JazzboCR wrote:
................what do you call the receivers that are tracking the object (the transponder)
They are called interrogators, a transmitter and receiver combo that is capable of recognizing the unique code (identifier) of the transponder.
A transponder is a receiver/transmitter that responds with a unique signal that identifies that transponder to interrogation signals (like EZ Pass).
A GPS receiver is actually a computer/receiver combo that calculates the position. A basic GPS
RECEIVER has no transmit function, and does not need it. The GPS satellite system
does not, ever receive any signals from the GPS receiver.
If you have additional equipment (like GPS tracking), then the calculated position can be transmitted elsewhere. This could be done by directly transmitting to a local receiver or to a dedicated satellite (not the GPS satellites), which will re-transmit the signal to a ground based unit.
Cell phones with GPS can re-send their calculated position over cell frequencies. Uplinks to satellites are less common, but never to GPS satellites..
FYI: I am trained in this field. I am an Electrical Engineer, have a FCC 1st class license and am experienced as a GPS instructor for airline pilots. This was in a previous life (pre CRT, for sure).
Trivia:
Civil aircraft use a 4096 code transponder system, which is 50's technology. The EZ pass transponder is significantly more sophisticated, with a true digital ID.
GPS signals used to have a built in random error for civilian users, so that the bad guys couldn't use it for precision bombing of friendlies. To give aircraft the accuracy needed to find a runway centerline required extra equipment, which involved additional land based stations. The stations had a fixed (known) position. This was the only use of a non-portable GPS receiver, and is now obsolete. The system transmitted the calculated error (the difference between the known fixed position and the GPS calculated position) to properly equipped aircraft so that position could be calculated precisely. This was called differential GPS. Our government eliminated the built in error in the 90's.