MGL Avionics User Forum

I have an interesting and inconvenient issue where my MX1 internal GPS and SP12 do not agree. I think this is the source of a few problems, probably the most important of which is that my ADSB broadcasts an incorrect altitude.

Not sure what points are the most valuable for troubleshooting, so I'll just write a list of observations from which I hope a big brain can figure this out.

1. SP12 is version 100119. MX1 is 00010010 and v 19.

2. Swapping the GPS antennas between the two devices make no difference.

3. With SP12 enabled on the CAN bus, RAIM is never higher than non-precision. If I disable the SP12 circuit, the MX1 immediately achieves RAIM Precision.

4. If I removed the antenna from the MX1 to prevent it from getting a fix, the SP12 acquires satellites but somehow has the wrong date and time. If I connect an antenna to the MX1, the correct date and time populates in both the INT and CAN data pages.

5. If both are connected, the SP12 altitude starts off agreeing with MX1, and gradually becomes more inaccurate as it acquires more satellites. HACC and VACC values changes to dashes, and it settles about 200-300 feet below my actual altitude. This is true in flight or on the ground with the engine off.

6. The MX1 typically finds twice the satellites twice as fast (or more) than the SP12.

Here's a couple pictures to help explain the issues. I realize that in the two pictures I'm not tracking many satellites... it's bad weather here today. The problems listed above are present when I'm tracking 12ish satellites, too.

LaneRM wrote: ↑Sat Nov 11, 2023 9:55 pm I have an interesting and inconvenient issue where my MX1 internal GPS and SP12 do not agree. I think this is the source of a few problems, probably the most important of which is that my ADSB broadcasts an incorrect altitude.

The manual on the SP-12 gives some hints. You need to tell us a bit more about the incorrect altitude though. Please make sure you set your transponder to "ALT". If you select "ON" the altitude information is flagged as not available and no mode-C transmission takes place (which contains the altitude) - only mode-A is allowed.
The reason for this is that you as pilot can effectively flag or switch off altitude transmissions if you are flying with an un-serviceable altimeter.

Not sure what points are the most valuable for troubleshooting, so I'll just write a list of observations from which I hope a big brain can figure this out.

1. SP12 is version 100119. MX1 is 00010010 and v 19.

2. Swapping the GPS antennas between the two devices make no difference.

3. With SP12 enabled on the CAN bus, RAIM is never higher than non-precision. If I disable the SP12 circuit, the MX1 immediately achieves RAIM Precision.

For your mode-s transponder transmissions - there are some rules to be followed - so your internal GPS even though it's usually at highest precision will transmit as "non-precision". Sad but true - rules are rules. For the SP-12 it does in fact go high precision as well - but ONLY if it is under coverage of WAAS satellites. Again - rules. On the SP-12 this condition is met is you see five flashes on the LED followed by a pause. In addition of course it needs to see enough satellites and the constellation needs to be favorable to get to the required position.
Due to the lower sensitivity antenna placement is vital - the SP-12 needs to be able to see an unobstructed view of the sky.
The GPS in your EFIS on the other hand is so sensitive you can basically use a wet noodle as antenna and it works.

4. If I removed the antenna from the MX1 to prevent it from getting a fix, the SP12 acquires satellites but somehow has the wrong date and time. If I connect an antenna to the MX1, the correct date and time populates in both the INT and CAN data pages.

Time and date can be used from internal GPS only (see the option in the GPS setup menu).

5. If both are connected, the SP12 altitude starts off agreeing with MX1, and gradually becomes more inaccurate as it acquires more satellites. HACC and VACC values changes to dashes, and it settles about 200-300 feet below my actual altitude. This is true in flight or on the ground with the engine off.

The SP-12 contains a certified GPS module. The internal GPS is a ublox device. In a nutshell - the ublox is 20 years ahead in technology. The certified GPS module is the same as used for most aviation applications. It's basically the only game in town.
Unfortunately the certification standard is based on early 1990's technology and demands a few things no modern GPS receiver would bother with.

HACC and VACC are horizontal and vertical accuracy values from the ublox system - these do not exist in the NexNav GPS receiver. As for altitude - this traditionally is a weak area of the GPS system. The uBlox is very good here (but still not as good as the horizontal). The SP-12 GPS receiver uses exactly the algorithms dictated by the certification requirements. As long as all airborne GPS receivers come to the same result FAA is happy.

6. The MX1 typically finds twice the satellites twice as fast (or more) than the SP12.

Yes indeed - the non-certified GPS in the EFIS is perhaps one of the best on the planet. That's why we use it.
It's far more sensitive and has anti-jamming technology which also helps it to see satellite signals in the presence of interfering signals.

Unfortunately it is not possible to update the certification requirements for GPS receivers to match how a modern GPS works as you cannot simultaneously change the GPS receivers in the World-wide aircraft fleet. The requirements where based on a very early GPS receiver and the way it calculated things and the way the data was sent. This was still the time we had single channel GPS receivers and thought this was a really smart thing. Basically the NexNav GPS receiver module is forced to implement the algorithms from those old days even if we know there are much better ways now. Some items have been modernized over the years (Like adding WAAS) but overall much of the old ways have to stay now for obvious reasons.

The reason you fit a SP-12 is not because you want a better GPS receiver - the reason you fit it is to be compliant to the FAA rules with respect to your ADS-B transmissions.

Just a bit of further information on this issue.

The SP-12 contains NexNav Micro-I GPS module which is a FAA approved device (it is used in practically all of the approved or certified GPS receivers compliant to TSO-C199 class B).

This device transmits GPS altitude relative to the ellipsoid which is what mode-s transponders should use (there are conditions where other forms are acceptable but for the highest accuracy broadcasts relative to elliptical is mandatory).
Every other GPS system gives you Height above mean sea level. This is a bit of a misnomer as sea level does not really come into it. Effectively it gives you height above the geoid which is a complex gravitational model and your actual altitude above or below the ellipsoid varies depending where you are. You can easily find maps using google showing your local geoid elevation above/below the ellipsoid.
The EFIS internal GPS gives height above sea level but also the geoid elevation at location. We display the height above sea level as given by the GPS as this is a bit more meaningful and anyway - that's what most GPS systems give.
For mode-s use - we then calculate the ellipsoid from the height above mean sea level and geoid - which depending on GPS can be either the actual geoid elevation or even the actual height above geoid - in any case the height above ellipsoid can be worked out.
Now - the NexNav does not have any idea what the geoid is - it only knows ellipsoid and gives geoid as zero. So the mode-s transponder still gets the correct and desired value but the EFIS now displays the height above ellipsoid and not height above geoid elevation + ellipsoid.
Where I am the geoid is about 100ft above ellipsoid so in my case the SP-12 reads about 100ft higher than the GPS in the EFIS. It's not exact as the method used to calculate height using the satellites is different as well - in case of the NexNav it is dictated by a very old way done in only one !!! GPS receiver in the early 90's from the company that Garmin bought out (Apollo). This old way (and a lot of other stuff) has been frozen in the aviation MOPS standards ever since.
It's not possible to change to more advanced, more accurate methods as you would have to then change all GPS receivers certified for aircraft use instantly as well.