sorry for my delayed response, got busy with hull inspection,
anyways here what i meant and i hope i clarify the gps thing with u all
The United States NAVSTAR Global Positioning
System (GPS) is a GNSS developed originally for
military purposes by the United States. It is now
extensively used in the non-military environment, and
particularly by mariners for navigational purposes. It
comprises a constellation of between 24 and
32 satellites that transmit precise signals which allow
GPS receivers to determine their current location, the
time and their velocity. Its official name is NAVSTAR
Obtaining a position
A GPS position fix is obtained by measuring the
ranges from a series of selected satellites to a
receiver. Ranges are determined by measuring the
propagation time of the satellite data transmissions.
However, it is not possible to precisely synchronise the
satellite and receiver clocks, the ranges measured are
not true ranges, but are termed "pseudoranges" since
they contain a receiver clock offset error. In order to
achieve a two-dimensional (2D) fix on the Earth's
surface at least three "pseudoranges" must be
obtained; the receiver microprocessor can then resolve
the three range equations to remove the effects of
receiver clock offset error. Similarly four
"pseudoranges" would be required to obtain a 3D fix.
2 Each satellite transmits data on two frequencies in
the L-band; L 1 = 1575-42 MHz and L2
1227 ?60 MHz. Both frequencies are integer multiples of
the basic 10?23 MHz clock frequency. Dual-channel
receivers are able to use both frequencies to correct
for the effects of ionospheric refraction. Data
transmitted on the L 1 and L2 frequencies is encoded
by a Pseudo Random Noise (PRN) modulation. A new
L5 frequency (1176-45 MHz) will benefit civilian? users,
providing better quality range measurements and
improving the tracking performance of the receiver.
GPS provides two levels of positioning capability;
the Precise Positioning Service (PPS) and the
Standard Positioning Service (SPS). The PPS is
derived from the Precise (P) code whilst the SPS is
derived from the Coarse Acquisition (CIA) code. The P
code which is primarily for military use is transmitted
on the L 1 and L2 frequencies. The CIA code is
transmitted on the L 1 frequency only at present but in
the future will also be transmitted on the L2 frequency
to provide the second civil signal. In order to protect
the military use of the L 1 and L2 signalS, the Y code
has been developed to be spectrally separate from the
CIA code. The Y code is an improvement on the P
code, broadcasting on a regional basis.
The accuracy quoted in SPS mode is conservatively
estimated at 95% (20 m) for a user equipped with a
typical single-frequency receiver. Differential GPS
(11.41) has been developed to improve the accuracy
of a determined position.
this is a professional gps/dgps device made by JRC, i took this shot today. and here where it is used.
do u think we use the gps we are using is equipped with PPS code receiver?.
If any 1 think like that than keep thinking because it is not.
btw i am a pro yesterday competed 20 years in this field :).
sailing as a master on this vessel.
take care and kind regards