Touko Hahkio, Doctor of Technology (Science)

Lepolantie 69 A, FI-00660 HELSINKI

Telephone +358 9 7247 655 or +358 400 770 606

**INPUT REQUIREMENTS OF ILS MAPPER PROGRAM**

**[ABSTRACT]{.underline}**

A new computer program for mapping scatterers that disturb ILS was
completed by the Author 1½ years ago and is available. It works with
Windows 95, 98 and NT and is fast and easy to use. It produces a map
often indispensable to the airport authorities for negotiations aiming
at the removal of a disturbing obstacle or at the improvement of ILS
antennas. In principle normal digitised ILS inspection flight recordings
form good input as such but for best results some additional
requirements should be taken into account when improving existing or
designing new data acquisition systems. The costs incurred are of minor
order. Requirements include termination of the approach sufficiently
near the antenna, high enough data acquisition rate, sufficiently
accurate distance measurement, not too high a time constant during
recording, and above all, recording of localiser receiver output in
parallel with the glide path when analysing the GP with ILS mapper. The
results obtained so far are both interesting and useful and can help the
interpretation of new "radio maps" in similar cases. Repeatability of
ILS maps is remarkable in comparison to mere plotted records.

**[GENERAL]{.underline}**

Regular inspection flights are necessary to verify compliance of ILS
with requirements set by ICAO and are performed either by authorities or
by other providers of flight inspection service. Their records are
useful as such in graphic form (although digitally recorded) for their
purpose of verification of essential parameters of ILS. However, with
small additions these records provide for excellent input data for
location by ILSmapper program of reflecting objects near the localiser
and glide-path antennas in case of excessive interference caused by
various structures, trees, etc. Location of scatterers requires
knowledge of the position aeroplane accurately enough, e.g. its distance
from the antenna and its lateral shift from the centreline. The
elevation affects the results only minimally but in the GP case the also
the elevation record is necessary in order to exclude the flight track
from the receiver output.

**[SCATTER OF ILS RADIO WAVES]{.underline}**

Radio wavelengths of roughly one metre create a fuzzy "optical" picture,
like the 2^nd^ world war metre wave radar where the aeroplanes appeared
as large blobs. However, even their type could be estimated due to their
propeller modulation and other behaviour. Likewise the relatively coarse
ILS maps can be interpreted more and more accurately with the wisdom
gained by cumulative experience.

Only large surfaces (walls) can direct reflecting metre waves
appreciably. As the mapping occurs in the relatively narrow front sector
of the approaching aeroplane, many scatterers behave like points in that
narrow sector i.e. the magnitude of the scattered wave does not vary
wildly.

The interference caused by scatter gives rise to a rhythmic wave
disturbance (bends) that appear in the inspection flight record. The
bend waves become shorter as the distance to the scatterer decreases
(except in some GP approaches far from the antenna).

### PRINCIPLE OF ILS MAPPER

ILSmapper performs a series of transforms of a kind (e.g. Fourier
Transform) that it annexes side by side to produce a map. The transform
is obtained by cross correlation between the unified flight record and
waveforms created by a point scatterer as correlating functions (whereas
Fourier transform uses sinuous waveforms). A similar method is used in
wide base interferometry of radio astronomy but in the ILS case the base
(the flight track) is at an end-fire aspect in relation to the area of
interest, not broadside as in the astronomy. The input data (already
subtracted by theodolite data) shall be unified i.e. the data interval
shall be made constant, the time factor of the receiver be compensated
for and the track be corrected (for each side separately) to represent
an approach along the centreline of the runway. The program accepts any
decimal separator being used in the host computer. The program steps are
in short as follows:

Unification of input data and calculation of curve forms:

-   RC correction i.e. emphasis of short bend waves

-   Cancellation of theodolite residue (the receiver is often delayed,
    the theodolite is not)

-   Calculation of Path and interpolation to 12m distance steps

-   Low-pass filtering of Path values

-   Quadratic Off CL phase correction

-   Linear shift of distance of data due to Path and interpolation to
    16.67m distance steps

-   Linear phase angle correction due to Path involving a filter effect
    of 1:9 bandwidth

-   Additional fine adjustment of Path phase (limits: LOC ±50m, GP ±29m)

Cross correlation transforms and display:

-   Cross correlation, results to 40x301 array, tapering of short bend
    wave ends

-   Calculation of results and display as plots plus provision with
    suitable scales

-   Interpolation and painting of Map (6 pixels per array value), level
    of black contour

-   Calculation of co-ordinates of Map point and display of them with
    array value by click of mouse

-   Marking of chosen points and their values on Map if so desired

-   Maximised/Normal window states

In two-frequency installations the ILSmapper program detects scatterers
of the main beam only. The program is not suitable for analysis of
disturbance caused by scatter on the clearance frequency.

