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Aspects of this invention include a downhole tool having an angular position sensor disposed to measure the relative angular position between first and second members disposed to rotate about a common axis. A plurality of magnetic field sensors are deployed about the second member and disposed to me

Aspects of this invention include a downhole tool having an angular position sensor disposed to measure the relative angular position between first and second members disposed to rotate about a common axis. A plurality of magnetic field sensors are deployed about the second member and disposed to measure magnetic flux emanating from first and second magnets deployed on the first member. A controller is programmed to determine the relative angular position based on magnetic measurements made by the magnetic field sensors. In a one exemplary embodiment, a downhole steering tool includes first and second magnets circumferentially spaced on the shaft and a plurality of magnetic field sensors deployed about the housing.

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1. A downhole tool comprising: first and second members disposed to rotate about a common axis with respect to one another;first and second circumferentially spaced magnets deployed on the first member the first and second magnets being configured to emit a magnetic field having a radial component t

1. A downhole tool comprising: first and second members disposed to rotate about a common axis with respect to one another;first and second circumferentially spaced magnets deployed on the first member the first and second magnets being configured to emit a magnetic field having a radial component that varies in strength substantially linearly with an angular position about the second member for a range of at least 30 degrees in angular position;a plurality of circumferentially spaced magnetic field sensors deployed on the second member, at least one of the magnetic field sensors in sensory range of magnetic flux emanating from at least one of the magnets; anda controller disposed to calculate an angular position of the first member with respect to the second member from magnetic flux measurements at the magnetic field sensors. 2. The downhole tool of claim 1, wherein the downhole tool is selected from the group consisting of directional drilling tools, rotary steerable tools, and drilling motors. 3. The downhole tool of claim 1, wherein the magnetic field sensors are deployed such that an axis of sensitivity of each of the sensors is substantially parallel with a radial direction. 4. The downhole tool of claim 1, comprising from about 5 to about 16 magnetic field sensors. 5. The downhole tool of claim 1, wherein the magnetic field sensors are selected from the group consisting of Hall-Effect sensors, magnetoresistive sensors, magnetometers, and reed switches. 6. The downhole tool of claim 1, wherein the plurality of magnetic field sensors and the controller are deployed on a circumferential array, the array being deployed in a ring shaped pressure resistant housing deployed on the second member. 7. The downhole tool of claim 1, wherein the magnetic field sensors are spaced equi-angularly about the circumference of the second member. 8. The downhole tool of claim 1, wherein the first and second magnets comprise cylindrical magnets, the first magnet having a magnetic north pole facing radially outward and the second magnet having a magnetic south pole facing radially outward. 9. The downhole tool of claim 1, wherein the first and second magnets are circumferentially spaced by an angle in the range from about 30 to about 180 degrees. 10. The downhole tool of claim 1, wherein the first and second magnets comprise arc-shaped magnets, the first magnet having a magnetic north pole on an outer surface thereof and a magnetic south pole an inner surface thereof, the second magnet having a magnetic south pole on an outer surface thereof and a magnetic north pole on an inner surface thereof. 11. The downhole tool of claim 1, wherein the controller is configured to calculate the angular position by calculating the circumferential location of a magnetic flux null. 12. The downhole tool of claim 11, wherein the controller calculates the circumferential location of the magnetic flux null by processing first and second magnetic flux measurements made at adjacent ones of the magnetic field sensors according to the equation: P=L⁡(x+AA+B)wherein P represents the location of the magnetic flux null, L represents an angular interval between said adjacent magnetic field sensors, A and B represent absolute values of the first and second magnetic flux measurements, and x represents a counting variable having an integer value representing the magnetic field sensor used to measure the first magnetic flux measurement. 13. A downhole tool comprising: a shaft deployed to rotate substantially freely in a housing;first and second arc-shaped magnets circumferentially spaced on the shaft, the first and second magnets being tapered, having a thin end and a thick end, such that a radial thickness of the magnets increases from the thin end to the thick end, the first magnet having a magnetic north pole on an outer surface and a magnetic south pole an inner surface thereof, the second magnet having a magnetic south pole on an outer surface and a magnetic north pole on an inner surface thereof;a plurality of circumferentially spaced magnetic field sensors deployed in the housing, at least one of the magnetic field sensors in sensory range of magnetic flux emanating from at least one of the magnets; anda controller deployed in the housing and disposed to determine a relative angular position between the housing and the shaft from magnetic flux measurements made by the magnetic field sensors. 14. The downhole tool of claim 13 comprising a steering tool including at least one blade disposed to extend radially outward from the housing into contact with a borehole wall. 15. The downhole tool of claim 13, wherein the thick end has a thickness at least four times a thickness of the thin end. 16. The downhole tool of claim 13, wherein the thin end of the first magnet is proximate to the thin end of the second magnet. 