DE1188150B - Device for measuring altitude based on the altitude angle and the distance to a very distant target with the help of reflected electromagnetic waves - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

H04p

German class: 21 a4- 48/44

Number: 1188 150

File number: A 35467IX d / 21 a4

Filing date: September 1, 1960

Opening day: March 4, 1965

The invention relates to a device for determining altitude on the basis of the altitude angle and the Removal of very distant targets with the help of reflected electromagnetic waves, in which one Stress proportional to the apparent height calculated taking into account the curvature of the earth is corrected to take into account the refractive properties of the atmosphere.

It is well known that a radar beam emitted by the earth's surface does not propagate in a straight line, but rather travels a curved path towards the earth as a result of the atmospheric refraction of rays.

Because of this refraction phenomenon, significant errors result when the height of with high Speed of flying aircraft or missiles at great distances and low altitudes, should be measured with a radar device at small angles of elevation.

If a rectilinear propagation of the electromagnetic waves is assumed, that is Altitude measurement based on the distance and altitude angle values measured by the radar device a purely geometric problem. In the case of great distances, the influence of the curvature of the earth must must be taken into account. Therefore, the apparent height is calculated using the following equation:

Hr =

In it is

Ir ₀

Hr = apparent height,
R = distance,
r ₀ = radius of the earth,
a. - Elevation angle.

However, the altitude calculated in this way is not correct if the radar beam is due to the refraction properties the atmosphere follows a curved path.

Recent research has shown that the refractive index of the atmosphere depends on the Temperature, pressure and dew point at the point concerned and therefore not for all Make the atmosphere a constant. Therefore the refraction or the total curvature is one Radar beam, which depends on the refractive index at each point of the beam path, not a constant, but changes according to the atmospheric conditions along the path of the jet. It is also succeeded in assuming the refractive properties of the entire atmosphere for certain weather conditions to be displayed depending on the height.

Device for height measurement based on the elevation angle and the distance of a very distant target with the help of reflected electromagnetic waves

Applicant:

Avco Corporation, Cincinnati, Ohio (V. St. A.)

Representative:

Dipl.-Ing. E. Prince and Dr. rer. nat. G. Hauser, Patent attorneys,

Munich-Pasing, Ernsbergerstr. 19th

Named as inventor:

George Brück, Wyoming, Ohio; Lee Chester Keene, Tulsa, Okla .; Robert John Schipper, Crestview Hill, Ky.

(V. St. A.)

In this way, one knows the refractive index

_a5 every point of the path of the radar beam, so can

Using Snell's law, derive an exact equation for the beam path as follows:

R =

Ah

1- -

cos ⁸ « ₀

Here is

R = length of the beam path to the target, H _t - true target height,

uh - change in height between neighboring points along the beam path,

n ₀ = 1 + JV ₀ · 10- ⁶ (according to definition)

= Refractive index on the earth's surface,

JV ₀ = refractive index at the earth's surface, η = 1 + JV · 10- ⁶ (according to definition)

= Refractive index at any point along the beam path, depending on from the height,

JV = refractive index at the relevant point, r ₀ = radius of the earth,

h = height at any point along the beam path and

K ₀ = angle of elevation of the beam at the transmission point.

509 517/312

3 4

However, this equation cannot be integrated and yields a small finite height H _{0 during the}

can therefore not by an analog computing device, the height information above the second part

be solved. To make an equation that gives H _{0 at height.} The height H ₀ depends on the

modern computing technology can be easily adapted to the required computational accuracy and amounts

can, the above equation is broken down into two parts, 5 practically less than 300 m. This is done as

the first part of which follows the altitude information up to one:

ysin ² « ₀ + 2H ₀ [ 1-sin OC ₀ \ ^r o fi /

d / z

". . . . _ h _ ". , 15 The amazing fact here is that the

fiieroei is r _x - _{(Ν <>} _ _{Ν) 10} -, - slope of the correction factor only from the elevation angle and not from

Refractive index curve on the earth's surface. depends on the distance and yet a very

It is easy to see that the emulation of the above gives the exact height correction.

