WO2007142532A1

WO2007142532A1 - Methods and arrangement for determining the direction to an emitter

Info

Publication number: WO2007142532A1
Application number: PCT/NO2006/000222
Authority: WO
Inventors: Per Atle VÅLAND
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2006-06-09
Filing date: 2006-06-09
Publication date: 2007-12-13

Abstract

The present invention provides a method for determining the position of continuously scanning radar emitters using two or more ESM/DF sensors (ESM1, ESM2). The method is based on traditional triangulation, with the additional steps of determining scan rate of the emitter (51), and the time that its main lobe illuminates the ESM sensors (ESM1, ESM2). Two sensors may use this information to estimate the angle (dDOA) between the sensors as seen from the emitter. This estimate may then be used to form improved range and bearing estimates to the emitter.

Description

METHODS AND ARRANGEMENT FOR DETERMINING THE DIRECTION TO AN

EMITTER

Technical field

The present invention relates to the field of direction finding, and in particular a method for determining the position of a radio emitter, such as a radar, using two or more Electronic Support Measures (ESM) /Direction Finder (DF) sensors.

Background

ESM/DF sensors receives signal from emitters, creates emitter description and determine bearing to emitters. An example of such a sensor system is shown in Fig. 1. The illustrated system includes a number of antennas 12 a-c, each with an individual receiver channel 13 a-c. In the processing unit 14, the phase differences between the signals received on the antennas are determined to find the direction to the emitter source 11. The "signature" of the signals from an emitter 11 may be used to identify the emitter.

This type of sensors is well known. When two or more sensors observe the same emitter, it is possible to determine the position of the emitter.

Two main methods of position determination have traditionally been used:

Triangulation

By utilizing each sensor (minimum two) for a direction of arrival (DOA) measurement, the position of the originating emitter may be determined, as illustrated in Fig. 2. Given the distance between the sensors ESMl and ESM2 is known, the ranges Rl and R2 from each sensor to an emitter 21 may be deducted from the bearing angles DOAl and D0A2 using simple trigonometry. While the emitter 21 is shown as a radar installation in the figure, this method may find the position of any type of radio emitter. The principle is also extendible to any emitter, e.g. optical or acoustical emitters.

Each emitter will have an uncertainty in the DOA measurement, E_doa. This error depends on the actual signal to noise ratio and various sensor errors, and is typically in the order of a few degrees. If the Emitter Position is expressed as a bearing and range from each emitter, the metric .error may be expressed as:

Seen from ESM 1 (Position: DOA 1; Ri) :

Cross bearing error: E_xi= Ri- E_dOa

Range error: E_ri= E_x2 / sin (DOAi-DOA₂)

Fig. 3 shows the uncertainty in the position measurements as wedge formed error fields 32, 33 around each bearing with DOAl and D0A2. The intersection between said error fields defines a diamond shaped uncertainty field 34. The various errors E_xi, E_X2, E_ri, E_r2 are indicated in the figure. The range error is dependent on the geometry of the measurement setup. Ideally, the two bearing measurements should intersect at 90° to minimize the size of the uncertainty field 34.

The main disadvantage of triangulation is the low accuracy. The bearing accuracy is given by the accuracy of the sensors, and the range accuracy is given by the sensor accuracy and the geometry. As the angle between the two bearings get smaller, the inaccuracy in range will increase . TDOA based positioning

By measuring exact time of arrival (TOA) of the same pulse to a number of ESM sensors (minimum three) , the position of the originating emitter may be determined, Fig. 4. The difference in time of arrival to two sensors places the originating emitter 41 on a hyperbola. The position may be determined by finding the crossing of two hyperbolas originating from a new set of sensors.

Each emitter 41 will have an uncertainty in the TOA measurement, E_toa- This error depends on the actual signal to noise ratio, the signal waveform, the time base and various sensor errors. Depending on the time base this error is typically in the range from few ns to a few tens of ns .

