blueflyer

blueflyer

Well Known Member
Im not sure how to clearly ask this question, but here goes....

I am curious about the GPS information a TSO'ed GPS unit receives (and ultimately displays) as compared to say a garmin 396.

Or put another way, is say a Garmin 155XL displaying more accurate information to my CDI than say a Garmin 396 would display on a CDI?

Clear as mud??
 
The TSO is more about reliability that accuracy I believe. The non-waas TSOd units meet standards regarding their ability to predict the integrity of their position calculation, called RAIM. WAAS verify their data differently. A non-tso unit may not be able to tell you when it's position calculation may be off.

Practically speaking, when a non-waas, non-tso unit is receiving adequate data, the navigation data would not be off enough horizontally to be noticeable I wouldn't think. Vertical data may be a good bit more precise with waas however, leading to reduced VNAV Minimums.

Chris
 
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To further clarify RAIM, the IFR gps will tell you when the signal is not reliable. A handheld unit will not.
When you need it the quality of your position makes a big difference
 
Good info about the reliability. Lets leave the reliability out of this question though. For this discussion, lets assume both the 155XL and the Garmin 396 are both receiving a good strong signal. I am more asking about the horizontal accuracy between the 155 box and the 396 box when displayed on the CDI.
 
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GPS.gov states that the "GPS signal in space will provide a "worst case" pseudorange accuracy of 7.8 meters at a 95% confidence level."

I think the main issue for us would be that 95% isn't good enough when you're in the clouds on final with a 600' ceiling. Generally, that 8m would not be noticeable on a CDI. Even on final approach with a full scale range of .3nm, that's over 100m per dot on the scale.

The only other variable is the gps hardware and also how it does the math, but these days even a cheapie gps chip can give you accuracy below 3m most of the time. My ipad tells me exactly when I cross onto an active runway. So I would bet that the position information from both units you mention would be very close to the same, MOST of the time. It's the fact that one may not be able to know when it ISNT right, that keeps it from being certified.
 
Two different things here.
WAAS = wide area augmentation system, it sends out corrections for ionispheric propagation delays, and other stuff, which makes the gps position computation more accurate. A WAAS receiver will nearly always be more accurate than a non-WAAS one.
Now, comparing TSO approved units to non-TSO ones (but both the same, either both WAAS, or both non WAAS), most likely the more accurate one will be the newer one. When a new chip comes along that improves things, the non TSO units are upgraded. But upgrading TSO umits is way too expensive(due to all the approvals needed) so you end up with, say, a 430W which is a bit less accurate than the newest WAAS hand held. But the differences are small.
 
Typically portable GPS's can only output their data to an EFIS via RS232 with the NMEA protocol.

Most modern panel mounted GPS's interface to the EFIS via ARINC429.

Most if not all portables have no ability to drive an external mechanical CDI so I assume your talking EFIS's.

Most modern panel mounted GPS's have the ability to automatically and progressively scale the CDI sensitivity over ARINC429. Portable units that do automatically scale the CDI as you get closer to the waypoint do it in distinct steps and not progressively.

The ARINC429 interface has more variables that it can communicate as well as more precision than the NMEA protocol. The rate at which all the variables update is better on ARINC423 than NMEA. In some cases, NMEA can take more than a couple seconds to update the variables.

The NMEA protocol was designed to drive ships around on AP (slow and easy). I really have no idea why it became popular for aircraft use since the "Aviation" protocol was already around and in some cases is better.
 
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