Equation by means of an analog device, if it is over- The course of the correction voltage as a function of the main is to be carried out, errors in the 20 speed of the elevation angle must of course be determined in the design The magnitude of the errors to be corrected can be determined once with the device itself. This offers the would bring. Professional no difficulties because of the previously

Attempts have therefore been made to achieve at least one of the approximated diagrams mentioned of the refractive properties correction in another way. A familiar of the atmosphere are available. The solution consists in calculating in the 25 of these diagrams given above for any equation for the height at straight-line waves - assumed the ratio of true propagation, introducing a constant correction factor c in height to apparent height at different heights as follows: angle. The curve obtained in this way already directly represents the course of the correction factor in

Ht = YR sin <x. 3 ° depending on the elevation angle for any

2 cr _{0 represents the} distance. The simulation of this correction

Here, H _{t is} the corrected height. factor, for example by a

adjusting device coupled potentiometer, offers

This equation can be very easily done with analog- no difficulty.

simulate computing devices, but it has been shown 35 In general, it is sufficient to use the correction factor for

that the achieved accuracy at great distances is a mean weather situation or, at greater accuracy

and low elevation angles was low. The outcome requirements, more typical of a limited number

did not get much better when you determine the con-weather conditions. In the latter case, then must

constant c changed depending on the height. the valid correction factor must be selected each time.

The aim of the invention is therefore to create a 4 ° This is the only required intervention of the operator

Radar device of the type specified above, in which with person; otherwise the altitude correction takes place

simple means a correction of the calculated completely automatically.

Heights is obtained which come very close to the theoretical. In it shows
large distances and very small elevation angles 45 F i g. Fig. 1 is a schematic representation of the refraction giving great accuracy in height measurement. problems,

According to the invention this is achieved in that F i g. 2 a schematic representation of a preferred

with the elevation angle adjustment device of the antenna supplied embodiment of the invention,

a device is mechanically coupled in such a way that F i g. 3 a family of curves to illustrate the

it emits a direct voltage, which as a function 5 ° known refraction properties at certain

of the altitude angle due to the prevailing atmospheric conditions at different altitudes

Refractive properties of the atmosphere known and

Ratio of true to apparent height proportionally- F i g. 4 to 9 curves for the correction factor in

nal, and that the amount of the apparent height as a function of the elevation angle, which is derived from the

proportional voltage with the magnitude of the 55 curves of F i g. 3 have been derived.

obtained DC voltage is multiplied by The influence of atmospheric refraction at a

a display voltage corresponding to the true height electromagnetic height measurement by reverse

to obtain. Beam location is shown in FIG. 1 shown. The radar

The essential idea of the invention consists in antenna generating a radar beam, which in the Entdarin that the calculated apparent altitude with a distance R hits a true target. Because of the correction factor is multiplied, which with the atmospheric ray refraction is the real prevailing refractive properties of the atmosphere ray path is curved, as by the solid (ie the prevailing weather conditions) is represented only by the line. Apparently, however, the beam follows the angle of elevation. Such a correction follows the dashed line, which is a height that can be implemented very easily because it practically only forms an angle α with the horizontal. This means that by providing an elevation angle dependent that the target's apparent altitude Hr is substantially DC and a voltage multiplication higher than the true altitude H ₁ . So it must be required. a correction factor can be introduced so that the

errors resulting from the refraction are compensated for.

The apparent height Hr is calculated on the basis of geometric relationships using the following equation:

H _R =

Ir ₀

+ R · sin / χ

(with r ₀ - earth radius).

According to the invention, the correction takes place in that this apparent height is multiplied by a correction factor K , which only depends on the elevation angle, so that the true height Ht is obtained according to the following relationship:

Ht

R ²

Ir ₀

+ R-sintx

N ₂₀ = fairly dry air, JV ₃₁ = dry air.