### INPUT CONSIDERATIONS

## [Length of approach]{.underline} 

Normally the inspection is limited just to the essential part of the
approach and terminated near the threshold, far from the transmitting
antenna. However, rapid scalloping, the most useful part of the bends
for positioning of objects, is often confined to the neighbourhood of
the antenna simply because objects there cause the strongest scatter.
Consequently the approach should not be terminated prematurely.

On the other hand, large structures, e.g. buildings, can be farther away
and still cause noticeable disturbance. The ILS mapping program in
question uses given bend frequency band for location of scatterers. It
requires e.g. a Localiser record spanning nine times the distance of the
scatterer from the centreline plus its distance from the antenna. For
instance a building situated 1km off the centreline and 10km away from
the antenna would require the approach to begin at 29km from the antenna
in the localiser case.

# [Data rate]{.underline}

A non-constant input data rate will do fine because the ILSmapper will
create constant data rate by interpolation. For best results the
distance between samples should be preferably 0.012km (0.00648NM) or
less and not greater than 0.0167km (0.0090NM). If there are some short
breaks in data communication to the inspecting aeroplane, the data will
still be useful because the program can interpolate over such gaps in
distance (although it cannot create missing data). The crucial data is
most often near the antenna as mentioned above so the data shall be
arranged in ascending order of distance for input to ILSmapper.

## [Distance measurement]{.underline}

Just for obtaining essential parameters of ILS installation the accurate
distance from antenna is not important. Sighting of two known points and
assumption of constant speed between and beyond them suffice in such
cases. However, the ILS mapping program requires distance values more
accurate than those based on just an assumption. The true distance is
namely the time integral of speed and can accumulate considerable errors
because of speed variations. Any direct distance measurement, e.g. DME,
will do. Measurement based on GPS is sufficient. Distance shall be given
from the antenna and the values shall form a continuously growing series
i.e. they shall be suitably filtered if necessary. Otherwise the program
will jump to next higher values and discard lower values in between,
thus seriously impairing the effective data rate. On the other hand,
this property allows large shift of effective starting point of data by
just copying and suitably modifying one row of data onto the top of the
record.

## [Tracking]{.underline}

The flight path or track is important for object positioning. Normally
the flight path is tracked by theodolite and its reading is transmitted
by a radio link to the aeroplane to be recorded along with the ILS
receiver output. For object positioning the lateral component of the
track is by far more important than the vertical one. Consequently the
localiser inspection flight data is sufficient as is whereas the
glide-path data often lacks the most important track component because
of the solely vertical tracking of the aeroplane. The lateral data
exists, however, in the localiser receiver output. It just has to be
recorded. The record of this additional component during GP approaches
is absolutely necessary for mapping of scatterers (the program contains
a filter that removes excessive roughness from the LOC data in the GP
case). In LOC approaches considerable accuracy is required from the
theodolite record if the scatterer is far from the antenna; for best
results the lateral error should not exceed a couple of metres. (N.B.
the swath for path correction by ILSmapper is only ±50m wide in LOC
case.)

The input data mentioned above is fed into the program as a text file
comprising three or four columns: 1) the distance; 2) the CDI reading
minus theodolite; 3) the lateral deviation (theodolite in the LOC case
or the LOC CDI reading in the GP case); 4) the elevation (theodolite),
if any.

## [Time constant]{.underline}

In inspection flights often a time constant typical for normal approach
is used. Such filtering causes delay and attenuates the high frequency
end of the bend spectrum, the most important frequencies for the object
positioning. However, ILSmapper can correct the frequency response
afterwards but with the cost of increasing noise. So the data should
rather be recorded with a small delay, if any, but could of course be
presented to the pilot with a time constant convenient for flying.

