Simple Radio Nav Q

[iflyhighinthesky]

This may be quite simple but i really don't get prf, pri stuff!

Could anyone go through it to the answer please?! Is Rnav a 2nd language to anyone else?!!!!

The maximum pulse repetition frequency (PRF) that can be used by a primary radar facility in order to detect targets unambiguously at a range of 50 NM is: (pps = pulses per second)

a.713pps
b.610pps
c.3240pps
d.1620pps

:confused:

[High Wing Drifter]

Max Range in Metres = C / 2 * PRF

1620 ~= ( 3x10^8 / (50*6080/3.28) ) / 2

[VC10 Rib22]

iflyhighinthesky,

From your post I fear that HWD's reply may presume too much of you. If I'm wrong I apologise. Maybe this would help:

The radar pulse must travel 185km [ 100nm (50nm to the target and 50nm back from the target)] before the next pulse transmission.

The time for this journey (Time = Distance/Speed) equals 185000(metres)/300000000 (seconds) (speed of light)
i.e. 0.000617 seconds which equals 617us [617 x 1/(10 to the power of 6)] i.e. the PRI = 617us

PRF(pps) = 1/PRI = 1000000/617 = 1620pps

Hope this helps

VC10 Rib22

[LewisS]

:sad: you guys are slowly putting me off becoming a comercial pilot, fairplay

Lew

[ATPMBA]

Answer D.

Your study manuals should cover that subject clearly. It's one of the more basic questions.

[FFP]

Very important this. You will use this alot in day to day Ops . . . . . .

Don't confuse the knowledge required to pass the exams with life in a big jet. Some of it is very useful, other bits you'll never see again.

[iflyhighinthesky]

Cheers guys,

I know it is a simpler one, it is just an area of rnav i am crap at, and unless you get the simple stuff sorted whats the chance of progress??!! We all have weaknesses,ATPMBA.

VC10, thats great thank you!

ifly:ok:

[CPL_Ace]

None of it is simple at first - sometimes it just "clicks" what you are doing. Sometimes it just pays to learn some answers.

A pulse must travel there and back at the speed of light (as they always do) so you work out the time it takes to do that using a simple

TIME = DISTANCE/SPEED
(Make sure that if you are using 3000000 m/s as your speed, that you are also using KILOMETRES or METRES FOR YOUR DISTANCE.

Your answer is the time it takes for one Pulse and will be a fraction of a second and so all you have to do is work out how many of these fractions fit into one second to give you Pulses per second.

I found it easier sometimes to relate these questions to something far less daunting than aviation because sometimes that's the main hurdle. This I likened to bouncing a ball. PPS is the same as Number of bounces per second! (Just don't tell your Training Captain!!)

[VC10 Rib22]

You are more than welcome. A question is only easy if you know the answer, and what one person considers easy another may consider difficult. Whilst ATPMBA is correct in stating that your manuals should cover this topic, I know the quality of manuals can vary significantly, which may explain your difficulty, and I'm sure he/she didn't intend to come across as condescending. RNav can be tricky, especially all the satellite stuff, but put the work in and you'll be fine.
VC10 Rib22
:ok:

[High Wing Drifter]

A question is only easy if you know the answer
Or if you know that the question is asking or know how to arrive at the answer. FWIW, I found simply memorising all the formulas/equations and conversions a general godsend. There are plenty of shortcuts that I didn't bother with, so long as you can convert metres, feet, miles, nm, km, Newtons, KG, lbs, etc and rearrange the terms of the equations then that must be a good 15% of effort required to pass many of the exams.