Author Topic: Mode-S Altitude  (Read 16347 times)

davemgill

Mode-S Altitude
« on: October 24, 2015, 04:43:45 pm »
I am in the process of getting all the parts I need to build a device. By entering my ICAO code in the config of PilotAware will it receive the transmission from my own Mode-S transponder and use that altitude? If it did I see no advantage in getting a barometric module.

Regards
David

Moffrestorer

Re: Mode-S Altitude
« Reply #1 on: October 24, 2015, 06:02:13 pm »
Hi Dave,

See Altitude Accuracy thread regarding necessity of the barometric module.

I believe if you set your aircraft ICAO code in the config page (rather than the auto generated one) it stops you from displaying as 2 plane symbols one on top of each other, generated from the P3i and the other from your ADSB, (assuming you have a GPS connected to your mode S for ADSB out. The P3i one is suppressed.

PAW utilises your GPS altitude for transmission. Anything it receives from a target is displayed as a relative height compared to your altitude. Your own ADSB transmission will be recognised by PAW and not be displayed on your device.

If any of this is wrong, I'm sure someone will put us both straight.

Chris



Admin

Re: Mode-S Altitude
« Reply #2 on: October 24, 2015, 06:05:34 pm »
I am in the process of getting all the parts I need to build a device. By entering my ICAO code in the config of PilotAware will it receive the transmission from my own Mode-S transponder and use that altitude? If it did I see no advantage in getting a barometric module.

Regards
David

Hi Dave,
Entering the ICAO code is used for 2 things
1. The ID code sent over P3I
2. Filter your own Transponder from display.

PilotAware has 2 sources of Altitude
1. GPS
2. Barometric

If you do not fit a barometric sensor, then you need to enter the QNH value in the web interface, otherwise vertical height differences to ADS-B traffic will not be accurate.

Hope this makes sense.

Thx
Lee

davemgill

Re: Mode-S Altitude
« Reply #3 on: October 30, 2015, 11:06:00 am »
Hi Lee,

It does make sense. However as my transponder will be transmitting the altitude based on true static and not internal cockpit pressure and this using the same reference altitude (1013) that other transponders are transmitting in their ADS-B outputs surely it would make sense to use this information. It would also allow a the option of a less complex PAW unit as the barometric module would not need to be included.

Regards
David

Moffrestorer

Re: Mode-S Altitude
« Reply #4 on: October 30, 2015, 12:30:46 pm »
Hi Dave,

I'm guessing only a low percentage of GA has a mode S transponder, even fewer have ADSB-out that will give a position that can be plotted by PAW. Mode C cannot be plotted either, even though QNE height is broadcast, because of no GPS position info.

The primary benefits of PAW, as I see it are that all GA can potentially see and be seen, using the P3i air-air transmissions, at low cost. Most of GA carry GPS and it seems sensible to use GPS height as a reference for PAW, because few will have encoding altimeters on board referencing 1013 hPa. Even if you have mode C and can be seen by ATC, simply by carrying PAW as well, the rest of us with PAW will be able to see you and your height relative to GPS height, plus you will be able to get the same info yourself regarding my trajectory.

As far as I'm aware, the whole reason for QNE (1013) was so airliners don't have to keep changing their altimeter pressure settings every few miles to take account of regional pressure settings, to maintain ATC vertical separation instructions.

Incidentally, I cheated and bought J. Curtis's shield with baro. module  already fitted.

Moffrestorer

Re: Mode-S Altitude
« Reply #5 on: October 30, 2015, 12:37:40 pm »
BTW Dave,

Do you fly a pressurised aircraft?

If not, I doubt whether the pressure difference between static line and cockpit pressure at the speeds most GA travel at will be very significant in terms of relative height for PAW purposes.

Admin

Re: Mode-S Altitude
« Reply #6 on: October 30, 2015, 03:53:09 pm »
It would also allow a the option of a less complex PAW unit as the barometric module would not need to be included.

Hi David,
Unfortunately not, because not all A/C using PAW will have a Mode S transponder fitted, in fact may not have a transponder at all. These A/C will need a Pressure Altitude reference in order to calculate relative vertical height difference to the surrounding ADS-B traffic

Thx
Lee

AdePrice

Re: Mode-S Altitude
« Reply #7 on: November 11, 2015, 05:36:53 pm »
Do you fly a pressurised aircraft?

If not, I doubt whether the pressure difference between static line and cockpit pressure at the speeds most GA travel at will be very significant in terms of relative height for PAW purposes.

Moffrestorer,

There can be a significant difference between the cabin altitude and the true altitude in some GA aircraft.  The CFM shadow usually has no static system and shadow pilot manual warns the IAS over reads by about 15%.  I own a Streak Shadow but did not realize straight away that the altimeter is also affected and it under reads by about 300' at 2500'.  I found out when I did some airborne trials with another a PA28, thinking I had 500' separation but the cold reality was that it was less than half that.

A bit of experimenting with a couple of PA28s showed there is little difference between cabin alt and static alt on a PA28.

Moffrestorer

Re: Mode-S Altitude
« Reply #8 on: November 13, 2015, 12:19:25 am »
Ade,

I find the situation with the Shadow quite surprising. If the altimeter under-reads, the cockpit must presumably be at a higher pressure than surrounding air, if it's getting its pressure source from the cockpit rather than a static line. I can imagine the cockpit can pressurise if for example, cabin heat is turned on, on some aircraft, or a fresh air vent is opened, and the air has no ready means of escape. I could envisage  a situation where the reverse could occur and the cockpit be at a lower pressure if a Venturi effect is created by airflow passing over the rear of an ill fitting canopy etc. I imagine most GA aircraft are not so well sealed so pressure equalises readily with ambient exterior air pressure, even when descending or climbing.

