Altitude - Is Mode C reported same as ADSB?

WDD

WDD

Final Approach
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Vintage Snazzy (so my adult children say)
Is your altitude pinged out by your Mode C transponder identical to what your ADSB pings?
 
Does the ADSB squawk pressure altitude or GPS altitude?

What does Flight Aware use/ report?

How does pressure altitude get translated into real altitude to whom ever is listening to Mode C?
 
WDD said:
Does the ADSB squawk pressure altitude or GPS altitude?

What does Flight Aware use/ report?

How does pressure altitude get translated into real altitude to whom ever is listening to Mode C?
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For ATC they do a +/- on their system so your pressure altitude of 4200 shows as 4500 on their scope.

For ADS-B to ADS-B everyone is on pressure altitude, so no need to convert to real just to convert back to pressure. You can tell the guys that haven't done an altimeter setting, or maybe they just suck at holding altitude.

FlightAware uses pressure altitude - because that's what ADS-B kicks out - which is why on occasion is has shown me flying 200+ feet below the level of Lake Michigan.
 
WDD said:
Does the ADSB squawk pressure altitude or GPS altitude?

What does Flight Aware use/ report?

How does pressure altitude get translated into real altitude to whom ever is listening to Mode C?
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ADS-out is sending pressure altitude via mode C. ATC’s system corrects that on their displays. I’ve heard Fore Flight corrects it as well. Flight Aware doesn’t correct for it and it’s stated on their website that altitudes indicated are pressure altitudes.
 
Velocity173 said:
ADS-out is sending pressure altitude via mode C. ATC’s system corrects that on their displays. I’ve heard Fore Flight corrects it as well. Flight Aware doesn’t correct for it and it’s stated on their website that altitudes indicated are pressure altitudes.
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Another belief shattered. I assumed Flight Aware corrected and showed real altitude, what ATC and the surface of lake Michigan would agree with. I also thought ADSB would be using WASS GPS altitude. If WASS GPS altitude is more accurate than altitude derived from pressure altitude, then not sure why the system wouldn't use that instead. Also not sure why ADSB would not use that to ping out ADSB Out altitude instead of uncorrected pressure altitude.
 
WDD said:
If WASS GPS altitude is more accurate than altitude derived from pressure altitude, then not sure why the system wouldn't use that instead.
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GPS altitude is considerably less accurate than pressure altitude.
 
LesGawlik said:
I've heard that for years, for I've noticed a strikingly good correlation between altitude and GPS altitude many times.
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The first problem is geometry, it is more difficult to calculate how high you are when all the satellites are above you. The second is due to the shape of the earth and depends where you are. GPS altitude is in reference to a hypothetical ellipsoid that only approximates the contour of the earth.
 
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dmspilot said:
The second is due to the shape of the earth and depends where you are. GPS altitude is in reference to a hypothetical ellipsoid that only approximates the contour of the earth.
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People say that a lot, but in practice the ellipsoid isn't the basis except in crappy memory starved systems. More elaborate geoids have been in use for a long time.

The satellite geometry is indeed telling. Vertical precisions are only about a third of what horizonals are.
 
My experience GPS altitude is generally off by at least 300-500 ft over indicated altitude with proper pressure correction when in the 8000 indicated range.
 
The difference between GPS and pressure altitude isn't because of the ellipsoid, geoid models, or inaccuracy...

It's because of the difference between standard atmosphere and real life atmosphere.

You adjust your altimeter based on the surface reading at a nearby airport. A 30.20 reading on a clear cold January day and a 30.20 reading on a hot July day should give the same altimeter reading at a given point on the ground. However, at let's say 12000' pressure altitude, your actual height above the ground at that same point would vary greatly between those January and July days. The actual height of that given pressure level varies greatly based on weather patterns. You can usually get these numbers in radiosonde data (skew-t).

