Avionics News August 2023 - 48

THEORY & PRACTICE
Continued from page 47
between the VHF navigation channels. For example, a
VDB may transmit on 115.525 MHz, exactly between
navigation channels on 115.50 MHz and 115.55 MHz.
When discussing GPS theory of operation, the reader will
benefit from remembering this is a Department of Defense
system. The design stressed difficulty of detection and high
resistance to jamming. The high bandwidth to data ratio
has made it into other technologies such as BlueTooth. The
technique of multiplexing, code division multiple access is
used on mobile phones, Bluetooth and other systems.
The simplest explanation of GPS navigation is the unit
measures the distance from as many satellites as possible
and then uses the distance data to determine its position. Of
course, the system is much more complex.
Determining the distance to a satellite is complicated
by the fact that satellites are moving, and unlike DME, the
receiver cannot transmit. The user's GPS receiver must
have an up-to-date almanac. The almanac tells the receiver
the satellite locations. Whenever the GPS is in use, it is
constantly downloading almanac data to stay up to date.
The longer a GPS is off, the more time it will need to
complete the data download. An aircraft that landed within
the hour will be ready to navigate again quickly. If the
aircraft has been parked for a week, the pilot may notice
the delay before the GPS is ready.
Once the GPS has determined the almanac is up to date,
it begins receiving time-stamped signals from the satellites.
Although the space vehicles have atomic clocks, the user's
GPS does not, so determining a proper time requires
statistical analysis. Once the analysis is complete, we will
have a distance measurement known as pseudo-range. We
call it pseudo-range because it is not an exact measurement,
but instead, a calculation.
Students of geometry may recall the intersection of
four spheres is a single point. Thus, the GPS must have
pseudo-range data from at least four satellites to determine
an unambiguous position. As more pseudo-ranges are
determined by receiving data from more satellites, the
accuracy of the position will increase.
The GPS signals broadcast by the satellites are
impossible to detect with conventional radio receivers.
The signals are spread-spectrum, meaning they are a
digital form of frequency modulation that deviates several
MHz while sending relatively little data. As a result, the
signals are below the noise floor of any conventional radio.
Furthermore, the signals are encrypted using the CDMA
technique. The signal bandwidth for the precision code sent
48 avionics news * august 2023
Valerie Inman stands in front of a
full-scale mockup of a GPS III A satellite.
by GPS is 20.46 MHz, which is over 2,400 times wider
than aviation communication transmissions.
The GPS signal is spread using a pseudo-random code,
which makes the signal look like noise unless you have the
key. The key is downloadable from the satellites; however,
the slow data rate, and the length of the code (20 trillion
bits), would make downloading a 37-week project.
When discussing GPS error, one will speak of error
budgets and dilution of precision. Errors caused by the
ionosphere, troposphere, clock drift, and noise are all part
of the error budget. In addition, the potential of receiving a
reflection, which takes longer to arrive, adds error. Finally,
there is the position of the satellite itself. Ephemerides are
differences between where we think the satellite is and its
actual location. Together, all these factors result in a user
equivalent range error up to 1.7 meters.
Dilution of precision is more commonly known as
geometric dilution of precision. Simply, the wider the variety
of satellite locations the user is receiving, the better the
GDOP (less is better) and the more accurate the calculated
position. A GPS antenna with a view of the whole sky, such
as the top of an airplane, will receive satellites overhead, on
the horizon, and several locations in between. This will result
in good GDOP and accurate position calculation. In contrast,
a hand-held GPS attached to a control yoke will only receive
satellites overhead and in front of the aircraft. The GDOP
in the case of the handheld will be poor, and the calculated
position will be less accurate.
Regarding the navigation systems discussed over the past
several months, there is more to learn. These articles are
merely an overview. Further study will be required to earn
a dependent navigation system endorsement to the AET
certification from ASTM International's NCATT. q
Avionics News August 2023

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