Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Chapter 3 / Sensors / NAVSTAR GPS
_ _
GPS receivers track four or more satellites, demodulating their carrier waves
to extract the PRBS that are correlated with stored PRBS sequences.
Autocorrelation techniques extract the time delay between the two PRBS
representing the distance of the receiver from each satellite. The time
delays contain a common receiver clock bias hence the term pseudo-range.
The receiver also tracks the Doppler shift in the carrier waves from each
satellite from which pseudo-range rate is extracted using tracking
bandwidths of the order of 5-6 kHz.
Each satellite transmits its navigation message on the carrier wave at 50 Hz,
the total transmission time being 12.5 minutes. Amongst other things, the
message contains Ephemeris data defining the satellite’s position with
respect to the earth, correction factors for the atomic clock data, and details
of the ionosphere's composition.
Modern receivers track all “visible” satellites, usually providing between 6-
11 pseudo-measurements using an Earth screening angle of 7.5°. These data
are processed in an over-specified least-squares solution that improves the
systems tolerance to high receiver dynamics and lower SNRs. Those that do
not, resort to optimum satellite selection procedures attempt to minimise the
Global Dilution Of Precision (GDOP), Kihara [K.8] , Stein [S.19] . Whichever
method of triangulation is used to fix the receiver’s position a minimum of 4
satellites is required to estimate the clock bias common to each pseudorange
measurement. Receivers capable of tracking both carrier waves
correct the pseudo-measurements for atmospheric transmission delays,
whereas civil equipment tends to track only a single frequency computing
the required correction from Ephemeris data. The receiver’s position with
respect to the Earth is converted to the WGS 84 ellipsoid using the
Molodenskii transform using pre-programmed incremental translation and
ellipsoidal parameters for the local area.
The PPS rms errors are typically 10-15 m and 0.1-0.2 m/s and those for CPS
are purposely degraded to give 50-100 m and 0.3-0.5 m/s when Selective
Availability (SA) is disabled. SA is the term used when the satellite clock
frequencies are jittered which reduces the accuracy of the CPS whilst still
meeting its stated design accuracy.
In “loosely coupled” INS/GPS systems both “sensors” work independently.
In “closely coupled” systems INS PVA data is used to speed up signal
acquisition and to reduce the carrier and code tracking loop bandwidths
making GPS less susceptible to jamming. Cold starts can take several
minutes whereas aided starts provide the first fix within about 30 s.
As the number of satellites increased to the full compliment of 24 in the
early 90s, GPS black box geodetic PVT measurements were combined with
INS and terrain data leading to the integrated TRN now used in modern
fighter aircraft and cruise missiles. No other sensor has had such an impact
on navigation and mid-course guidance, however, several factors have
prevented GPS from replacing TRN entirely:
3.11-2