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tso'd vs non tso'd gps receivers

2johns

Active Member
Question. Are non TSO'D GPS receivers unable to accurately receive WAAS signals. The local WAAS here is very accurate and we use it for seeding, spraying, and harvesting using our non TSO'D John Deere auto steer receivers that give us 12" accuracy. Wouldn't the non TSO'D units such as Garmin 496, 696 etc. be accurate enough for ADSB operations?
 
If the GPS position information transmitted by your transponder does not meet government specs, then the ground transmitter will not transmit all of the traffic information unless another aircraft that is properly equipped is located nearby.
 
As Dave pointed out, "accuracy" and "integrity" are two different animals. What's required for an ADS-B position source is BOTH accuracy and integrity.

With my old Lowrance 2000c "VFR Only" GPS I could navigate down my driveway and into my garage. Would I trust it to allow me to do this trick blindfolded, EVERY time I drove in the laneway? Ummm, I think I like my car and my house too much to take that risk!

Most of us have never seen GPS signals go totally wacky. I've seen it happen, and seen what would happen to an airplane blindly flying along using that incorrect GPS position. That's why we need the integrity combined with accuracy - we want our GPS receivers to be able to tell us when they think their reported position falls below a specific accuracy level.
 
Here is my understanding. Your ADS-B out signal has to match the FAA performance standards if it is not TSO'd and you are EAB or ELSA. However, let me share my experience.

I installed a TSO'd FreeFlight XVR-978 transceiver in my ELSA. I got a compliance report from the FAA revealing an ICAO hexiidecimal ID issue and a BDELT deviation. In plain English I screwed up the ICAO identifier when I entered it into the unit and my ADS-B out was reporting glitches in my vertical speed about 0.3% of the time. I fixed the ICAO code issue,but the BDELT issue persisted at the 0.3% rate, so I contacted FreeFlight and Dynon who said anything less than 1% was OK. Next I asked the FAA guy what the FAA spec was for BDELT error, and it's been three weeks with nothing but crickets --- I guess it's true, we're not happy until you're not happy.
 
Cynical viewpoint:

Accuracy requires a WAAS receiver capable of receiving and interpreting the GPS signals and giving high accuracy results to the operator.

Integrity is the manufacturer justifying charging an extra $1000 so they can put an additional keyword in the data stream to tell your ADS-B out transmitter that yes, the receiver is "trusted."
 
Cynical viewpoint:

Accuracy requires a WAAS receiver capable of receiving and interpreting the GPS signals and giving high accuracy results to the operator.

Integrity is the manufacturer justifying charging an extra $1000 so they can put an additional keyword in the data stream to tell your ADS-B out transmitter that yes, the receiver is "trusted."

Rob - until the day you see your GPS head off into nah-nah land, it's easy to be cynical. The big $$$ charged for the certified receivers is a huge incentive to push us in the direction of cynicism.

Then one day you'll see that trusty GPS produce a position fix that's hundreds of miles off. That's when you have the "aha!" moment and realize you really wouldn't want that to have happened on an LPV approach to minimums. :rolleyes:
 
TSO has nothing to do with using WAAS or including integrity monitoring as a function in a GPS receiver. WAAS provides a correction value for the receiver measurements of the L1 GPS signal, which is how it achieves higher accuracy than a non-WAAS/non-differential GPS receiver. Differential GPS receivers, of which WAAS is a sub-set, are typically used in ground based applications that require high position accuracy such as surveying and construction.

The cost of the TSO certification testing and paperwork are what you are really paying the extra money for, not the actual hardware. Happily, under EAB rules you do not need a TSO'ed WAAS GPS for 2020 ADS-B compliance as Dynon recently showed with the SV-GPS-2020.

John Salak
RV-12 120116
N896HS
 
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