GLOBAL POSITIONING SYSTEM (GPS)

by

Mathew Jose

Introduction


The GPS is a satellite-based radio navigation system, which broadcasts a signal that is used by receivers to determine precise position anywhere in the world. The receiver tracks multiple satellites and determines a measurement that is then used to determine the user location. Satellite navigation systems are unaffected by weather and provide global navigation coverage that fully meets the civil requirements for use as the primary means of navigation in oceanic airspace and certain remote areas.

external image gns480large.jpg

Figure 1. A typical example (GNS 480) of a stand-alone GPS receiver and display.


Basic concept of GPS

GPS consists of three distinct functional elements: space, control, and user.

1. Space Segment


The space element consists of over 30 Navstar satellites. This group of satellites is called a constellation. The space element consists of 24 Navigation System using Timing and Ranging (NAVSTAR) satellites in 6 orbital planes. The satellites in each plane are spaced 60° apart for complete coverage and are located (nominally) at about 11,000 miles above the Earth. The planes are arranged so that there are always at least five satellites in view at any time on the Earth. Each satellite is installed with four atomic clocks that are extremely accurate(+-3 nS). Satellites also download ALMANAC data; this is a set of orbital parameters status for all satellites in constellation. The receiver uses almanac data during initial acquisition of satellite signals. EPHEMERIS data is also down-linked by each satellite; this data contains current satellite position and timing information.

external image ConstellationGPS.gif

Figure 2. A visual example of the GPS constellation in motion with the Earth rotating.

2. Control Segment

The control element consists of a network of ground-based GPS monitoring and control stations that ensure the accuracy of satellite positions and their clocks. In its present form, it has five monitoring stations, three ground antennas, and a master control station.
The control segment is composed of
  1. A master control station (MCS) located at Schriever Air Force Base in Colarado springs, USA
  2. An alternate master control station,
  3. Four dedicated ground antennas and
  4. Six dedicated monitor stations

3. User Segment


GPS installed on aircraft comprises two receivers and two antennas located in a forward position on the top of the fuselage. The receiver and satellite generate identical pulse coded signals at precisely the same time; these signals are compared to provide the time delay. This time delay is then calculated with the known position and orbit of satellite to provide line of position. Since the clocks of the receiver and the satellite may not match. The resultant range is called pseudo-range. The true range can be calculated by removing the time bias error. Most receivers have a digital display and have a user friendly method of getting inputs an providing outputs.
external image gps.jpg

Figure3.Location of GPS antennas on aircraft.

GPS Signals


Each satellite transmits low power (20-50 watts) signals on two carrier frequencies: L1 (1575.42 MHz) and L2 (1227.60 MHz). Two signals are sent to provide corrections caused by ionosphere refraction. There are three sets of data modulated on the L1 and L2 signals:
1. Course acquisition code (C/A) used by commercial GPS receivers and modulated at 1.023MHz.
2. Precise (or Protected) code (P-code) used by military and modulated at 10.23MHz.
3. Navigation/System data.

GPS Testing


The testing for most GPS is very simple, since its all just self tests. Let us take the Garmin GNS480 for example (refer to figure1):

Starting Up

The GNS 480 performs internal checks and shows the status of the tests during start up. The startup screen,
owner name (if entered), testing, position, and database information shows on the screen for several seconds
and then shows the first Map page. It is not generally necessary to enter a GPS seed position unless the unit
has either been moved several hundred miles or been unused for six months or so with the power off. A seed
position should have been entered the first time the unit was turned on during installation.

Power Up

1. Push the PWR/VOL knob in to turn on power.
2. When the position display appears, you can press CHG to manually enter your present position or just
wait a few seconds for the GNS 480 to establish your position.
3. The GNS 480 performs a number of tests at startup to ensure proper operation. You may press SKIP to
bypass the startup tests, however, completing these tests is required for IFR flight. Any failures will be
noted by a message.

Self-Tests

The GNS 480 performs internal checks and shows the status of the tests during start up. After these internal
checks, the GNS 480 is ready to navigate.

Database Check

The GNS 480 verifies the integrity and expiration date of the database. Up to two database cycles are supported.
The GNS 480 will load the appropriate current database cycle and also let you know if a database is
not current (dates invalid).

References


1. "Aircraft Communications and navigation systems" by Mike Tooley and David Wyatt.
Butterworth-Heinemann publications.

2. Wikipedia: http://en.wikipedia.org/wiki/Global_Positioning_System
[Accessed:] December 15, 2011

3. Garmin: http://www.garmin.com/manuals/GNS480_PilotsGuide.pdf
[Accessed:] December 15, 2011