Automatic antenna pointing system based on global positioning system (GPS) attitude information

• US 5,347,286 A
• Filed: 03/19/1993
• Issued: 09/13/1994
• Est. Priority Date: 02/13/1992
• Status: Expired due to Fees

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Babitch, Daniel
• Primary Examiner(s): Issing, Gregory C.
• Application Number:
  US08/033,953
• Time in Patent Office: 543 Days
• Field of Search: 342/359, 342/424, 342/423, 342/422, 342/56, 342/74, 342/75, 342/76, 342/77, 342/367, 343/757, 343/725
• US Class Current: 342/359
• CPC Class Codes: G01S 19/54 using carrier phase measurements; using long or short baseline interferometry
• CPC Class Codes: H01Q 1/3275 mounted on a horizontal surface of the vehicle, e.g. on roof, hood, trunk
• CPC Class Codes: H01Q 3/10 to produce a conical or spiral scan
• CPC Class Codes: H04B 7/0814 based on current reception conditions, e.g. switching to different antenna when signal level is below threshold

Abstract

An embodiment of the present invention is a system for automatically pointing a directional antenna. The system comprises two GPS antennas mounted at horizontally opposed extremities of the directional antenna, the placement is such that the GPS antennas lie on a line having a normal vector approximately parallel to a boresight of the directional antenna. The outputs from the GPS antennas are downconverted to a measurement frequency and differentially phase compared for each of a plurality of radio visible GPS satellites. A measurement controller that receives the phase comparisons. A navigation computer receives measurements from the measurement controller and uses a microprocessor to calculate the GPS latitude and longitude and the attitude angles of the pair of GPS antennas. A servo points the directional antenna at a particular target communications satellite, based on the computed azimuth, elevation, latitude, and longitude, as provided by the navigation computer. One of these servos is used to move the GPS antennas in roughly a circle to average out multipath effects to improve accuracy and to reduce integer ambiguities.

Claims

1. A communications system, comprising:

a directional antenna supported by a mobile platform subject to continuous irregular motion;

a plurality of GPS patch antennas mounted non-collinearly apart from one another and attached to said mobile platform at points where said platform shadows the GPS antennas from GPS satellite visibility to a minimum extent;

attitude determination means including maximum likelihood estimations (MLE) optimization means for resolving integer ambiguities and multiplexing means for coupling all of the GPS patch antennas into a single GPS receiver downconverter for calculating a platform attitude based on the phases of a plurality of GPS signals received by the GPS patch antennas from at least three GPS satellites; and

servo means mechanically connected to the directional antenna and for automatically positioning the directional antenna according to a pointing direction obtained by the attitude determination means wherein a radio communication may be established.

2. The system of claim 1 wherein:

the attitude determination means and said MLE optimization means further comprise;

means for collecting a plurality of carrier phase measurements from the GPS patch antennas based on a reception of GPS signals from a plurality of GPS satellites such that each of said carrier phase measurements adds a range of possible integer values according to the whole-cycle ambiguity of a GPS carrier signal;

means for forming a decision tree expressing each of said possible combinations of said possible integer values;

means for computing a weighted-fit error at one or more stages of said decision tree, such that a determination may be made that none of said integer combinations connected to a current branch of the tree can be optimum and said current branch may accordingly be cut thereby speeding up a search through said decision tree; and

means for selecting a final integer combination on the basis of a best fit with said measured phase data, as indicated by its weighted-fit error.

3. A portable satellite communications system including automatic antenna pointing for rapid field setup by relatively unskilled operators, the system comprising:

a portable carrying case;

a parabolic dish antenna supported by the portable carrying case for satellite communications with a communications satellite;

a pair of GPS patch antennas mounted apart from one another on the parabolic dish antenna;

attitude determination means for calculating a pointing direction for the parabolic dish antenna to said communications satellite with a known orbital position base on GPS carrier signals received by the GPS patch antennas from a GPS satellite that is apart from said communications satellite and that is based on an earth position derived for the GPS patch antennas from information included in said GPS signals;

servo means for automatically positioning the parabolic dish antenna according to a result obtained by the attitude determination means such that a radio communication may be established between said communications satellite and the parabolic dish antenna; and

multipath interference reduction means for improving an attitude solution result and including sampling means for moving the parabolic dish antenna for collecting several sets of the relative GPS signal phase data by the pair of GPS patch antennas and for use by the attitude determination means to calculate an ultimate pointing direction.