JV ₃₁ correspond to the fact that the resistor 14 is replaced by one of the resistors 12, 13, 15, 16 by means of a changeover switch, which result in the differences between the various correction curves. The output voltage at tap 11 of the K potentiometer 10 is therefore a function of the elevation angle and also depends on the correction curve set: K = K (x, N).

To calculate the true height Ht of the target, the given correction formula is transformed as follows:

H _t = \ K (%, N) sin χ +

K (x, N) R

R.

It is therefore necessary to determine the course of the correction factor K («) as a function of the elevation angle.

This can be based on the in F i g. 3 shown well-known curves happen, which the decrease the refractive index with increasing altitude in different weather conditions. On the abscissa is the The refractive index JV is plotted, which is defined as follows:

n = 1 + N- 10 ~ ^a .

Here η is the refractive index. Five curves are shown for different weather conditions:

iV " ₁₀ = air containing water,

N ₁₉ - moist air,

No = mean value,

35

From these known curves one can easily determine the ratio of true height to apparent height Calculate the height at any assumed distance as a function of the elevation angle α. This gives the curves of F i g. 5 to 8 for the corresponding curves of Fig. 3. Three these curves are shown in FIG. 4 shown again on an enlarged scale.

These curves immediately show the correction factor K (x) by which the apparent height is multiplied. It can be seen that in all cases no correction is required for elevation angles greater than 5 °. .

In Fig. 2 shows an arrangement which automatically carries out the specified correction.

A K potentiometer 10, which is fed by a precisely regulated DC voltage source, is used to simulate the correction factor K (x). Connected in series with a resistor 14, the K potentiometer has a resistance curve that corresponds to that shown in FIG. 5 corresponds to the reference curve N _r shown, which has the lowest mean error for all five curves. The rotor 11 of the K-potentiometer 10 is mechanically coupled to the elevation angle adjustment device of the radar antenna in such a way that the output voltage of the K-potentiometer is proportional to the corresponding value of the correction factor at each elevation angle of the antenna. The adaptation of the output voltage of the K potentiometer 10 to the various correction factors that correspond to the other curves JV ₁₀ , N _{1 a} , N ₂₀ , To simulate the element K (a, N) sin α, the output voltage of the K potentiometer 10 becomes a Sine potentiometer 20 supplied, the resistance curve of which corresponds to a sine function. The tap 21 of the sine potentiometer is also mechanically coupled to the elevation angle adjustment device of the antenna, so that its output voltage represents a direct voltage which is proportional to the sine of the elevation angle multiplied by the input voltage K (pc, N) . This output voltage is thus proportional to K (x, N) sin α. If negative elevation angles are also to be detected, the lower end of the sinus potentiometer 20 can be connected to a precisely regulated negative voltage in the manner shown. For negative angles, the same correction factors K (x, N) are applied as for the elevation angle 0 °.

In order to reproduce the link - ~ —-—, the

Output voltage of the K potentiometer 10 also via a resistor 23 to a distance integrator 25 supplied. The distance integrator 25 is constructed in a known manner and generally includes a circuit for generating a sawtooth voltage, its shape and amplitude in direct Relationship to the correction factor can be changed. The sawtooth voltage generated in the distance integrator 25 provides the range deflection for a cathode ray tube (not shown) on which in known way the target is shown according to distance and altitude.

The output voltage of the distance integrator is now proportional to the value K (oc, N) R. This output voltage is changed in circuit 27 to take account of the curvature of the earth. This circuit is constructed in a known manner so that? its input voltage is divided by a value proportional to 2r ₀ . At the output there is thus a voltage proportional to - ^ ---—.

