Module 5 Flashcards
GPS Nominal Constellation
- 24 satellites in 6 Orbital Plates
- 4 Stellites in each Plane
- 20,200 km altitudes
- 55 degree inclination
Siderial Day
23h 56m 04s
Need to have a siberial day slightly shorter than a solar day, that shortness is 3 min and 56 sec per day
A siderial day is the bases of the gpa system, it is set up to make 2 orbits per siderial day,
so every day 2 orbits off set
Sources of GPS Error
Standard positioning Service (SPS):
Selective Avaliability (1-100m) no longer a problem Ionosphere (5-7m) Troposphere (0.5-0.7m) Satellite clocks (1 to 3.6 meters Orbital errors (
Basic Functions of GPS
- Positioning
- Distance and direction between your positon and a waypoint (computed vector)
- Velocity and travel log
- Time measurements
Three Segment GPS
- Space segment
- Control segment
- User segment
Control Segment
Master Control Station (US Space Command)
Monitor Station
Ground Antenna
Pseudo Range R
Radius distance from satellite including clock bias.
lousy estimation of distance of satellite and location that includes clock error
Ephemerous
- Ephemeris data describes short sections of satellite orbits
- A receiver gathers new ephemeris data each hour
- The ephemeris parameters are use to compute the SV position for any time within the period of the orbit described
short segments that tell unit where to expect satellite. frequency signal used to calculate Delta t (clock correction),what satellite is sending signal, location
(What orbit each satellite is), The trajectory for the next 4 hours
phase line loop
Misclosure
error in benchmark (M)
Misclosure Correction 2 ways:
-look at total horizontal distance (h), M*h/ perimeter
-break up error by number of segments (s) M/s
below elevation-add
above elevation-substract
Why use one misclosure correction instead of the other?
- No making horizontal measurement
- long survay- mixture of short shots and long shots, then there are greater errors on long shots
gps
- we can make elevation and horizontal position measurements
- access to satellite, sweeping away much of the old methodologies for surveying
- One instrument isolate allows you to make very precise measurement
Autonomous gps
no ssistance from any other ground measurement (if we turn off WAAS)-
Why gpa measurement from garmin worst for elevation than horizontal positioning?
A Hemisphere allows to make measurements of horizontal position with satellite around you. So if we had errors associated from one is offset by erros by another one to better locate horizontal
For vertical position we have satellite above our head to calculate error, but all in one direction there are not satellite below to balance your measurement. the one that fall below horizon are not available, only working working with half the satellite, half a view (180 degrees)
First satellite
1979-in a decade period put in their orbit
Do We want satellite set sun-earth frame that changes every day?
No, orbital plane are held on reference to distante start that are not changing to deal with the problem of the sun changing orientation to the satellites.
ex. between summer soltice and winter soltice, same time a day but reference has switch 180 degrees
Orbital frame held static
earth turning, satellite moving but orbital frame remaining static.
For every satellite in view there is a green light going to black dot, when sattelite sits below horizon garmin cant pick it up (not available to you), turn in color red. Available on other side of world but not to you
Selective availability
Before Bill clinton, high precestion only available to military, only degraded gps capability available until 1998 (100m measurement)
Unless US at war the whole constellation can be turn off in selective regions
atmosphere (Ionosphere, Troposphere)
- Big sources of error are distortion in upper atmosphere
- Troposphere 1/10 of the error associate with ionosphere
satellite clock
gps Clock from satellite, up to 3.6m error
Orbit error
satellite position might be off
Radio signal noise
call receiver noice
Multipath
devise looking at constellation, trees…filter view to the sky, reflect/ bouncing signal off the ground that has a stronger signal than the received by the unit itself. There is sheilding
Use gps for
velocity
stationary gps mounted permently concrete plataform can measure tectonic velocities-big uses of high precition gps units is to understant tectonic movement
gps for time measurement
very precise time, for research, use gps to get highly precise time stamp for data log
Monitor segment
24 gps satellites, and other that sit out waiting to be move into place if the other satellite go out or deorbital. Fly together only few meter apart. Master station (Colorado) of from ground antennas transmitting info to satellite to move into orbit US system 35 year old US Space Command (colorado spring) satellite control in US airforce academy, they receive signal from monitoring station that tells that what each satellite is doing. every 4 hours send adjustment to get it into proper orbit
Orbits do not cover
antartica or north artic. Big problems, degrade measurement there
From way form Receiver can determine
how long is being since it left satelite (delta light), Times speed of light gives a range, cannot get position from one sattellite
why we cannot get position from 3 satellites?
becasue they create 2 outcomes on either side where the spehre intersect, one positive and one negative
Need 4 satellite to know which of those two intersecting point is the real one
There is a unique situation where 3 satelitte could solve position, where they pecfectly intersect, without overlap. Right/place/time.
clock bias
everys sattelite estimating the range using the same error. Need to factor out clock bias
ephemeral orbital
satelite correction-traveling small fraction in speed of light causing a shift on time, (relativety ) making things younger on saturday than earth by 14 sec
Relativety Needs to be factor in calculation of distances using gps
Real? Range -
clock bias corrected range
clock error represented as distance error
Reference frame-WGS84
Data frames
Satellite correction
Ephemeric data
for the next 4 hour, info about ionosphere, universal time, and generel map of all satellite
GPS/GNSS
GPS
Nastar-gllobal satellite positioning
GPS-only refers to US system
GNSS- global navegation satellite system
Galileo
operational on 2020.can take up Russian and US frequency
PRN code
binary wave land, phase line loop use to align the wave form sent by satellite and recevied by receiver to determine delta T.
correltion coefficient of 1. One is perfectly aligned
Time offset determination by phase-lock loop
Distance = c*t(offset)
