Small GPS Dilemma

[lolachampcar]

Being the nerd that I am, I just had to replace my GPS with the new 2020 but there is a small rub.

I fly aerobatics a few times a week next to a Class C.  A few years back, there were some conflicts which caused the C some heartburn.  I was fine on separation but my rapid altitude changes drove three TCAS RAs in ten minutes.  Apparently controllers have to write up every RA thus the heartburn.

I met with the management team and we decided it would be best if I checked in and offered to participate.  Adding ADSB-In to that and now things are much safer.

Now to the reason for the post.  I've noticed a change in ADSB traffic performance with the 2020 receiver.  Position lock is lost often when inverted and is slow to return.  Traffic is lost during most of these events which makes the whole system much less useful and, in some instances, I can fly a whole 20 minute routine and not have traffic.

My question is one of how V14 handles GPS data.  If I return to the previous Dynon GPS receiver will the pre V14 functionality return?  I ask because V14 brings a change to accuracy reporting for the non-2020 compliant receiver (GPS-250?).  Does this change for the non-compliant receiver cause it to "drop out" or cease to provide data in the same way as the GPS-2020?

Any insight into how ADSB-Out reporting is affected by a drop out in GPS position would be helpful.

Thanks!
Bill

 

[dynonsupport]

Bill:

What you're observing is the reality of the SV-GPS-2020 having to be more "deliberate" in reporting its position to the system. This is one of the reasons that in Revision W of the SkyView System Installation Guide, we recommended that if you have an SV-GPS-250 already installed, that you consider leaving the SV-GPS-250 in place, and installing the SV-GPS-2020 in ADDITION to the SV-GPS-250, and simply demote the SV-GPS-250 to "POS2". That way you can be compliant with the FAA 2020 ADS-B Out Mandate, and your SkyView can fail down to the SV-GPS-250 if the SV-GPS-2020 loses lock.

Being the nerd that I am, I just had to replace my GPS with the new 2020 but there is a small rub.

I fly aerobatics a few times a week next to a Class C.  A few years back, there were some conflicts which caused the C some heartburn.  I was fine on separation but my rapid altitude changes drove three TCAS RAs in ten minutes.  Apparently controllers have to write up every RA thus the heartburn.

I met with the management team and we decided it would be best if I checked in and offered to participate.  Adding ADSB-In to that and now things are much safer.

Now to the reason for the post.  I've noticed a change in ADSB traffic performance with the 2020 receiver.  Position lock is lost often when inverted and is slow to return.  Traffic is lost during most of these events which makes the whole system much less useful and, in some instances, I can fly a whole 20 minute routine and not have traffic.

My question is one of how V14 handles GPS data.  If I return to the previous Dynon GPS receiver will the pre V14 functionality return?  I ask because V14 brings a change to accuracy reporting for the non-2020 compliant receiver (GPS-250?).  Does this change for the non-compliant receiver cause it to "drop out" or cease to provide data in the same way as the GPS-2020?

Any insight into how ADSB-Out reporting is affected by a drop out in GPS position would be helpful.

Thanks!
Bill

 

[lolachampcar]

Fantastic!
Does the new V14 software put more stringent requirements on the 250's position as well or will it continue to operate as it did before V14?

 

[dynonsupport]

Bill:

I think we explain this pretty well in the SkyView System Installation Guide, Revision W, page 8-13:

(US only) Additional configuration required for SDA/SIL=1 required for continued reception of ADS-B Traffic (early 2016)

The following is only applicable to US pilots whose aircraft is equipped with an SV-GPS-250 (but not an SV-GPS-2020), SV-XPNDR-261/262 (or other Mode-S / ES transponder), and SV-ADSB-470 (ADS-B In receiver).

Prior to 2016, the FAA transmitted TIS-B to aircraft in US airspace who transmit SIL/SDA=0 (GPS integrity), allowing those aircraft to receive targeted “ADS-B Traffic” specific to their aircraft. In early 2016, the FAA will discontinue transmitting TIS-B to aircraft in US airspace who transmit SIL/SDA=0.

Prior to SkyView firmware v14.0, SkyView systems equipped with an SV-GPS-250 transmit only SIL/SDA=0. In SkyView firmware v14.0 and later, SkyView can be configured to transmit SIL/SDA=1 or SIL/SDA=0. When set to SIL/SDA=1, this allows aircraft in US airspace equipped with an SV-GPS-250 to continue to receive TIS-B / ADS-B Traffic through December 31, 2019. If you are a US pilot and have an SV-GPS-250 (but not an SV-GPS-2020) on your aircraft, information on configuring your SkyView system for SIL/SDA=1 is found in SV-XPNDR-261 / 262 Installation, Configuration, and Testing.

Fantastic!
Does the new V14 software put more stringent requirements on the 250's position as well or will it continue to operate as it did before V14?

 

[Dynon]

Put another way, though - the SV-GPS-250 will still maintain lock in the same situations as it always has, whether set to SIL 1 or SIL 2. So the shortest answer to your question is "no".

 

[KRviator]

Bill:

What you're observing is the reality of the SV-GPS-2020 having to be more "deliberate" in reporting its position to the system. This is one of the reasons that in Revision W of the SkyView System Installation Guide, we recommended that if you have an SV-GPS-250 already installed, that you consider leaving the SV-GPS-250 in place, and installing the SV-GPS-2020 in ADDITION to the SV-GPS-250, and simply demote the SV-GPS-250 to "POS2". That way you can be compliant with the FAA 2020 ADS-B Out Mandate, and your SkyView can fail down to the SV-GPS-250 if the SV-GPS-2020 loses lock.

