WO2009067754A1

WO2009067754A1 - A guidance system

Info

Publication number: WO2009067754A1
Application number: PCT/AU2008/001762
Authority: WO
Inventors: Robert Tucker
Original assignee: Agriculture Guided Implement Systems Pty Ltd
Priority date: 2007-11-30
Filing date: 2008-11-28
Publication date: 2009-06-04
Also published as: AR069506A1

Abstract

A guidance system (10) is disclosed for guiding a guidable device relative to a set of markers (38). Each marker (38) has a plurality of detectable marker elements (39) and the marker elements together define a detectable signature sequence. The guidance system comprises a marker detection device (12, 14, 16, 18) arranged to detect the signature sequence of a marker, and to provide a position signal indicative of the relative position of the marker detection device relative to each detected marker using the markers. The system also comprises a steering controller (24) arranged to receive the position signal and to control the position of a guidable device in response to the position signal when the detected signature sequence corresponds to a valid signature sequence. A corresponding method is also disclosed.

Description

A GUIDANCE SYSTEM

Field of the Invention

The present invention relates to a guidance system and, in particular, to a guidance system of the type suitable for use in guiding agricultural machinery.

Background of the invention

It is known from applicant's co-pending International Patent application No. WO 01/05213, the contents of which are hereby incorporated by reference, to provide a locating system for determining the location of an article relative to a crop row. The system includes several markers disposed at spaced intervals along a line substantially parallel to the crop row, and a sensor unit responsive to the markers for generating a signal indicative of the location of the sensor unit relative to the markers. In this way, it is possible to determine the location of the markers and thereby the crop row relative to the sensor unit, and to use the signal generated by the sensor unit to guide agricultural machinery relative to the crop row, such as weeding apparatus, fertilizer disposal apparatus, and so on.

However, while this type of system is effective in guiding agricultural machinery relative to a crop row, the system is nevertheless prone to false movements which are potentially damaging to the crops because the system is also responsive to magnetized objects other than markers which are present in the ground, and potentially to markers previously disposed in the ground from previous seasons . Summary of the invention

In accordance with a first aspect of the present invention, there is provided a guidance system for guiding a guidable device relative to a set of markers, each marker having a plurality of detectable marker elements and the marker elements together defining a detectable signature sequence, the guidance system comprising: a marker detection device arranged to detect the signature sequence of a marker, and to provide a position signal indicative of the relative position of the marker detection device relative to each detected marker using the markers ; and a steering controller arranged to receive the position signal and to control the position of a guidable device in response to the position signal when the detected signature sequence corresponds to a valid signature sequence.

In one embodiment, the system comprises a set of markers, each marker having a plurality of detectable marker elements, the marker elements together defining a detectable signature sequence, and each marker in the set having substantially the same signature sequence.

Each marker element may be arranged to generate an electromagnetic field.

In one embodiment, the marker detector device is arranged to generate a primary magnetic field and each marker element is arranged to modify the primary magnetic field.

The marker detection device may be arranged to derive the signature sequence by detecting peak values of signals detected from the markers . Alternatively, the marker detection device is arranged to derive the signature sequence by detecting magnetic field strength of the marker elements .

Alternatively, the marker detection device is arranged to derive the signature sequence by detecting spacings between adjacent marker elements.

Alternatively, the marker detection device is arranged to derive the signature sequence by detecting RFID signatures.

In one embodiment, the marker detection device comprises at least two sensors, the steering controller being responsive to a position signal based on a difference between the respective sensor signals produced by the at least two sensors .

In one arrangement, the marker detection device further comprises an integrity detector arranged to compare at least two peaks produced by a sensor so as to determine whether the detected peaks have been influenced by an external object based on a comparison of the peaks, wherein the integrity detector is configured to ignore a marker when a difference between the peak values produced by the sensor is beyond a predetermined threshold.

In one embodiment, the marker detection device comprises at least one electromagnetic field sensor. The marker detection device may comprise magnetometer type sensors.

In one embodiment, the system is arranged to store at least one reference signature sequence corresponding to a valid signature sequence, and the system is arranged to compare a detected signature sequence with the at least one reference signature sequence so as to determine whether the detected signature sequence corresponds to a valid signature sequence.

In one arrangement, the system is arranged such that a detected signature sequence is stored and used subsequently by the system as a reference signature sequence .

