Method for correcting navigation errors due to water currents

• US 4,335,433 A
• Filed: 06/16/1980
• Issued: 06/15/1982
• Est. Priority Date: 06/16/1980
• Status: Expired due to Term

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Bauer, Russell H., Heidkamp, Madeleine
• Primary Examiner(s): Gruber, Felix D.
• Assistant Examiner(s): Chin, Gary
• Application Number:
  US06/160,049
• Time in Patent Office: 729 Days
• Field of Search: 364/443, 364/447, 364/449, 364/450, 364/451, 364/452, 364/453, 364/454, 364/457, 364/459, 73/178, 343/112 R
• US Class Current: 701/200
• CPC Class Codes: G01C 21/203 Specially adapted for sailing ships

Abstract

In a satellite navigation system for an ocean vessel, a major source of error is unknown ocean currents when the navigation is done by dead-reckoning with respect to the waters. The present method for determining and correcting for ocean navigation error which is caused by unknown ocean currents provides a highly accurate position of the vessel with respect to the earth. The position error reflects not only the difference between a dead-reckoned position and a satellite fix position which up-dates the dead-reckoned position but also accounts for the motion of the vessel due to the ocean currents during the time between the satellite lock-on and the actual error computation and correction.

Claims

1. In a system having a satellite fix position determining means and a dead-reckoning means for continuously estimating position between satellite fixes, a method for correcting ocean vessel navigation errors due to essentially constant water currents, comprising the steps of:

periodically locking on to a satellite and determining present vessel position in latitude (φ

_F.sbsb.2) and longitude (λ

_F.sbsb.2) using satellite fixes at lock-on,

comparing satellite fix latitude (φ

_F.sbsb.2) and longitude (λ

_F.sbsb.2) with dead-reckoned latitude (φ

_DR) and longitude (λ

_DR), respectively, at the time of fix to yield update corrections in latitude (φ

_u) and in longitude (λ

_u),

defining differential functions (A_ij) which relate respective changes in current satellite fix position in latitude (φ

_F.sbsb.2) and longitude (λ

_F.sbsb.2) to respective changes in velocity in the north (v_N) and east (v_E) directions,

3. A method as in claim 1 wherein the determining of the velocity north error (v_N) and the velocity east error (v_E) comprises the step of:

approximating v_N to a value of φ

_u divided by the time τ

between satellite fixes and

approximating v_E to a value of λ

_u divided by the time τ

between satellite fixes.

4. A method as in claim 1 wherein the function determining the velocity north error (v_N) and velocity east error (v_E) is:

##EQU3## and where τ

is the time between successive satellite fixes and where A_ij '"'"' corresponds to A_ij at the previous satellite fix.

6. A method as in claim 4 comprising the further steps of:

storing previous water velocities X'"'"' and Y'"'"' in the north and east directions, respectively,

subtracting the velocity north error (v_N) from the previous north water velocity (X'"'"') and subtracting the velocity east error (v_E) from the previous east water velocity (Y'"'"'), and

vectorially combining the two difference values to provide a present water velocity output v_w.

2. partially predicting satellite fix errors in latitude (φ

_E) and longitude (λ

_E) at current and previous satellite updates as a function of the velocity changes (v_N and v_E) and the differential functions (A_ij),

determining the values for the differential functions A_ij, comprising the steps of introducing known velocity errors in the north and east directions and measuring variations in satellite fix latitude (φ

_F.sbsb.2) and longitude (λ

_F.sbsb.2) positions resulting from the introduced known velocity errors,

measuring the time (T) between satellite updates, and

determining the velocity north and velocity east errors (v_N and v_E) as a function of;

time between satellite updates, update corrections (φ

_u and λ

_u), and differential functions (A_ij).

5. A method as in claim 2 or 3 comprising the further step of:

removing a position error buildup due to the velocity errors v_N and v_E comprising the step of;

altering the dead-reckoned latitude and longitude positions φ

_DR and λ

_DR, with φ

_DR and λ

_DR defined as;

##EQU4## where φ

_DR '"'"' and λ

_DR '"'"' are the previous dead-reckoned latitude positions;

space="preserve" listing-type="equation">A.sub.11 =∂

φ

.sub.F.sbsb.2 /∂

v.sub.N

space="preserve" listing-type="equation">A.sub.12 =∂

φ

.sub.F.sbsb.2 /∂

v.sub.E

space="preserve" listing-type="equation">A.sub.21 =∂

λ

.sub.F.sbsb.2 /∂

v.sub.N

space="preserve" listing-type="equation">A.sub.22 =∂

λ

.sub.F.sbsb.2 /∂

v.sub.E

comprise the A_ij functions;

RNAV is the distance to the center of the earth; and

TSLLK represents the time between the satellite lock-on and the time when the position error buildup is removed.