GPS technical tutorial
- technical summary, overview or tutorial about GPS satellite navigation
system giving details of the satellites, signals transmitted and receiver
technology required.
Today the Global Positioning System or GPS is well
established for military, commercial and private use, and although complicated,
the GPS technical aspects are well understood and the levels of performance are
high. Small GPS receivers are available at very reasonable prices. They are
available at these prices because the Navstar satellite system that forms the
basis of GPS is owned and run by the US Department of Defense. A further reason
that GPS receivers are available at such low prices is because of the
significant advances made in integrated circuit and digital signal processing
techniques.
The fully operational GPS satellite system consists of a
constellation of 24 operational satellites with a few more in orbit as spares in
case of the failure of one. The GPS satellites are in one of six orbits. These
are in planes that are inclined at approximately 55 degrees to the equatorial
plane and there are four satellites in each orbit. This arrangement provides the
earth user with a view of between five and eight satellites at any time from any
point on the Earth. When four satellites are visible, sufficient information is
available to be able to calculate the position on Earth.
GPS control network
The GPS satellites need to be monitored and controlled from
the ground and it is necessary to be in contact with each satellite for most of
the time to be able to maintain the level of performance required. To achieve
this there is a master station located at Falcon Air Force Base, Colorado
Springs, USA. However there are other remote stations are located on Hawaii,
Ascension Island, Diego Garcia and at Kwajalein. Using all these stations the
satellites can be tracked and monitored for 92% of the time. This results in two
1.5 hour periods each day when the satellite is out of contact with the ground
stations.
Using the network of ground stations the performance of the
GPS satellites is monitored very closely. The information that is received at
the remote stations is passed to the main operational centre at Colorado Springs
and the received information is assessed. Parameters such as the orbit and clock
performance are monitored and actions taken to reposition the satellite if it is
drifting even very slightly out of its orbit, or any adjust the clock if
necessary or more usually provide data to it indicating its error. This
information is passed to three uplink stations co-located with the downlink
monitoring stations at Ascension Island, Diego Garcia and Kwejalein.
GPS operation
GPS operates by a process of triangulation. Each GPS
satellite transmits information about the time, and its position. By comparing
the signals received from four satellites the receiver is able to deduce how
long it has taken for the signals to arrive and from knowledge of the position
of the satellites it can calculate its own position.
The GPS satellites transmit two signals on different
frequencies. One is at 1575.42 MHz and the other at 1227.6 MHz. These provide
two services, one known as course acquisition (C/A) and the other is a precision
(P) signal. The precision signal is only available for the military, but the C/A
elements of GPS are open to commercial use, although initially a random "wobble"
was put onto this to degrade its accuracy for civilian use. This facility known
as Selective Availability (S/A) was discontinued in May 2000.
Both signals are transmitted using direct sequence spread
spectrum (DSSS), and this enables all the satellites to use the same frequency.
They can be separated in the GPS receiver by the fact that they use different
orthogonal spreading codes, and this works in exactly the same way as the CDMA
cell phone systems. The spreading codes are accurately aligned to GPS time to
enable decoding of the signals to be facilitated.
The coarse acquisition signal at 1.5 GHz uses a 1.023 MHz
spreading or chip code, while the precision signal is transmitted at 1.2 GHz
using a 10.23 MHz code. This precision signal is encrypted and uses a higher
power level. Not only does this assist in providing a higher level of accuracy,
it also improves the reception in buildings.
All the GPS satellites continually transmit information. This
includes what are termed ephemeris data, almanac data, satellite health
information, and clock correction data. Correction parameters for the ionosphere
and troposphere are also transmitted as these have a small but significant
effect on signals even at these frequencies.
The ephemeris data is information that enables the precise
orbit of the GPS satellite to be calculated. The almanac data gives the
approximate position of all the satellites in the constellation and from this
the GPS receiver is able to discover which satellites are in view. Although each
satellite contains an atomic clock, they all drift to a small extent and as a
result details of the clock offsets are transmitted. It is found that it is more
effective to measure the error and transmit this data than maintain the clock
exactly on time.
Transmitted data
The data transmitted by the GPS satellite is formatted into
25 frames, each 1500 bits in length. The frames are divided equally into five
sub-frames. At a rate of 50 bits per second data transmission rate it takes six
seconds to transmit a sub-frame, 30 seconds to transmit a frame and 12.5 minutes
to transmit the complete set of 25 frames.
Sub-frames 1, 2, and 3 are the same for all data frames and
contain critical satellite specific information. This allows the receiver to
determine a single satellite clock correction and ephemeris within 30 seconds.
Sub-frames 4 and 5 contain less critical data that applies to the complete
satellite constellation and this data is distributed throughout the 25 frames.
GPS reception
The strength of the GPS signal that is received on the
surface of the earth is very low. Typically it is around -127 dBm although there
is a variation on this arising from the elevation of the GPS satellite. This may
reduce the signal level by up to about 3 dB. To receive the signal a receiver
bandwidth of around 2 MHz is often used even though a chip rate of 1.023 MHz is
used. The reason for the use of the wide bandwidth is that it reduces
differential group delay that would cause positional errors.
With the wide bandwidth and low signal levels this means that
the actual received signal is below the thermal noise level. The only way to
recover the signal is by correlation over a large number of chips. Commonly, the
correlation is done over a complete code cycle of 1023 chips, giving a
correlation time of 1ms. Using these techniques the best receivers may receive
signals down to levels of around -142 dBm.
From a cold start, the GPS receiver chooses a satellite to
look for and tries all possible code phases to see if correlation is achieved.
The problem is compounded by the Doppler shift on the satellite which forces the
receiver to look in a number of Doppler 'bins' for each code phase, thus
increasing the search time. If no satellite is found then the search is repeated
for the next satellite. Modern receivers speed up this search by using large
numbers of correlators in parallel. Tens of thousands of correlators are
typically used.
Once the first signal has been correlated, the GPS receiver
can then demodulate the data the signal carries. With the almanac data available
the GPS receiver is able to deduce which satellites are visible, and hence which
ones to receive. In addition to this it enables the receiver to correlate the
signals more quickly.
The receiver measures the relative phases of the signals from
each of the satellites to provide what are termed "pseudoranges". Then the GPS
receiver uses the emphemeris data and also compensates for elements such as the
clock offsets, effects of the ionosphere and troposphere and even relativity.
The receiver uses all of this information to calculate its own clock error and
position. The overall calculation is somewhat involved and uses iterative
processes to reach the final result.
In view of the time taken to correlate with the GPS
satellites, as well as the time taken to transmit the data, the Time To First
Fix (TTFF) is usually in excess of 12.5 minutes. Faster TTFF times from what is
termed a cold start are often achieved by using a vast number of correlators,
and by using this approach it is not always necessary to wait until all the data
has been received before the first fixes can be made.
Summary
The Global Positioning System, GPS, has been in use for a
number of years now and has proved to be very successful, with the GPS technical
aspects being well understood by the various companies designing, manufacturing
and selling GPS or satnav systems.
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