Global Navigation Satellite Systems (GNSS) — Page 324, Lesson 319

Let’s start with the accuracy standard, because that sets the bar for everything else in GNSS. The ICAO specification for the Standard Positioning Service—that’s the basic, un-augmented GPS signal—requires an accuracy at the 95% confidence level. That means 95 out of 100 position fixes must fall within these limits. Horizontally, that’s ±13 metres. Vertically, it’s ±22 metres. And for time, the requirement is 40 nanoseconds. A nanosecond is 10⁻⁹ of a second—one billionth of a second. So the system must time-stamp within 40 billionths of a second. Now, accuracy is one thing, but integrity is the real safety issue. ICAO has a specification for how quickly a radio navigation system must warn you if it fails. For precision systems—like the Instrument Landing System, ILS—you need a warning within 2 seconds. For non-precision systems, you get 8 seconds. Here’s the problem: when you’re using four satellites to compute a 3D position, the receiver has no way to detect that any one satellite’s data has degraded. If a satellite starts broadcasting bad data, you could be flying with errors of hundreds of miles and never know it—unless you cross-check the GNSS position against another system. That’s why differential systems are being developed: they detect the degradation and give you a timely warning. Let me define Differential GPS, or DGPS. The core problem is that a standalone GPS receiver cannot tell if satellite data has degraded, and that can seriously endanger the safety of flight. DGPS improves accuracy by monitoring the integrity of the satellite data and warning the user of any errors. It does two things: it either prevents or minimises the effect of errors, or it provides a failure warning and improves the accuracy of the deduced position. There are three kinds of DGPS in use or under development: Air Based Augmentation Systems—ABAS; Ground Based Augmentation Systems—GBAS; and Satellite Based Augmentation Systems—SBAS. Let’s take ABAS first. To determine at the receiver whether any satellite’s data is in error, you need a fifth satellite. With five satellites, you can compare positions generated by different combinations of them. That lets you detect that an error exists and identify which satellite is the rogue one. You can then deselect that satellite. But here’s the catch: once you’re back down to four satellites, you’ve lost the monitoring facility. The CAA recommends a minimum of six satellites be available, so that if one is deselected, integrity monitoring continues. The GPS term for this is Receiver Autonomous Integrity Monitoring—RAIM. RAIM currently has only limited availability; it would need at least 30 operational satellites to achieve continuous global availability. And note this limitation: RAIM only provides failure warning and either prevents or minimises errors in the computed position. It does not improve accuracy beyond that. Now GBAS. This is a local area DGPS—LADGPS—implemented through a Local Area Augmentation System, or LAAS. In aviation, GBAS provides both failure warning and enhancement of the GPS receiver position. It does this by removing ephemeris errors and satellite clock errors, and by minimising ionospheric and tropospheric errors. But it will not remove errors from receiver noise or multipath reception, because those errors are particular to the receiver itself. GBAS is specifically established to provide precision runway approaches. So the key contrast: ABAS uses extra satellites at the receiver to detect a bad satellite. GBAS uses a ground station to correct the signal and is built for precision approaches. SBAS we’ll cover when we get to it. Those figures show the PDOP geometry and the LAAS ground system—worth a look on screen as we go.