There was some discussion a few weeks ago about the source of altitude information for the Arnav ICDS 2000 unit (esp. for it’s Terrain Obstruction Proximity System, or TOPS). Does it ‘read’ the transponder, or the GPS?
All I’ve spoken to at Cirrus, Wings Aloft and Arnav, now agree: Altitude information gets to the ICDS display from the Garmin GNS 430.
Just want to be sure we are clear here. "Getting altitude from the Garmin 430 can mean one of two things.
If the GPS is in 2D mode (see pg 43 of GNS 430 Pilot Guide and Reference), the 430 is getting altitude data from an altitude serializer (probably the transponder).
If the GPS is in 3D mode, the 430 is caldulating altitude using satellite data.
On the surface, 3D mode sounds “better”, but because of the geometry, current non-WAAS GPS have a very difficult time calculating altitude accurately. Errors of a couple of hundred feet are not uncommon.
Thus, to “really” answer the question below, need to know if the 430 in the SR20/22 is reading data from the transponder (or other altitude serializer) or computing it. Believe the better answer would be the altitude serializer. I have a dedicated altimiter/serializer for my Apollo at present. Work great.
A current owner can tell from the Satellite Status Page, NAV page #5. STATUS should show either 2D Navigation or 3D Navigation. Which is it ? ?
There was some discussion a few weeks ago about the source of altitude information for the Arnav ICDS 2000 unit (esp. for it’s Terrain Obstruction Proximity System, or TOPS). Does it ‘read’ the transponder, or the GPS?
All I’ve spoken to at Cirrus, Wings Aloft and Arnav, now agree: Altitude information gets to the ICDS display from the Garmin GNS 430.
- Mike.
A current owner can tell from the Satellite Status Page, NAV page #5. STATUS should show either 2D Navigation or 3D Navigation. Which is it ?
Bob,
Great catch, and great question. I don’t have my GNS430’s (with SR20 attached) yet - 16 days to go! - but I’d bet that the 2D/3D isn’t fixed. What I mean is, it depends on how many satellites (and in what configuration) it is receiving at any given moment as to whether it CAN provide 3D info or not. At least, that’s how it works on my handheld GPS 295. (By the way, 99% of the time, it is in 3D mode. Generally speaking, the only time I even see 2D is shortly after power-up, while it is still acquiring satellites.)
Here’s what the GNS 430 manual says on the subject:
2D Navigation – The GPS receiver is in 2D navigation mode. Altitude data will be provided by an altitude serializer.
3D Navigation – The GPS receiver is is 3D navigation mode and will compute altitude using satellite data.
It could be that somewhere in the GNS 430 is a menu item to FORCE 2D mode, and thereby force the use of the serializer. I don’t know; I also don’t know if we even HAVE a serializer; or is the serializer part of the GPS?
Clyde, other owners: What’s the real answer?
Mike.
It could be that somewhere in the GNS 430 is a menu item to FORCE 2D mode, and thereby force the use of the serializer. I don’t know; I also don’t know if we even HAVE a serializer; or is the serializer part of the GPS?
The 430 does get altitude information from the altimeter (same source as your transponder - uncorrected pressure altitude). It’s a certification requirement in Australia for an IFR GPS to have this input connected.
However, the altitude data provided by the 430 to the Arnav is GPS altitude, and as has been pointed out, can be in error by a few hundred feet. If I watch the altitude readout on the Arnav when at say 8000’, I have typically found it to read high, by anything up to 500’. Of course this could also be accounted for by the fact that the altitude input is uncorrected. I was comparing it with corrected altitude. I’ll make a note next time of what the altimeter setting is.
OTOH when sitting on the ground, it seems to give an answer much closer to the field elevation.
2D mode is essentially a non-issue, because you will rarely see less than 7 satellites in view. I have seen the 430 indicate loss of RAIM on only one occasion, it would drop out for 30 seconds or so then come back, this lasted in total for about 5 minutes. The second GPS was doing the same thing, though in the opposite phase.