Compact single-aperture antenna and direction-finding navigation system
A radio-based navigation system uses a small multi-mode direction-finding antenna and a direction-finding receiver to determine platform position, velocity, attitude and time while simultaneously providing protection against narrowband and broadband sources of interference. Global Navigation Satellite System (GNSS) signals such as those from a Global Positioning System (GPS) provide attitude measurements with a compact multi-mode direction-finding antenna (e.g., a small two-arm spiral with improved angle-of-arrival performance over the entire hemisphere enhanced through use of a conductive vertical extension of the antenna ground plane about the antenna perimeter and/or conductive posts placed evenly around the antenna perimeter) that provides protection against jammers. The multi-mode spiral may be treated as an array of rotationally-symmetric antenna elements. The GPS receiver architecture may be modified for direction-finding and thereby attitude determination by increasing the number of input signals from one to at least two while minimizing the required number of correlators and mixers.
1. A single-aperture direction-finding antenna comprising:
a. at least one conductive driven element located above a conductive ground surface;
b. a plurality of feed ports driving said at least one conductive driven element, wherein the feed ports and the driven element are rotationally-symmetric about a common center above said conductive ground surface and support at least two direction-finding antenna modes while sharing a single radiating aperture; and
c. at least one rotationally-symmetric conductive member electrically connected to said conductive ground surface at a predetermined distance from said common center and extending away from the ground surface toward said at least one conductive driven element while leaving a gap between said at least one conductive driven element and said conductive member, wherein the height of said conductive member and said gap are selected to maximize the efficiency and improve the rotational symmetry of the at least two direction-finding antenna modes,
whereby azimuthal phase and magnitude variations of said antenna modes are reduced resulting in improved angle-of-arrival accuracy and direction-finding performance over a hemisphere above said conductive ground surface.
2. The direction-finding antenna of claim 1 , wherein said at least one conductive driven element comprises spiral arms.
3. The direction-finding antenna of claim 1 , wherein said conductive member comprises a conductive wall located along an outer perimeter of said at least one conductive driven element.
4. The direction-finding antenna of claim 1 , wherein said conductive member comprises a set of conductive posts distributed in a manner that preserves rotational symmetry of the direction-finding antenna about an outer perimeter of said at least one conductive driven element.
5. The direction-finding antenna of claim 1 , wherein:
a. said at least one conductive driven element comprises spiral arms; and
b. said conductive member comprises:
(1) a cylindrical conductive wall located along a circular perimeter with a radius approximately equal to an outer radius of the spiral arms having a wall height less than the distance between the ground surface and the spiral arms; and
(2) a set of conductive posts distributed evenly about the circumference of said conductive wall in a manner that preserves rotational symmetry of the direction-finding antenna, and wherein the posts are electrically connected to the wall and vertically extend to approximately the same height as the spiral arms.
6. A method for determining angle of arrival of received radio-frequency signals at a receiving single-aperture direction-finding antenna, said method comprising:
a. receiving radio-frequency signals via a plurality of feed ports for at least one rotationally-symmetric conductive driven element emanating from a common center above a conductive ground surface, wherein said at least one conductive driven element and the feed ports support at least two direction-finding antenna modes while sharing a single radiating aperture;
b. providing at least one rotationally-symmetric conductive member electrically connected to said conductive ground surface at a predetermined distance from said common center and extending away from the ground surface toward said at least one conductive driven element while leaving a gap between said at least one conductive driven element and said conductive member, wherein the height of said conductive member and said gap are selected to maximize the efficiency and improve the rotational symmetry of the at least two direction-finding antenna modes; and
c. determining angle-of-arrival of the received radio-frequency signals over a hemisphere above said conductive ground surface.
7. The method of claim 6 , wherein said at least one conductive driven element comprises spiral arms.
8. The method of claim 6 , wherein said conductive member comprises a conductive wall located along an outer perimeter of said at least one conductive driven element.
9. The method of claim 6 , wherein said conductive member comprises a set of conductive posts distributed in a manner that preserves rotational symmetry of the direction-finding antenna about an outer perimeter of said at least one conductive driven element.
10. The method of claim 6 , wherein:
a. said at least one rotationally-symmetric conductive driven element comprises spiral arms; and
b. said conductive member comprises:
(1) a cylindrical conductive wall located along a circular perimeter with a radius approximately equal to an outer radius of the spiral arms having a wall height less than the distance between the ground surface and the spiral arms; and
(2) a set of conductive posts distributed evenly about the circumference of said conductive wall in a manner that preserves rotational symmetry of the direction-finding antenna, and wherein the posts are electrically connected to the wall and vertically extend to approximately the same height as the spiral arms.
11. The direction-finding antenna of claim 5 , wherein said spiral arms are mechanically supported by a rotationally symmetric antenna substrate located about said conductive posts.
12. The direction-finding antenna of claim 10 , wherein said spiral arms are mechanically supported by a rotationally symmetric antenna substrate located about said conductive posts.
13. A single-aperture direction finding antenna as in claim 1 , wherein said at least one conductive driven element comprises a multi-arm spiral antenna element contained within a circular element space having a diameter
d
≈
m
max
π
ɛ
r
λ
where m max is the maximum operating resonant mode number utilized, λ is the wavelength at the lowest antenna operating frequency, and ∈ r is the effective relative dielectric constant of the antenna.
14. A method as in claim 6 , wherein said at least one conductive driven element comprises a multi-arm spiral antenna element contained within a circular element space having a diameter
d
≈
m
max
π
ɛ
r
λ
where m max is the maximum operating resonant mode number utilized, λ is the wavelength at the lowest antenna operating frequency, and ∈ r is the effective relative dielectric constant of the antenna.