F

F1R

Well Known Member
Here is a question for the synthetic vision manufacturers. In light of the recent developments that may compromise GPS signal reliability, how possible would it be to develop an inertial navigation failover mode for periods of lost GPS signal? I am thinking that the better quality AHRS sensors should be capable of providing the raw data needed for processing. Perhaps some systems already are capable of providing an accurate image and nav data with a lost GPS signal?

If not it could be a good opportunity for enhancing safety, reliability and market share.
 
If you're talking something like an AHRS derived 3D positioning, there is no way. Even the most compact laser ring systems are hundreds of thousands of dollars. You would need something like that at a minimum to avoid short term precess that would result in any level of accuracy. Even the BEST INS systems are backed up positionally by GPS, VOR/DME database x/c, etc. The INS in the jet I fly at work is one of the best in the world, and the acceptable level of precess is still 1nm/hour. Not NEARLY accurate enough for precision navigation in the weather.

A cheaper alternative, is a map and the window, and backing your position up per sound pilotage and not being lazy. For IFR flight, an en route and a VOR/DME.
 
Sig600 said:
If you're talking something like an AHRS derived 3D positioning, there is no way. Even the most compact laser ring systems are hundreds of thousands of dollars. You would need something like that at a minimum to avoid short term precess that would result in any level of accuracy. Even the BEST INS systems are backed up positionally by GPS, VOR/DME database x/c, etc. The INS in the jet I fly at work is one of the best in the world, and the acceptable level of precess is still 1nm/hour. Not NEARLY accurate enough for precision navigation in the weather.

A cheaper alternative, is a map and the window, and backing your position up per sound pilotage and not being lazy. For IFR flight, an en route and a VOR/DME.
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I've been thinking about this for a while. I don't know why the EFIS guys havent built it in yet. You could use this http://www.sparkfun.com/products/9623 continuously update it with GPS (if available). Basically make an EGI. Then when you lost GPS you would still have NAV info. BTW, I don't know what INS you are talking about but the Mil Std is 0.8/hr and the RLGs I've flown were usually more like .1/hr. I think we wrote them up at 0.9/hr or greater.
 
Like Sig600 said, cost is the prohibitive factor with navigation-grade INSs (like the military and airliners use) going in the range of hundreds of thousands of dollars. With the Sparkfun thing, even though its cheap, without the aiding of the GPS it's accuracy goes down the tubes very quickly. In fact I would venture a guess that it's only slightly better than the INS in your inner ear when your eyes are closed.

You could aid it with GPS but that would take your overall position accuracy from say 3 meters to 2.9 meters. Once you lost GPS, the position would get off in more or less 30 seconds.
 
cguarino said:
I've been thinking about this for a while. I don't know why the EFIS guys havent built it in yet. You could use this http://www.sparkfun.com/products/9623 continuously update it with GPS (if available). Basically make an EGI. Then when you lost GPS you would still have NAV info. BTW, I don't know what INS you are talking about but the Mil Std is 0.8/hr and the RLGs I've flown were usually more like .1/hr. I think we wrote them up at 0.9/hr or greater.
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Anything less than a 1.5/hour we accept to the INS update before shutdown. >1.5 warrants maint action. Assuming you don't have GPS aiding (which is kind of the point of this discussion.)
 
I've always thought a better backup to GPS would be a system w/ a NAV radio that automatically tunes and idents VORs and triangulates the aircraft's position. I know little about jets but my understanding is that some of them have the ability to do this.

Edit: ok, triangulate isn't the word since you really only need two VORs, but you get my point.
 
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Also, for those that may not know, fighters generally have a larger acceptable INS error per hour due to the yanking and banking they do compared to a transport aircraft or airliner. Sig600 flies fighters, his numbers are accurate for non-GPS aided INS in his aircraft. I know because I flew the same ones. That same INS is likely to have only 0.1-0.3 NM error on a cross country flight. Even most yanking and banking flights it is only 0.5-0.7.
 
Jamie said:
I've always thought a better backup to GPS would be a system w/ a NAV radio that automatically tunes and idents VORs and triangulates the aircraft's position. I know little about jets but my understanding is that some of them have the ability to do this.

Edit: ok, triangulate isn't the word since you really only need two VORs, but you get my point.
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My knowledge/experience is about 10 years old, but the FMS system in the CRJ does just that. There is a hirearchy of what the system uses to determine / cross check positioning. Monitoring/XC of VORTACS was 2nd or third tier, GPS was like 5th in the list for the system to use.
 
After looking at the SL-30 install manual (which explains all of the RS-232 communications available to the SL-30) it appears to be possible to tune the active and standby NAV frequencies on the radio and of course receive the station identifier (derived from morse code in the SL-30) and all other pertinent information.

If there was sufficient demand it would be possible for EFIS manufacturers to interface with this radio (and possibly others) to provide auto VOR tuning en-route. The units could then fail over to VOR-based navigation in case of GPS interference, etc.

Obviously accuracy would be proportional to the distances from the VORs.
 
Cross thread

I already have an SL30 on full time duty with a TSO'd CDI for VOR and ILS . It works great and at the moment I am quite happy I have not spent a bundle on a 530W or it's new replacement due to the compromised GPS reception several pilots have recently encountered.

To the original question, it may well not soon be possible or economical to pursue an INS built into the modern SYN VIS EFIS systems we know today. Since both deal with lat. and long. numbers, it appeared doable at first consideration. From the experts and manufacturers I am told the quality of inertial sensors to make it a reality are cost prohibitive.

Looking outside the canopy will remain the most enjoyable way to fly.:)
 
JPL developments

I was at the JPL open house last week and with the advances in their MEM's gyro's in just the last three years I would not be surprised to see the cost and accuracy hit the consumer market in the next two or three years.:rolleyes:
 
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