I've noticed a lack of subject matter on the loss of SBAS as compared to RAIM.

In the ACS there is mention of RNP, SBAS (WAAS), and RAIM, which are all related, but the available content for RAIM and WAAS is much heavier on losses of RAIM than losses of WAAS.

This really became apparent to me when I was searching Garmin G1000 documentation to find annunciations regarding the loss of SBAS, and I noticed that there essentially isn't one other than "APR DWNGRADE" which loosely translates in the manual to, "you lost WAAS capability".

However, the loss of RAIM has a much more clear and direct annunciation, "RAIM UNAVAIL": that needs no translation and is immediately clear that you've lost RAIM.

Why would Garmin not have an "SBAS UNAVAIL" or "WAAS UNAVAIL" annunciation in the same manner as they do for RAIM? It's almost as if the industry as a whole largely discounts the possibility of SBAS failures.

Here's a list of G1000 NAVIII Annunciations

Here is where WAAS is mentioned in the annunciation list. Note that the column on the left is the annunciation displayed on the flight deck, but the column on the right is the manual's referenceable extrapolation:

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Notice above that neither of the flight-deck-displayed texts on the left which have extended explanations on the right that include "WAAS" also contain "WAAS" or "SBAS" in their in-flight annunciation messages' texts.

Here is where RAIM is mentioned in the annunciation list:

enter image description here

Notice above that the flight-deck-displayed text does contain "RAIM" (twice) for a RAIM loss.

I included the above information to demonstrate my point, and I essentially have three closely-related questions:

Why is Garmin so indirect about the loss of SBAS in their annunciations?

Why do pilots not also perform SBAS predictions like they do for RAIM?

Why is the loss of RAIM given so much more emphasis in training than the loss of SBAS?

  • 2
    $\begingroup$ Note that LOI is also a loss of RAIM, but it does not mention RAIM explicitly. RAIM UNAVAIL means that RAIM is still available, but does not offer the level of protection needed for the final approach segment. So much for clear interface design... $\endgroup$ – DeltaLima Dec 2 '20 at 9:36
  • $\begingroup$ Your question doesn't define any of the acronyms it uses. Fortunately (since this question made it into Hot Network Questions), the answers expand them and give enough context to understand the title question for people that know anything about GPS. Still, the question body is mostly opaque to anyone curious what this is about. $\endgroup$ – Peter Cordes Dec 3 '20 at 13:29
  • $\begingroup$ Acronyms are addressed. aviation.meta.stackexchange.com/questions/343/… $\endgroup$ – Ryan Mortensen Dec 3 '20 at 21:57
  • $\begingroup$ FAQ for acronyms: aviation.meta.stackexchange.com/questions/347/aviation-acronyms $\endgroup$ – Ryan Mortensen Dec 3 '20 at 21:57

From a safety perspective RAIM is more critical than SBAS.

If you are flying using GPS, you always rely on RAIM to keep you safe, sometimes you need SBAS too.

Receiver Autonomous Integrity Monitor is a function within an aviation grade GPS receiver that monitors the integrity of the GPS system. Without RAIM, a malfunction in the GPS system can cause large position errors that go unnoticed. These position errors can lead to nuisances such as controlled flight into terrain that really ruin the day.

SBAS on the other hand is (merely) an augmentation to GPS. It improves the vertical accuracy by a few feet, improves the RAIM function so that it is quicker to detect position errors and can detect smaller position errors. But basically, GPS without SBAS is safely usable for the vast majority of applications.

So, if you loose SBAS, you may not be able to perform certain approaches or use certain routes, but RAIM is still there guarding the system integrity.

But if you loose RAIM, the GPS position shown may be correct, or it may be 5 miles off, or 10 miles, or more; it's anybody's guess. The GPS receiver simply has no way to determine whether the calculated position is correct if RAIM is not available.

Now to your questions: 1) Why is Garmin so indirect about the loss of SBAS in their annunciations?

I can't speak for Garmin as to why they emphasize RAIM more than SBAS in their annunciations.

In case of case of the SBAS loss, the annunciation focuses on the operational effect (Approach Downgrade). In case of the RAIM loss, the annunciation focuses on the cause / technical description (RAIM UNAVAIL and LOI).

My guess is that SBAS was added later to their product line as RAIM, and their interface design philosophy had changed from focusing on the technical cause to focusing on the operational effect. For system commonality reasons they did not update their pre-existing RAIM annunciations when adding SBAS.

2) Why do pilots not also perform SBAS predictions like they do for RAIM?

SBAS depends mainly on the availability of a healthy SBAS satellite. The SBAS satellites are predicted to be healthy, and they cover a fixed area of airspace (they are geostationary satellites). The availability status of SBAS in not dynamic; SBAS is predicted to be available.

