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Stratus vs IFD/SkyTrax ADS-B weather
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paulr View Drop Down
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    Posted: 30 Apr 2017 at 11:27pm
I had a planned flight KMRB-KDCU today, but ended up having to divert to KCHA for weather. It was pretty ugly. We waited out the weather for a bit until the biggest line of storms passed. The ground-based radar in Foreflight showed light to moderate precipitation to the west, so I decided to launch along a route that would keep me away from the bad stuff.

Once I took off and got on course, I noticed that I was seeing very different results from two sources that I would expect to be the same: my Stratus 1 driving Foreflight and my SkyTrax 100 driving the IFD540. This is what I mean:


I know there are differences in how FF and the IFD540 display radar data. I leave FF set on "radar (composite)" and have "four-color radar" turned off. I (think I) understand how FF's radar layer settings work, but those shouldn't have any effect when you're using a Stratus 1 because it's just displaying the same ADS-B NEXRAD data that everyone else gets: composite only.

The issue I have here isn't that the color presentation is different, it's that one of these is W-R-O-N-G. Either there is bad weather north and south of my path, or there isn't. (I'm ignoring the stripe of white in FF where radar data is apparently missing). Look between the ownship marker and 4A9: the two views depict very different situations. 

Even accounting for the latency inherent in collecting, processing, and transmitting ground radar data over the ADS-B network, I can't explain why two ADS-B In devices in the same point in space/time should have such different results unless they are somehow getting completely different data streams from ground stations.

In the event, the Foreflight presentation was much closer to the actual conditions. It was also much closer to the more detailed NEXRAD view I was seeing on my phone. No convection, maybe a 20kt headwind, and some light to moderate rain falling from a higher layer.

Can one of you smart people explain to me what might have caused this difference? 
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MysticCobra View Drop Down
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Part of your disconnect was that you had "4 color weather" turned off.  Scroll down to the bottom of this FF blog post for some insight into how ADSB weather intensity data is delivered and displayed:  http://blog.foreflight.com/2016/07/14/new-radar-layer-updates/

From that post:  "If you are flying with airborne radar, you may want to look at the new low resolution four-color NEXRAD mosaic now available in ForeFlight Mobile. The colors depicted in this radar mosaic match the standard color-to-dBZ mapping defined by the RTCA as documented in Section 3.8.2 (Table 3-2) of RTCA DO-267A (shown below). This standard is also used for airborne radar displays."


Edited by MysticCobra - 01 May 2017 at 7:54am
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paulr View Drop Down
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Thanks. Looks like I interpreted the four-color setting as exactly the opposite of what it truly does. I thought it would blend down the higher resolution radar that the app downloads from the Internet into the same format as an airborne radar; I didn't realize it had any effect when your "airborne radar" is really ADS-B In data.
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mfb View Drop Down
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I went to a very interesting talk at Oshkosh last year given by Dr. David Strahle, who is a radiologist and an aviation weather radar expert. (It turns out that human radiology and weather radar are basically the same thing. The weather is just on a slightly bigger scale.) 

He talked a lot about Foreflight and its depiction of the weather. Bottom line - MysticCobra is exactly right.

If you look at the weather depiction scale in the ForeFlight "Mobile Legends" manual, you'll see that with the 4-color weather depiction turned off (page 3), ForeFlight breaks weather returns into 16 colors. ADS-B yellow doesn't start until you get above 40 dBZ return intensity. However, with 4-color turned on (page 6), ADS-B yellow starts at 30 dBZ, which is a lot lower. (At least 10 times lower, if I recall my logarithms correctly.)

I couldn't find a table for the IFD540 that shows radar depiction as a function of dBZ, but let's assume that it's the same as the ForeFlight 4-color table. That means that if you were flying in 35 dBZ rain and had ForeFlight set with 4-color to off, the rain would show yellow on the 4-color IFD540 and green on the 16-color ForeFlight. But if you turn 4-color to on, the ForeFlight and the IFD will look the same.

Dr. Strahle also said to use the Radar-Composite setting in ForeFlight, not Radar-Lowest Tilt. The lowest tilt setting just shows very low rain. Composite is a combination of all levels, including the one where you might be flying.

Hope this helps.

Mike
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paulr View Drop Down
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Thanks, Mike. Now that you mention it, I remember reading your original post about the Oshkosh seminar, but apparently I didn't remember what it said or I wouldn't have asked that question! I *did* remember the tip about using composite instead of lowest-tilt. 
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pburger View Drop Down
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Even if there is a disconnect on the colors being used, there is still a mismatch on the DATA being displayed.  The scale of the screens is similar.  On the IFD there are several RED blocks between KBGF and the plane, where there are no such indications on the ForeFlight screen.  I notice the IFD shows "regional radar 11 min" so I suppose it could be a latency issue, but I assume the data is coming from the same source (i.e. - ADS-B towers).
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Catani View Drop Down
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Originally posted by pburger pburger wrote:

Even if there is a disconnect on the colors being used, there is still a mismatch on the DATA being displayed.  The scale of the screens is similar.  On the IFD there are several RED blocks between KBGF and the plane, where there are no such indications on the ForeFlight screen.  I notice the IFD shows "regional radar 11 min" so I suppose it could be a latency issue, but I assume the data is coming from the same source (i.e. - ADS-B towers).
Don't think it's a data mis-match, it's because different color schemes are use to display the same data, as MysticCobra stated.  If you select four-color on ForeFlight, the IFD and FF colors and displays should match.  If you deselect FF four-color, the IFD's yellow becomes dark green on FF.  Also, the IFD's red becomes either yellow, orange or red on FF, depending on intensity.  In the OPs case, given that his experience showed the weather was not too significant, it appears that the IFD's red became yellow on FF, and the IFD's yellow was dark green on FF.

The IFD's colors should be the same as XM's for the same intensities, assuming XM data agrees with ADS-B broadcase weather data.

The IFD is following the FAA's requirements for weather display colors on color onboard weather radars. FF gives you the option to choose a color scheme that provides more information using more colors, with differences apparent on the OP's post - a better option but then FF is not certified and need not comply with FAA color schemes for certified equipment.  Any remaining differences there may be comparing the two should be minor, after making these adjustments, unless there is something wrong with the equipment or the signal.

The best way to display returns I think is using continuously variable gradations of gray, depending upon the return received by the dish. This was how on board weather radar was displayed in the pre-color days.  I think four-color is a big step backwards from monocolor days.  Having different color schemes is even more backwards.


Edited by Catani - 01 May 2017 at 6:15pm
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pburger View Drop Down
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Catani - Are we looking at the same picture?  In the OP's picture, there are RED blocks on the IFD where there are either no blocks or green blocks on FF.  One doesn't jibe with the other. 
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oskrypuch View Drop Down
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I think that was a rectangle that had not populated yet with data.

* Orest
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Catani View Drop Down
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Originally posted by pburger pburger wrote:

Catani - Are we looking at the same picture?  In the OP's picture, there are RED blocks on the IFD where there are either no blocks or green blocks on FF.  One doesn't jibe with the other. 
As Orest says, the FF display, as noted by the OP, is missing a big chunk of data in the middle of the display (the large rectangular area - precip doesn't make large precise rectangles).  Taking that into account, and the different color standards, I think they are displaying essentially the same info, just using different colors.
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