Last month I spent time examining some of the logistic difficulties connected to the incorporation of IFR GPS equipment into the general aviation (GA) fleet.
This time, it's only fair to look at the positive features offered by the system. To do that, just as with any other nav aid, the first undertaking must be to gain a thorough understanding of operational principles.
For some, much of this will be old hat, but since the goal is to use the system for guidance in non-visual conditions, it is best to assure full understanding of strengths and weaknesses in advance of use. Much of what follows comes straight from the AIM, the last word on aerial nav aids.
GLOBAL POSITIONING SYSTEM
GPS is a satellite based navigation system that provides accurate position and velocity information to an unlimited number of users over a world-wide grid, unaffected by weather.
Wow. It's no wonder folks got excited. That simple sentence amounts to a statement that all the really thorny problems connected with electronic navigation have been solved: system inaccuracy, weather interference, limited range, trans-oceanic coverage, and international consistency.
BEFORE GPS…
NDB/ADF: In the earliest days of electronic navigation, the already vague accuracy of low frequency nav aids was further compromised by lightning and precipitation static, turning severe weather into a major barrier to reliable signal reception (somewhat ironic, in that most nav aids are developed with the intent of neutralizing the effects of weather). GPS solves this.
VOR/DME: Most weather related electronic problems vanished with the adoption of the VHF and UHF spectra in the 1950s and 60s; at the same time, accuracy jumped ahead tenfold. These advances, however, were accompanied by severe limitations imposed by line-of-sight restrictions: effective signal range in many cases dropped as low as 40 miles. This characteristic made it necessary to carpet the nation with hundreds of VOR stations to assure unbroken coast-to-coast coverage. When one adds in the fact that there are only 160 available VOR frequencies to play with, it is easy to appreciate the difficulties and expense faced by the FAA as the system expanded. GPS solves this.
Restrictions to range also means that VOR navigation coverage is completely unavailable on trans-oceanic flights. GPS solves this.
World-wide restrictions: The expense and complexity of developing VOR networks have had the effect of limiting full implementation to only the most affluent nations--and even there, difficulties of physical access in some locations make it impossible to establish uninterrupted webs. GPS solves this.
After a brief flirtation with LORAN, we come to the1990s and GPS. Weather problems? Negligible. Restrictions to range? Laughable. Expense? Already paid for (well, sort of, governments being what they are).
THE MECHANISM
There is a web of 24 GPS satellites that orbit the planet at a height of approximately 11,000 miles. Their physical location is precisely monitored at all times by a ground-based net managed by the Department of Defense. The system is designed so that a minimum of five satellites are always visible to a user anywhere on earth.
Clock accuracy is one of two critical components of GPS, and the satellites are a virtually flawless time-keeping system, each containing a remarkably precise atomic clock. The second critical component of the system is the almanac of precise satellite position ("ephemeris") contained in each orbiter. This almanac is updated as necessary when errors from predicted position are noted by the ground based monitoring system.
Each satellite broadcasts a specific code that contains its ephemeris and the precise time of the moment of transmission. When a GPS receiver reads this code from a satellite (A), it compares the time of transmission to the time of reception and makes a basic distance calculation based on the travel speed of the radio waves: approximately 186,000 miles per second. Thus, if the elapsed time for transmission was .06 seconds, the satellite to airplane distance would be a little over 11,000 miles.
Armed with this information and the transmitted ephemeris, the GPS receiver can make a basic assumption of position: it is located somewhere on the surface of a bubble, or a sphere, centered around satellite A with a radius of 11,000 miles.
THE SECOND AND THIRD SATELLITES
A second calculation made with a second satellite (B) produces a second sphere. The two spheres intercept like two touching soap bubbles, and the result of that intersection is a circle. At every point on that circle, the receiver would be at the correct computed distance from both satellites.
With location narrowed down from the huge three-dimensional surface area of the original sphere to the perimeter of a two-dimensional circle, the receiver is well on its way to determining position. A third satellite (C) almost completes the job: although it may be a bit difficult to visualize, the C bubble cuts the AB circle at only two locations,
one of which is somewhere deep in space and logically improbable. The receiver has now located itself.
THE FOURTH SATELLITE
Or, it has until you realize that the estimated size of each bubble and the accuracy of all the resulting computations depend on the reliability of the receiver's internal clock--a timepiece probably closer to Mickey than Atom. This is a serious source of potential error: a timing discrepancy of .001 second translates to an almost 200 mile discrepancy.
In order to delete receiver clock error, GPS incorporates a very clever piece of logic. The process requires a fourth satellite (D). When the receiver computes position from a new trio of orbiters, say ABD, clock error guarantees the result will be different from the ABC position. And, again, when the BCD position is computed, that one ends up differing from the first two. Finally, ACD produces a fourth result.
Since the difference in computed position among the four comes from an internal timing error, the receiver next needs to find what adjustment in time is needed to bring all four into line. Once that constant is found, the internal clock is reset and the receiver proceeds with horizontal accuracy of 100 meters or less. Brilliant.
ACCURACY
At this point, GPS results are generally good enough for VFR operations. However, the increased accuracy required for non-visual flight adds several layers of complexity.
One set of errors that must be handled is created when the radio signals beamed by the satellites pass through the atmosphere. Because these errors are predictable, they can be largely filtered out.
A second error set is created when signals bounce off terrain and structures, creating "ghosts" for the receiver. Again, sophisticated electronic filtering works to reduce the problem.
Finally, and this one has no obvious solution, there are potential errors of geometry that crop up when the intersecting angle between two satellites is too flat (at least one satellite too low on the horizon) or too sharp (satellites too high). Because of the orbital pattern built into the entire system, this problem becomes unmanageable as aircraft approach either of the poles.
In general, at more moderate latitudes, geometric errors can still produce unreliable position computations even when it seems certain that enough satellites are in view to provide accurate reference. IFR receivers must take these potential errors into consideration when determining if present or future accuracy is adequate.
RAIM
In order to deal with all potential problems of GPS accuracy, IFR receivers must be capable of a continuous accuracy check, a process somewhat clumsily called Receiver Autonomous Integrity Monitoring (RAIM). RAIM must be able to judge current and future reliability and must be additionally accompanied by an alert system to ensure pilots are made aware of system inaccuracies.
The first issue handled by RAIM is simple redundancy. While four valid satellites are good enough for accurate positioning, five are required for IFR operations. A barometric altimeter can substitute for the fifth, but it is important to note that accuracy depends on entering the current altimeter setting into the receiver.
Geometric errors also enter into the RAIM computation: the requisite number of satellites must be high enough above the horizon to ensure accuracy for the predicted duration of the flight. Many GPS receivers have the capability of receiving six and removing corrupt signals from the navigation solution.
RAIM WARNINGS
IFR GPS use has progressed to the point that it is no longer necessary to monitor alternate nav sources when using GPS, provided the receiver uses RAIM and provides appropriate warnings. In general, the AIM advises there are two types of RAIM warnings. The first alerts the pilot that insufficient satellites are available. The second warns of less specific accuracy problems that "exceed the limit." GPS based IFR operations must cease once a RAIM problem surfaces.
Yep. You heard right: GPS is not approved as a stand-alone IFR nav system. Unless your aircraft is equipped with at least one other approved IFR navigational aid appropriate to the planned flight, you cannot use GPS to fly IFR.
Wait a minute! I thought VOR was dead, long live GPS….
Nope. Not true. You cannot file GPS unless you have the capability of continuing at any point by more prosaic means. Among other things, this means you cannot file a GPS alternate unless that airport also offers a conventional approach you are equipped to use.
There are even more surprises in store. Tune in next time…
NEXT
