SmilingJack

SmilingJack

Well Known Member
Hello! I have an AFS 4500 with SV with a G3X feeding it the GPS signal, etc...

When I line up for take off I see the runway, but it is off to the right.

My question is how accurate is your GPS for those using SV?

When I talked to Garmin they tell me since I am not WAAS that I can expect +or - 45 feet. My car nuvi is more accurate then that!

Thanks!
 
Your car Nuvi, or any other automobile GPS, is probably NOT more accurate than that. What the Nuvi does is ASSUME that you are on the road, and so depicts your position smack dab on the road on the map.

Until you stray pretty far off the road, that is.

If your aviation GPS were to do the same thing, it would assume you are about to takeoff on the runway, and shift the map several feet to show the runway straight ahead.
 
Last edited:
My 50-dollar Garmin WAAS GPS was a lot more accurate than 45ft. Why is that one not WAAS?
 
What exactly do you mean by "G3X feeding it the GPS signal?"


Are you feeding the NMEA signal out of a G3X into the AFS? My guess is that you mean something else. Perhaps one of the portable garmins or a "puck GPS" like the GPS 18x...

AFAIK the G3X is WAAS capable.

If it is a G3X, which SV screen is showing the deviation, or are both the G3X and the AFS4500 showing the same thing?
 
James, my GDU375 has a GA57x gamin antenna (non-waas)hooked into it and then the GPS signal feeds into my AFS 4500 using NMEA/AVTN.

When I fly directly over an airport I look at my GDU375 screen and the airport symbol is not exactly under me.

When I am on the runway at an airport my AFS4500 SV is showing the runway, but it is not right under me and is off to the side showing I am off in the weeds.

Both displays seem to be giving me the same accuracy.

I'm not sure what PChunt was telling me about the Car Nuvi, it does seem to me more accurate then my airplane GPS system at this point.

Thanks all for the input so far.
 
SmilingJack said:
James, my GDU375 has a GA57x gamin antenna (non-waas)hooked into it and then the GPS signal feeds into my AFS 4500 using NMEA/AVTN.

(snip)

Both displays seem to be giving me the same accuracy.

(snip)
Thanks all for the input so far.
Click to expand...

Ahh, OK.

The GDU375 has a waas receiver, so my understanding is that you should be getting better accuracy even without the TSOd antenna.

What does the satellite page on the GDU say?
 
The problem is most likely not with the GPS or the Antenna. In your configuration the AF-4500 is getting the GPS position from the G3X using RS-232 NMEA messages. The accuracy of the AF-4500 is dependent on how many decimal places the Latitude and Longitude is being sent from the G3X.

The circumference of the Earth at the equator is 40,075,161.2 meters. The equator is divided into 360 degrees of longitude, so each degree at the equator represents 111,319.9 meters or approximately 111 km. As one moves away from the equator towards a pole, however, one degree of longitude represents a diminishing number of meters, approaching zero at the pole. The number of decimal places required for a particular accuracy at the equator is:

Decimal Place Degrees Distance

3 .001 111 Meters - 364 Ft
4 .0001 11.1 Meters - 36.41 Ft
5 .00001 1.11 Meterrs - 3.64 Ft

Trouble Shooting runway position errors:

1. Is there a setting in the GPS that allows you to configure the number of decimal place that are sent in the NMEA message? How many decimal places are being sent?

2. Are you running the latest software and database in your AF-4500 EFIS?

3. The current position that the AF-4500 EFIS is receiving from the GPS can be displayed from the following menu: CHECK -> MAINT -> ABOUT

An interesting test is to take a picture of this page and then use Google earth to verify the actual runway position. Send me a picture of this page and I will help you figure out the location of your problem :)

Rob Hickman
Advanced Flight Systems Inc.
N401RH RV-4 N402RH RV-10
 
Hi John,

Did you receive my e-mail last week about this issue? I think we need to try increasing the baud rate sent from the GDU-375 to 9600bps.


Rob had a good suggestion regarding the ABOUT page. Write down that LAT/LONG (which we are receiving from the GDU-375) and put it in Google Earth to see exactly where that position is. It could be that we aren't receiving detailed enough position information, could be the non-WAAS antenna you are using, or it could be an error in our airport database. Some pictures would be great to help mitigate this issue.
 