17. The downhole tool of claim 13, wherein the first and second magnets each subtend a circular angle greater than an angular spacing between adjacent ones of the magnetic field sensors. 18. The downhole tool of claim 13, wherein the first and second magnets are configured to emit a magnetic field having a radial component that varies in strength substantially linearly with an angular position about the housing for a range of at least 30 degrees in angular position. 19. The downhole tool of claim 13, wherein the shaft and the housing are fabricated from at least one magnetic material. 20. The downhole tool of claim 13, comprising from about 5 to about 16 magnetic field sensors deployed equi-angularly about the circumference of the housing. 21. The downhole tool of claim 13, wherein the plurality of magnetic field sensors and the controller are deployed on a circumferential array, the array being deployed in a ring shaped, pressure resistant housing, the pressure resistant housing being deployed in the steering tool housing. 22. The downhole tool of claim 13, wherein the controller is configured to calculate the angular position by calculating a circumferential location of a magnetic flux null by processing first and second magnetic flux measurements made at adjacent ones of the magnetic field sensors according to the equation: P=L⁡(x+AA+B)wherein P represents the location of the magnetic flux null, L represents an angular interval between said adjacent magnetic field sensors, A and B represent absolute values of the first and second magnetic flux measurements, and x represents a counting variable having an integer value representing the magnetic field sensor used to measure the first magnetic flux measurement. 23. A method for determining a relative angular position between first and second members of a downhole tool, the method comprising: (a) deploying a downhole tool in a borehole, the downhole tool including first and second members disposed to rotate about a common axis with respect to one another, first and second circumferentially spaced magnets deployed on the first member, a plurality of circumferentially spaced magnetic field sensors deployed on the second member wherein the first and second magnets are configured to emit a magnetic field having a radial component that varies in strength substantially linearly with an angular position about the second member for a range of at least 30 degrees in angular position;(b) causing each of the magnetic field sensors to measure a magnetic flux; and(c) processing the magnetic flux measurements to calculate the relative angular position between the first and second members. 24. The method of claim 23, wherein (c) further comprises calculating a circumferential location of a magnetic flux null by processing first and second magnetic flux measurements made at adjacent ones of the magnetic field sensors according to the equation: P=L⁡(x+AA+B)wherein P represents the location of the magnetic flux null, L represents an angular interval between said adjacent magnetic field sensors, A and B represent absolute values of the first and second magnetic flux measurements, and x represents a counting variable having an integer value representing the magnetic field sensor used to measure the first magnetic flux measurement. 25. The method of claim 23, wherein the first and second magnets comprise tapered, arc-shaped magnets, having a thin end and a thick end, such that a radial thickness of the magnets increases from the thin end to the thick end, the first magnet having a magnetic north pole on an outer surface thereof and a magnetic south pole an inner surface thereof, the second magnet having a magnetic south pole on an outer surface thereof and a magnetic north pole on an inner surface thereof. 26. The method of claim 23, further comprising: (d) repeating steps (b) and (c) at a time interval in the range from about 10 to about 100 milliseconds. 27. The method of claim 23, further comprising: (d) repeating steps (b) and (c); and(e) processing the angular position measurements calculated in (c) and (d) and a time interval between said angular position measurements to calculate a relative rotation rate between the first and second members. 28. A downhole tool comprising: first and second members disposed to rotate about a common axis with respect to one another;first and second arc-shaped magnets circumferentially spaced on the shaft, the first magnet having a magnetic north pole on an outer surface and a magnetic south pole an inner surface thereof, the second magnet having a magnetic south pole on an outer surface and a magnetic north pole on an inner surface thereof;first and second tapered, arc-shaped magnetic lenses deployed radially between the first and second magnets and selected ones of the magnetic field sensors, the magnetic lenses being fabricated from a magnetic material;a plurality of circumferentially spaced magnetic field sensors deployed on the second member, at least one of the magnetic field sensors in sensory range of magnetic flux emanating from at least one of the magnets; anda controller disposed to calculate an angular position of the first member with respect to the second member from magnetic flux measurements at the magnetic field sensors. 29. A downhole tool comprising: first and second members disposed to rotate about a common axis with respect to one another;first and second circumferentially spaced sets of discrete magnets deployed on the first member, each set including a plurality of discrete magnets circumferentially spaced about a unique circumferential portion of the first member, a magnetic strength of the discrete magnets increasing from one end of each set to an opposing end;a plurality of circumferentially spaced magnetic field sensors deployed on the second member, at least one of the magnetic field sensors in sensory range of magnetic flux emanating from at least one of the magnets; anda controller disposed to calculate an angular position of the first member with respect to the second member from magnetic flux measurements at the magnetic field sensors.