from the station, 212 feet at 1 mile away. Cross-track error (in miles) equals the distance from the station multipled by the tangent of the radians off course (convert degrees to radians).

ILS signals are even more sensitive, with one dot of deflection equating to about 500 feet at the outer marker and 150 feet at the inner marker.

GPS has no such notion of angular distance, by its nature instead giving crosstrack error in terms of horizontal distance abeam the desired track. A GPS receiver with a display or electrical output that drives a CDI will indicate 5 miles of crosstrack error for a full-scale needle deflection, the required sensitivity for enroute operations. This is roughly equivalent to a VOR full scale deflection at 28 miles from the station.

Moving to the terminal area (within 30 miles of the airport) required sensitivity drops to 1 mile for full scale deflection, and for approach mode (within the Final Approach Fix) that sensitive must become .3 miles for the receiver to be IFR certified. This sensitivity is called Required Navigation Performance or RNP. A sensitivity of 0.3 miles is called RNP 0.3.

Unfortunately, in FS2002, the default is fixed at 5 mile sensitivity, so it is not accurate enough for terminal and approach operations, and without add-on products, we won't be shooting any GPS or RNAV procedures any time soon, at least not legally!

So what can you do with the GPS? You can use it to aid in situational awareness when flying VFR. You can use it for IFR enroute navigation, including off-airway navigation. You can use it freely as a DME substitute and as a substitute for ADF on ADF-required approaches. DME substitution also applies to precision approaches. If you have the Reality-XP GL50 or other GPS capable of RNP 0.3, then and only then may you fly a GPS or RNAV approach under IFR.

GPS Approaches -

There are two kinds of GPS approaches, both of which are non-precision at this time. An "overlay" procedure refers to the GPS which is overlaid on top of an existing non-precision procedure. For example, VOR or GPS-B at KCNO. This simply means that if you have an IFR-certified GPS, you may use it to navigate the approach in lieu of the VOR. It is not necessary to have operational VOR onboard to fly this approach. Waypoints on this approach will be ground navaid-based.

GPS only approaches may only be used if you have an IFR-certified GPS receiver onboard. These will have waypoints which are not based on ground navaids like the VOR. An example is the GPS RWY 8 at KBUR. Once exclusively called GPS approaches, these are now being called RNAV approaches, meaning they can be flown provided you have avionics capable of achieving the required navigation performance (RNP), not just limited to GPS such as multi-sensor FMS. GPS only approaches add IFR capability to those airports without other NAVAIDs, and allow for more flexibility in design to achieve obstruction clearance since a GPS waypoint may be placed anywhere. Waypoints on this approach will often be RNAV style and not be based on ground-based navaids whatsoever. You must have these waypoints in your GPS database.

Both of these are flown much like you would fly a VOR approach, except using your GPS coupled to the CDI instead of a VOR receiver. Vertical navigation is not precision and minimum altitudes apply to waypoints much as in a VOR approach.



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