Radalt characteristics

[gravity32]

Can anyone tell me what is meant by "Tracking Capability"? For a Rockwell Collins LRA-900 it is stated to be 330ft/sec. Is this the limit of speed vertically or horizontally (descent rate or groundspeed)? If it is horizontal, what happens to accuracy if 330ft/sec is exceeded?

[mono]

I would try to use some common sense.

330ft per second is a little over 195kts. Well within the capability of the majority of a/c that have Rad Alt fitted.

It is also however a little under 20,000ft per minute. Which even for a fighter a/c is a pretty astonishing rate of climb/descent.

Civilian Rad Alts start to display at 2500ft (less than 7.5 seconds at 20,000ft per minute descent rate). Rates in excess of the tracking capability produce No Computed Data (NCD) indication.

Now which do you think it is?

[gravity32]

Thanks Mono, I am a great believer in common sense. And certainly common sense tells us that 330ft/sec must be the vertical speed limit. I have been arguing this point with some people who say it is the horizontal speed. They point out that while 200 knots is well within the speed of a typical commercial plane, it is substantially above the speed of a normal approach, where the radalt would be used. So they are not convinced. You don't happen to have a handy reference I could quote?

[grounded27]

You provided a good argument by math performed, allthough not usual, aircraft operate at 250kts below 2500ft at times and that RA better damb work. To limit RA operation to 195 kts velocity is a rediculous notion.

[gravity32]

Thanks grounded 27. Unfortunately those who dispute this common sense argument say that by the time a plane has descended to decision height it will be going less than 150 knots. They also say that Radalt should not be used earlier because there is no guarantee the ground does not slope and give a false indication. What I really need is a reference to an instrument manual, or text book which discusses it.

[grounded27]

Rad alt is required on most aircraft to report 1500ft and loc cap before G/S cap. on a coupled approach.

Also terrain avoidance, I can think of many times I have decended through terrain at high altitudes (baro) but were within close enough proximity to the terrain to necessitate it's use above 10k at or possibly faster than 250 kts.

[gravity32]

grounded27, I am not sure I understand your second paragraph:
"Also terrain avoidance, I can think of many times I have descended through terrain at high altitudes (baro) but were within close enough proximity to the terrain to necessitate it's use above 10k at or possibly faster than 250 kts." Are you saying that there were many times when you could not be sure of terrain clearance just from baro altimeter and position, so needed radalt? Does this mean you flying into a region where the terrain height was not known?

[grounded27]

No terrain height is known (computed and baro ref) but the rad alt will come alive and display height at speeds at or above 250kts, egpws would not exist w/o it to further prove the point.

Here is wiki ref for egpws, it would not function in all modes without radalt. Certainly a heavy jet is not climbing out or decending over terrain at speeds less than 195kts.

The wikiref has refrences to solid publications that should end your dispute.

Sorry I am not allways a clear as I could be, I hope I was clear enough this time.


Ground proximity warning system - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Ground_proximity_warning_system)

[mono]

As grounded has said. Rad Alt is used for more than a landing aid.

An a/c can be clean and at a considerable altitude and still be in proximity to terrain. The crew need to know this. I've been in an a/c when the GPWS has triggered (Mode 2 on initial approach into KTM) and I can tell you we were doing more than 195kts at the time (ground speed and TAS are a lot higher at altitude)

Sorry I don't have a reference but you may find one on the net.

[dClbydalpha]

This is marketed as a high precision low-range device, for use during final approach and flare. The unit is declared as certified under TSO - C87

This TSO calls for test under the conditions below

(1) Lateral velocities from 0 to 50 feet per second.

(2) Longitudinal velocities from 0 to 300 feet per second.

(3) Pitch angel (sic) range of 0 to ±15 degrees.

(4) Roll angel (sic) range of 0 to ±20 degrees.

(5) Vertical velocity from 0 to 15 feet per second up to 100 feet and 0-20 feet per second above 100 feet.
I think that makes it clear what the minimum design aims were for this unit. What it actually achieves I cannot comment on.

[mono]

That's only the test criterea. The accuracy should be within certain limits within the ranges shown.

The Rad Alt tracks altitude not ground speed. Therefore this is what the spec refers to.

I'll see if I can check with the workshop to confirm.

[dClbydalpha]

Unless Mono can get the tech spec out of the workshop, or someone else has the equivalent I suspect you won't get a definitive answer. But as this appears to be a crew room discussion, rather than an operational question, I'll throw my general opinion in. Hopefully it will help you construct an informed argument for your friends.

A radalt is a radar that tracks the ground, the altitude is an output from the range gate. If the radalt loses lock, for whatever reason, then it will declare unlocked and attempt to re-acquire, this could cause the output to become NCD. Similarly some radalts calculate a track quality, and decalre the data as NCD when the track quality drops below a certain threshold. Most radalts have a memory flag to avoid going NCD whilst attempting to re-acquire.

In level flight, over perfectly flat reflective ground, then the ground speed would have little effect. But as this is not the case then we have to consider it. The radalt will have a large footprint in an attempt to even out scatter effects, and more importantly detect a leading edge from the closest point of the ground under the aircraft. It will also, probably, have a means of eliminating doppler effects from the calculation. Amongst the possible solutions is using a saw tooth waveform and calculating on both the up and down slope. As such there are limitations to the processing. Ultimately the dominant factor will be the velocity along the radalt's range track direction. As the aircarft pitches up the radalt will begin to see a significant component of the groundspeed along this axis. Although the actual range track will stay fairly constant as the distant to the ground's closest point remains steady. Without knowing the details of the tracking algorithm it is impossible to say when the radalt will lose track. Similarly without knowing how track quality is used, it is impossible to say at what point that would cause a loss of data output. The TSO points to a set of max orthogonal velocities and a max attitude of the aircraft for which the radalt has to keep lock, and retain sufficient quality of track to meet the accuracy requirements. Outside of that I'm afraid I can't tell you what would happen.

