Good GPS article/book to prep for Oral portion of IFR Checkride

[ruthsindelar]

I have a student taking a checkride with an examiner that everyone loves but warns that he very big on GPS questions (ie, how many satellites do you need to use GPS enroute? What if you lose one satellite enroute? Could a satellite be operational but somehow be "lost" to your unit?, etc).

Any suggestions on a good article or book that covers these types of questions that my student could read prior to her checkride? This examiner actually helped design the GPS system for the FAA so his knowledge is quite deep. Very fair examiner but really likes the students to know the GPS inside and out....

Thanks

 

[gismo]

I have a student taking a checkride with an examiner that everyone loves but warns that he very big on GPS questions (ie, how many satellites do you need to use GPS enroute? What if you lose one satellite enroute? Could a satellite be operational but somehow be "lost" to your unit?, etc).

Any suggestions on a good article or book that covers these types of questions that my student could read prior to her checkride? This examiner actually helped design the GPS system for the FAA so his knowledge is quite deep. Very fair examiner but really likes the students to know the GPS inside and out....

Thanks

The basics are pretty well covered in the appropriate chapter of the AIM. If you want a more complete reference John Ekalbar's book: "Instrument Flying Update".

http://www.mypilotstore.com/mypilotstore/sep/4568

 

[poadeleted20]

In addition, most of those are inappropriate questions. All you should need to know about that stuff to pass the test (in addition to how and when to do a RAIM prediction and understand GPS NOTAMs) is how to interpret the warnings your box gives you and what to do if you get one. You are only required to know what time it is, not how the clock's parts are made.

 

[TMetzinger]

Have him bring this to the checkride, just to see if the Examiner remembers it:

Inertial Navigation (IN) Theory Made Easy
The following explanation, in simplified terms, is typical of all IN systems:

1. An aircraft knows where it is at all times; it knows this because it knows where it isn't.
By subtracting where it is from where it isn't, it obtains a difference, termed deviation.
2. The IN system uses deviation to generate a corrective command to drive the system from
a position where it is, to a position where it isn't, arriving at a position where, if it wasn't,
it is now. Consequently, the position where it wasn't follows the position where it was to
the position where it isn't.
3. Futhermore, the position where it is now is not the position where it wasn't, and the system
has therefore acquired a variation. A variation is difference between where the aircraft is
and where it wasn't, and is considered to be a significant factor which needs correction.

The logic of the system is as follows:
Because variation has modified some of the information obtained by the aircraft, the
aircraft is not really sure where it is; however, it is sure where it isn't (within reason),
and it knows where it was. The IN system now subtracts where it should be from where
it wasn't (or vice versa) and by differentiating this from the algebraic difference between
its deviation and variation, (which is called error), it computes the correct information to
compensate for all factors.
​

 

[dmccormack]

Have him bring this to the checkride, just to see if the Examiner remembers it:

Inertial Navigation (IN) Theory Made Easy
The following explanation, in simplified terms, is typical of all IN systems:

1. An aircraft knows where it is at all times; it knows this because it knows where it isn't.
By subtracting where it is from where it isn't, it obtains a difference, termed deviation.
2. The IN system uses deviation to generate a corrective command to drive the system from
a position where it is, to a position where it isn't, arriving at a position where, if it wasn't,
it is now. Consequently, the position where it wasn't follows the position where it was to
the position where it isn't.
3. Futhermore, the position where it is now is not the position where it wasn't, and the system
has therefore acquired a variation. A variation is difference between where the aircraft is
and where it wasn't, and is considered to be a significant factor which needs correction.

The logic of the system is as follows:
Because variation has modified some of the information obtained by the aircraft, the
aircraft is not really sure where it is; however, it is sure where it isn't (within reason),
and it knows where it was. The IN system now subtracts where it should be from where
it wasn't (or vice versa) and by differentiating this from the algebraic difference between
its deviation and variation, (which is called error), it computes the correct information to
compensate for all factors.
​

Interesting, but if an IR student spouted that off during an oral, my guess is the DE will mutter, "Let's see how well you can actually fly, brianiac..."

