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Which GPS does dead reckoning?

I have read various reports over the years that some models will continue providing navigation, based on last GS, track etc, following the loss of the GPS signal.

Crucially it would be interesting if say the autopilot (analog deviation or ARINC429) outputs continue to work.

What happens if you are flying an arc or some similar procedure?

Administrator
Shoreham EGKA, United Kingdom

The G1000 does Dead Reckoning.

The autopilot still works in DR mode.

However, for nav it won’t use GPS as a source. (of course)

VOR/LOC/NDB/DME tracking is fine though. Hdg & alt modes are unaffected as well.

Last Edited by Hodja at 17 Dec 16:05

Our GPS (and DME/DME) based Honeywell system also does DR when it loses all sources of radio navigation.

What happens if you are flying an arc or some similar procedure?

It just continues normally as if nothing has happened, but displays a “DR” annunciator and a flashing “MSG” light to inform you that it has switched to DR mode. Precision is quite good as long as you fly at constant altitude (i.e. no major change in wind).

EDDS - Stuttgart

Garmin GNS, GTN and G1000 at least support dead reckoning mode. Others might also. You can find detailed information in de pilot’s guide on how your equipment performs. On these Garmin products, the CDI stays active based on the dead reckoning. Important note: Dead reckoning is only available during enroute fase and that dead reckoning mode is time limited to 20 minutes. Offcourse it is not that accurate and it’s accuracy degrades over time. Within 20 minutes it either receive the GPS signal again, switches back to normal GPS mode, or it says GPS LOST after 20 minutes. When it displays GPS LOST the CDI (in GPS mode) is invalid.

Some analog autopilots don’t check the CDI flag, so it could be that it just centers → pilot interaction needed!

Aspen displays support two panel mount GPS systems. If they fail, it can resume on it’s own emergency GPS. Using the flight plane information from the panel mounted GPS. If the emergency GPS also fails, it is no longer able to produce position data

Last Edited by Jesse at 17 Dec 18:33
JP-Avionics
EHMZ

So what exactly happens in the DR mode?

It cannot work at all usefully unless the GPS has a heading input (X/Y/400Hz or ARINC429), and AFAIK most don’t – they have no need for it. I have just looked up the GNS430 IM and it can accept a heading input but it says it needs it only if you are using a stormscope or TCAS (which is normal – an MFD displaying such data needs heading too).

Ideally it should also have a TAS input, which is yet another complication, but IAS would be good enough.

Administrator
Shoreham EGKA, United Kingdom

The G1000 does Dead Reckoning.

Can a G1000 also determinate a position based on DME or VOR ?

DR mode is part of the WAAS TSO as defined in RTCA DO 229-C/D. It is not vendor specific.

KUZA, United States

So what exactly happens in the DR mode?

The GPS displays DR, and assumes you fly the aircraft as close to the values indicated on the GPS (heading + speed)
It calculates with this values to determine you new position, just as you would fly dead reckoning yourself.

JP-Avionics
EHMZ

Can a G1000 also determinate a position based on DME or VOR ?

No it won’t do that. I don’t know if they use their AHRS as aid using dead reckoning, as the AHRS itself also uses GPS as an aid.

JP-Avionics
EHMZ

It cannot work at all usefully unless the GPS has a heading input …

Either that – ours has heading and air data computer connections, i.e. IAS, TAS, temperature and barometric altitude, so it can even compute vertical profiles in DR mode – or as Jesse writes, it must assume that you fly contant track and ground speed. I think I’ve seen this with some Garmin GNS430/530 units briefly.

EDDS - Stuttgart
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