GaryP1007

Stratus Disappointment

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TCAS, ADS-B, TIS and ATC all use baro altitude no GPS altitude. On an ADS-B or Mode C out transponder (either 1090 or 978 MHz) all reported altitude data comes from a barometric altitude encoder. This normalize the system and insure that everybody is on the same reference.

 

José

So I asked the question about relative altitudes and he display of traffic info from a gdl39 into a garmin handheld. I got a good response from Garmin and I wanted to share it here. I would assume this info would be relevant to other portable adsb receivers as well.

ADS-B Out equipment transmit both pressure and geometric (GPS) altitudes. The relative altitude of these ‘Out’ targets can be computed by the 39 from the geometric altitude. Traffic uplinked from GBTs do not have geometric altitude. These targets will not have relative altitude but should show up on the traffic page (assuming you have TIS-B uplink from a GBT).

Also, the GDL 39 has an internal pressure setting for determining its own altitude. The pilot can tell the GDL 39 if it is being used in a pressurized aircraft. Setting the Aircraft Type to ‘Pressurized’ will tell the 39 to not use the pressure sensor reading. This will disable the display of relative altitude and traffic alerting. If the Aircraft Type is set to Not Pressurized’ and the GDL39 is used in a pressurized aircraft, the relative altitude will be inaccurate.

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 I was all ready to switch to ADS-B last Fall, even suspended my XM account. Well, I ended up reactivating XM & will stick with it for a while. Just a reminder, ADS isn't 'free' either. You can divide the $800 by $37 worth of monthly XM costs to get a ways down the road.

 

  It's also easy to "suspend" your XM account if you won't be doing much longer distance flying for a few months at a time.

It still takes 800$ worth of equipment to receive XM, so the cost is a wash.  you can run the GDL-39 on an iphone or iPad, so you are all in for 800$ including the phone. Or use it with an aera 500.   The XM takes at minimum a garmin GPS that can recieve XM, which is the Aera 510,  thats 900$.    The difference is the music and the ongoing 500$ a year in subscriptions.

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