RAIM depends mainly on the number of GPS satellites visible from the GPS receiver, and how the GPS satellites are distributed over the sky (from the GPS receiver's viewpoint). This is very dynamic, depends on the total number of GPS satellites in orbit, their health status, their distribution in orbit, the position of the GPS receiver etc. etc. For RAIM to work, the GPS receiver needs at least GPS 5 satellites to be visible (a few degrees above the horizon) and they need to be nicely distributed over the sky. So for RAIM, the situation is very dynamic, but it is predictable. That is why RAIM prediction is done.

3) Why is the loss of RAIM given so much more emphasis in training than the loss of SBAS?

Because, if not properly handled, the loss of RAIM has a much higher probability of killing you than the loss of SBAS.


The RAIM (Receiver autonomous integrity monitoring) is what allows you to fly using a GPS. If you lose RAIM, by regulations you cannot use the GPS to navigate. So, it should not be a surprise for you to have regulators speak more of RAIM than SBAS.

The RAIM basically monitors the accuracy of the navigation. It requires at least five satellites for operation. What it does is it drops one of the satellites at a time and uses the remaining four to calculate the position of the aircraft. This calculated position is then compared to the dropped satellite's measured distance. If this comparison shows an error the GPS or the flight management system warns the pilot that it can no longer accurately give out the position of the aircraft. The RAIM essentially compares the error to the RNP (Required Navigation performance) value. To give you an example, the typical enroute RNP is 2.0. If the error is greater than 2.0 the pilot is alerted. On approach for instance the RNP 0.30. If the error is more than 0.30, the pilot is again alerted and the aircraft can no longer perform the approach. Long story short, RAIM is essential to ensure an accurate GPS navigation.

enter image description here

How Airbus aircraft show loss of GPS accuracy to pilots (Source: https://safetyfirst.airbus.com/gnss-interference/).

The SBAS (Satellite based augmentation system) on the other hand is not something within the aircraft. The SBAS requires ground stations, which measures errors in the GPS signals and provides corrections. This gives a very accurate navigational performance. However, SBAS is not required for most of the GPS based approaches. There is only one type of approach that requires it and that is LPV approach. In an LPV approach you a have both a lateral and vertical guidance generated by the GPS. And it allows you to do approaches up to a traditional ILS category I minima. Normally, you will come across GPS approaches with LNAV and LNAV/ VNAV approaches. In LNAV approaches, only the lateral guidance is given by the GPS, with the vertical guidance expected to be managed by the pilot manually. So, it naturally has a higher minima. In an LNAV/ VNAV approach, still the lateral is managed by the GPS. However, the vertical is now performed by the baro-VNAV. It uses the aircraft altimeter to compute the vertical profile of the approach. This allows LNAV/ VNAV minima to be lower than LNAV only minima. These types of approaches do not require an SBAS. But it requires RAIM. And one last thing. WAAS is purely a US based system and most countries do not receive WAAS signal. There are SBAS stations from many other countries but they are not as good or as widely available as WAAS. So, outside of the States you will rarely find LPV approaches. So, SBAS is not really something of a concern for most airline operators and pilots.

To summarise, SBAS is only required if you want to perform LPV approaches. In all other GPS approaches and other GPS navigation you do not need to have a signal from SBAS. In contrast, RAIM is something within the aircraft systems and it monitors the integrity of the GPS signals. It is important because without such a monitoring system you cannot guarantee your position on the earth. And knowing your position is necessary if you are a pilot flying an aircraft.

I will also answer your questions one by one:

1. Why is Garmin so indirect about the loss of SBAS in their annunciations?

I think Garmin is very direct. It says approach APR DWNGRADE for loss of WAAS. And that is the consequence of losing WAAS. You can no longer perform LPV approaches. Hence, there is an approach downgrade. And if your aircraft does not have an approved baro-VNAV, your minima is that of LNAV only minima. Loss of SBAS has no other real negative affect on your GPS navigational performance.

2. Why do pilots not also perform SBAS predictions like they do for RAIM?

RAIM predictions or monitoring is crucial for navigating using GPS. On the contrary, SBAS availability is not as important as RAIM because it is purely necessary for only one type of GPS approach (LPV).

3. Why is the loss of RAIM given so much more emphasis in training than the loss of SBAS?

The same answer as that for the 2nd question.

  • 1
    $\begingroup$ "There are SBAS stations from many other countries but they are not as good or as widely available as WAAS"? EGNOS (European SBAS) is considered to have slightly better availability and similar accuracy. $\endgroup$ – Zeus Dec 3 '20 at 23:52

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