Getting target coordinates

[foxtheancient]

Hello guys.

 

I found really interesting to play with my friends who fly Viggen and finding targets for them. As I can find the target and count the target's position with informations I have. Viggen can input those coordinates in the computer and guide its weapons to it.

 

At the internet I am not able to find much about how to count the coordinates, though. I found just this:

 

TargetUTMNorth = cos(bearing+declination)*(range/10)+CurrentUTMNorth

TargetUTMEast = sin(bearing+declination)*(range/10)+CurrentUTMEast

 

The problem is, that in some cases it works, in some cases it works roughtly and in same cases it does not work at all (point's coordinates counted are kilometers away). I created excel for this, so it is not counting error issue. I usually double check inputs and outputs so it is not typo issue. I don't understand why it is no unreliable. Does anyone have any experience with this to share?

[lemoen]

A few questions:

 

 

1. Are you using the cos/sin that works with degrees when using bearing and declination. If not you need to use radians, which is angle/180*pi.

 

2. Why is it range / 10? Make sure that your range/10 is the same unit as UTM (metres)

[foxtheancient]

I am not very familiar with all those coordinate systems. I belive Gazelle is using DMM (dd°mm.m') and UTM. There is a button on navigation panel to swap between display of those two (GEO/UTM). In that formula I use those UTM numbers, like f.e. N 463481 E 73148 (translates into N 41°49.9' E 41°47.2').

 

My procedure is:

- get my current coordinates in UTM from navigation computer

- find my target with camera

- read bearing from the camera (0-359)

- read distance with help of laser rangefinder (meters)

- find declanation in navigation computer (f.e. 6°02', which I round to full number for the formula, because I am not sure how to work with those mintues here)

- fit it all in the formula

- read new coordinates in UTM

- put them in the navigation computer as a waypoint

- check if the waypoint is heading at least roughly the same direction and distance

- if so, switch back to DMM as Vigen needs DMS

- if not, being confused

 

I am using Win10 calc in scientific mode for sin/cos calculations.

Edited April 24, 2017 by foxtheancient

[lemoen]

I don't know the Windows 10 calculator. Do this to check your calculations:

 

Get the answer for: cos(180)

 

If the answer is -0.59846006... the calculator expects radians in which case you should use (bearing*pi/180) instead of bearing.

 

If the calculator expected degrees then the answer would be -1, in which case do nothing.

 

 

Secondly, UTM coordinates are in metres right? Try using your range as is, in those formulas (instead of dividing by 10). See of that makes a difference.

[Nero.ger]

have you considert your altitude?

unless both you and your target are on the same altitude the distantance will not be correct.

Since the laser is measuring throu 3d-space while coords are one a 2d plane.

Pythagoras might help you here ;)

[lemoen]

have you considert your altitude?

unless both you and your target are on the same altitude the distantance will not be correct.

Since the laser is measuring throu 3d-space while coords are one a 2d plane.

Pythagoras might help you here ;)

 

That'll make a difference but not kilometers, assuming he's staying close to the ground.

[drPhibes]

2. Why is it range / 10? Make sure that your range/10 is the same unit as UTM (metres)

 

The NADIR system in the Gazelle doesn't use the last digit (1m) in the UTM coordinates, so it only has a 10m resolution.

 

I made a UTM calculator spreadsheet for the Gazelle some time ago. I'll post it here when I get home from work.

 

As for the elevation causing errors, that's technically true, but for low elevation angles (i.e. low altitude difference), the adjacent leg of the triangle is practically equal to the hypotenuse.

[drPhibes]

My excel sheet is way too messy to be presentable, but here's a step-by-step guide to making your own:

 

Inputs:

UTM N

UTM E

Distance to next WPT (in meters)

Bearing to next WPT (in degrees)

Magnetic declination - in either decimal degrees (e.g. 6.216°) or in degrees and arc minutes (6° 13' etc.).

 

Calculation:

First of all, if your magnetic declination is in degrees + arc min, convert to decimal degrees. Let's use the example above (6° 13'):

 

Decimal degrees = 6° + (13'/60) = 6,216°

All angles need to be in radians (at least by default in Excel):

 

Radians = angle*PI()/180

You then calculate the UTM coordinates of your next waypoint:

 

New UTM N = cos(bearing+mag decl.)*(distance/10)+UTM N

New UTM E = sin(bearing+mag decl.)*(distance/10)+UTM E

[foxtheancient]

EDIT

 

Sorry I was accidently writing in wrong language :D

 

I now know where the problem was. W10 calc does the math of sin/cos "correctly" - no need to modify the input. However Excel does not, it needs to have the inputs modified before calculation. As I was sometimes calculating by hand (w10 calc) and sometimes by Excel, I got sometimes wrong and sometimes correct results.

 

Thanks for advice how to convert degrees to correct format.

Edited April 26, 2017 by foxtheancient

[WildBillKelsoe]

 

there ya go..