1. TITLE OF THE INVENTION

A novel method of obliquely marking a ground based target for obtaining its position coordinates.

2. FIELD OF THE INVENTION

The present invention relates to aircraft navigation, and in particular it relates to a method for acquiring obliquely the position of a ground based target.

3. PRIOR ART AND DRAWBACKS OF PRIOR ART

The marking procedure enables the pilot to acquire the coordinates (latitude, longitude and elevation) of any ground based target. Prior to this invention the pilot needs to fly overhead of the ground target to mark it accurately. But if the ground target is situated in the enemy territory, it is not safe for the aircraft to fly over and mark it.

4. AIM OF INVENTION

The main objective of this invention is to provide a novel methodology for acquiring the position coordinates of the ground based target without the need to fly over the target.

5. SUMMARY OF INVENTION

Refer Figure 1:

The marking procedure is a method for acquiring the position coordinates of a ground based target.

There are two phases of operation during marking procedure, acquisition phase and track phase. The pilot select the acquisition phase by selecting the Mark button on Up Front Control Panel (UFCP)(5). The Flight Management Computer (FMC) (4) obtains the present position (latitude and longitude of aircraft) and attitude angles (roll, pitch and true heading) from Inertial Navigation Global Positioning System (INGPS) (2).During acquisition phase the FMC computes the position coordinates (latitude and longitude) of the ground target based on the position of fixing cross (refer figure 3) symbol on HUD (5).The fixing cross on HUD represents the position of ground target to be marked and its position on HUD is decided by the deflection of Hands On Throttle And Stick top(HOTAS)(3) slewing inputs in the vertical and horizontal direction.

The FMC sends the initial position coordinates to Laser Pod (1) in acquisition phase if Laser Pod is selected as range sensor. The FMC obtains the slant range and depression angle (figure 2) to the ground target from Laser Pod and compute the Height above the target with respect to the aircraft datum. With the obtained Height above the target FMC computes the position of fixing cross. During the unavailability of Laser Pod FMC only computes the height above target based on available range sensor.
In track phase the Laser Pod track the ground target. The FMC obtains the position coordinates (latitude and longitude) of the ground target from Laser Pod directly in this phase. If Laser Pod is not available FMC stabilizes the movement of aircraft and fixing cross. The pilot can slew the cursor to the ground target and FMC computes the position in terms of latitude, longitude and elevation. The computed position coordinates will be stored as mark point coordinates in the non volatile memory and is displayed on UFCP.

6. BRIEF DESCRIPTIONS OF DRAWING

Figure 1 is the block diagram of Flight Management Computer interacting with different
Sub systems for obtaining the ground target position coordinates.

Figure 2 represents the mark point on a terrain with various height parameters.

Figure 3 depicts the positioning of fixing cross symbol on HUD.

Figure 4 illustrates the flow chart of marking procedure.

7. DESCRIPTION OF THE INVENTION

Refer figure 1:

The FMC (4) obtains the position coordinates of ground based target by using the present position, attitude (pitch, roll and true heading) angles and baro inertial height above mean sea level of aircraft from INGPS (2), slant range and depression angle of ground target from the present position from Laser Pod (1) and slewing inputs from Hands on Throttle And Sticktop (HOTAS)(3). The position coordinates of the ground target to be acquired called the mark point can be designated by fixing cross symbol on HUD (refer figure 3). The pilot slews the fixing cross on HUD until it will exactly overlap the ground target for marking and the FMC compute the position (latitude and longitude) of fixing cross as given below:

Refer figure 4: Step 101:
The marking procedure is initiated by pressing Mark push button on UFCP and the FMC goes to acquisition phase after the mark procedure is initiated. During acquisition phase the FMC obtains the azimuth and elevation deflection of HOTAS slewing input as analog signals and converts the deflections in terms of degrees. The computed deflection in the azimuth direction is the azimuth_cross (figure 3) and the deflection in the vertical direction is the depression_cross (figure 3) of the fixing cross symbol on HUD. The position of the fixing cross symbol on HUD in turn represents the ground target whose position is to be acquired.

The FMC receives the slant range and depression angle to the ground target from Laser Pod in acquisition phase and compute the height above target (Height_Above_Target) with respect to the aircraft datum(refer figure 2).
Height_Above_Target = slant range * sin (depression angle).
During the non availability of Laser Pod ,the height above the target is obtained from other employed range sensor modules in FMC.

