Determining X, Y, Z and T coordinates with GPS satellites
X = latitude
Y = longitude
Z = altitude
T = time
A minimum of four (4) orbital satellites is necessary to determine 3D position
and time. Three (3) satellites are necessary if you are only wishing to calculate
your 2D position.
Control Segment Tracking Stations for GPS
There are five Monitor Control Stations located on the surface of the Earth. The
Master Control Station is located in Colorado Springs, home of the USAF
Academy and near NORAD, reflecting the DOD influence in the birth of GPS.
Note how the remaining control stations are all located in the tropics on remote
islands in each respective ocean. This enables maximum monitoring ability
24/7/365 of the GPS orbital satellites.
Ephemeris Data
The following chart displays a truncated list of ephemeris data. Ephemeris data
is updated every ten minutes. You should always wait a few minutes after
turning on your GPS so that it may collect the most recent ephemeris data.
Standard Orbital Tracks of GPS Satellites in 3D and 2D
Dilution of Precision (DOP)
Good satellite geometry is essential to ensuring accurate readings of position and
date collection. An ideal DOP will be having ample satellites from which to read
that are both high and low in the sky, with 45° angles considered to be optimal.
The following charts display both good and bad DOP values. FYI: VDOP =
Vertical DOP, HDOP = Horizontal DOP, GDOP = Geometric DOP, PDOP
= Percentage DOP
Determining PDOP
An ideal DOP value will be low (PDOP <= 6 or HDOP <= 4). Most GPS units
today come with software that will assist in satellite tracking to determine where
and when you will have good PDOP values.
The best combination will be ample satellite coverage and low PDOP values. The
following charts will display most and least optimal coverage for Lexington,
Kentucky on June 17, 2003.
The greatest total number of satellites is found between 12:00AM to 12:30AM
(0000h to 0030h) and from 11:15PM to 12:00AM (2315h to 0000h). There is
also good coverage from 7:00AM to 9:15AM (0700h to 0915h). The greater the
number of satellite, generally the lower the PDOP value, but this is not always
the case. You may have cases where half a dozen satellites may be clustered in
one area of the sky, giving good satellite coverage but poor PDOP. This will
often occur at very high latitudes (ex. Greenland or Antarctica) or in equatorial
regions.
The lowest PDOP values are found from 12:00AM to 1:00AM (0000h to 0100h)
and from 10:15PM to 12:00AM (2215h to 0000h). There are also low PDOP
values from 7:00AM to 930AM (0700h to 0930h). These times are also the times
when you have your maximum satellite coverage. Note that your highest PDOP
values (2:30AM or 0230h) coincide with your lowest satellite coverage.
Your choices for optimal coverage and low PDOP are going to be just after
midnight, a couple of hours after sunrise, and the hour just before midnight.
You may also calibrate your software package develop filters that will
automatically eliminate times of day in which the satellite coverage is too low
and/or the PDOP values are too high, as seen in the diagram below. The filter
has been set to eliminate all times in which satellite coverage is below 4 and to
eliminate all times when PDOP values are above 6. This filter does not display
the most optimal times to conduct GPS readings, but it does eliminate the worst
times and gives the user a much larger window of opportunity to conduct
fieldwork.
Important – these filters do not take into account terrain features or man-made
obstructions. Good satellite coverage and low PDOP values can change
dramatically depending upon site-specific features on the surface of the Earth
(i.e. tall buildings, mountains, steep gorges, etc.)