Refining the usefulness and safety of satellite navigation

More accurate GPS flightpaths, curved RNP approaches, lower minimums—all coming soon to an airport near you.

By Bill Gunn
ATP/CFII. Citation, Sabreliner, Westwind

Honeywell's Smartpath precision landing system, approved by FAA in 2009, clears the way for increased safety and efficiency at airports by providing precise GPS-based navigation service.

Dominating air navigation today is the global navigation satellite system positioning system (GNSSPS)—otherwise known as the global positioning system (GPS). US GPS satellites provide navigation coverage worldwide.

The US wide area augmentation system (WAAS) provides coverage for much of the western hemisphere—currently the US, Canada and Mexico have or will have WAAS infrastructure. Using a mix of systems, long-range business aircraft enjoy reliable enroute, terminal and approach navigation worldwide.

Summing up the basics

Basic GPS uses a theory similar to centuries-old celestial navigation, which measures star angles to determine the long side of a right triangle. GPS, referencing satellites instead of stars, uses time.

At 186,282 mps, the speed of light is the constant for GPS positioning. If you measure the time of a radio frequency signal transmission and the time of its reception, you have distance.

Based on this distance and the known elevation and orbit of the GPS satellite, a right triangle is created. Visualize this scribing a circle in near-Earth atmosphere, add several satellites, and the common point of intersection is your position in the sky.

Timing must be precise to 1 billionth of a second—or 1 nanosecond (0.000000001 sec)—for highest accuracy. Most GPS navigators do not have clocks accurate to this level, so the timing demand is resolved in part by using several satellites to compare positioning.

Deriving 3D position fixes from 24-plus GPS satellites orbiting Earth (in 6 orbits), your moving aircraft, and the rotating Earth, is a demanding and complex task. One large issue is that Earth's ionosphere causes GPS signal timing delays that vary by area of the country.

Compounding the problem is the fact that there is no absolute value for time. As strange as this may seem, it has been proved conclusively that clocks operating under the influence of gravity and acceleration run at different rates.

The GPS satellites orbiting in curved orbits around the Earth, the difference of gravitational pull on your aircraft in the air and on the ground, the GPS satellites, and WAAS communication satellites, all make this business of accurate navigation very challenging.

The GPS signal direct from the satellite to your aircraft is called the coarse acquisition (C/A) code or signal. GPS satellites can drift slightly from their planned orbit—the timing and other issues already discussed mean that C/A GPS is accurate to 300 ft laterally and 460 ft vertically 95% of the time.

Accuracy here must be measured by not only how close the positioning is but the integrity, availability and continuity of the GPS C/A signal. Integrity refers to usability of the satellite signal—it means that the signal has not been corrupted.

Integrity is also the ability of a system to provide timely warnings in the cockpit when the system should not be used for navigation as a result of errors or failures in the system. Availability refers to the percentage of time that the signal is expected to be received and usable.

Continuity differs from availability in that it refers to the continuous reception of the signal. A signal could have high availability but suffer numerous short outages that cause disruption to your navigation solution.

Knowing when a signal is skewed is also a large part of the issue. At the inception of GPS for civil air navigation, this known error, combined with an intentional random error injected by the Dept of Defense—select availability—meant that GPS was adequate for enroute, terminal and nonprecision laterally guided approaches only.

Because the DoD suspended the select availability error signal in 2000, this is no longer a factor in GPS navigation.


  TBL 1-2-1
US Standard RNP levels
RNP Level Typical Application Primary route width (NM) - Centerline to boundary
0.1 to 1.0 RNP SAAAR Approach segments 0.1 to 1.0
0.3 to 1.0 RNP approach segments 0.3 to 1.0
1 Terminal and En Route 1.0
2 En Route 2.0

Table showing required accuracy for aircraft GPS nav systems and the precision to which the aircraft must be flown. "RNP" in the cockpit translates to CDI capture, while 0.3 RNP means the CDI will deflect fully from center in 0.3 nm.

GPS today is placed in the family of RNAV. This is important in that GPS may be augmented with other RNAV systems to present a navigation solution. Required navigation performance (RNP) is the means by which FAA sets the levels of accuracy, navigation solution integrity and availability, as well as its continuity and functionality for different navigation legs. In the US, RNP for different flight segments is shown in Table 1-2-1 in the AIM. (See above.)

SAAAR signifies "special aircrew and aircraft authorization required." Additional onboard equipment and aircrew training are necessary to fly these approaches. The norm for an instrument approach to no lower than 200 and 1/2 minima is ap­proach RNP 0.3.

Terminal RNP is 1.0, and enroute RNP is 1.0–2.0. SAAAR RNP approaches can require RNP accuracy as refined as 0.1 depending on the protected airspace footprint for the approach, obstruction clearances, runway environment and other factors.

Enroute, terminal and approach

"Enroute" is defined as more than 30 nm from the departure or arrival airport reference point (ARP). "Terminal" is within 30 nm of the departure point or destination ARP but outside the FAF and "approach" is from the FAF to the MAP.

Your IFR GPS navigator selects the correct RNP level of navigation accuracy automatically, based on your position relative to the departure or arrival airport. In the cockpit, RNP may be described as full deflection of the CDI. Center of the display to full deflection is 2.0, 1.0 and 0.3 nm, for enroute, terminal and ap­proach modes, respectively.


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