The marker detection device may comprise a plurality of sensor elements arranged to line up in use with respective marker elements of a marker so that the marker elements are simultaneously detectable by respective sensor elements .

In accordance with a second aspect of the present invention, there is provided an agricultural machine comprising a guidance system according to the first aspect of the present invention.

In accordance with a third aspect of the present invention, there is provided a set of detectable markers, each detectable marker comprising: a plurality of detectable marker elements, the marker elements together defining a detectable signature sequence; each marker being disposable in the ground, being detectable by a marker detection device and being usable to provide a position signal indicative of the relative position of the marker detection device relative to the marker; the signature sequence being usable to identify whether the marker is valid; and wherein each marker in the set has substantially the same signature sequence.

In one embodiment, each marker element comprises a magnetizable material or a magnetized material. In one embodiment, the signature sequence is defined by providing magnetic marker elements and arranging the orientations of the magnetic poles of the marker elements .

In an alternative embodiment, the signature sequence is defined by providing magnetic marker elements and arranging the marker elements so that the peak magnetic field strength varies across the marker.

In an alternative embodiment, the signature sequence is defined by providing magnetic marker elements and varying the distance between marker elements .

In an alternative embodiment, the signature sequence is defined by providing a plurality of RFID marker elements.

In one arrangement, at least one marker is arranged such that the detectability of the at least one marker reduces over time.

In one embodiment, at least one marker is arranged so as to corrode when disposed in soil .

In one arrangement, at least one marker comprises a corrosion accelerating material.

In one embodiment, at least one marker comprises particulate detectable material disposed in a shell, the shell being formed of material arranged so as to degrade when disposed in soil.

In one embodiment, at least one marker comprises detectable corrosion susceptible material disposed in a shell, the shell being formed of a frangible material, whereby fracture of the shell renders the detectible material susceptible to corrosion. In accordance with a third aspect of the present invention, there is provided an elongate carrier for guiding a guidable device, the carrier comprising a set of markers according to the second aspect of the present invention.

In one embodiment, the carrier comprises a degradable substrate on which the set of markers are disposed.

In one embodiment, the substrate comprises paper.

In one arrangement, the carrier comprises a seed tape comprising a plurality of markers and a plurality of seeds.

In an alternative arrangement, the carrier comprises a rope .

In one embodiment, the carrier comprises an irrigation pipe.

In accordance with a fourth aspect of the present invention, there is provided A method of guiding a guidable device, the method comprising: providing a set of markers, each marker having a plurality of detectable marker elements, the marker elements together defining a detectable signature sequence, each marker in the set having substantially the same signature sequence; disposing the markers in the ground; detecting the signature sequence of each marker; identifying the markers using the detected signature sequence; using each marker to provide a position signal indicative of the relative position of the marker detection device relative to the marker; and controlling the position of a guidable device in response to the position signal when the detected signature sequence corresponds to a valid signature sequence .

Description of the Drawings

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a block diagram of a guidance system in accordance with an embodiment of the present invention;

Figure 2 is a block diagram illustrating example operative components of a control unit of the guidance system shown in Figure 1;

Figure 3 is a schematic diagram illustrating magnetic characteristics of an example marker element of a marker of the guidance system shown in Figure 1, and an example response to the marker element produced by a magnetic field sensor of the guidance system shown in Figure 1;

Figure 4 is a schematic diagram illustrating example magnetic characteristics of a marker and an example response produced by a magnetic field sensor when the sensor passes over the marker;

Figure 5 is a schematic diagram illustrating an example distribution of markers during use and including a magnetized object capable of distorting the response produced by the guidance system; Figure 6 is a schematic diagram illustrating the response produced by a magnetic field sensor of the guidance system when a magnetized object is disposed adjacent a marker;

Figure 7 is a schematic diagram illustrating an example distribution of markers during use and showing an arrangement of magnetic field sensors in accordance with an embodiment of the present invention; and Figure 8 is a diagrammatic representation of a subsurface irrigation pipe including markers according to the present invention.

Description of an embodiment of the invention

Referring to the drawings, in Figure 1 there is shown a guidance system 10.

The guidance system 10 is configured to operate in association with agricultural machinery so as to maintain at least a portion of the machinery at a predetermined location relative to the ground, in particular relative to a crop row. Such machinery includes weeding apparatus, fertilizer disposing apparatus, farm vehicles, and so on.