Step 102:
The FMC converts the mark point (fixing cross) position on HUD into aircraft axes by using the following formulae.
rt = rk'' tan(depression cross) rj = r; tan(azimuth _ cross)
The direction cosines used for the conversion to various axes are gi'
110 = cos (pitch angle)
I20 = 0.0
I30 = -sin (pitch angle)
m10 = sin (pitch angle)*sin (roll angle)
m20 = cos (roll angle)
m30 = cos (pitch angle)*sin (roll angle)
n10 = sin (pitch angle)*cos (roll angle)
n20 = -sin (roll angle)
n30 = cos (pitch angle)*cos (roll angle)

Step 103:
The converted fixing cross position represents the mark point position on ground with respect to the aircraft axes. The aircraft axes coordinates of mark point are converted with respect to ground axes coordinates as given below: Rex = 110 * ri + m10 * rj + n10 * rk
Rcy = I20 * ri + m20 * rj + n20 * rk
Rcz = Height Above Target

Step 104:
The distance in the north and east direction from the present position to the mark point is computed by FMC as given below:
Ren = Rex * cos(True Heading) - Rcy * sin(True Heading)
Rce = Rcy * cos(True Heading) + Rex * sin(True Heading)
The computed Ren and Rce is used for the computation of the latitude and
longitude of the mark point as given below:
Latitude = (Ren / R) + Latitude of present position
Longitude = (Rce / (R * cos (Latitude of present position))) + Longitude of
Present position Here R is the radius of Earth.
During the acquisition phase, if LDP is available as range sensor, the FMC sends the computed ground target position in terms of latitude and longitude to Laser Pod which operates in slave mode to accept the target position coordinates.
Step 105:
The pilot slews the HOTAS slewing input until the fixing cross is positioned on the desired ground target to be marked and FMC continuously computes the position coordinates of the slewed ground target as per the steps from 101 to 104.

Step 106:
The pilot changes operational phase to track phase by pressing the phase change trigger on HOTAS after the fixing cross is exactly slewed onto the ground target. During the track phase the Laser Pod goes to the track mode and it starts tracking the ground target position.

Step 107:
If LDP is available as a range sensor follow step 109 else follow step 108.

Step 108:
During Track phase, if Laser Pod is not available as the Range sensor, the FMC obtains the velocities in the north (Vn), east (Ve) and vertical(Vv) directions from INGPS and compute the component of velocities with respect to the ground axes system as given below:
Vgx = Vn * cos( True Heading) + Ve * sin( True Heading )
Vgy = Ve * cos( True Heading ) - Vn * sin( True Heading )

The FMC stabilizes the position of fixing cross (Mark point) with respect to HOTAS slewing input deflection and movement of aircraft in ground axes system as given below:
Rex = Rex - ((Vgx - hex) * 0.040256 ) + Ay * Rcy
Rcy = Rcy - ((Vgy - hey) * 0.040256 ) - Ay * Rex
Rcz = Rcz + Vv * 0.040256 Here A\\i is the difference in the true Heading between two consecutive cycles of execution of FMC and hex and hey are the deflections of HOTAS slewing input.

Step 109:
During the track phase, the Laser Pod tracks the ground target and it obtains the position coordinates of the tracked ground target. FMC obtains the position coordinates of the ground target directly from the Laser Pod.

Step 110:
The FMC compute the position coordinates of the fixing cross in terms of latitude and longitude as given in step 104 if Laser Pod is not available as Range sensor. This step will be repeated until the ground target is slewed exactly.

Step 111:
The FMC obtains the height of the aircraft with respect to sea level (baro inertial altitude) from INGPS and computes the height above the target as given in step 101. The elevation of the mark point (refer figure 2) is computed as given below:
Elevation = baro inertial altitude - height above the target.

Step 112:
If the desired ground target is not tracked follow the steps from 107 to 111, otherwise follow the step 113.

Step 113:
The pilot press the store command from UFCP/HOTAS to acquire the coordinates of the ground target. The obtained latitude, longitude and elevation are stored in the non volatile memory of FMC as mark point and the acquired position coordinates are displayed on UFCP for pilot's inspection.

CLAIMS

We claim
1. A method for acquiring the position coordinates (latitude and longitude) of a ground target obliquely.

2. The method claimed in 1 interacts with INGPS, Hands on Controller, Up front Control panel System for its input.

3. The method claimed in 1 computes position coordinates of ground target using slant range, depression angles, attitude angles, aircraft velocities and HOTAS slew rates.

4. The method claimed in 1, stores the computed position coordinates of ground target in the non volatile memory of Flight Management computer.