1 September 2002
Engineering marvel saves miners
Somerset, Pa.—When an elite team of rescue professionals pulled the last of nine miners from the Black Wolf coal mine in the early morning of 28 July, not many people in North America thought this was anything but a miracle.
The crew was 240 feet below the surface and nearly a mile and a half from the mine entrance when it drilled into an adjacent water-filled shaft, which released a tidal wave that chased the men through the coal catacombs.
Rescuers immediately knew they had to drill an exploratory shaft to locate the miners and start pumping air into the hole. The obvious and important question was where to drill.
A bevy of mining, safety, and civil engineers leveraging guile, experience, and space-based technology set about to answer that question and extract nine lives from deep underground.
Using mine maps, they figured out the high ground underneath was where the miners would most likely be able to survive—if, indeed, they had. They then translated that point on the map to a spot on the ground where they would drill. They performed this all-important translation with a satellite global positioning system (GPS).
The GPS, according to Peter Dana at the University of Colorado, consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites, as new ones regularly launch to replace older satellites.
The satellite orbits repeat almost the same ground track as the earth turns beneath them once a day. The orbit altitude is such that the satellites repeat the same track and configuration over any point every 23 hours and 56 minutes.
There are six orbital planes, with nominally four space vehicles (SVs) in each, equally spaced and inclined at about 55° with respect to the equatorial plane. This provides the user with between five and eight SVs visible from any point on the earth.
Using a receiver(s), the GPS user converts SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of x, y, z (position), and time.
Receiver costs vary depending on capabilities. Small civil standard positioning service (SPS) receivers are available for under $200. Some of these can accept differential corrections.
Receivers that store files for postprocessing with base station files cost more ($2,000 to $5,000).
Receivers that act as differential GPS reference receivers (computing and providing correction data) and carrier phase tracking receivers (two are often required) can cost up to $40,000.
Military precise positioning service (PPS) receivers, of the type that performed the mine calculations in Somerset, cost more and are difficult to obtain.
As to performance, low-cost, single-receiver standard positioning projects have 100-meter accuracy. Medium-cost, differential SPS code positioning has 1- to 10-meter accuracy. High-cost, single-receiver PPS projects obtain 20-meter accuracy. High-cost, differential carrier phase surveys perform at 1-millimeter to 1-centimeter accuracy.
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