By Jonny Lupsha, Wondrium Staff Writer
A court-appointed ankle monitor led to a man’s implication and arrest for murder. After lying to police about his whereabouts during the crime, they saw he had gone to a forest with the victim but left alone. GPS measures distance to determine location.
A New Mexico man was arrested for murdering his wife when his ankle monitor, which had been assigned by a court after a previous crime, provided police with key evidence. Armando Zamora, 35, gave police a false alibi for the time that his wife had been killed, but a review of the Global Positioning System (GPS) device in his ankle monitor showed police that he had traveled to a woodcutting site with the murder victim, then left alone.
Police were then able to locate Zamora using the same ankle monitor and arrest him, after which he confessed to beating her to death with an axe in the forest and leaving her body.
GPS uses a series of orbiting satellites, which monitor the distance from multiple satellites to any GPS receiver, such as Zamora’s ankle bracelet, to determine the receiver’s precise location. In his video series Physics in Your Life, Dr. Richard Wolfson, the Benjamin F. Wissler Professor of Physics at Middlebury College, said radio signals travel between a satellite and receiver to do so.
Eyes in the Skies
According to Dr. Wolfson, the Global Positioning System is primarily a space-based system involving a constellation of 24 satellites orbiting Earth at 12,000 miles above the surface.
“The orbital shape is circular, and the orbital period is about 12 hours, so these are going around about twice as fast as the Earth is rotating underneath them,” he said. “The exact orbits are monitored from the ground, so we know exactly how the satellites are moving.”
From the ground, people monitoring the GPS frequently send information to each satellite to let it know exactly where it is. Each satellite lasts between seven and 15 years and is eventually replaced, usually by a newer and more accurate model. So how does it work?
“Fundamentally, it works by measuring the distances to […] four of the satellites, and uses the measured distances to determine exactly where on Earth an observer is,” Dr. Wolfson said. “The constellation of satellites is configured so that a given observer can always see at least four satellites.
“The question is, how far is it to each of those satellites? If you can answer that question, you can pin down your position with incredible accuracy.”
Early navigation systems on phones and in cars used triangulation, which was far less accurate. It would use your position to determine distances to different objects. GPS is the opposite: It uses distances to determine location. This process is called trilateration.
“Distances are determined, ultimately, by timing radio signals coming from the satellites, and using the known satellite to calculate the distances to the satellite,” Dr. Wolfson said. “The speed of light is fast, but we can nevertheless use it for timing.”
It’s possible to gauge the timing of a radio signal’s journey to a billionth of a second. If only one satellite were being used to gauge the distance, the satellite would become like the tip of a cone while the receiver could be at any point around the edge of the base of the cone, since all those points are equidistant from the tip. Fortunately, GPS uses multiple satellites at once, so the location of the receiver is pinpointed by comparing several distances.
Zamora’s murder victim, Erica Zamora, 39, had filed for divorce in June.