Wednesday, December 10, 2008

What does a GPS Unit Track? Proponent & Opponent

Imagine the situation: Your client is in interrogation, you (his attorney) by his side, law enforcement are asking questions nicely. Your client, sounding very reasonable, has explanations of his actions for the entire course of law enforcements time frame interest. Suddenly, they produce a map with a line on it, with little time stamps, which they tell you tracks the exact location of your client to the crime scene, and places him at the crime at the time it occurred! You look, and it seems to be what they are telling you. What do you tell your client?
First, if he looks as white as a sheet, you may be in trouble. However, if he's indignant, and maintains his innocence, look again at the sheet.
Many questions arise in such a situation. First, did the police actually have a GPS on your client's vehicle? Or person? Did your client have a GPS that evidence could be extracted from? Was the alleged crime the type law enforcement would have had either the warning (a tip) or the time (continuing investigation of one type or another) where a GPS could have been placed in advance? Is there any indication your client was under previous surveillance?
If the answers to all these questions are "no," you may have to consider that the "track" was constructed by the police to deceive your client into a confession. Remember, the police can lie.
How could they construct a credible deception? First, they probably know about the time the crime occurred. They probably know where your client lives, works, etc. Because GPS typically records all its data tracks in GMT (Greenwich Mean Time) rather than local time, both GPS units and programs to interpret the data from them "shift" their time data by a user selected number of hours (plus or minus GMT) to display the data in local time. Therefore:
  1. An officer can attempt to "recreate" a GPS track any time up to 24 hours (less the local adjustment) after the crime
  2. Then, the officer "fools" the GPS display software by giving it a false GMT offset number, and
  3. The officers print the resultant track and hand you something that looks like it took place both before, during and after the crime, and it has a real "feel" to it, as it is a real track, just "shifted" in time from the real crime timeline.
Its essential to remember that a GPS track shows where the GPS UNIT went, not necessarily where the person went, where the vehicle went, etc. To that end:
  • It is always essential to establish a foundation of 1) Who placed the GPS 2) when they placed the GPS 3) How they know the GPS was not disturbed, and (probably overlooked) 4) How does the court know that the GPS track offered came from this GPS, that is, who did the downloading, and how did they do it?
  • This offers plenty of defenses: What if the GPS was moved? What if a third-party moved a police attached GPS, committed the crime, and reattached it? How do we know who was driving the GPS-tracked vehicle, as GPS does not answer this.
  • I will later consider other defenses for an otherwise valid track, but don't forget them now.
Now, are potential counterfeit GPS tracks a reason to exclude AUTHENTIC GPS tracks? Absolutely not. However, courts and counsel must be sure to get the RAW, GMT data, with a chain of evidence, like burning from the GPS to a non-changeable read-only Compact Disk (CD-R), then do their own display of the data to ensure nothing inappropriate is going on.
Finally, if you get that raw data, get a display, check with an expert that it all looks right, the authentication steps are all there, and your client is still looking white as a sheet, it may be time to cut a deal - a valid GPS track is difficult to beat!

NEXT: Checking the "validated" GPS track

Saturday, December 6, 2008

Is GPS evidence gathering something new? PROPONENT

For centuries, man has been asking and answering the question, where am I, and corollary questions, where are they (who I am seeking), and where am I in relation to them. To do this, man devised maps, co-ordinate grid systems on those maps, and methods to take position information and record it to determine where the person was on those maps. The process involves four activities:

  • Finding a reference point. One who is lost has no reference point, finding a reference point is the start of finding out where one is. In the past, mountains, the moon, the stars and the sun all were used as reference points. Today, GPS satellites provide the reference points.

  • Getting a "fix"- "Fixing" your position usually requires using two or more reference points in conjugation to determine your location related to some other desired end, usually, your destination.

  • Plotting the "fix"- Taking the information about the reference point and calculating a position, usually involving reference to tables calculated from known information. Plotting the fix usually involves a time component.

  • Recording the "fix" recorded or plotted for future reference, including the time component.

  • Finding a "course" - Additional fixes are plotted and recorded, then connected to find a course, or your path in a general direction. Together with the time component, you can chart both your direction towards a goal and your speed towards that goal.

For centuries, man has been doing manually what a GPS does automatically. Lewis & Clark carried a sextant, "shot" the sun and the stars, fixed their current location on a grid, and found their course across the Louisiana Purchase to the Pacific.

From these observations we reach the first truth: The Global Positioning System does nothing new-it simply automates the ancient process of finding out where we are. It does it faster and more accurately, and automatically, yes, but it does nothing that is significantly new.


  • The GPS "constellation" of satellites is a smarter version of the sun, moon and stars, but it provides the same reference point as the stars.

  • The GPS "fix" calculates position using the constellation in the same fashion. In addition, the satellites provide a uniform time reference, which greatly increases accuracy, but does not change the procedure. A timed satellite code pulse provides the same information as a pocket watch on the ground-time reference. A GPS receiver calculates the position using a computer, which, while faster than Lewis & Clark's tables, performs the same function.

  • A GPS "fix" is recorded in computer memory, which serves the same function as Lewis & Clark's notebook.

  • Course calculation, although done by computer, uses the same type of computation applied to the information in memory that Lewis & Clark performed to plot their course.

The speed, accuracy and automation of GPS generated evidence do not provide a legal basis for excluding them from trial proceedings. As long as a foundation for the basis of operation of each of the above items is introduced, there should be no bar to the GPS generated evidence of locations- the GPS "track."

Because by observation a person could do the same reference point location, fix, plot, and course tracking to a third observed party or vehicle manually, because a GPS can do so more quickly and efficiently should not bar third party tracking simply based on the automated process. If the manual process would be barred, the GPS automated process should be barred.