This voltage is fed to a height integrator 31. The output voltage of the sine potentiometer 20 also reaches the height integrator 31 via a resistor 21 and becomes the output voltage there the circuit 27 is added, so that a voltage results which the following value proportional is:

", _To . . K (x, N) R

K (<x, N) sin α - \ -.

Ir ₀

The height integrator is also constructed in a manner known per se. So it contains a sawtooth oscillator, whose output voltage is a function of distance and elevation angle. the

The output voltage of the height integrator is accordingly a sawtooth voltage, its amplitude and Shape in direct relation to the added output voltages of the sine potentiometer 20 and the curvature of the earth correcting circuit 27 is modified. At the output of the height integrator 31 thus occurs a voltage proportional to the desired true height:

Ht = K (oc, N)

2r ₀

Ssin «

This voltage is fed to the elevation deflection circuits of the cathode ray tube.

To start up the device, the operator connects one of the resistors 12 to 16 with the K-potentiometer 10 to set the correct correction curve according to the prevailing weather conditions to choose.

To determine the correction curve, the operator first measures the dew point, the temperature and the air pressure on the surface of the earth near the radar device and calculates the Refractive index JV at the bottom from the following equation:

77 P, f 2.26 -10 «

-jexp 5369

XI3

In here is

Td = dew point temperature ( ^g K);

T = air temperature ( ⁰ K);

P = air pressure (millibars);
exp = the sign for an exponential function (e = 2.71828).

The refractive index JV determined by this equation then becomes 0 with the values of JV for the height at the various in FIG. 3 curves shown. From Fig. 3 it can be seen that the following Values on the ground apply:

JV = 387 for profile JV ₁₀ (water-containing air),

JV = 355 for profile JV ₁₉ (humid air),

JV = 343 for profile JV _r (mean value),

JV = 318 for profile JV ₂₀ (fairly dry air) and JV = 294 for profile JV ₃₁ (dry air).

proven in practice; but you could just as easily use separate K-potentiometers from those each corresponds to a correction curve. Circuits other than the one shown can also be used to solve this problem of the given equation can be used. Furthermore, the number of correction curves used is not limited to five.

Claims (4)

Patent claims: 1. Device for determining altitude due to the altitude angle and the distance very far distant targets with the help of reflected electromagnetic waves at which a voltage that is proportional to the apparent height calculated taking into account the curvature of the earth, is corrected to take into account the refractive properties of the atmosphere, thereby characterized in that a device with the elevation angle adjustment device of the antenna is mechanically coupled in such a way that it emits a DC voltage, which as a function of the elevation angle due to the prevailing refractive properties of the atmosphere known ratio of true to apparent as the amount is proportional, and that the amount of the voltage proportional to the magnitude of the direct voltage thus obtained is multiplied to obtain a display voltage corresponding to the true height. 2. Apparatus according to claim 1, characterized in that the device consists of a potentiometer (10), which is fed by a fixed voltage, the pick-up of which is connected to the elevation angle adjustment device the antenna-coupled element and its resistance curve is the ratio of true to apparent height than Corresponds to the function of the elevation angle. 3. Apparatus according to claim 2, characterized in that additional potentiometers are optional can be used whose resistance curves correspond to different refraction properties. 4. Apparatus according to claim 2, characterized in that to take into account different Refraction properties Resistors (12 to 16) optionally in series with the potentiometer (10) can be switched on. The correction curve that comes closest to the calculated refractive index is then switched on one of the resistors 12 to 16 is set. Various possible changes are immediately apparent to the person skilled in the art. For example, in FIG. 2 circuit shown with a K-potentiometer 10 and five resistors 12 to 16 Considered publications:
U.S. Patent Nos. 2,444,770, 2,591,698;
Telecommunications Journal, 8 (1955), 11 (November), pp. 578 to 586;
Principles of Radar, New York - London, 1946, Pp. 9-107 to 9-116. For this purpose 2 sheets of drawings 509 517/312 2.65 © Bundesdruckerei Berlin