For planes cruising well above the ground this isn't a big deal as long as you're all calibrated to the same altimeter reading. For example if you're all showing 12,000 indicated doesn't matter if you're actually at 11,600 or 12,500 you're all at the same level. For instrumented planes, say for aerial mapping, it can become a big deal as being off a few hundred feet vertically can effect the resolution of the mapping.
 
Some ADSB transponders also broadcast “geometric” (GPS) altitude.

You can see this in the fields on the left hand side when you select an aircraft on https://globe.ADSBexchange.com.
 
My aircraft allows setting the altimeter via GPS altitude on the ground or baro. Push the knob to set via GPS or twist for baro. It’s not as accurate as using the barometer setting. Generally I see GPS altitude off from 10 to 40 feet and rarely 60 feet. If set via baro it’s always right on.
 
Todd82 said:
The difference between GPS and pressure altitude isn't because of the ellipsoid, geoid models, or inaccuracy...

It's because of the difference between standard atmosphere and real life atmosphere.

You adjust your altimeter based on the surface reading at a nearby airport. A 30.20 reading on a clear cold January day and a 30.20 reading on a hot July day should give the same altimeter reading at a given point on the ground. However, at let's say 12000' pressure altitude, your actual height above the ground at that same point would vary greatly between those January and July days. The actual height of that given pressure level varies greatly based on weather patterns. You can usually get these numbers in radiosonde data (skew-t).

For planes cruising well above the ground this isn't a big deal as long as you're all calibrated to the same altimeter reading. For example if you're all showing 12,000 indicated doesn't matter if you're actually at 11,600 or 12,500 you're all at the same level. For instrumented planes, say for aerial mapping, it can become a big deal as being off a few hundred feet vertically can effect the resolution of the mapping.
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You can get significant differences much lower than 12,000. That’s why some Approach’s have the Snowflake symbol on them.
 
Jeff767 said:
My aircraft allows setting the altimeter via GPS altitude on the ground or baro. Push the knob to set via GPS or twist for baro. It’s not as accurate as using the barometer setting. Generally I see GPS altitude off from 10 to 40 feet and rarely 60 feet. If set via baro it’s always right on.
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I can’t remember the last time my altimeter read field elevation when I set the altimeter. But then I’m a renter and don’t fly airplanes less than about 40 years old.
 
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dans2992 said:
Some ADSB transponders also broadcast “geometric” (GPS) altitude.

You can see this in the fields on the left hand side when you select an aircraft on https://globe.ADSBexchange.com.
Click to expand...

I think maybe ADSBexchange may be doing the Fore Flight thing instead of the Flight Aware thing as described in @Velocity173 ‘s #6 above. I’m pretty sure the Transponder isn’t ‘broadcasting’ geometric altitude.
 
So much misinformation. ForeFlight does not convert pressure altitude to MSL. ADS-B and transponders are required by regulation to use the same pressure altitude source (91.217). ADS-B broadcasts both pressure altitude and geometric altitude (a form of GPS altitude that uses a spheroid model of the earth surface (91.227(d)(3) and (14)). GPS altitude or GSL is a GPS altitude where the local variances between geometric altitude is accounted for. Geometric altitude is not used by ATC, only pressure altitude, although their computer corrects the pressure altitude for the barometric setting when displaying altitude to the controller. GPS or GSL altitude is more accurate than pressure altitude, especially the higher up one flies in the atmosphere, because GPS/GSL altitude, the error does not vary with altitude, whereas a barometric altimeter varies, particularly with temperature and at 10,000 feet can exceed 1000 feet of error at temperatures significantly different than standard temperature. Vertical separation of aircraft is based on pressure altitude.
 
When I see traffic altitude in ForeFlight am I seeing pressure altitude or gps geometric altitude? Does ForeFlight log pressure or gps geometric?
 