Put another way, though - the SV-GPS-250 will still maintain lock in the same situations as it always has, whether set to SIL 1 or SIL 2

Putting these two together, I have a GPS-2020 and a GPS-250 in the RV. Naturally, the GPS-2020 is primary and is selected for the GPS positon source for the transponder, but if that loses lock, will the transponder revert the GPS-250 to broadcasting SIL=1 for ADS-B integrity?

ISTR a comment "somewhere" - that I can't find anymore, naturally -  that if the position source failed it had to be manually changed, but the comment above suggests otherwise. Or is it only for mapping and SynVis that the GPS will automatically fail over to the -250?

 

[lolachampcar]

Put another way, though - the SV-GPS-250 will still maintain lock in the same situations as it always has, whether set to SIL 1 or SIL 2. So the shortest answer to your question is "no".

I did read the manual so thank you for the second post
My concern was that changing from a sda/sil of zero to one would have the 250 loosing lock in V14 where it would not have in previous firmware revisions.

Thank you for the clarification.  Two receivers it is for me!

 

[dynonsupport]

No, the transponder GPS SOURCE will not "fail down" from the SV-GPS-2020 to the SV-GPS-250.

For its use of GPS data, such as MAP and Synthetic Vision, SkyView will fail down from SV-GPS-2020 to the SV-GPS-250 (but that doesn't include passing position data to the transponder).

If you'd prefer to use the SV-GPS-250 as the position source for SkyView AND your transponder, that's a legal option up until 2020 - just select the SV-GPS-250 as the GPS source in TRANSPONDER SETUP and SV-GPS-250 as POS 1 in SERIAL PORT 5 SETUP.

Putting these two together, I have a GPS-2020 and a GPS-250 in the RV. Naturally, the GPS-2020 is primary and is selected for the GPS positon source for the transponder, but if that loses lock, will the transponder revert the GPS-250 to broadcasting SIL=1 for ADS-B integrity?

ISTR a comment "somewhere" - that I can't find anymore, naturally -  that if the position source failed it had to be manually changed, but the comment above suggests otherwise. Or is it only for mapping and SynVis that the GPS will automatically fail over to the -250?

 

[lolachampcar]

Ooops.........
Back to the 250 for me if the ADSB-Out source does not revert to the 250 when the 2020 looses lock.

To be absolutely clear to anyone reading this thread not familiar with Dynon's GPS', they are excellent. It is only the aerobatics that are driving my issue, not Dynon's hardware or system software.

 

[swatson999]

So this is a pretty good question for the post-2020 world...what *are* the requirements on position reporting viz-a-viz aerobatic flight? Would one be in violation of an FAR (post-2020) if their ADS-B stopped reporting for some short period of time while the GPS recovered?

 

[lolachampcar]

The regs say you have to have it.  They do not say anything about the "up" time

It is unreasonable to think a single antenna GPS will maintain lock while it can not view sats. I guess the real question is "does an aerobatic aircraft have to have dual antennas with antenna diversity to be legal?".

 

[swatson999]

The regs say you have to have it.  They do not say anything about the "up" time

It is unreasonable to think a single antenna GPS will maintain lock while it can not view sats.  I guess the real question is "does an aerobatic aircraft have to have dual antennas with antenna diversity to be legal?".

That was kind of what I was getting at...dual antennae, top and bottom, for those who like to go inverted from time to time?

I imagine there is some sort of "escape clause" in the regulations about system reliability "during normal flight", for example, or some statement about during flight where bank angles don't exceed X, etc. Anyone know?

 

[airguy]

Think you can design a GPS antenna that can establish a lock, to FAR 91.227 requirements, in the split second required as the belly comes up above the horizon and the top receiver breaks lock?

Good luck with that...

 

[swatson999]

Think you can design a GPS antenna that can establish a lock, to FAR 91.227 requirements, in the split second required as the belly comes up above the horizon and the top receiver breaks lock?

Good luck with that...

Thus my question...although from an engineering standpoint, that's quite a challenge, indeed

 

[dynonsupport]

DO-229 is the performance document for WAAS GPS sensors called out by TSO C145. It says:

2.1.2.5 Dynamic Tracking
When the aircraft has dynamics within normal maneuvers defined by the maximum
ground speeds and accelerations shown below, the equipment shall meet the accuracy
requirements of 2.1.2.1 and the satellite acquisition time requirements of 2.1.1.8 and the
satellite reacquisition requirements of 2.1.1.9. Note that g = acceleration of gravity = 9.8
m/s2.

Ground Speed: 800 kt
Horiz Accel : 0.58g
Vertical Accel: 0.5g
Total Jerk 0.25 g/s

Abnormal maneuvers are defined to be maneuvers whose accelerations/jerks exceed these
values, up to the maximum ground speeds and accelerations shown below.

Ground Speed: 800 kt
Horiz Accel : 2g
Vertical Accel: 1.5g
Total Jerk 0.74 g/s

During abnormal maneuvers, the equipment shall not output misleading information.
When the aircraft returns to normal maneuvers from abnormal maneuvers, the equipment
shall meet the steady-state reacquisition requirements of Section 2.1.1.9. During the
abnormal maneuver period, loss-of-navigation capability and loss-of-integrity monitoring
alerts and outputs shall function as specified.

So you can see from the above that there is no expectation that the GPS will keep up with an aircraft in much more than standard IFR procedures.

2.1.1.8 and .9 aren't perfectly clear what to do when you lose the complete solution as you go upside down, but allow between 20 and 134 seconds to re-acquire.

The ADS-B regs (91.225 and 91.227) don't say anything about your position source outputting valid position. What they actually say is you can't output misleading position. So it's a requirement for a GPS to stop the instant it isn't sure anymore.

I've argued a brick meets the requirements of 91.225 and 91.227 before, since it never outputs a misleading position

 

[lolachampcar]

Thanks.. Fantastic information.