Antenna system and method for measuring the azimuth and elevation angles of an active, signal sending radiosonde

An antenna system and method for measuring azimuth and elevation angles of an active, signal sending radiosonde. The system includes a first passive antenna group having at least two antenna arrays, the direction pattern of which is wide at least in elevation plane for measuring azimuth angle based on the phase differences between the antenna arrays, a second passive antenna group having at least two antenna arrays, the direction pattern of which is wide at least in elevation plane for measuring the elevation angle based on the phase differences between the antenna arrays and the rotational position of the antenna field, and at least one third antenna having high gain for receiving the telemetry signal, the direction pattern of which element is narrow in azimuth plane and wide in elevation plane. The first and second antenna groups form a solid antenna field which is fixedly tilted in a predetermined elevation position.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a National Stage application of PCT/FI03/00963, filed Dec. 16, 2003, which claims priority to Finnish Application No. 20022202 filed Dec. 16, 2002, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna structure and to a method for measuring the azimuth and elevation of an active, signal sending radiosonde.

The present invention is related to atmospheric sounding systems, in which the properties of the atmosphere are measured in-situ by independent, self powered active components typically called radiosondes including a radio transmitter. Typical features for this technique are inactive (non-sending) receiving antennas and the fact that the measuring device (radiosonde) is either lifted or dropped through the space to be measured.

Parameters measured in-situ by sensors of the radiosonde, such as air pressure, temperature and relative humidity, are transmitted through a telemetry link to a receiving station. Other parameters of interest are wind speed and wind direction that can be measured by using navigation aid networks such as GPS or Loran-C, by a primary or secondary radar, or by a passive (non-sending) and independent (no navigation aid networks used) radiotheodolite. Height of the radiosonde can be calculated from the air pressure, temperature and humidity data.

The object of the invention is to determine the azimuth and elevation angles of an active radiosonde in a three dimensional space with a passive (non-sending) antenna structure independent of navigation aid networks. A typical application of the invention is to locate a radiosonde launched into the atmosphere with the help of a balloon filled with hydrogen or helium. Radiosonde azimuth and elevation angles are determined from the received radiosonde signal.

Wind speed and direction can be calculated from consecutive azimuth and elevation angles and height of the radiosonde.

2. Description of Background Art

Prior solutions in the 1680 MHz meteorological frequency band track the radiosonde mechanically both in the azimuth and elevation directions. The disadvantage of this solution is the complicated and expensive mechanical receiving antenna structure.

Another disadvantage of the prior solutions is the disability to attenuate ground reflections enough when the radiosonde signal is received from a low elevation angle.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the present invention to overcome the problems of the prior solutions and to provide an entirely novel type of antenna structure and a method for determining the azimuth and elevation angles of a radiosonde.

The goal of the invention is achieved by a fixedly backwards tilted antenna field, in which the antenna elements are assembled on an antenna frame. In one typical embodiment the antenna field is rotated around vertical axis approximately to the direction of the radiosonde while the elevation angle remains essentially constant.

In another embodiment of the invention there are at least three such fixedly tilted antenna fields that point to different fixed azimuth directions. This solution has no moving parts.

The invention offers significant benefits.

By attenuating the ground reflection the azimuth and elevation angles of the radiosonde can be measured more precisely especially when the radiosonde is in a low elevation angle.

The mechanics of the antenna structure can be simplified and manufactured at a lower cost. Furthermore the reliability of the system is increased as there are less moving parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance withFIGS. 1aand1b, the essentially planar antenna field1comprises a vertical antenna group12and a horizontal antenna group13. The vertical antenna group12comprises at least two antenna arrays10aand10bpositioned above each other. In this solution each array comprises three antenna elements9. The direction pattern of these arrays10aand10bis wide in elevation plane.

The vertical antenna group10aand10bis used for determining the elevation angle of the radiosonde based on the phase differences of the received radiosonde signal between the antenna arrays10a,10b.

Respectively horizontal antenna group13comprises two horizontal antenna arrays11aand11bpositioned at least essentially symmetrically around the vertical center line of the antenna field1. In this solution each array comprises two or more antenna elements9. The direction pattern of these arrays11aand11bis also wide in elevation plane.

The azimuth angle of the radiosonde is determined with arrays11aand11bbased on the phase differences between the antenna arrays11a,11band the rotational position of the antenna field1.

