Radar vs. GPS
 

Noticed that in most tests listed that GPS speed is normally anywhere from right on to 1mph slower than radar....what gives.. Wasn't there a thread about the accuracy of GPS's on land vs water? Would this factor into the equation???? Which IS more accurate GPS or Radar???/ Food for thought......

"Bad-Habit"

THIS MAY START THE WHOLE DEBATE OVER AGAIN, BUT HERE GOES.

GPS is the best way to measure your speed unless you can run in a certified Kilo course. When you use your radar to measure your speed on the water there are many factors. Radar uses the same basic principal as GPS for the calculation. SPEED = DISTANCE / TIME. Radar measures this. The radar cannot account for the speed of waves and current moving towards or away from the boat as it measures speed. This is why you see a slight variance between the two methods of measurement. They both measure the same thing, but the radar would need to use correction factors to get Radar speed to equal GPS speed. If you could have a stationary radar clock a moving boat, then that would eliminate most of the errors / correction factors and be more accurate. Bob Teague wrote an article on this last year in Powerboat.

In my opinion a stationary laser measuring a moving boat would be the most accurate method of calculating speed.

Well GPS it is then........

"Bad-Habit"

Good Topic,
 

I would have to agree and disagree with Dredgeking. I understand your point of a stationary radar clocking a boat, the gun is not moving with the current etc, but I would need someone to explain how a cop car can radar you coming at him or going away from him. The gun must be able to distinquish if it
is moving or not.

I am not quite sure how a radar gun works with movement, but I am sure that it does.

I think the police radar works with the certified speedometer in the police car so that it knows how fast the police vehicle is moving and it can use that correction when calculating your speed.

Go to www.valentineone.com and read Mike Valentine's articles on moving radar. The radar in the cop car is measuring the speed of the cop car against stationary objects and comparing it to the moving objects being measured. Lot's of room for error since the radar can't distinguish between relatively close but different objects. Such as the truck moving at 10mph down the access road in the areas the radar signal is bouncing off of.

I believe that both are accurate within an acceptable standard of care. Since the radar uses doppler (radio) waves there may be some variations due to subtle movements of the transmitter/receiver and it's ability or inability to lock on and correct for those subtle movements.

Ray - relax, it's only a message board, it's o n l y a m e s s a g e b o a r d. . .

On open water the radar gun can be shot off the waters surface from the boat which will give you the boats speed in ref. to the water. The GPS will give you your speed measured across the ground. They can agree 100% or differ considerably. I will let the experts explain why these variances occur. ;)

Relax? can't figure out what you're talking about.

John, you agree with the fact that the radar has some "issues"?

OK here we go again: This is the article from PowerBoat Magazine..
I just copied and pasted it.

We have had several inquiries regarding the difference in the speed readings in traditional speedometers, a radar gun, and GPS. Many of our readers are experiencing different readings listed in Powerboats's Performance reports. Let me see if I can clear this up.
One of the most common complaints is that a newly installed GPS digital or analog speedometer is redaing "slow". Most likely, this is not the case. It is more likely that the conventional speedometer is reading high.
Depending on the manufacturer, some conventional water pressure speedometers read high. A few are pretty accurate. Pitot placement and style also play a major role in speedo accuracy. The most accrate pitots are transom mounted and receive clean water. Pitots integral to Bravo drives are pretty accurate depending on drive height, but trim angle can affect the pitot angle and speedo reading accordingly. If you are testing a new or used boat, don't rely on the speedo for accuracy. Bring along a GPS.
A GPS tells you how fast you are traveling around the circumference of the Earth, which is why the mph is really speed over ground. It doesn't always accurately tell you how fast you are traveling across the water. Currents, waves and navigational zig-zags all affect the GPS reading. But, if you are on dead calm water, there is no current and you are running in a straight line, the GPS will give you accurate overall readings.
Radar guns present another accurate option. Most modern radar guns can be used from inside the boat and provide a pretty accurate reading, which is reflected from the water ahead. Older guns could only record a reading from a land mass or had to be held from a stationary point to radar a passing boat. The only accurate way to radar a passing boat is to have the boat coming directly toward you or away from you. Coming at you is usually more accurate because sometimes the radar gun picks up the spray from the rooster tail from the rear. If you aim the gun at a boat that is passing by you, the reading will be low because of the angle. A radar gun used from inside the boat is reflecting off the water, it tends pick up wind waves. So, if you are going against the wind, it may read higher than if you are going with the wind.
During performance trials, we record the indicated speed on the speedo provided by the manufacturer. We install a seperate Nordskog Perfomance Products GPS with a memory function. And we use radar to record peak speed and acceleration tests.
Looking at our data, you will note that certain tests show all the speeds approximatley the same. Most of the time the radar is a little higher tha GPS. Generally speaking, this is because the radar records the absolute instant of peak speed while the GPS is an average of the distance traveled each second. As far as the speedos go, we record the readings for comparison only. Many of the boats tested last year had GPS speedos or a GPS system in place of a conventional speedo. When it comes to tournament ski boats, speedo readings are insignificant because speedos are adjustable.
In other tests, you will note a fairly significant difference between radar and GPS readings. This usually has to do with where the tests were conducted. In other words, if the tests were performed where the current was high, the wind was high, the water was rough or a combination, the readings will differ. The GPS top-speed readings we report are best achieved speed during the test. We also use the GPS to verify effieciency readings, for the boat and fuel economy. The radar readings are a comination of a two-way pass, ideally one into the wind and one with the wind.
Certain types of boats react differently to the wind. Usually, cats run faster into the wind because they get more lift, and V-bottoms run faster downwind because there is less wind resistance. The radar will read higher into the wind because it is picking up wind waves coming at you and conversely, will read slower downwind because the waves are going with you.
If the current is running the same as the wind, the GPS will read higher than the radar and speedo but is not an accurate indicator of water speed. And if the current is running upwind, the radar and GPS could be the same. If you are running against the wind and up current in a V-bottom, the radar will read high and GPS low. The point is, there are many variables.

NOW THATS AN ANSWER!!!!!!! I don't think I could think of anything else to ask if I wanted to....

A+ my lad A+

"Bad-Habit"