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Heisenberg legal uncertainty principle: Court says prosecutor could use parolee's GPS device to show where he was when a car was shot up, but not how fast he was moving around at the time

A man convicted of trying but failing to kill another man in a car at Quincy Street and Baker Avenue in Dorchester in 2015 will get a new trial because prosecutors used speed data from the GPS device strapped to his ankle - a condition of his parole on a federal drug charge - without proving that the GPS system was actually reliable for seeing how fast he was moving before and after the shooting.

Since a key part of the Suffolk County prosecutor's case was to use speed data from the GPS to show Matthew Davis, already classified as an armed career criminal, drove up to the scene, ran up to the car, fired at least seven shots, then ran away, the data from the device was enough to prejudice the jury and so a new trial is warranted, the Supreme Judicial Court ruled.

Under Massachusetts law, prosecutors have to prove that a technology is reliable enough to be introduced as evidence, in a hearing before the trial judge. GPS by itself is no longer new, and Davis's attorney did not question its use in general at a pre-trial hearing, but on appeal argued that what was different in this case was the use of speed data, in addition to location data - in part because the Colorado-based system used in this case collects speeds differently than location information. The court said this was a crucial issue in part because simply being at a crime scene is not, of itself, enough to prove somebody is guilty of the crime.

The justices agreed that the trial judge erred in not making prosecutors prove in the pre-trial hearing that the system's speed data could be held to be reliable enough to warrant using it to help make the case that Davis fired at another man, who was not hit but who then crashed his car and ran away on the morning of Sept. 15, 2015.

Davis was wearing the device as part of a release condition on his federal conviction for a 2008 crack case, which netted him seven years in prison and another five years on federal probation.

In an argument before the SJC, a Suffolk County prosecutor argued the speed data, even if wrongly introduced, was not really a big enough of a deal to possibly sway the jury.

But the court disagreed, noting the prosecutor in the case mentioned speed ten times in a closing argument, using the speed information to match Davis's positioning with testimony from a witness who saw a man matching his description run past her a couple blocks away and with the timing of a video from a surveillance camera.

The defendant's speed was not merely duplicative of his location. It was crucial evidence used to correlate the defendant's movements to those of the shooter in the video and to the man [the witness] saw running. Without the speed, a jury would have only been able to infer that the defendant was in the area where the shooting took place. With the speed, however, the jury could match the defendant's movements to those of the shooter in the video and the man [the witness] saw, thereby presenting a compelling narrative that the defendant was the shooter. Thus, we cannot say that it "did not influence the jury, or had but very slight effect." Dargon, 457 Mass. at 399, quoting Flebotte, 417 Mass. at 353. We hold that admitting the speed data was prejudicial error and the defendant's convictions must be reversed.

The jury convicted Davis of armed assault with intent to murder and related charges; he was sentenced to six years in state prison.

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