Calculating a Future Position

I've got this information about an entity:

Lat/Long Position Heading (In Radians) Speed (In knots)

How do I calculate where the entities lat/long position will be at an arbitrary point in time (or figure out its position at intervals) assuming that the e开发者_StackOverflowntity is moving in a straight line at a constant speed at a constant altitude?

(As you can tell, my math skills are weak)

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First, you cannot move on a straight line and at the same time at a constant altitude. Your Lat/Long coordinates are over a reference ellipsoid (most likely WGS84). Their are various libraries or DB extensions that have a bunch of functions to do exactely these kinds of problems. Postgis for instance is one of them. Most of them are free and tested and do work. I would not recommend to try to implement this yourself (assuming the earth is a perfect sphere with radius r) as you must take into consideration the reference ellipsoid you are using.

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Taking into consideration the answer provided by Hyperboreus, there is the aviation formulary here: http://williams.best.vwh.net/avform.htm that has formulas for calculating a new lat/lon given a starting point, and angle, and a distance. You would have to calculate the distance yourself. It sounds like you have everything you need to calculate the distance though, as speed is just distance / time, and if you multiply that by the time offset you want to use, you've got your distance.

1 knot = 1 NM / hr distance = yourTimeInDecimalHours * speed;

code sample to illustrate this (copied from the above referenced link):

double lat1 = 0, lon1 = 0;                      // NOTE: these are in radians - remember PI/2 radians = 90 degrees
double d = timeInDecimalHours * speedInKnots;   // so a half hour at 35 knots would be .5 * 35
double tc = usersTrueCourse;                    // aka user's heading
double lat=                                     // this will be in radians!!
    Math.asin(
        Math.sin(lat1) 
        * Math.cos(d)
        + Math.cos(lat1)
        * Math.sin(d)
        * Math.cos(tc)
    );
double lon = 0;
if (Math.cos(lat) == 0) {
    lon = lon1      // endpoint a pole
} else {
    lon = (lon1 - Math.asin(Math.sin(tc)*Math.sin(d)/Math.cos(lat)) + Math.PI % (2*Math.PI))-Math.PI;
}