For the error analysis, if the distance to the emitter is greater than the distance between sensors, the emitter position may be approximated by a bearing line from a point midways between the sensors. The angular error resulting from inaccuracies in the tdoa-estimate is in the order of Δd/L where Δd is the range difference error (tdoa multiplied with the speed of light) and L is the baseline length between the sensors. An error in tdoa of 30 ns and a baseline length of 10 km results in an angular error in the order of 0.05°.

Thus, this method is 10-100 times better in accuracy compared to triangulation in the typical case. The main drawback of the method is that the same pulse must be received on two different sensors. Since radar emitters use high-gain antennas with a narrow main lobe, the measurement must be based on detection in the transmitter side lobes. Typically, this implies that the signal strength is reduced by 25-40 dB as compared to main lobe detection. Thus the range is reduced by a factor of 20 to 100 with receiver characteristics unchanged. Additionally, the method is based on a minimum of three sensors instead of two in the triangulation case.

TDOA based measurements are far more accurate than triangulation due to the possibility of long baseline bearing estimate. The main disadvantage is short range, since the method requires simultaneous reception of the same pulse on two receivers placed far apart. This requirement implies that in order to obtain good geometry, at most one receiver will be in the emitter main lobe at one time. Thus, at least two receivers will at any time only receive signals from the emitter sidelobes.

An additional disadvantage of a TDOA based position determination is the need of three sensors that is able to observe the emitter as opposed to two in the case of triangulation. In an area of coverage this means 50% more receivers in the network. With surface based emitters, this also implies increased coverage gaps, since line of sight from the emitter to three sensors are needed at all time.

Summary

It is an object of the present invention to provide a new emitter positioning method with an improved accuracy over prior art triangulation methods.

Another object is to combine the new method with prior art methods for emitter positioning in order to exploit the strong points of all methods.

These objects are achieved in a method as defined in the appended claims .

In particular, according to a first aspect the present invention claims a method for determining the position of a scanning emitter with constant scan rate. Said method includes observing when the emitter is illuminating a first sensor, observing when the emitter is illuminating a second sensor, and estimating a first angle between the first and the second sensor as seen from the emitter. This is used for estimating the range and/or bearing to the emitter.

According to a second aspect the invention claims a method for determining the position of an emitter, first using traditional triangulation. This includes observing the emitter from a first sensor and determining a first direction of arrival in said first sensor, observing the emitter from a second sensor and determining a second direction of arrival in said second sensor, and using said directions of arrivals for estimating a first range and first bearing to said emitter. The improvement over prior ^' art including the steps of testing if the emitter is a scanning emitter with constant scan rate, and if so observing when the emitter is illuminating the first sensor, observing when the emitter is illuminating the second sensor, estimating a first angle between the first and the second sensor as seen from the emitter, and forming a second estimate of the range to the emitter, said second range estimate replacing said first range estimate.

An advantageous embodiment of this method includes the additional steps of observing when the emitter is illuminating a third sensor, estimating a second angle between the second and third sensor as seen from the emitter, and using said first and second angles when forming said second range estimate.

Another embodiment of this method includes the additional steps of testing if a pulse emitted by the emitter is received by all three sensors, and if so determining the time of arrival of one and the same pulse in all three sensors, determining a first difference in the time of arrival between the first and second sensor, determining a second difference in the time of arrival between the second and third sensor, forming a second bearing estimate using said first and second differences in the time of arrival, said second bearing estimate replacing said first range estimate.

Brief description of the drawings

Below, the invention will be described in detail in reference to the appended drawings, of which

Fig. 1 is a schematic view of a typical ESM/DF unit (prior art),

Fig. 2 illustrates position determination using triangulation (prior art) ,

Fig. 3 illustrates how errors in the sensor measurements affect the position estimate,

Fig. 4 illustrates position determination using TOA (prior art),

Fig. 5 illustrates position determination using the inventive method,

Fig. 6 illustrates another embodiment of the invention,

Fig. 7 is a flow diagram illustrating an aspect of the invention.