John Collins said:
So much misinformation. ForeFlight does not convert pressure altitude to MSL. ADS-B and transponders are required by regulation to use the same pressure altitude source (91.217). ADS-B broadcasts both pressure altitude and geometric altitude (a form of GPS altitude that uses a spheroid model of the earth surface (91.227(d)(3) and (14)). GPS altitude or GSL is a GPS altitude where the local variances between geometric altitude is accounted for. Geometric altitude is not used by ATC, only pressure altitude, although their computer corrects the pressure altitude for the barometric setting when displaying altitude to the controller. GPS or GSL altitude is more accurate than pressure altitude, especially the higher up one flies in the atmosphere, because GPS/GSL altitude, the error does not vary with altitude, whereas a barometric altimeter varies, particularly with temperature and at 10,000 feet can exceed 1000 feet of error at temperatures significantly different than standard temperature. Vertical separation of aircraft is based on pressure altitude.
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If the Transponder is 'replying' with Geometric Altitude, what is interrogating it. Or does it literally Broadcast without Interrogation?

EDIT: Think I got it now. ADSB doesn't use the Transponder. How am I doing?
 
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luvflyin said:
If the Transponder is 'replying' with Geometric Altitude, what is interrogating it.
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?????

Transponders have been transponding pressure altitude when interrogated since long before GPS was an even a gleam in its inventors eyes. That has not changed. Not one bit.The old dumb transponders like my KT76 don't even have a way of getting the Geometric altiturd from GPS. It's like it's still the 1970's.

 
off topic, but when I see Jackson 5 and listen to the music -- Michael was an amazingly talented performer.
 
Capt. Geoffrey Thorpe said:
?????

Transponders have been transponding pressure altitude when interrogated since long before GPS was an even a gleam in its inventors eyes. That has not changed. Not one bit.The old dumb transponders like my KT76 don't even have a way of getting the Geometric altiturd from GPS. It's like it's still the 1970's.

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Ok. I thought somewhere above it was said that ADSB uses the Transponder to output it's info.
 
luvflyin said:
Ok. I thought somewhere above it was said that ADSB uses the Transponder to output it's info.
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ADSB sends the same pressure altitude and code as the Mode C and adds the Geo altitude and a bunch of other really useful stuff like your wingspan.
 
Capt. Geoffrey Thorpe said:
ADSB sends the same pressure altitude and code as the Mode C and adds the Geo altitude and a bunch of other really useful stuff like your wingspan.
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Ok. Got it now. ADSB does it's thang with it's gadgets and antennas. And the Transponder does it's thing with it's gadgets and antennas. No connection? I think I'm all caught up now. But whadda The Jackson Five got to do with it?
 
I don't think you got it.

the transponder and altitude encoder do tell information to the ads-b device. In my case the GTX327 is sending squawk and pressure altitude to the Tailbeacon that sends the whole mess to the ground. I'm not sure it's techinically correct which one is sending which, but I do know that in Anon mode if I'm squawking 1200 it randomizes the ADS-B data but if I'm on a discrete it send my actual tail number. So the two are talking somehow.
 
Ravioli said:
I don't think you got it.

the transponder and altitude encoder do tell information to the ads-b device. In my case the GTX327 is sending squawk and pressure altitude to the Tailbeacon that sends the whole mess to the ground. I'm not sure it's techinically correct which one is sending which, but I do know that in Anon mode if I'm squawking 1200 it randomizes the ADS-B data but if I'm on a discrete it send my actual tail number. So the two are talking somehow.
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Ok. Here's another stab at it. The ADSB and Transponder have some connection, even if it's just to let the ADSB know if yer squawkin a dozen or sumpin else. And the altitude encoder talks directly to ADSB to give it pressure altitude. brb. Just googled GTX327. It is a Transponder. So the encoder probably sends it to the Transponder which then sends it to the ADSB??? What's the Tailbeacon. Shirley can't be the flashing red light thing.
 
@luvflyin - in my particular setup. Dynon D-100 is the Alt Encoder GTX327 is the transponder, and uAvionics Tail Beacon is the ADSB out device (mounted on the tail with a solid white light).
 
