Radio Navigation queries

[Jinkster]

It has come to my attention that having got 74% in Radio navigation last time "every stone needs to be unturned" and I am a little confused on some of these questions.

If anyone can help me bearing in mind I find maths a pretty challenging subject I would be much appreciated.



1) Two NDBs, one 20nm from the coast and the other 50nm further inland. Assumning coastal error is the same for each, from which NDB will and aircraft flying over the sea recieve the greatest error?

Answer = NDB at 50nm (can I ask why because I thouhgt it was the NDB at 20nm)




2)The time taken from transmission of interrogation pulse to the reciept of the reply from DME groundstation is 2000 microseconds (ignore delay at the DME) Slant range is?

Answer = 165nm (was never sure how to do these questions)



3) The Maximum PRF required for range of 50nm is?

Answer = 1620pps (was never sure how to do this question either :rolleyes: )



4) Maximum theory range for the PRF of 1200pps is?

Answer = 67nm



5) PRF of a radar is 450pps, spped of light is 300,000kps. Maximum range of the radar?

Answer = 333km



6)To double the range of a primary radar, the power must be increased by a factor of ?

Answer = 16 (I thought this was 4)




Thanks all.

Jinkster

[FlyingForFun]

Question 1 - draw a diagram, and all will become clear. Draw the coast, the two NDBs, and the paths the radio waves will follow. No need for maths, but try to get the bend at the coast roughly the same for each NDB. Remember that the ADF doesn't know about the coast, so after you've drawn the path the radio waves follow, go back and draw the path the ADF think they follow, i.e. start from the aircraft position, in the same direction as the line you've already drawn, but don't bend it at the coast. All will become clear.

Question 6 - primary radar signals need to get to the aircraft and back again. To double the distance to the aircraft, multiply the signal strength by 4 (as you rightly say). To double the distance back to the radar head, multiply by 4 again. 4 * 4 = 16 - there's your answer.

The others I'll leave for someone else, or for when I've got more time.

Good luck!

FFF
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[quidam]

Jinkster,

Agree with above for your first and last questions.

My memory is swamped as I'm off to BGS in two weeks but.....


2) Figure that this is just a straight foward speed,distance time calulation. Speed=dist divided by time

The transmitted signal takes 2000 micros seconds to get there and back and so will only take 1000 micro seconds to cover the actual one way distance. Radio waves travel at speed of light. Sort the units to give speed in meters per second and time in seconds. That will give dist in metres. Convert that dist to nm (3280 feet to a kilometer or 1m = 3.28 feet). As they give no aircraft height they can only ask for slant range. If they wanted 'flat' range you'd then need to use pythagoras.


To my great shame I can't remeber the formula but I think you'll find that within your notes there is a max and min range formula for radar that uses prf/prp. Using that formula all you need to do is be careful with the units.

I'll look it up when I get home.


Q

[Alex Whittingham]

Q2. A DME signal travels from the aircraft to the ground station and is then re-transmitted back to the aircraft.

The time given of 2000 microseconds is 2000 millionths of a second, or 2/1000, two thousandths of a second.

At the speed of light the signals travel 162NM in the first thousandth of a second to get from the aircraft to the ground station, then another 162NM in the second thousandth of a second to get back. The range is 162NM. Lord knows where 165NM comes from but it is exactly like that in the exam.

Q3. The formula to find the maximum range of a pulse radar is c ÷(2 x PRF), where c is the speed of light If you turn this around you get PRF = c ÷ (2 x max range). Substituting thevalues given. PRF = 162,000 ÷ (2 x 50) = 1620 pulses a second.

Questions 4 and 5 use the same formula.

[Jinkster]

Thanks all for the help.

Jinkster

[going round]

I was at Bristol for the June/July period and every one of the questions you raised was covered there. I'll take question 4 as its a quickie.

To answer this question you have to appreciate the difference between a primary and a secondary radar. For a secondary radar, to double the range you would indeed have to up the power by a factor of 4. But in a secondary radar you are only interested in getting the signal out to the aircraft. The return signal is provided by the equipment on the aircraft using its own power.

The question however relates to a primary radar which has to reley on its own power to get the signal out to the a/c and back again with no assistance from an a/c transponder, hence it needs to be increased by a factor of 16.

I'm sitting in August. Wish me luck.

My applogies jinkster, that answer was for Q 6