Detailed description

According to a first aspect of the invention, there has been developed an improved method of determining the position of an emitter source by triangulation, in the following called emitter scan based positioning. Emitter scan based positioning

Radars typically use scanning high gain antennas, with narrow main lobe. Typical navigation radars have 3dB beam width of 1° to 5°, military radars often less than 2°. If the radar uses constant scan rate, either rotational or reversing, the ESM sensor will be able to precisely determine scan rate and the time that the main lobe illuminates the ESM-sensor . Two sensors ESM₁, ESM₂ may use this information to estimate the angle between the sensors as seen from the emitter 51, as shown in Fig. 5.

In most cases the DOA difference may be estimated to an accuracy of 0.1-0.2 times the emitter main lobe. With two sensors the resulting DOA difference may be improved drastically as compared to a triangulating system. Using three sensors, as illustrated in Fig. 6, the emitter position may be determined using the two DOA differences alone, typically achieving an angular error of 0.1° to 0.5°. The range error improves correspondingly.

As shown in Fig. 6, three sensors ESMi, ESM₂ and ESM₃ are observing the scanning emitter 61. The DOA differences dDOAχ2 and dDOA₂3 between the sensor pairs ESMi-ESM₂ and ESM₂-ESM₃, respectively, are used for determining the position of the emitter 61.

This method is only usable with continuously scanning 5 radars (rotating or oscillating) .

Combination of methods

The methods for positions determination described above may all be used in a network of ESM/DF sensors. If the sensors automatically associates remote emitter observations with o own observations, the process may be entirely algorithmic. Using multiple ESM/DF sensors capable of estimating accurate bearing to the emitter in a cooperative manner provides the possibility of estimating the Emitter position. Such a system may easily provide the emitter position by automatically triangulating the emitter from two or more positions. This provides a robust if not very accurate position estimate. The accuracy may be improved by introducing one ore both of the alternate methods. Two cases apply, namely the use of two sensors and the use of three or more sensors.

Two ESM/DF sensors

With two sensors, only the triangulation method is able to estimate emitter position. The two other methods may be used to improve either range or bearing estimate whenever the circumstances allow (simultaneous reception of same pulse on both sensors in the case of TDOA and constant emitter scan rate in the case of the emitter scan based method)

Triangulation only is used for initial position estimate. Angular accuracy is given by ESM/DF sensor accuracy (E_doa) .

Cross bearing error: E_xχ= Ri- E_dOa

Range error : En= E_x2 / sin ( DOAi-DOA₂ )

Er2= E_xi / sin ( DOAi-DOA₂ )

Emitter Scan Based method may be used to improve range accuracy if the emitter has a constant scan rate (either oscillating or rotating) . The range error now becomes dependent on the error in the emitter scan DOA difference error (E_dDoft) which is typically 10 to 30 times smaller than the ESM/DF sensor accuracy:

Range error: E_ri= R₂- ECJD_OA / sin(dDOA)

Er2= Ri- EdDOA / sin(dDOA) TDOA based positioning may be used to improve the bearing estimate if one or more pulses are received and processed on both ESM/DF receivers. Note that, in practice multiple pulses must be received on both sensors in order not to reduce the robustness of the system. The bearing error is dependent on bearing angle. When the ranges from the emitter to each of the sensors are equal, the bearing error is minimal and equal to the TDOA error (in meters) divided by the baseline length (Δd/L) .

Cross bearing error: E_xI= Ri- Δd/L

Example:

Assume two ESM/DF sensors placed 20 km apart, each with a bearing accuracy of 3°. A radar emitter is positioned so that the distances to each of the sensors are approximately 20 km. The radar has a rotating antenna with a main lobe beam width of 2°.