Ravioli said:
@luvflyin - in my particular setup. Dynon D-100 is the Alt Encoder GTX327 is the transponder, and uAvionics Tail Beacon is the ADSB out device (mounted on the tail with a solid white light).
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Gonna google uAvionics Tail Beacon. That'll probably make it click for me. Thankyou
 
The answer to all this is it depends.
There are two ways to be ADS-B compliant.
1. Use the new 9 something frequency plus traditional Mode C/S on 1090.
2. Use ADS-B on 1090 only.

You can have a transponder which is ADS-B out on 1090 which handles both the ADS-B and the Mode C/S information all in one. This is required for planes based in Canada and those which fly over an altitude in the USA (I think 18k but not positive).

Or you can in many cases keep your existing Mode C/S transponder and add a second device like Tail beacon or GDL88 {I think} which then piggy backs off the Mode C/S and broadcasts the additional data on the UAT frequency (which is only used in the USA). How these two devices work together depends on the vendor solution. There is now defined rule stating how it has to work. Just that they do.

Tim

Sent from my HD1907 using Tapatalk
 
luvflyin said:
You can get significant differences much lower than 12,000. That’s why some Approach’s have the Snowflake symbol on them.
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I know, I was just using an example.
 
WDD said:
When I see traffic altitude in ForeFlight am I seeing pressure altitude or gps geometric altitude? Does ForeFlight log pressure or gps geometric?
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With ForeFlight, when you are viewing ADS-B traffic, you see a relative altitude based in hundreds of feet between the the target aircraft with your aircraft. In most cases, the target altitude is a pressure altitude and ForeFlight extracts the pressure altitude from your ownship ADS-B Out broadcasts that it also receives. In this instance the Target Aircraft Pressure Altitude from its ADS-B Out broadcast minus your ownship Pressure altitude is used for the comparison, a +01 means the target is 100 feet higher. In some instances the Target Aircraft does not broadcast a pressure altitude (like when the pressure altitude encoder isn't working), and in that case, the Target Geometric Altitude is compared with Ownship Geometric Altitude, so the relative altitude should be the same, that is ForeFlight makes an apples to apples comparison, either both are pressure altitudes or both are Geometric Altitudes. If you tap on the target, the data tag will display the pressure altitude of the target.

If by log, you mean the ownship track log, the track log has nothing to do with ADS-B and the altitude is based on GPS altitude. GPS altitude is derived from the GPS position source you are using. GPS altitude is based on Geometric Altitude and is adjusted for the local difference between sea level and the WGS84 Spheroid surface. The local difference between sea level and the WGS84 model is called the geoid and it accounts for the fact that gravity is not uniform on our planet and that denser material increases local gravity. So sea level is not really the same all around the world. In the US, this correction is within +/- 200 feet of the spheroid surface. Some have termed Geometric Altitude +/- geoid corrections as GSL or GPS corrected to a Sea Level equivalent. At my base airport, one needs to add 103 feet to the Geometric Altitude broadcast by ADS-B to match the altimeter reading assuming it has the correct altimeter setting on the surface. Most GPS systems doe this adjustment and what is reported is GSL. ADS-B does not make the adjustment, no need to, it just uses the raw Geometric Altitude. You will normally get excellent correspondence between the altimeter reading and a GSL value on the surface of the airport. As one climbs above the surface, the baro altimeter gets more and more error introduced by non standard temperatures. The GSL stays within +/- 30 feet. On hot days the baro altimeter will show a lower altitude than the GSL and on very cold days, the baro altimeter will show a higher altitude than GSL. I guarantee you will want to believe the GSL in the mountains if you want to avoid hitting the rocks on a cold day.
 
luvflyin said:
Ok. Got it now. ADSB does it's thang with it's gadgets and antennas. And the Transponder does it's thing with it's gadgets and antennas. No connection?
Click to expand...

Not necessarily. My installation runs coax (which originally ran direct to the antenna) from my transponder, through my GDL 82 and then to my old transponder antenna. The only new antenna is a dedicated GPS antenna for the ADSB unit. So the GDL 82 is receiving pressure altitude from my encoder via my transponder.

From the install manual:

51165921142_dfc5e3a821.jpg
 
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