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Why Not WAAS?


In the context of car GPS devices, frequently people will ask me about WAAS. If WAAS improves the accuracy of GPS, isn’t it good for car GPS devices? Well, maybe… but probably not. It is time for a discussion about why WAAS was developed, what WAAS is good for and what it does, and when you should look for it in a GPS device.

FAA, Airplanes

WAAS (Wide Area Augmentation System) was designed primarily for aircraft in mind as a project of the FAA. In most all definitions of WAAS you may come across, nearly all of them describe it as an “air navigation aid”. That certainly doesn’t mean it doesn’t provide benefits elsewhere, but keep in mind that the design goal was for aircraft.

What Does WAAS do?

(As the Mythbusters would say, “Warning- Science Content”.) Here is the simplest way I know to describe it. The GPS satellites are zipping around at around 14,000 km/hr and a bit over 20,000 kilometers away. They are not always exactly where they should be, and changes in the earth’s atmosphere can impact the apparent distance. Using dozens of locations on the ground, we can calculate any drift error of the satellites. Knowing the amount of error, we can send correction information to GPS devices so they can account for the calculated error. So we can tell the GPS “hey, this satellite thinks it is here, but it really isn’t, use this location instead”. I’m oversimplifying things here quite a bit– but you will get the gist of how it works.

So this brings me to my first point– if everything was working exactly as expected in a “perfect world” scenario, there would be no need for WAAS. The satellites would be where they are supposed to be, the atmosphere would be stable, and things would be good. To say it another way, WAAS doesn’t increase the accuracy of a perfectly running GPS system, it corrects for deficiencies.

Accuracy with WAAS

First, let’s not confuse GPS accuracy with map accuracy. The accuracy we are talking about here is the accuracy of the GPS at calculating your coordinates. A map might have a road or stream mapped slightly incorrectly– but that isn’t relevant (yet) to this discussion. While specs differ slightly, most figures put regular old GPS at being accurate to within 10 meters, 95% of the time. With WAAS that confidence will improve to within 5 meters, 95% of the time.

While not technically correct, for practical purposes I tend to think of WAAS like this… WAAS doesn’t improve the accuracy of your GPS; instead it improves the confidence that you really are where the GPS estimates you are. Again– that isn’t technically true, but for civilian non-airplane applications I believe it is a better way to look at it.

Battery Life

WAAS can have a major impact on battery life. Why? Because the data being sent down to your device from the WAAS satellites contains quite a bit of information that your GPS will spend a long time listening to. Without WAAS, the radio receiver in your GPS can take micro-second naps when the signal is good and stable to conserve battery power without impacting the user experience. When you add in the large amount of data coming down from the WAAS satellites, those cat naps are just a wish as your GPS sucks down the battery hoping they’ve got the energy of a double shot espresso.

Measuring it out

Okay, so let’s look at some real-world values. Say your GPS without WAAS is giving you coordinates that are accurate to within 8 meters. (Actual accuracy, not the silly meaningless “estimated accuracy” field from your GPS.) Now let’s say you turn on WAAS, wait for the data download, and you can now achieve 4 meter accuracy. You’ve improved the accuracy by four meters (13ft). A four meter improvement isn’t that much unless you are flying an airplane towards a runway in bad weather.

At highway speeds four meters goes by in 0.14 seconds. Even on a city street, those four meters will go by in 0.35 seconds. (In case you are wondering, most sources cite the blink of an eye as 0.4 seconds… so those four meters go by faster than the blink of an eye.) Here is another way to look at it. At the map scale and screen resolution of most auto GPS devices, four meters covers about 1.3 pixels on the screen.

Final Fix

But none of that matters much anyway, because the underlying map won’t be nearly that accurate anyway. So for cars, I suggest that WAAS is completely irrelevant and won’t make any difference in your driving experience, sans reduced battery life.

If on the other hand you are out geocaching or trying to do rudimentary survey work, four meters might make a difference. However even here, good technique of averaging waypoints using different times of day might improve accuracy as much as WAAS does. If you’re in an airplane, you obviously will require WAAS.

3 Responses

  1. Good summary Tim.

    In addition most in-car GPS’s have a lock on road feature enabled. I’m certain that the snap to road nature of this feature completely negates any benefit of having WAAS enabled.

    Scott - February 5th, 2010
    • Good point, Scott. Snap-to-road is certainly changing the apparent position as much or more than WAAS.

      Tim - February 5th, 2010
  2. Hi Tim,

    Thanks for the informative (with actual figures) article. We plan to develop a mixed-reality (cyber-physical) game based on GPS location of the cellphone at our university and need very accurate GPS location. How heavy would it be for a cellphone to receive WAAS data. The maximum speed at which the cellphone will move would not be more than 5 meters/sec.

    Thanks and Regards,

    Abhijeet - January 25th, 2012

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