Title: NAVSTAR GPS
1NAVSTAR GPS
GPS BASICS
Mike Mickelson KD8DZ 08 Dec. 2009
2GPS Global Positioning System Satellites are used
by the NAVSTAR Global Positioning System
- The first NAVSTAR GPS Satellite (NAVSTAR 1) was
launched on Ferbruary 22nd 1978. The oldest
functioning GPS Satellite dates from 1990. - The GPS satellite constellation is operated by
the 50th Space Wing of the USAF.
3Constellation of 24 to 32 GPS Satellites. Which
are monitored and controlled by ground stations.
These Orbit the earth once every 12 hours,
Which allows satellites to be updated twice a
day. In addition there is the Wide Area
Augmentation System of Geostationary Satellites,
WAAS which are updated every 5 seconds from
ground stations.
4- Satellite numbers
- Block Launch Period Satellites
launched Currently in service - I 19781985 10 1 1 0
- II 19851990 9 0
- IIA 19901997 19 11
- IIR 19972004 12 1 1 12
- IIR-M 20052009 8 7
- IIF 20092011 0 10 2 0
- IIIA 2014? 0 12 3 0
- IIIB 0 8 3 0
- IIIC 16 3
- Total 58 2 110 236 3 30
- 1 Failed
- 2 In preparation
- 3 Planned
- Last update 24 November 2009
5Satellite Complement, Software, and Hardware
- The basic GPS system consists of a constellation
of between 24 to 32 satellites in medium Earth
orbit. Medium Earth orbit lies between 1243
miles and 22,236 miles. - GPS Orbits at around 12,552 miles (20,200 km.
- Geostationary orbit at 22,236 miles (35,786 km.
- Each GPS satellite contains several cesium atomic
clocks (usually 4) having a precision on the
order of a few nanoseconds. One nanosecond is
the time it takes light in a vacuum to travel
30cm or about one foot.) - Each satellite also contains its ephemeris and an
almanac. The ephemeris contains the coordinate
information for the satellite. The almanac
contains other information about all the
satellites. - The satellite is also equipped with transmitters
and receivers to allow for making corrections to
the satellites clock, ephemeris, and almanac.
These corrections are received from ground
stations and rebroadcast to the GPS Receivers.
6GPS RADIO FREQUENCIES
- L1 (1575.42 MHz) Mix of Navigation Message,
coarse-acquisition (C/A) code and encrypted
precision P(Y) code. - L2 (1227.60 MHz) Military access only.
- L3 (1381.05 MHz) Used by the Nuclear Detonation
Detection System Payload for signal detection of
nuclear detonations and other high-energy
infrared events. Used to enforce nuclear test ban
treaties. -
- L4 (1379.913 MHz) Being studied for additional
ionospheric correction. - L5 (1176.45 MHz) Proposed for use as a civilian
safety-of-life (SoL) signal. This frequency
falls into an internationally protected range for
aeronautical navigation, promising little or no
interference under all circumstances.
7How Does NAVSTAR GPS Work?
Five Concepts
- Triangulation for satellites is the basis of the
system. - To triangulate, GPS measures distance using the
travel time of a radio signal at the speed of
light as modified by the Ionosphere and the
Earths atmosphere. - 3. To measure the travel time, GPS needs very
accurate clocks. - 4. Once you know the distance to a satellite, you
then need to know where the satellite is in space
using the on board ephemeris and almanac. - As the GPS signal travels through the Ionosphere
and the Earths atmosphere, corrections need to
be made.
8Triangulation Steps
X
Surface of a Sphere of Radius R.
R
R c ?t
X GPS Satellite R distance from GPS
Satellite to GPS receiver. c speed of
light (radio signal) ?t time difference between
GPS Satellite clock and GPS Receiver clock.
9SIGNALS GENERATED BY THE SATELLITE
Pseudo Random Code
Navigational Message
10Clock from GPS Satellite
?t
GPS Receiver Clock
11ERROR PROPAGATION
- For example, an timing error of 0.001 seconds
(one ms) results in a distance error of
approximately 186 miles. - Code Phase error. Up to 1 microsecond results in
about 1000 feet. GPS receivers easily correct to
1, or about 10 to 20 feet.
12INTERSECTION SPHERES FROM 2 SATELLITES
Sphere of Radius R1
Sphere of Radius R2
133D Diagram of the intersection of two spheres of
radii R1 and R2. The GPS Receiver is anywhere on
the Circle of intersection of the two spheres.
14- The addition of a third sphere reduces the
uncertainty to two points. At least one of which
is on the surface of the Earth. - The addition of a fourth sphere pinpoints the
location of the GPS Receiver uniquely within the
inherent errors of the system. - The fourth satellite allows clock
synchronization. Which allows the GPS unit to
have Atomic Clock Accuracy.
15CORRECTIONS
- Most GPS Receivers track at least 12 GPS
Satellites (12 channel receiver) if visible above
the horizon. (For example the DeLorme LT-40 has
a 16 channel receiver.) - DGPS Differential GPS comes in two flavors.
- Fixed station at a known geographic location with
several Rovers. Corrections are real time or
post processed. - SBAS Satellite Based Augmentation Systems such
as WAAS (Wide Area Augmentation System) augmented
GPS Receivers include real time corrections from
the Geostationary WAAS Satellites above the
continental US and Hawaii or EGNOS in Europe.
These systems provide improved accuracy and are
another form of Real Time Corrections.
16Programs such as the Delorme Street Atlas and GPS
units with NMEA 0183 Serial Protocol can be used
to provide real time geographical location.
17GPS Accuracy
- Older GPS units with Selective Availability, (SA)
ON. 30 ft. 10m - Older GPS unit with SA OFF, as of 2000, 15 feet
(3-5m) - Differential GPS Uses post processed data from a
GPS Ground Station of known precise location.
1 ft. (0.3m) Data is usually post processed. - GPS-WAAS. Typical 3 to 6 ft. (1 to 2 meter
accuracy). - Carrier Phase wave GPS. Combines data from
precisely located base station. Sub inch
accuracy (few millimeters) used these days by
surveyors. -
18SOME USEFUL WEB SITES
- http//www.trimble.com/gps/sub_phases.shtml
Trimble gps Tutorial. - http//www.gps.gov/ This site describes some of
the 21st Century GPS Augmentation on the horizon