The theodolite reading shall be subtracted from the receiver output in
order to present just the interference caused by the terrain and various
objects there. However, the two series of data do not match in case of
the receiver output being delayed because the delay of the theodolite
record is very small. The delay leaves a residue of the track in the
difference. The residue easily swamps the wanted difference near the
antenna if the track oscillates rapidly that is often the case in GP
records. However, the ILSmapper program is able to fairly suppress the
residue caused by the delay difference.

The residue caused by the delay mismatch is clearly visible in **Figure
1**.

## [Bend wave bandwidth]{.underline}

Scattering from the terrain converts the otherwise straight flight
record along the centreline to an oscillating curve, consisting of
repetitive waves called bends. Oscillations near the antenna have
generally shorter wavelength than oscillations farther away. The bend
wavelengths used to calculations in ILSmapper are about 50 to 450 m in
the localiser case; the bend frequency bandwidth would be from 0.11Hz to
1.0Hz when approaching with a speed of 100 knots. In the glide-path case
the wavelengths are somewhat longer due to a correction depending on the
offset of the GP antenna. The ILS receiver used should not strongly
attenuate these bend frequencies during data recording.

## [Lack of sign convention]{.underline}

There is no international agreement on the signs of outputs of the ILS
receiver and of the theodolite. The default used in the ILSmapper is 150
Hz side being positive, also in case of the theodolite. Signs differing
from this rule are taken care of by using minus signs in front of
certain parameters in the program. Such signs do not generally turn
those parameters negative; they rather act as switches.

Because the polarity of receiver outputs and theodolite readings has not
been internationally agreed, the program utilises as default the Finnish
custom (150 Hz is positive). Anyhow, any opposite sign can be complied
with by providing certain input values with minus signs, see the
following tables:

## LOC

+-----------------------------------+-----------------------------------+
| # What is of opposite sign        | # Where to put minus sign         |
+-----------------------------------+-----------------------------------+
| Theodolite                        | DR 1/s (data rate)                |
+-----------------------------------+-----------------------------------+
| CDI minus theodolite              | DR 1/s and LOCw                   |
+-----------------------------------+-----------------------------------+
| Theo. and CDI - theo.             | LOCw. (LOC width)                 |
+-----------------------------------+-----------------------------------+

**GP**: If LOC CDI is of opposite sign, provide LOCw. with minus sign.
In addition:

+-----------------------------------+-----------------------------------+
| # What is of opposite sign        | # Where to put minus sign         |
+-----------------------------------+-----------------------------------+
| Theodolite                        | GPwidth                           |
+-----------------------------------+-----------------------------------+
| CDI minus theodolite              | GPwidth & GPangle                 |
+-----------------------------------+-----------------------------------+
| Theo. and CDI - theo.             | GPangle                           |
+-----------------------------------+-----------------------------------+

### Site and other parameters

The size and layout of various airports as well as the parameters of
their ILS installations differ, so certain distances, angles and angle
widths, the side of interest etc. form an important group of input data
and shall not be forgotten. The program contains some choices for input
and output units of distance as well.

The program has also controls for the adjustment of the time constant,
the map area displayed and the value at the contour of the black blip
visible on the map.

**Figure 2** displays an overall view of the ILSmapper window and the
various parameters involved.

#### CONCLUSION

Several requirements have been imposed on inspection flight records if
used as input for the ILSmapper program that is available and working.
The requirements do not stem so much from the very program that has been
described than from the need of all co-ordinates of the position of the
aeroplane that would apply to any such calculation program. In addition
some requirements derive from the likely area of strongest scatter that
is relatively near the antenna. The importance of high bend frequencies
to positioning of scatterers has been explained. Lack of international
agreement on signs has been dealt with. All data required is generally
available but perhaps not always fully recorded. Anyhow, the
requirements mentioned do not generally involve costly improvements in
the flight inspection equipment currently in use.

### REFERENCES

1.  Touko Hahkio: Mapping of ILS Scatterers and Suppression of Scatter
    Errors, Thesis for the Degree of Doctor of Science of Technology.
    ISBN 951-22-4173-0, ISSN 1239-7350, Helsinki University of
    Technology, 16 Oct. 1998.

2.  Touko Hahkio: Terrain Images from ILS Records. Paper presented at
    the 9^th^ International Flight Inspection Symposium, Braunschweig,
    June 3 -- 7, 1996.

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