Assuming accuracy in determining DOA difference of 0.2¹ and a pulse TDOA accuracy of 30 ns (9m) we have:

Three ESM/DF sensors (or more)

With three or more sensors, all the methods are able to estimate emitter position whenever the circumstances allow (simultaneous reception of same pulse on three sensors in the case of TDOA and constant emitter scan rate in the case of the emitter scan based method)

Triangulation only is used for initial position estimate. Angular accuracy is given by ESM/DF sensor accuracy (E_doa) based on the two sensors with best geometry, (three estimates are provided and the computed result should be an optimum combination of the three)

Cross bearing error: E_xi= Ri* E_doa

Range error: E_rχ= E_x2 / sin (DOAi-DOA₂) E_r2= E_xi / sin (DOAi-DOA₂)

Emitter Scan Based method may be used to estimate position if the emitter has a constant scan rate (either oscillating or rotating) . The range error now becomes dependent on the error in the emitter scan DOA difference error (E_dDca) which is typically 10 to 30 times smaller than the ESM/DF sensor accuracy:

Cross bearing error: E_xi= Ri- E_dD0A

Range error: En= R₂- E_dD0A / sin(dDOA) E_r2= Rr E_dDo_A / sin(dDOA)

TDOA based positioning may be used to estimate the position if one or more pulses are received and processed on two ESM/DF receivers at a time. Note that, in practice multiple pulses must be received on all three sensors in order not to reduce the robustness of the system. The bearing error is dependent on bearing angle. When the ranges from the emitter to each of the sensors are equal, the bearing error is minimal and equal to the TDOA error (in meters) divided by the baseline length (Δd/L) . The general error equations are:

Cross bearing error: E_xI= R₁- Δd/L

E_x2= R₂- Δd/L

Range error: En= R₂- Δd/L / sin (DOA_a-DOA_b)

E_r2= R₁- Δd/L / sin (DOA_a-DOAb)

where DOA₃ and DOAb are the bearings from a point centred between adjacent ESM/DF sensors.

Example:

Assume three ESM/DF sensors placed 10 km apart, each with a bearing accuracy of 3°. A radar emitter is positioned so that the distances to each of the sensors are approximately 20 km. The radar has a rotating antenna with a main lobe beam width of 2°.

Assuming accuracy in determining DOA difference of 0.2° and a pulse TDOA accuracy of 30 ns (9m) we have:

The figures are conservative in that the error has not been assumed to be independent, that is, it is assumed that the error is not reduced by exploiting multiple measurements. Summing up the results

The three principles described above all have significant characteristics. The relative advantages of each method and the described combinations may be summarized as in the following table:

Legend: low performance + good performance ++ best performance NA not applicable Algorithm

The last table shows that each method has its merits, and in a given situation a combination of the methods available may provide optimum results.

The selection of position determination method is based on the emitter measurements and may be visualized as shown in Fig 7.

The algorithm shown in the figure starts at step 70. In step 71 the algorithm enters a continuous loop initially testing if there any emitters present. If an emitter is . found, the algorithm continues to step 72 where a crude measurement of bearing and range is made using a conventional triangulation method. In some cases this may be the end result, but hopefully this measurement may be improved. In step 73 a test is made to determine if the emitter is a source with constant emitter scan rate (i.e. scanning radar) . If this is the case, a new range estimate is made in step 74 using the inventive emitter scan method. The range estimate from step 72 is substituted with the new estimate. Then, a new test is made in step 75 to see if two or more sensors are receiving the same pulse from the emitter. If so, a TDOA based estimate of bearing is made in step 76, and the bearing estimate obtained in step 72 is substituted with the new estimate. At this stage, the position of the emitter source has been determined with full accuracy and the loop returns to step 71 and waits for another emitter.

The inventive method has been described in a setting for monitoring continuously scanning radar emitters. However, the method as such may be used in other fields using signals from sound to light, provided the emitter has the proper characteristic. The method may be used for determining positions of beacons, e.g. lighthouses or radio beacons. The term beacon may include an emitter that is placed on an object for tracking purposes. The invention may also be used for determining the position of space crafts, if they are carrying beacons or may othervice act as continuously scanning emitters. Another filed of use is in astronomy observing objects in space, e.g. so called "pulsating" stars.

Claims

C l a i m s

1. A method for determining the position of a scanning emitter with constant scan rate, the method being characterized in observing when the emitter (11, 21, 31, 41, 51, 61) is illuminating a first sensor (ESMi) , observing when the emitter (11, 21, 31, 41, 51, 61) is illuminating a second sensor (ESM₂) , estimating a first angle (dDOA, dD0Ai₂) between said first and second sensor as seen from the emitter (11, 21, 31, 41, 51, 61), estimating the range and/or bearing to the emitter (11, 21, 31, 41, 51, 61).