While the present embodiment is described in relation to guidance of agricultural machinery, it will be understood that the system may be used for other applications wherein precision guidance is required.

The guidance system 10 includes first and second sensors 12, 14 arranged to detect markers disposed during use in the ground, and a marker detection device arranged to analyse and identify detected objects as markers. In this example, the marker detection device comprises a peak detector 16 arranged to derive peak values from signals produced by the sensors 12, 14, and a control unit 18 arranged to identify a signature associated with a marker, in this example by analysing the peak values produced by the peak detector 16, comparing the sequence of peak values produced by the peak detector 16 with reference sequences 20 stored in a storage device 22, and making a determination as to whether a detected marker is genuine when the detected peak sequence substantially matches a reference sequence 20 stored in the storage device 22. The control unit 18 is also arranged to control and coordinate operations in the guidance system 10, in this example using data 23, which may include software usable by the control unit 18, stored in the storage device 22.

As an alternative, instead of comparing a detected peak sequence with a stored reference sequence in order to identify the sequence as genuine, the first detected sequence may by stored in the storage device 22 as a reference sequence. In this way, it is not necessary for the guidance system 10 to have knowledge of all possible sequences which may be associated with a set of markers.

The guidance system 10 also includes a steering controller 24 arranged to receive instructions from the control unit 18 indicative of whether an agricultural machine including the guidance system 10 needs to move in order to maintain alignment with a crop row based on the respective peak values produced by the first and second sensors 12, 14. In particular, it is expected that each sensor 12, 14 will produce substantially the same response signal as the sensors 12, 14 move across the marker when the marker is disposed centrally of the sensors 12, 14. A difference between the response signals produced by the sensors indicates that the marker is not disposed centrally of the sensors and, accordingly, lateral movement of the sensors, and thereby in this example of the agricultural machinery, is required.

In this embodiment, the markers comprise a magnetizable material or a magnetized material such as iron and as such have associated magnetic field characteristics which can be detected by appropriate magnetic field sensors, in this example magnetometer type sensors 12, 14 or hall effect type sensors. The magnetizable material or magnetized material may be formed by applying a liquid magnetizable substance onto a suitable carrier, and allowing the liquid material to subsequently harden.

It will be understood that by using magnetized markers, a passive guidance system is provided since it is not necessary for an active electromagnetic field to be generated in order to detect the markers. Such a passive system enables the guidance system to be reliable, robust and relatively simple.

However, it will be understood that alternative markers and detection arrangements are envisaged.

In one alternative arrangement, metallic markers are used which require an active primary magnetic field and metal detector type sensors arranged to sense changes in the primary field which occur when the metallic material is disposed in the primary field.

A further alternative arrangement includes RFID type markers and associated detection sensors tuned to the markers.

The guidance system comprises sets of markers, each set of which has an associated predefined signature which is detectable such that the markers in the set can be identified as genuine using the detected signature. In the present example, this is achieved by providing each marker with magnetic material arranged such that a variable magnetic field pattern is created around the marker, and by detecting peaks in a magnetic response signal produced by each sensor 12, 14. In this example, the signature is achieved by including a plurality of magnetic marker elements in a marker and arranging the orientations of the magnetic poles of the marker elements so as to define a magnetic field pattern. The number of marker elements is variable and in this example 3 marker elements are included in each marker.

However, it will be understood that other arrangements are envisaged. For example, in an alternative embodiment, the markers may be configured such that by varying the magnetic field strength across the marker, a detectable signature is produced, for example by providing each marker with a plurality of marker elements and configuring the marker elements so that the peak magnetic field strength varies across the marker.

In a further alternative example, the distance between adjacent marker elements is varied so as to define a detectable signature associated with the marker.

Example operative components of the control unit 18 are shown in more detail in Figure 2. The operative components include a processor 30 which may include a microprocessor controlled by appropriate software 23 stored in the storage device 22, a sequence identifier 32 arranged to analyse a peak sequence derived from the signal response produced by each of the first and second sensors 12 , 14 , and to determine whether the peak sequence matches a reference sequence 20 stored in the database 22.

The operative components also comprise an integrity detector 34 arranged to compare at least two peaks produced by the same first and/or second sensor 12, 14 and to make a determination as to whether the detected peaks have been influenced by an external object based on a comparison of the peaks. For example, if a marker is arranged so that in absence of a foreign influencing object at least two peaks of substantially the same size are produced, the integrity detector 34 may be arranged to ignore a marker when a difference between peak values produced by a sensor 12, 14 is beyond a predetermined difference .

