Quote:
With a laser gun like the police uses to measure speeders you can measure a car that is standing still to well above 50 kph. How? You move the laser with your hand when you measure(The laser must be mounted on a tripod or the reading is not valid).
Since your GPS is mounted on your head, what heapen if you suddenly bend forward? You get a quick spike in speed. The board doesn’t slow down because you compensate with your body weight to stay in balance(your feet does not move).
I suggest the GPS must be mounted on the board to measure board speed accuratly.
regards,
Håvard
As I hope to show below, comparing the speed measurements from a laser “gun” (laser speedometer) with GPS speed measurements is, as the saying goes, “like comparing apples with oranges”.
Here’s some comments about the performance characteristics of the laser speedometer from tests conducted by the Home Office[UK] - Dept. of Research and Science and summarized in the report:
“The potential ‘slip error’ of the LTI 20.20 laser speedometer (Fri Jan 27 00:00:00 GMT 2006)”.
According to the abstract, these tests show that:
“If the device is used in accordance with the ACPO (Association of Chief Police Officers) guidance, it will produce accurate readings within the limits set in the Speedmeter Handbook. Our tests prove that to be the case. While the device does detect when it is not held sufficiently steady, it can not detect when it is moved deliberately in order to generate an inaccurate reading.”
A little farther along in the abstract it warns:
“The guidance advises against carrying out measurements for enforcement purposes at the extremity of the range due to the difficulty of holding it sufficiently steady to obtain a reading.”
A basic question to be asked is: “What is the source of these false speed readings when the unit is held in an unsteady hand?”
Although I have not (yet) found a detailed description of the laser speedomenter, I have learned that it not only measures speed, but also distance:
"An LTI 20-20 laser speedometer is a small handheld device, which aims a laser beam at the target vehicle. When the beam returns to the device’s receiving set, the device translates the data for the speed of the vehicle and for the distance at which the vehicle was measured. The police use the LTI 20-20 laser speedometer, which is carried by hand, to enforce the traffic laws on urban and interurban roads. The device does not photograph the vehicle; the policeman stops the offending driver on the spot.
Tests have shown that the laser speedometer is extremely reliable. The results of measurements using this device constitute admissible evidence in court; test findings have successfully stood up in the Supreme Court a number of times."
http://police.gov.il/…eeding_accidents.asp
From this information, I would suspect that speeds are computed from two (or more) measurements of distance versus the elapsed time between them.
Given this information, what is a mechanism by which the instrument could produce a non-zero speed reading from a stationary car–but only if held in a shakey hand (i.e not on a tripod)?
My hypothesis is that the erroneous readings are related to the fact that the shaking hand causes the “aimed” laser beam to move from one location on the vehicle to another (or to an object other than the vehicle) as the alignment of the laser beam wanders in accord with the shakey movements. As this occurs, the beam would be reflected back from objects at varying distances from the central target of the beam. Different distances at different times computes out to a speed. Note that in accord with the UK study summary, this problem would be exacerbated as the distance increases between the instrument and the car. Also big differences in distance (e.g. front of vehicle vs a tree next to the vehicle) lead to high speeds.
This is a totally different situation from GPS speed measurements–independent of whether they are made using the Doppler shift of the frequencies sent from the satellites to the receiver, or from time/distance measurements. In the GPS case, the satellites positions, trajectories, and velocities–and how those quantities are changing with time–are known to a high precision. Furthermore, these signals are detected at the location of the receiver antenna (dimensions on the order of a few inches or less). Hence there is no effect equivalent to, or analogous to, the “shaking hand” and it’s effect on the separation between the source and the object.
That’s not to say that there can’t be some additional speed associated with (in the present case) the movement of the board rider’s head. But there are some constraints on how large that error may be. Given the dirth of information available from the manufacturers of the GPS units on their internal algorithims and computations, it is difficult to say how “instantaneous” the “max speed” and the “real time speeds” reported by the unit really are. However, by experimentation, I know that the “real time speeds” are apparently averaged over a time longer than 1 second since if one moves the unit at a constant speed, and then abruptly stops (or vice-versa goes from zero speed to some constant speed) the change from one steady speed to the other takes more than 1 second (the default updating time interval). Hence the error from short-term movements will be related to the (vector-averaged) velocity over some time like a second, or longer. If we assume the motion of the head changes it’s position (relative to the center-of-mass of the body) by 1 foot during one of these motions, we can get an idea of the associated possible error.
The “extra” speed reported by the GPS that is associated with this movement will be:
v = 1 ft /(the elapsed time to move that 1 ft distance)
If the motion takes 1 second, then the associated “error” is 1 ft/sec, or about 0.7 mph. If it takes only 1/2 a second, the real speed of the motion will be 2 ft/sec, or 1.4 mph. But…the GPS apparently averages over a period of 1 second (possibly more), so during the other half second of the averaging period the speed will be 0. Hence the speed error (for this example) will never exceed 0.7 mph for fast motions, and it will be the actual speed of the motion if that is less than 0.7 mph.
Moreover, moving the GPS receiver to the board can introduce some additonal inaccuracies since the solid angle of sky “visible” to the GPS receiver is reduced (it is now lower in the “valley” formed by the wave). That means that there is an increased likelihood that the number of satellites it can “track” is reduced. According to the GPS literature, generally the more satellites tracked, the greater the accuracy (many of the errors are random and tend to cancel out better as the number of measurements increases).
Apart from the accuracy of the measurements, the helmet-mounted GPS has some other advantages–one of which is that it spends less time submerged in the water, so any slow leaks (in the containment bag or box) are less likely to be damaging to the unit (the “waterproof” units sold to the general public are typically only guaranteed to be waterproof for 30 minutes at a depth of 1m in quiescent water–a far cry from surfing conditions). Another factor to consider is that with a helmet mounted unit in some areas the GPS probably won’t get damaged if one’s leash breaks and the board goes into the rocks.
Hence all things considered, my vote is for a helmet-mounted GPS, rather than a board mounting.
mtb
