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

I want to walk you through the accuracy and integrity side of GNSS now. We've covered how the system works, but the real question for a professional pilot is: how good is it, and how do we know when it's lying to us? Let's start with the ICAO accuracy specification. The standard for the Standard Positioning Service, the SPS, is stated at a 95% confidence level. That means 95% of the time, your position will be within these tolerances. Horizontally, that's ±13 metres. Vertically, it's ±22 metres. And for time, the specification is 40 nanoseconds. Now, a nanosecond is 10 to the power of minus 9 seconds — that's one billionth of a second. So the system is extremely precise on time, but you can see the vertical accuracy is noticeably worse than horizontal. That's a fundamental characteristic of satellite geometry, and it's why we treat vertical guidance with extra care. Now, accuracy is one thing, but integrity is the safety-critical part. 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, you need a warning within 2 seconds. For non-precision systems, you get 8 seconds. Here's the problem with GNSS: when you're using 4 satellites to compute a 3D position, you have no way to detect if the data from any one of those satellites has degraded. If a satellite starts broadcasting bad data, you could be looking at errors of hundreds of miles, and you'd have no idea — unless you cross-check the GNSS position against another system. That's why differential systems are being developed. Their job is to detect any degradation in accuracy and give you a timely warning of failure or degradation in the information being provided. Let me make that concrete with Differential GPS, or DGPS. The core problem is that a GPS receiver has no internal means of detecting when satellite data degrades. DGPS is the solution: it improves accuracy by monitoring the integrity of the satellite data and warning you of any errors. It does two things — it provides a warning of failure in the satellite data and prevents or minimises the effect of such errors, or it provides that failure warning and improves the accuracy of the deduced position. There are three kinds of DGPS currently in use or under development, and I want you to know all three acronyms cold. Air Based Augmentation Systems, or ABAS. Ground Based Augmentation Systems, GBAS. And Satellite Based Augmentation Systems, SBAS. Let's look at ABAS first, because that's the one that works inside the receiver itself. To determine at the receiver whether any satellite's data is in error, you need a fifth satellite. Here's the clever part: by comparing positions generated by the different combinations of those five satellites, you can detect errors in the data, and hence identify which satellite is the rogue one. That rogue satellite can then be deselected. But there's a catch — once you're back down to 4 satellites, you've lost that monitoring facility. That's why the CAA recommends a minimum of 6 satellites be available. That way, if one satellite is deselected, you still have enough left to continue integrity monitoring. The GPS term for this whole process is Receiver Autonomous Integrity Monitoring, or RAIM. Now, RAIM has only limited availability at present. To achieve continuous global availability, you'd need at least 30 operational satellites. And note this limitation carefully: RAIM will only provide failure warning and either prevent or minimise errors in the computed position arising from erroneous satellite data. It does not improve accuracy — it only protects you from bad data. Now let's move to GBAS, the Ground Based Augmentation System. This is a local area DGPS, often abbreviated as LADGPS, implemented through something called a Local Area Augmentation System, or LAAS. In aviation, GBAS does two jobs: it provides failure warning, and it enhances the GPS receiver position. How does it enhance it? By removing ephemeris errors and satellite clock errors, and by minimising ionospheric and tropospheric errors. But here's the critical limitation — it will not remove errors arising from receiver noise and multipath reception, because those errors are particular to the receiver itself. They're local to you, not to the satellite or the signal path in the sky. And GBAS is specifically established for one purpose: to provide precision runway approaches. That's its design goal. So let me tie this together. You have three layers of augmentation. ABAS works inside your receiver with RAIM, using extra satellites to detect and deselect bad data — it's a warning system, not an accuracy enhancer. GBAS works from the ground, removing the satellite and atmospheric errors to give you precision approach capability, but it can't fix your own receiver's noise or multipath. And SBAS, which we'll get to, works through satellites. Each one has its own role, its own limitations, and its own failure warning characteristics. That's the integrity picture for GNSS.