As shown in Figures 3 and 4 , in this example each marker 38 includes several marker elements 39, each of which has associated magnetic field characteristics 40 which produce a sensor response 42 at each sensor 12, 14 as the sensor 12, 14 moves relative to the marker element 39 in a direction indicated by Arrow A.

The sensor response 42 comprises a central peak 44 and two smaller side troughs 46. During use, the central peak is analysed by the peak detector 16 and a peak value representative of the height of the central peak 44 is determined and supplied to the control unit 18.

As shown in Figure 4, by appropriate orientation of the marker elements 39 and thereby of the magnetic field characteristics 40 of the marker elements, a sensor response 56 which has a recognizable magnetic field signature comprising a first relatively large peak 58, a relatively large inverted peak (referred to as a trough 60) and a second relatively large peak 62 is produced. The sequence of the relatively large peaks and trough 58, 60, 62 is detectable using the sensors 12, 14 and peak values representative of the height of the first and second peaks 58, 62 and the depth of the trough 60 are generated by the peak detector 16. The peak values and sequence of the peaks and trough are compared with reference sequences 20 stored in the database 22, and if the detected sequence matches with a reference sequence, the detected marker is considered to be genuine. If the detected sequence does not match with a reference sequence, the object associated with the detected sequence is considered to be non-genuine.

As shown in Figures 5 and 6, during use when a non-marker influencing object 70 is present adjacent a marker 48 and the sensors 12, 14 are moved relative to the markers in a direction indicated by arrow C, an example distorted sensor response 71 is produced at one or both of the sensors 12, 14.

The example distorted sensor response 71 comprises a first relatively large peak 72, a relatively large inverted peak (referred to as a trough 74) and a second relatively large peak 76 which is significantly greater in height than the first peak 72. The sequence of the relatively large peaks and trough 72, 74, 76 is detected using the sensors 12, 14 and peak values representative of the heights of the first and second peaks 72, 76 and the depth of the trough 74 are generated by the peak detector 16.

While the peak values and sequence of the peaks and trough are compared with reference sequences 20 stored in the database 22 in order to determine whether the detected marker is genuine, the control unit 18 does not instruct the steering controller 24 to effect any movement based on a difference between the peak magnitudes of response signals produced by the respective sensors until the integrity detector has analysed the response signals and provided an indication that the marker is free of influence by external objects.

In the present example wherein the signature comprises two peaks and one trough of substantially the same height when not affected by external objects, the integrity detector 34 compares the height of the first and second peaks and calculates a value indicative of the difference in height between them. In this example, since an object 70 is present adjacent the marker 48, the difference value is greater than the predetermined difference value and as a consequence the integrity detector issues a signal to the control unit 18 indicative that the marker should be ignored.

In one embodiment shown in Figure 7, each marker 90 comprises first and second marker elements 92, 94. Each sensor 12, 14 further comprises sensor elements 12a, 12b and 14a, 14b respectively. In this embodiment, the sensors 12, 14 are moved in a direction relative to the markers indicated by arrow D so as to detect markers 90. The distance between sensor elements 12a and 12b is substantially the same as the distance between sensor elements 14a and 14b and the distance between marker elements 92 and 94. In this way, if sensor element 12a is in line with sensor element 14a, and sensor element 12b is in line with sensor element 14b, then marker elements 92 and 94 can be detected by separate sensor elements simultaneously.

The arrangement described above has the advantage wherein each sensor element 12a, 12b, 14a, 14b can utilise a respective sample and hold circuit so that the marker elements 92, 94 can be detected simultaneously. Detection of the markers 90 can therefore be done at a faster rate compared to an arrangement wherein each sensor 12, 14 comprises only a single sensing element, particularly when analogue sample and hold circuits are used. If a single sensing element and respective sample and hold circuit is used in each sensor 12, 14, detection of the second marker element 94 will need to be undertaken at a time after the sample and hold circuit has been reset subsequent to reading the first marker element 92. For example, if the sample and hold circuit is dependent on a capacitor discharging before a fresh reading can be taken, then the time taken to do this will be a limiting factor on the speed that the sensors 12,14 can move relative to the markers 90.