4. A portable satellite communications system including automatic antenna pointing for rapid field setup by relatively unskilled operators, the system comprising:

a portable carrying case;

a parabolic dish antenna supported by the portable carrying case for satellite communications with a communications satellite;

a pair of GPS patch antennas mounted apart from one another on the parabolic dish antenna;

attitude determination means for calculating a pointing direction for the parabolic dish antenna to said communications satellite with a known orbital position based on GPS carrier signals received by the GPS patch antennas from a GPS satellite that is apart from said communications satellite and that is based on an earth position derived for the GPS patch antennas from information included in said GPS signals, and comprises maximum likelihood estimation (MLE) integer resolution means for;

collecting a plurality of carrier phase measurements from the GPS patch antennas based on a reception of a plurality of GPS satellites such that each of said carrier phase measurements can add a range of possible integer values according to the whole-cycle ambiguity of a GPS carrier signal;

forming a decision tree expressing each of said possible combinations of said integer values;

computing a weighted-fit error at one or more stages of said decision tree, such that a determination may be made that none of said integer combinations connected to a current branch of the tree can be optimum and said current branch may accordingly be cut thereby speeding up a search through said decision tree; and

selecting a final integer combination on the basis of a best fit with said measured phase data, as indicated by its weighted-fit error; and

servo means for automatically positioning the parabolic dish antenna according to a result obtained by the attitude determination means such that a radio communication may be established between said communications satellite and the parabolic dish antenna.

5. The system of claim 4, further comprising:

motor means for spinning the parabolic dish antenna such that the GPS patch antennas may collect a plurality of GPS signals for the attitude determination means such that a three-dimensional pointing solution may be obtained for application to the servo means.

6. The system of claim 4, further comprising:

at least one additional GPS patch antenna attached to the parabolic dish antenna such that at least three GPS patch antennas are included in a collection of a plurality of GPS signals for the attitude determination means such that a three-dimensional pointing solution may be obtained for application to the servo means.

7. The system of claim 4, further comprising:

at least one additional GPS patch antenna attached to the parabolic dish antenna and coupled to the attitude determination means; and

motor means for spinning the parabolic dish antenna such that at least three GPS patch antennas are included in collecting a plurality of GPS signals for the attitude determination means wherein a unique three-dimensional pointing solution may be obtained for application to the servo means.

8. The system of claim 4, further comprising:

data storage means for saving a previous pointing solution for use by the attitude determination means such that the time to compute a current pointing solution is reduced by recalling said saved previous pointing solution.

9. The system of claim 4, further comprising:

multiplexer means for multiplexing said GPS signals from the GPS patch antennas to the attitude determination means which includes a single downconverter for all GPS carrier phase measurements.

10. A satellite communications system, comprising:

a platform subject to continuous motion while the system is in operation;

a directional dish antenna supported by the platform for satellite communications with a communications satellite;

a set of three of GPS patch antennas non-colinearly mounted apart from one another on the directional dish antenna and having a known geometric relationship to the directional dish antenna;

continuous attitude determination means for calculating a series of pointing directions for the directional antenna from a GPS satellite that is apart from said communications satellite and that is based on an earth position derived for the GPS patch antennas from information included in said GPS signals;

servo means mounted on the platform for automatically positioning the parabolic dish antenna with respect to the platform according to a result obtained by the attitude determination means to establish a radio communication between a communications satellite and the parabolic dish antenna;

multipath interference reduction means for improving an attitude solution result and including sampling means for moving the parabolic dish antenna to collect several sets of relative GPS signal phase data by the pair of GPS patch antennas and for use by the attitude determination means to calculate an ultimate pointing direction; and

maximum likelihood estimation (MLE) integer resolution means for;

collecting a plurality of carrier phase measurements from the GPS patch antennas based on a reception of a plurality of GPS satellites such that each of said carrier phase measurements adds a range of possible integer values according to the whole-cycle ambiguity of a GPS carrier signal;

forming a decision tree expressing each of said possible combinations of said integer values;

computing a weighted-fit error at one or more stages of said decision tree, and determining that none of said integer combinations connected to a current branch of the tree are optimum and said current branch may accordingly be cut thereby speeding up a search through said decision tree; and

selecting a final integer combination on the basis of a best fit with said measured phase data, as indicated by its weighted-fit error.