One preferable embodiment 1 of the invention includes only one rotatable support frame divided in upper6and lower parts14. The antenna field1with its frame2is mounted on a stationary tripod3having circular support plates4at the end of its legs5. An independent antenna8is for radiosonde telemetry. The antenna frame is rotatable around the vertical axis7for directing the antenna field1approximately to the direction of the radiosonde. The azimuth angle is measured with the horizontal antenna group13on the lower part14of the frame and elevation angle with help of the vertical antenna group12positioned on the upper and lower parts6and14of the frame. A simplified version of the antenna groups required for the angle measurement is presented is theFIG. 1b. The tilting angle α is typically 30°. The term “fixed tilting” or “fixed tilting angle” in this context means also solutions, where a small vibrational deviation of the tilting angle is allowed for example due to the wind.

Due to the antenna group13for azimuth measurement, the antenna field1forms an inverted T- or L-shape. With this solution a low center of gravity and wind load can be achieved. Obviously, the azimuth antenna group13can be positioned also in the upper part6or a center of the antenna field1within the scope of the inventive idea, whereby a T-, inverted L- or plus (+) shape is formed. The invention does not limit the azimuth and elevation antenna groups to be perpendicular to each other or the ground, thereby allowing, for instance, also an X-shape antenna field.

Radiosonde telemetry reception is independent of azimuth and elevation measurements. The telemetry signal is received by a separate high gain directional antenna8. The direction pattern of the antenna8is typically narrow in azimuth plane and wide in elevation plane.

FIG. 1brepresents a simplified version of the antenna structure ofFIG. 1a. In this version each antenna array is replaced by single antenna elements9.

FIG. 2represents another embodiment of the invention in a form of a fixed pyramid shaped antenna with four tilted antenna fields14. The azimuth angle is measured with horizontal antenna group20comprising two antenna arrays18aand18bat the bottom of the pyramid. Arrays include two or more antenna elements16. Elevation angle is measured with vertical antenna group19comprising two vertically positioned antenna arrays17aand17bin the upper and lower parts of the pyramid. The telemetry signal is received by a separate directional antenna15positioned on the top of the pyramid.

In both of the before described solutions the azimuth angle is determined from the measured phase difference of at least two antenna elements or arrays in the horizontal direction (horizontal groups13or20), and from the direction of the antenna field14.

The elevation angle is determined from the measured phase difference of at least two antenna elements or arrays essentially in the vertical direction (vertical groups12or19).

In accordance withFIG. 3, the purpose of the antenna system34is to obtain a direct radio signal32from the radiosonde31. When the radiosonde31is in a low elevation angle, ground reflection30coming from the (negative) mirror angle has been a major factor degrading the performance of prior solutions. The present invention decreases this problem by aligning a gain pattern minimum35(null) of the radiation pattern33to the direction of the ground reflection30. The direction is typically determined experimentally for different elevation angles by aligning the main beam by phased array techniques such that the ground reflection is minimized.

In accordance withFIG. 4, the gain pattern minimum (null) is formed by an antenna array (10a,10b,11a,11b, or17a,17b,18a,18b) which consists of at least two antenna elements (9or16). Gain pattern minimum (null)30is directed by modifying the signal phase and amplitude of each antenna element in the array (beamforming).

According toFIG. 5, the sum of the modified signals represents the antenna array that can now be regarded as a single antenna element with a more suitable radiation pattern. A gain pattern minimum (null) is formed separately for each of the antenna arrays in the horizontal and vertical groups (12,13or19,20).

Phase shift is designed experimentally for different elevation angles (radiation patterns). Antenna beam forming is explained in more detail e.g., in reference Robert J. Mailloux, Phased Array Antenna Handbook, Chapters 2 and 3, 1994 Artech House, Inc, ISBN 0-89006-502-0.

In accordance withFIG. 6, the angle of arrival can be measured with two identical antennas A1and A2using interferometric principle explained in more detail e.g., in reference Englar, Mango, Roettcher, Watters, FINAL REPORT FOR THE MININTRACK TRACKING FUNCTION DESCRIPTION, Volume 1, March 1973, NASA-TMX-66213. If the base length (b) is less or equal than half of the wavelength (λ/2) the unambiguous angle of arrival (−90°<α<90°) can be measured. When the phase difference (φ) between antenna A1and A2has been measured (−180°<φ<180°), the direction of arrival (DOA) can be calculated as:

Instead of the planar antenna field1or14shown inFIGS. 1a,1band2the antenna field may be also convex, concave or for example stepped. In the rotatable embodiments ofFIGS. 1aand1ball the antennas, antenna arrays and antenna elements are positioned on this uniform rigid antenna field1regardless of the shape of the antenna field. In the embodiment ofFIG. 2the telemetry antenna15is not included to this antenna field14.

In this application with wide beam is meant beam widths greater than 120°.

Respectively narrow beam means beam widths smaller than 30 °.