WOW! That is a very cool reply from Rob! Still digesting it.

I'll take a picture tomorrow and email it to you.

As for upping the baud rate, yes I did. But, I had to get creative since the G3X has limitations on what selections you can apply.

In order to et the AFS4500 to a baud rate of 9600 I had to select in my G3X TIS In/ NMEA+VHF Out.

If you remember, when I used Aviation In/NMEA+VHF Out I kept getting a "RS-232 error on the G3X since I don't have an "IN" to it.

So, using the TIS IN/NMEA+VHF Out tricks the G3X into not looking for the RS-232 and I therefore don't get an error message and I am able to use 9600 baud.

more to follow on the Long/Lat decimals.

Thanks!
 
Rob,

I dug into the G3X setup menu and found I have several options:
HDDD.DDDDD
HDDD MM.MMM'
HDDD MM'SS.S"

Also, when I plotted the long and Lat from my "About" screen on the AFS4500 it located me just outside my hangar - which is where I tested it tonight.

After I get your recommendation on which format would be the most accurate I will move this test to the runway and see what I get for Long/Lat.

Seems like I am getting closer.
 
G3X Data Output Errors

It looks like the G3X is not outputting the same position as it is displaying, it might be the Map Datum setting in it.

The discrepancy in lat/lon between what was shown on the GDU-375 vs. what was shown on the AF-4500 display. We ran a Serial Port Raw Capture utility so we could see what was actually sent to us via Serial output on the GDU-375 (configured for NMEA +VHF output).

Below is a transcript from the G3X serial output.

$GPRMC,174621,A,4137.7482,N,07352.6953,W,0.0,327.8,221210,13.4,W,D*21
$GPRMC,174622,A,4137.7484,N,07352.6953,W,0.0,327.8,221210,13.4,W,D*24
$GPRMC,174623,A,4137.7485,N,07352.6954,W,0.0,327.8,221210,13.4,W,D*23

This -4137.7482,N,07352.6953,W – translates to N 41 37 44.892 W 73 52 41.718

At 17:46:21 the GDU-375 display displayed the lat/long – N 41 37 48.0 W 73 52 43.9.
Our screen was showing the following lat/long (received from the serial port) N 41 37 44.83 W 73 52 41.63
As you can see, we are displaying exactly what is coming out of the serial port. Yet the GDU-375 is not outputting the same lat/long shown on the screen through the serial port. The lat/long shown on the screen is the correct one for the actual aircraft position.

The actual lat/lon of the 24 numbers on the runway are indeed what the GDU-375 is displaying (N 41 37 48.0 W 73 52 43.9). So the display on the GDU-375 is correct, however it is not outputting the same lat/long on the serial port.

Rob Hickman
Advanced Flight Systems Inc
N401RH RV-4 and N402RH RV-10
 
Last edited:
G3X GPS Settings

You should verify that you have the following settings in your G3X:

Location Format
hddd mm.mmm'

Map Datum
WGS 84

Email me your phone number if you need help over the weekend.

Rob Hickman
Advanced Flight Systems Inc.
N401RH RV-4 N402RH RV-10
 
Last edited:
Thanks Rob, You and Trevor have been great helping me with this interface issue. I'll try that and let you know.

By the way, the beta software with the "Six Pack" is very cool! I'm glad you let me try it out while trouble shooting this GPS deal.

Thanks,
 
Solution Found!

To close out this thread I am posting the findings.

In order for the G3X to give the AFS4500 the data it needs to properly display accurate SV the G3X com must be set to "Normal NEMA" speed. If it is set like mine was to "Fast NEMA" the Long/Lat data is digested by the AFS, but the GPS Altitude doesn't make it in, so it is not read.

With Trevor's persistence at AFS and Matt's attention at Garmin we were abel to solve the issues.

It would have been quicker, but as all of you can relate...snow, chores and oh...work get in the way!

I am just happy to deal with 2 top notch companies that won't give up! Beyond EXCELLENT customer support! Thanks Guys!
 
You must log in or register to reply here.