Well, you can derive speed when given distance and time.
Speed = Distance/Time
Take one distance reading, mark off, oh.. 5 seconds. Take another. Subtract both to get the distance traveled. Take that distance traveled over the time and you have speed. (of course, you'll have speed in Meters/Second, so you'll have to convert Meters/second to knots - according to a quick Google search,
knots meters per second 0.5148)
Now, unless you're stationary, you also have to factor in how much closer you got to the position as well to keep accurate.
Of course, the more readings you take, the more accurately you can plot their current speed.
Dunno the specifics (like if you have to be 90deg from the ship or if both will work, etc) - I was never very good at math.