In the embodiment shown in Figure 7, each marker element 92, 94 is detected simultaneously. As such, there is no need to wait for capacitors to discharge so as to reset the sample and hold circuit . It should be noted that each sample and hold circuit will need to be reset before a subsequent marker 90 is detected, however the distance between successive markers 90 will be substantially larger than the distance between marker elements 92, 94 of an individual marker 90.

In addition, the sensors 12, 14 may be arranged so that the leading sensing elements, in this case sensing elements 12b and 14b, will detect the first marker element 92 and 'prime' the sensors 12, 14 so that they are ready to detect both marker elements 92, 94 simultaneously when sensing elements 12a and 14a are in line with marker element 92 at the same time that sensing elements 12b and 14b are in line with marker element 94.

In the case where the marker elements 92, 94 produce a sensor response comprising a peak and a trough, the sensors 12, 14 don't necessarily need to be primed. For example, if marker element 92 produces a peak and marker element 94 produces a trough, then sensing elements 12a, 14a can detect the peak produced by marker element 92 at substantially the same time that sensing elements 12b, 14b detect the trough produced by marker element 94.

It will be appreciated that the above markers may be disposed in the ground in any suitable way, such as separately at the same time that seed is disposed in the ground, by incorporating the markers into a tape which may or may not also comprise seed, or in any other suitable way. It will also be appreciated that in order to prevent a gradual reduction in the efficiency of the guidance system over time because of the presence of an increasing number of markers in the ground, it is necessary to remove, destroy or render markers undetectable so that the guidance system responds only to the intended marker set and the markers are not influenced by the presence of residual metallic and/or magnetic material.

In one arrangement, a suitable soil disturbance implement such as a plough is used to separate the marker elements in each marker and thereby destroy the detectable sequence which forms the signature associated with the marker.

In order to increase the likelihood that marker elements will separate from each other, the marker elements may be disposed on a substrate which is susceptible to breaking down when placed in the ground. For example, the substrate may be formed of paper material.

However, while this method is effective in destroying the marker signature, magnetic material which has the potential to affect subsequently laid markers remains in the soil.

In order to minimize the amount of influential material present in the soil, a relatively powerful magnet may be used to remove residual magnetic material, although this method is only suitable for ground conditions wherein the soil is sufficiently loose to permit free movement of the magnetic material.

In an alternative arrangement, the markers are configured such that the magnetic field strength reduces over time with the duration of detectability being determined according to specific combinations of compounds used in the marker and the sensitivity of the sensors 12, 14. With this arrangement, in order manage the effective detectable life of the markers, the markers may be magnetized shortly before disposal in the ground, for example by including a magnetizer device on a seed and marker laying apparatus .

In a further alternative arrangement, markers may be arranged so as to physically deteriorate over time, for example by configuring the markers so as to corrode when disposed in the soil. In order to accelerate the corrosion process, the markers may be provided with corrosion accelerating materials, for example by laminating the material with corrosion accelerating material.

In a further alternative arrangement, the markers may be formed of particulate magnetic material bound together inside a shell, with the shell being formed of a material which breaks down over time when disposed in the soil. With this arrangement, when the shell breaks down, the particulate magnetic material is dispersed in the soil and in particular after ploughing is rendered undetectable by the guidance system.

In a further alternative arrangement, the magnetisable markers, for example in the form of spheres, are encapsulated to prevent corrosion. The encapsulant is subsequently destroyed by mechanical or other means thereby exposing the magnetic material to corrosive elements in the soil .

In a further alternative arrangement, the markers may be demagnetized by applying an oscillating magnetic field to the markers disposed in the ground. This causes the magnetic particles in the magnetic material to be randomized thereby reducing the overall strength of the magnetic field produced by the marker.

It will also be appreciated that the markers may as an alternative be incorporated into an irrigation pipe 80, for example as shown in Figure 8.

As shown in Figure 8, the pipe 80 comprises apertures 82 evenly distributed along the pipe 80, and annular markers 84 which may be formed on or integrally with the pipe 80. With this example, it will be understood that a machine comprising a guidance system shown in Figure 1 is able to maintain an accurate position relative to the pipe 80 by detecting and locating the machine relative to the markers 84 incorporated into the pipe 80.

It will be appreciated that it is possible to use subsurface pipes to guide machinery, for example so as to guide crop sowing apparatus relative to irrigation pipes so that seeds are appropriately located relative to the irrigation pipes for optimum water take up by the subsequent crops. It is also possible to accurately detect the location of the subsurface pipes and thereby ensure that operative components of an agricultural machine such as tillage points of a ground tillage apparatus are kept away from the subsurface pipes so that the pipes are not damaged.

