Monday, November 30, 2009

GPS Evidence-Two Sources are better than one

As I've stated in a previous post here, GPS track evidence admissibility may hinge on whether it is relevant to the issue in the case, and the GPS evidence relevance will, in turn, hinge on the frequency the track information is recorded. A high frequency track (like one track-point per second) is like a very high resolution digital picture, razor sharp in every detail for the time frame it covers. A low frequency track (like once every thirty seconds), although each track point is accurate, is "fuzzy" and "indistinct" in between track points, like a lower resolution picture.

A high frequency track would be important to pinpoint actual speed as accurately as possible, to defend against speeding tickets or charges of excessive speed in an accident situation. Low frequency tracks, although of general interest, may not be legally relevant, and thus not admissible in court, in cases needing greater accuracy.

Like photographs, there is a trade-off for both high frequency tracks and high resolution photos- available data storage space. High frequency tracks burn through available memory and use it up quickly, just as high resolution photos quickly use up memory cards. Even if you use a GPS that transmits to a third party, high frequency tracks (like a one track point per second track) are much more expensive than a low frequency track (like 30 or 40 second track point spread.) So what do you do if you both the protection of a high frequency track and the economy of a low frequency track? Simple. Get two GPS systems. Set one to record at a low frequency track rate, to generally track travel, etc. and set the other GPS system to the highest track recording frequency the unit allows, such as once a second or better. While the high frequency GPS unit would over-write data very frequently, such data wouldn't be needed unless there is an accident or speeding stop. In both situations, power would likely cut off the GPS tracking and preserve the evidence, or the operator could shut the unit down manually until it could be properly preserved.

Imagine an over-the-road trucker with ticket problems. Although he drives within the limits, police radar does make mistakes, sometimes tracking trees or other vehicles. This driver could loose his livelihood if such an error happens to him. GPS forms a potent defense to radar speed traps. His employer uses a GPS tracking service to track the trailer and truck, and generally check his driving, but it only records his location once every 30 seconds. Therefore, he carries a portable GPS set to one second tracking.

An officer pulls him over in a rural setting. He's sure he wasn't speeding, but takes the ticket. He shuts off the GPS and contacts his attorney. His attorney locates a local private investigator who can download and preserve the GPS track from high frequency GPS. The low frequency GPS info is already preserved at the headquarters of the remote tracking company; They need only verify and send it to the attorney, and testify by phone if necessary. The attorney takes the case to court and wins. Its not just the trucker's word, its his word and two GPS's.

Remote tracking companies can also develop (if they haven't already) secondary high frequency tracking units that don't send any data until 1) either they are triggered by the operator or 2) set off by the g-forces of an accident. Such a unit, like a blackbox flight recorder, would continuously over-write previous data for 20-30 minutes, preserving the GPS information in high detail for the small time frame it was necessary.

Some say a man with two watches never knows what time it is, because the two never agree. Is this a risk with two GPS's? No, because like two camera taking the same picture, one high resolution and one low, they don't record exactly the same thing, but close enough to fairly and accurately represent the scene. I doubt a low resolution track would ever cause the more accurate high resolution track to be excluded from evidence.