In summary it is not so easy as saying at groundspeed x or vertical speed y the output will become invalid, there is a lot more to it than that, including aircraft manouevre and nature of the terrain. As I have said to my colleagues recently, it is all about when the radalt loses track, and it is wrong to assume that at a certain speed radalt data will automatically show as NCD on an FDR.

[grounded27]

Bottom line is that NCD is set by manufacturer at 2500ft agl for most commercial airliners. I am sure this unit is capable of more but the caa's and mfgr's agreed upon this.

[dClbydalpha]

The original question is not about what altitude the output is declared NCD, but how groundspeed may effect it.

The initial "common sense" responses hinted that groundspeed has nothing to do with it. I've tried to point out that it is not as simple as that. Particular when you bring pitch angle into it. As Mono pointed out, the radalt's primary output is altitude, and so a clear limit can be set at which you declare NCD. However, various other factors can cause the output to become NCD, but it is not as straight forward as saying that it's this particular vertical speed or that particular groundspeed. It is a set of conditions that cause a loss of "track", and the OP is unlikely to get a specific answer to that question without the tech spec.

There has been much recent internet discussion about whether a specific radalt would go NCD at a certain groundspeed, it made this thread seem very familiar to questions I have been asked by colleagues.

[grounded27]

The original question is not about what altitude the output is declared NCD, but how groundspeed may effect it.



Like I said 2500feet agl, be it at 135 kts and 50 feet or .86 mach at 42,000. this is what the unit and more importantly aircraft is certified for. This little discussion about angle and speed is a non sequitur.

[dClbydalpha]

Sorry you don't find my statements logical grounded.
I don't understand your statement about certification. The basis of certification I believe is the TSO and that is quite clear on the matter. Irrespective of aircraft.

I will repeat, in my experience, a Radalt will flag data as NCD whenever in Search mode (after mem has timed out.) This can be caused by many things, not just altitude. Speed may not be a direct cause, but is part of the set of causal factors, particularly during manouevre. So to say whether a radalt will go NCD is not always straight forward. The specification tells us when it won't under normal conditions. That is why the velocities are stated in the specification.

Is tracking capability "groundspeed", No. Is it "vertical speed", Not exactly. What will happen outside this limit of 330 ft/s. Outside the certification limits the accuracy is not guaranteed to be as per specification.

Grounded, you clearly have more detailed knowledge about this unit in particular so that is where I will leave it.

[mono]

or .86 mach at 42,000.

Actually grounded no! The whole point of this discussion is that if the tracking capability relates to a groundspeed rather than an altitude rate (i.e 330 ft/sec) then your statement above won't be true. Even if the the a/c were to be within 2500ft of the ground at 42,000ft its speed would exceed the tracking capability and therefore be NCD because 330ft/sec equates to only about 195kts.

Please RTFQ.

[grounded27]

You are saying that EGPWS is not capabile of responding at max airspeed and altitude of the aircraft that it is certified for?

[Meikleour]

Gravity32: With the older types of RADALT it was quite common to see the display briefly trigger from an opposite direction aircraft flying 2,000ft below when in the cruise! Modern ones don`t do this.

[gravity32]

dClbydalpha,

I have been looking at graphs of descent, comparing the radalt with calculated True Alt. While True alt shows a very steady decline, the radalt shows some irregularity. From what you are saying it appears that this may be caused, not by excessive speed, but by irregularity on the ground confusing the return signal. It seems reasonable that the likelihood of confusion would be greater at high speed, and also greater at high altitude where the footprint of the signal would be wider.

Would it be right therefore to conclude that radalt would be highly accurate at low altitude over relatively smooth terrain no matter how high the ground speed?

Do you have any information about the angle of the cone of the signal, so that it would be possible to calculate from the height how far off line an object like a building might be picked up and interfere?

[dClbydalpha]

Do you have any information about the angle of the cone of the signal


The radalts I have worked with are comfortable with a composite attitude angle of around 45°. This would roughly correspond to the half cone angle. I have to say that this is a general observation and is very much dependant on individual units, and their antenna designs and placements.

Would it be right therefore to conclude that radalt would be highly accurate at low altitude over relatively smooth terrain no matter how high the ground speed?


I think that is generally correct, in level flight over a "smooth" reflective surface the ground speed would have little to no effect.

Errors come from a variety of sources, modern radalts are very good at coping with scatter and multi-path effects. They are also largely doppler immune. By design they look for the leading edge of a valid return signal to try and give the closest point and not the slant angle. Therefore if the ground is closer than the slant range to the building then it will still track the ground. There are also filters that prevent sudden large changes of altitude, such as when passing over another aircraft. Accuracy is generally given as a percentage so therefore the lower you are the more accurate the reading. Although there is a lower cutoff due to the limit of technology that gives an absolute measurement accuracy usually around ±3 ft. Because a descent rate is seen directly by the altimeter excessive descent rates can degrade the accuracy as well.

[gravity32]

Thanks for that. It seems I need to refine the angle question a bit. The footprint may not be circular. It is the roll angle that I was wondering about. Do you know the angle from vertical of the outer edge of the beam, side-on to the direction of travel?

[dClbydalpha]

Unfortunately I don't, sorry.
For an approximation of the radalts I know, I'd assume it is circular and at 45°. The tech specs that I have don't specify individual angles, just an overall angle limit. But looking at the TSO it sepcifies a test limit in roll that is greater than in pitch, which makes sense in operation.