© 2007 Cypress Solutions
Installation and Operation Manual: CTM-130 / CTM-132 (Revision 1.2)
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The Global Positioning System (GPS) is a worldwide radio-navigation system operated by the US
Department of Defence (DoD) and formed from a constellation of 24 satellites orbiting at an altitude of
10,900 nautical miles and with an orbital period of 12 hours.
GPS uses these satellites as reference points to calculate positions accurate to a matter of metres. This
is achieved by measuring the time it takes for a radio signal to travel from each satellite to the receive
unit. Multiple measurements are then used to accurately triangulate the receiver location.
The signals received from the orbiting satellites are based on pseudo random codes. The code for each
satellite is unique so that the receiver can determine which satellite it is receiving from. Additionally,
these pseudo random codes are extremely complex in order to minimize the possibility of incorrectly
picking up another signal or background no ise with the same code. The receiver is able to generate the
same pseudo random code for every satellite it is tracking. The difference in time be tween the locally
generated code and the code received from the satellite is used to calculate the distance from the
satellite to the receiver. The receiver keeps track of the exact location of every satellite by using an
almanac stored in its local memory. In this way the receiver is able to triangulate its location by
calculating the distance from at least three satellites. But this calculation assumes that the receiver
generates its pseudo random code starting at exactly the same time as the satellites, which would
require extremely accurate (and expensive) atomic clocks in every receiver! The receiver overcomes the
need for such an accurate clock by triangulating its position from at least four satellites instead of only
three. The additional satellite allows the receiver to calculate its own clock error and to apply this
correction to all calculated distances.
There are a number of error sources that can introduce inaccuracies into the position calculation.
Understanding these error sources can be helpful in applying the position information provided by the
GPS receiver.
Although the satellites are in extremely accurate orbits they can be influenced by solar “winds” of
cosmic radiation and also by gravitational forces from the sun and moon. These errors are called
"ephemeris” errors". The satellites position is regularly monitored by the US DoD and then passed to
the satellite for passing on to the receivers as part of the pseudo random code information. The
receiver is then able to account for these ephemeris errors. However, between these position
updates there may be a slight error in the calculated signal.
As a GPS signal passes through the charged particles of the ionosphere and then through the water
vapour in the troposphere it gets slowed down, and this creates a timing measurement error which
may be different for each satellite and so cannot be taken into account.
Close to the ground the signal may bounce off various local o bstructions before it reaches the
receiver. This is called multipath error and is similar to the ghosting on a TV.
There are often more satellites available than a receiver needs to fix a position, so it will choose a
few and ignore the rest. If it chooses satellites that are close together in the sky the intersecting
circles that define a position will cross at very shallow angles. This increases the grey area or error
margin around a position. If it chooses satellites that are widely separated then the circles intersect
at almost right angles which minimizes the error region.