CONS 340 Post-Midterm – Complete Questions & Answers Study Guide

CONS 340 POST-MIDTERM

, a col l ecti on of government satel l i tes desi gned to provi de accurate posi ti oni ng
GPS
and navi gati on i nf ormati on anywhere on the surface of the earth




1 - space segments - satel l i tes
Components of GPS 2- control segment - ground stati ons
3 - user segment - recei vers




-uses 24 satel l i tes (21 acti ve, 3 i nacti ve)
space segment -each makes a compl ete orbi t ever 12 hours
-thi s provi des 5-8 satel l i tes above hori zon f rom any poi nt on earth




control segment -5 ground based control stati ons




-consi sts of GPS recei vers and the user communi ty
user segment -recei vers use satel l i te si gnal s to compute posi ti on, vel oci ty and ti me esti mates
-these esti mates are used pri mari l y for navi gati on




-i t i s a rangi ng system (tri angul ati on)
1) a GPS recei ver ("the user") detects 1-way rangi ng si gnal s f rom several satel l i tes
-each transmi ssi on i s ti me-tagged

how the GPS system works (i n 5 easy steps) -each transmi ssi on contai ns the satel l i te's posi ti on
2) the ti me-of -arri val i s compared to ti me-of -transmi ssi on
3) the del ta-T i s mul ti pl i ed by the speed of l i ght to obtai n the range
4) each range puts the user on a sphere about the satel l i te




-1 satel l i te puts us anywhere on one sphere
-2 satel l i tes puts us anywhere that the two spheres overl ap
tri angul ati on for l ocati on -3 satel l i tes puts us on one of two poi nts where al l three spheres meet. general l y
onl y one i s reasonabl e
*more satel l i tes used to tri angul ate equal s hi gher accuracy

, Di stance = Rate x Ti me
-the whol e system revol ves around ti me
di stance measuri ng
-Rate = 186,000 mi l es per second (speed of l i ght)
-Ti me = ti me i t takes si gnal to travel f rom the SV to GPS recei ver




SV cl ocks
-$100,000 - $500,000 each
-made of Cesi um and Rubi di um
measuri ng travel ti me Recei ver Cl ock
-si mi l ar to quartz watches
-al ways an error between satel l i te and recei ver cl ocks (del ta-t)
-requi re 4 satel l i tes to sol ve for x,y,z, and del ta-t


*broadcast on 2 L-band carri ers
-L1: 1575.42 MHz - modul ated by C/A-code and P-code
-L2: 1227.6MHz - modul ated by P -code

GPS si gnal s *most unsophi sti cated recei vers onl y track L1
*I f L2 tracked, then the phase di f ference (L1-L2) can be used to f i l ter out
i onospheri c del ay
-thi s i s true even i f the recover cannot decrypt the P-code




-combi nati on and code
-L1 si gnal carri es navi gati on message and SPS (standard posi ti oni ng servi ce) code
NAVSTAR Carri er Si gnal s (L1 & L2) si gnal s
-L2 si gnal i s used to measure i onospheri c del ay by PPS (preci se posi ti oni ng
servi ce) equi pped recei vers




-Coarse Acqui si ti on code - i s a repeati ng 1 MHz pseudo random noi se (PRN)
code
NAVSTAR si gnal s (C/A) -thi s i s a bi nary number that repeats every mi l l i second
-Carri er Phase GPS i s based on L1 & L2 si gnal s
-Code Phase GPS i s based on C/A code




-the i nf o each SV broadcasts i nf o about ALL other SVs i n addi ti on to i ts own nav
data (i n a reduced-accuracy format)
-permi ts recei ver to predi ct, f rom a col d start, "where to l ook" for SVs when
The Al manac
powered up
-GPS orbi ts are so predi ctabl e that the al manac may be val i d for months
-data i s l arge: 12.5 mi nutes to transf er i n enti rety




-for exampl e: the PRN code i s sent f rom a satel l i te, and generated by the GPS
recei ver
Ti me i s Di stance -code f rom satel l i te reaches recei ver
-di f ference between code recei ved and code generated by recei ver = ti me =
di stance