In a further alternative embodiment, the markers are incorporated into an elongate carrier such as a seed tape or rope type carrier. With the latter variation in particular, during use the rope would be laid in the ground and used during a season to guide agricultural machinery. After the season has ended the rope may be removed easily by applying a force to one end of the rope. Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

Claims

1. A guidance system for guiding a guidable device relative to a set of markers, each marker having a plurality of detectable marker elements and the marker elements together defining a detectable signature sequence, the guidance system comprising: a marker detection device arranged to detect the signature sequence of a marker, and to provide a position signal indicative of the relative position of the marker detection device relative to each detected marker using the markers; and a steering controller arranged to receive the position signal and to control the position of a guidable device in response to the position signal when the detected signature sequence corresponds to a valid signature sequence.

2. A guidance system as claimed in claim 1, comprising: a set of markers, each marker having a plurality of detectable marker elements, the marker elements together defining a detectable signature sequence, and each marker in the set having substantially the same signature sequence .

3. The guidance system of claim 2, wherein each marker element is arranged to generate an electromagnetic field.

4. The guidance system of claim 2, wherein the marker detector device is arranged to generate a primary magnetic field and each marker element is arranged to modify the primary magnetic field.

5. The guidance system of any one of the preceding claims, wherein the marker detection device is arranged to derive the signature sequence by detecting peak values of signals detected from the markers .

6. The guidance system of any one of claims 1 to 4, wherein the marker detection device is arranged to derive the signature sequence by detecting magnetic field strength of the marker elements.

7. The guidance system of any one of claims 1 to 4 , wherein the marker detection device is arranged to derive the signature sequence by detecting spacings between adjacent marker elements.

8. The guidance system of any claim 1 or claim 2, wherein the marker detection device is arranged to derive the signature sequence by detecting RFID signatures .

9. The guidance system as claimed in any one of the preceding claims, wherein the marker detection device comprises at least two sensors, the steering controller being responsive to a position signal based on a difference between the respective sensor signals produced by the at least two sensors.

10. The guidance system of any one of the preceding claims, wherein the marker detection device further comprises an integrity detector arranged to compare at least two peaks produced by a sensor so as to determine whether the detected peaks have been influenced by an external object based on a comparison of the peaks, wherein the integrity detector is configured to ignore a marker when a difference between the peak values produced by the sensor is beyond a predetermined threshold.

11. The guidance system of claim 9 or claim 10, wherein the marker detection device comprises at least one electromagnetic field sensor.

12. The guidance system of claim 11, wherein the marker detection device comprises magnetometer type sensors .

13. The guidance system of any one of the preceding claims, wherein the system is arranged to store at least one reference signature sequence corresponding to a valid signature sequence, and the system is arranged to compare a detected signature sequence with the at least one reference signature sequence so as to determine whether the detected signature sequence corresponds to a valid signature sequence.

14. The guidance system of claim 13, wherein the system is arranged such that a detected signature sequence is stored and used subsequently by the system as a reference signature sequence.

15. The guidance system of any one of the preceding claims, wherein the marker detection device comprises a plurality of sensor elements arranged to line up in use with respective marker elements of a marker so that the marker elements are simultaneously detectable by respective sensor elements.

16. An agricultural machine comprising a guidance system as claimed in any one of the preceding claims .

17. A set of detectable markers, each detectable marker comprising: a plurality of detectable marker elements, the marker elements together defining a detectable signature sequence; each marker being disposable in the ground, being detectable by a marker detection device and being usable to provide a position signal indicative of the relative position of the marker detection device relative to the marker; the signature sequence being usable to identify whether the marker is valid; and wherein each marker in the set has substantially the same signature sequence .

18. The set of detectable markers of claim 17, wherein each marker element comprises a magnetizable material or a magnetized material .

19. The set of detectable markers of claim 18, wherein the signature sequence is defined by providing magnetic marker elements and arranging the orientations of the magnetic poles of the marker elements .

20. The set of detectable markers of claim 18, wherein the signature sequence is defined by providing magnetic marker elements and arranging the marker elements so that the peak magnetic field strength varies across the marker.