 

[gismo]

Interesting, but if an IR student spouted that off during an oral, my guess is the DE will mutter, "Let's see how well you can actually fly, brianiac..."

I was thinking that the DE would be laughing so hard he'd forget all about asking stupid GPS questions (maybe that's what you meant?).

 

[dmccormack]

I was thinking that the DE would be laughing so hard he'd forget all about asking stupid GPS questions (maybe that's what you meant?).

Yeah, that too.

 

[ruthsindelar]

In addition, most of those are inappropriate questions. All you should need to know about that stuff to pass the test (in addition to how and when to do a RAIM prediction and understand GPS NOTAMs) is how to interpret the warnings your box gives you and what to do if you get one. You are only required to know what time it is, not how the clock's parts are made.

Thanks for your reply Ron. Would you be willing to share what you tell students regarding those points? We all have our own styles and what we feel is important and I would value hearing what you share. Sounds like you do a great deal of IFR training and your perspective would be greatly appreciated.

Best

 

[poadeleted20]

Thanks for your reply Ron. Would you be willing to share what you tell students regarding those points?

Whew -- that's usually a couple of hours of training, discussing all the issues surrounding GPS operations. However, to the specific points you raised...

how many satellites do you need to use GPS enroute?

I do teach them the concepts of the four satellites for position and the fifth for RAIM, but I tell them their GPS takes care of all that internally, so they don't need to worry about how many satellites they're currently receiving. If there aren't enough, the GPS will tell them, and then they must fall back to an alternate means of navigation.

What if you lose one satellite enroute?

Unless you're monitoring the satellite data page (and I don't teach that), you'd never know you "lost" one satellite while enroute unless it was the one which dropped you from five to four (in which case you get a RAIM light) or from four to three (in which case you get a "don't use the GPS" alarm and revert to VOR's). Either way, it's not something to watch -- the GPS takes care of that for you.

Could a satellite be operational but somehow be "lost" to your unit?

Not an issue I cover, as it's not something you control or even need to monitor. Hence, an inappropriate question for an IR applicant. And if asked, I'd say the answer should be "I don't know, and it doesn't matter -- if there's a problem, the GPS will tell me and I'll take appropriate action."

Remember -- it's a practical test, not a test on GPS theory and certification standards.

 

[Capt. Geoffrey Thorpe]

What if you lose one satellite enroute? Could a satellite be operational but somehow be "lost" to your unit?, etc).

Depending on how far you go, satellites can disappear over the horizon.

And, if you've ever used a GPS other than in an aircraft, you can watch the satellite display and see them come and go all the time as you go under trees, change directions so they are blocked, etc. I would assume the same would happen if you have a yoke mount - the satellites you get at any moment will depend on the direction you are heading.

 

[poadeleted20]

And, if you've ever used a GPS other than in an aircraft, you can watch the satellite display and see them come and go all the time as you go under trees, change directions so they are blocked, etc. I would assume the same would happen if you have a yoke mount - the satellites you get at any moment will depend on the direction you are heading.

That's true, but we're talking installed IFR GPS here, and their antennas are supposed to be better positioned than that. And if trees are in the way, you've got other, larger problems.

 

[NelsonMinar]

Wikipedia's articles on GPS are good for background. Start with GPS, skip the math. Some other good articles are linked from there. Particularly WAAS, RAIM. Usual caveats about Wikipedia accuracy, but the GPS articles I've read have been a cut above the usual Wikipedia standard.

I'd also be sure to be up to speed on the recent WAAS satellite outage.

 

[ruthsindelar]

If you want a more complete reference John Ekalbar's book: "Instrument Flying Update".

===

Thought I would report back that I am finishing up Ekalbar's "Instrument Flying Update"....It is EXTREMELY well done. Lots of useful information. My GNS430 is non-WAAS but I now have a strong background in WAAS now. Highly recommend it.