2. A method as claimed in claim 1, the method including the additional steps of observing when the emitter (11, 21, 31, 41, 51, 61) is illuminating a third sensor (ESM₃) , estimating a second angle (dDOA₂₃) between said second and third sensor as seen from the emitter (11, 21, 31, 41, 51, 61) .

3. A method as claimed in claim 1 or 2, the method including the additional step of determining the scan rate of the emitter.

4. A method as claimed in claim 3, the method including 5 the additional step of determining the time each sensor is illuminated by the emitter.

5. A method as claimed in claim 1 or 2, wherein said emitter is a radar.

6. A method for determining the position of an emitter, o the method including: observing the emitter (11, 21, 31, 41, 51, 61) from a first sensor (ESMi) and determining a first direction of arrival (DOAi) in said first sensor, observing the emitter (11, 21, 31, 41, 51, 61) from a second sensor (ESM₂) and determining a second direction of arrival (DOA₂) in said second sensor, estimating a first range and first bearing to said emitter, c h a r a c t e r i z e d i n testing if the emitter ( 11 , 21 ,

31, 41, 51, 61) is a scanning emitter with constant scan rate, and if so observing when the emitter (11, 21, 31, 41, 51, 61) is o illuminating the first sensor (ESMi) _r observing when the emitter (11, 21, 31, 41, 51, 61) is illuminating the second sensor (ESM₂) , estimating a first angle (dDOA, dDOAi₂) between said first and second sensor as seen from the emitter (11, 21, 31, 41, s 51, 61), forming a second estimate of the range to the emitter (11,

21, 31, 41, 51, 61) , said second range estimate replacing said first range estimate.

7. A method as claimed in claim 6, the method including o the additional step of determining the scan rate of the emitter .

8. A method as claimed in claim 7, the method including the additional step of determining the time each sensor is illuminated by the emitter.

5 9. A method as claimed in claim 6, the method including the additional steps of testing if a pulse emitted by the emitter (11, 21, 31, 41, 51, 61) is received by both sensors (ESM_x, ESM₂) , and if so determining the time of arrival of one and the same pulse in both sensors (ESMi, 0 ESM₂) , determining a first difference in time of arrival (dr_X2) between said first and second sensor (ESMi, ESM₂) , forming a second bearing estimate using said difference in time of arrival, said second bearing estimate replacing said first bearing estimate.

10. A method as claimed in claim 6-10, wherein said emitter (11, 21, 31, 41, 51, 61) is a radar.

11. An arrangement for determining the position of a scanning emitter with constant scan rate, the arrangement including first and second Electronic Support

Measures/Direction Finder sensors (ESMi, ESM₂) , each sensor including a number of antennas (12a-c) with individual receiving channels (13a-c) and a processing unit (14), characterized in that the processing unit (14) in said first sensor (ESMj) is adapted to observe when the emitter (11, 51, 61) is illuminating the first sensor (ESMi) i the processing unit (14) in said second sensor (ESM₂) is adapted to observe when the emitter (11, 51, 61) .is illuminating the second sensor (ESM₂) , the processing unit in said first and/or second sensor being adapted to estimate a first angle (dDOA, dDOAi₂) between said first and second sensor as seen from the emitter (11, 51, 61), and estimate the range and/or bearing to the emitter (11, 51, 61) .

12. An arrangement as claimed in claim 11, the arrangement including a third sensor (ESM₃) , said third sensor including a number of antennas (12a-c) with individual receiving channels (13a-c) and a processing unit (14), the processing unit (14) in said third sensor being adapted to observe when the emitter (11, 51, 61) is illuminating the third sensor (ESM₃), the processing unit (14) in said first, second or third unit being adapted to estimate a second angle (dDOA₂₃) between said second and third sensor as seen from the emitter (11, 51, 61) .