I guess in your installation, the PAW pressure sensor module (baro) could be engineered to be in a separate sealed box and be vented to the outside via poly tubing exiting at 90 degrees to the airflow, the electrical connections being made via plug and socket mounted on the PAW. 

neilmurg

Re: Mode-S Altitude
« Reply #9 on: November 13, 2015, 10:55:57 pm »
This is one area where I don't see the logic. Can someone help me? The PAW receives GPS data, either from the ipad/android, an external GPS or it's own which surely includes altitude. Why isn't this used instead of a baro? I'm just asking for clarification, I'm happily adding a barometer sensor as that will work for me.

Ian Melville

Re: Mode-S Altitude
« Reply #10 on: November 13, 2015, 11:27:49 pm »
GPS  altitude is not as accurate as long/Lat positions. it is also with reference to the GPS model of the world, which is imperfect and not the same as the pressure altitude used by transponders. So that we can make a valid comparison between aircraft at different altitudes, we need to sing from the same sheet, hence we fit a baro.

neilmurg

Re: Mode-S Altitude
« Reply #11 on: November 14, 2015, 09:18:30 am »
...we need to sing from the same sheet, hence we fit a baro.
I was expecting something more like "Yes it's possible to use GPS/transponder Mode S but it's not at the top of the todo list/we've got more important stuff to do" -or- that's not possible because... -or- we chose to do it this way

brinzlee

Re: Mode-S Altitude
« Reply #12 on: November 14, 2015, 09:35:17 am »
GPS Altitude Readout > How Accurate? (rev. 2/10/01)
New GPS buyers are frequently concerned about the accuracy (or lack of it) of the altitude readout on their newly purchased GPS. Many suspect their equipment may even be defective when they see the altitude readout at a fixed point vary by many hundreds of feet. This is NORMAL.

With most low cost GPS receivers, the horizontal error (without SA now that it is off) is specified to be within about +/- 15 meters (50 feet) 95% of the time. Most users find this is a conservative specification and that their modern GPS receivers routinely perform better than this worst case specification. But.. Users should expect that SOMETIMES they may see the error approach the specification limits.   AND..  5% of the time,  the error may be "any value" from zero to whatever".  Note: Unless you have a CLEAR AND UNOBSTRUCTED view of the sky (on your dash or looking out of an airplane window with no externally mounted antenna, or similar obstructed view does not count!)  you can count on your error excursions to be much greater than the above numbers.   Your GPS <depends> on this clear and unobstructed view or it cannot make accurate range measurements to the satellites.

Generally,  Altitude error is specified to be 1.5 x Horizontal error specification.  This means that the user of standard consumer GPS receivers should consider +/-23meters (75ft) with a DOP of 1 for 95% confidence.  Altitude error is always considerably worse than the horizontal (position error). Much of this is a matter of geometry. If we (simplistically) consider just four satellites, the "optimum" configuration for best overall accuracy is having the four SVs at 40 to 55 degrees above the horizon and one (for instance) in each general direction N, E, W, and S.  (Note:  You will get a very BAD DOP if the SVs are at the exact same elevation.  Luckily,  this is a rare occurrence.)  See:  DOP demonstration site by Norris Weimer> How SV geometry affects GPS accuracy(Java Required)..  The similar "best" arrangement for vertical position is with one SV overhead and the others at the horizon and 120 degrees in azimuth apart. Obviously, this arrangement is very poor from a signal standpoint. As a result, of this geometry the calculated solution for altitude is not as accurate as it is for horizontal position.  Almost any calibrated altimeter will be more stable at reading altitude  than a GPS.

GPS altitude measures the users' distance from the center of the SVs orbits. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent.  What this means is that if you are walking on the seashore,  and see your altitude as -15 meters,  you should not be concerned.  First,  the geodetic model of the earth can have much more than this amount of error at any specific point and Second,  you have the GPS error itself to add in.  As a result of this combined error,  I am not surprised to be at the seashore and see -40 meter errors in some spots.

DGPS operation (where available) will dramatically improve the performance of even low cost GPS receivers. Horizontal accuracy of +/- 5 meters and altitude accuracy of +/- 10 meters (relative to the WGS-84 geode) with suitable DGPS receivers and low cost GPS receivers such as the Garmin GPS-12XL can be expected.

In any case, it is extremely unwise to overly depend on the altitude readout of a GPS. Those who use GPS altitude to aid in landing their small plane should have their insurance policies paid up at all times.

Joe Mehaffey

Admin

Re: Mode-S Altitude
« Reply #13 on: November 14, 2015, 09:54:48 am »
Just to give clarification on this....

Firstly, remember the altitude reported on your navigation device, will be GPS based, not barometric.
PAW, uses 2 references, it uses GPS and Barometric.
The barometric could be calculated automatically if you have a sensor fitted, or manually, by dialling in the QNH figure on the config page

When comparing relative vertical difference to ADSB traffic, barometric altitude us used, because this is the reference the transponder is using
When comparing to other pilotawares, GPS altitude is used for the comparison, this was an early design decision before the addition of a barometric sensir

Thx
Lee

neilmurg

Re: Mode-S Altitude
« Reply #14 on: November 14, 2015, 11:23:54 am »
That is outstanding feedback, thankyou both (and Joe Mehaffey). I forgot to ask about the Mode C/S altitude encoding, I suppose we're not listening to our own squawk.