21. The set of detectable markers of claim 18, wherein the signature sequence is defined by providing magnetic marker elements and varying the distance between marker elements.

22. The set of detectable markers of claim 18, wherein the signature sequence is defined by providing a plurality of RFID marker elements .

23. The set of detectable markers of any one of claims 17 to 22, wherein at least one marker is arranged such that the detectability of the at least one marker reduces over time.

24. The set of detectable markers of claim 23, wherein at least one marker is arranged so as to corrode when disposed in soil.

25. The set of detectable markers of claim 24, wherein the at least one marker comprises a corrosion accelerating material .

26. The set of detectable markers of claim 23, wherein at least one marker comprises particulate detectable material disposed in a shell, the shell being formed of material arranged so as to degrade when disposed in soil.

27. The set of detectable markers of claim 23, wherein at least one marker comprises detectable corrosion susceptible material disposed in a shell, the shell being formed of a frangible material, whereby fracture of the shell renders the detectible material susceptible to corrosion.

28. An elongate carrier for guiding a guidable device, the carrier comprising a set of markers as claimed in any one of claims 17 to 22.

29. The elongate carrier as claimed in claim 28, wherein the carrier comprises a degradable substrate on which the set of markers are disposed.

30. The elongate carrier as claimed in claim 29, wherein the substrate comprises paper.

31. The elongate carrier as claimed in claim 28 or claim 29, wherein carrier comprises a seed tape comprising a plurality of markers and a plurality of seeds.

32. The elongate carrier of claim 28 or claim 29, wherein the carrier comprises a rope .

33. The elongate carrier of claim 28 or claim 29, wherein the carrier comprises an irrigation pipe .

34. A method of guiding a guidable device, the method comprising: providing a set of markers, each marker having a plurality of detectable marker elements, the marker elements together defining a detectable signature sequence, each marker in the set having substantially the same signature sequence; disposing the markers in the ground; providing a marker detection device to detect the markers; detecting the signature sequence of each marker; using each marker to provide a position signal indicative of the relative position of the marker detection device relative to the marker; and controlling the position of a guidable device in response to the position signal when the detected signature sequence corresponds to a valid signature sequence .

35. The method of claim 34, comprising arranging each marker element so as to generate an electromagnetic field.

36. The method of claim 34, comprising generating a primary magnetic filed and arranging each marker element so as to modify the primary magnetic field.

37. The method of claim 35 or claim 36, comprising detecting the signature sequence by detecting peak values of signals detected from the markers.

38. The method of claim 35 or claim 36, comprising detecting the signature sequence by detecting magnetic field strength.

39. The method of claim 34, comprising detecting the signature sequence by detecting spacings between adjacent marker elements.

40. The method of claim 34, comprising detecting the signature sequence by detecting RFID signatures.

41. The method as claimed in any one of claims 34 to 40, comprising providing at least two sensors, and controlling the position of a guidable device in response to a position signal based on a difference between the respective sensor signals produced by the at least two sensors.

42. The method of claim 37, comprising comparing at least two peaks produced by a sensor so as to determine whether the detected peaks have been influenced by an external object based on a comparison of the peaks, and ignoring a marker when a difference between the peak values produced by the sensor is beyond a predetermined threshold.

43. The method of any one of the claims 34 to 42, comprising storing at least one reference signature sequence corresponding to a valid signature sequence, and comparing a detected signature sequence with the at least one reference signature sequence so as to determine whether the detected signature sequence corresponds to a valid signature sequence.

44. The method of claim 43, comprising storing a detected signature sequence and subsequently using the stored signature sequence as a reference signature sequence.

45. The method of any one of claims 34 to 44, comprising disposing a plurality of sensor elements so as to line up in use with respective marker elements of a marker so that the marker elements are simultaneously detectable by respective sensor elements.

46. The method of any one of claims 34 to 45, comprising arranging at least one marker so as to corrode when disposed in soil.

47. The method of claim 46, comprising providing the at least one marker with a corrosion accelerating material .

48. The method of claim 46, comprising disposing particulate detectable material in a shell formed of material arranged so as to degrade when disposed in soil.

49. The method of claim 46, comprising disposing detectable corrosion susceptible material in a shell formed of a frangible material, whereby fracture of the shell renders the detectible material susceptible to corrosion.

50. A guidance system for guiding a guidable device relative to a set of markers substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

51. A method of guiding a guidable device relative to a set of markers substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings .