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B Multipath Elimination Technology
MiLLennium Command Descriptions Manual 65
NovAtel’s Internal Receiver Solutions for Multipath Reduction
The multipath antenna hardware solutions described in the previous paragraphs are capable of achieving varying
degrees of multipath reception reduction. These options, however, require specific conscious efforts on the part of
the
GPS user. In many situations, especially kinematic, few (if any) of the above solutions may be effective or even
possible to incorporate. By far, the best solutions are those which require little or no special efforts in the field on
the part of the
GPS user. This is what makes NovAtel’s internal receiver solutions so desirable and practical.
NovAtel has placed long term concerted effort into the development of internal receiver solutions and techniques
that achieve multipath reduction, all of which are transparent to the GPSCard user. These achievements have led
to Narrow Correlator Technology.
It utilizes innovative patented correlator delay lock loop (
DLL) techniques. As it is beyond the scope of this manual
to describe in detail how the correlator techniques achieve the various levels of performance, the following
paragraphs will provide highlights of the advantages of this technology.
NARROW CORRELATOR TECHNOLOGY
NovAtel’s MiLLennium GPSCard receivers achieve a higher level of pseudorange positioning "performance" vs.
standard (wide) correlator, by virtue of its celebrated Narrow Correlator technology. By utilizing Narrow
Correlator techniques, the MiLLennium GPSCard is capable of pseudorange measurement improvements better
than 2:1 when compared to standard correlation techniques. As well, the Narrow Correlator inherently reduces
multipath reception (approaching a factor of eight compared to standard correlator) by virtue of its narrower
autocorrelation function.
Standard correlators are susceptible to substantial multipath biases for
C/A code chip delays of up to 1.5 chip, with
the most significant
C/A code multipath bias errors occurring at about 0.25 and 0.75 chip (approaching 80 m error).
On the other hand, the Narrow Correlator multipath susceptibility peaks at about 0.2 chip (about 10 m error) and
remains relatively constant out to 0.95 chip, where it rapidly declines to negligible errors after 1.1 chip.
While positioning in single point mode, the multipath and ranging improvement benefits of a Narrow Correlator
receiver vs. standard correlator are overridden by a multitude of
GPS system biases and errors (with or without an
antenna choke ring ground plane). In either case, positioning accuracy will be in the order of 40 metres
CEP (SA
on, no multipath). However, the benefits of the Narrow Correlator become most significant during pseudorange
DGPS operation, where the GPS systematic biases are largely cancelled.
Receivers operating
DGPS with standard correlator technology typically achieve positioning accuracies in the two
to five metre
CEP range (low multipath environment and using choke ring ground plane), while NovAtel’s Narrow
Correlator receivers are able to achieve positioning accuracies in the order of 0.75 metre
CEP (low multipath
environment and using choke ring ground plane). The Narrow Correlator achieves this higher accuracy through a
combination of lower noise ranging measurements combined with its improved multipath resistance when
compared to the standard correlator.
SUMMARY
Any localized propagation delays or multipath signal reception cause biases to the GPS ranging measurements that
cannot be differenced by traditional
DGPS single or double differencing techniques. Generally speaking, single
point positioning systems are not too concerned with multipath reception, as the system errors are quite large to
begin with. However, multipath is recognized as the greatest source of errors encountered by a system operating
in differential mode. It has been discussed that careful site selection and good antenna design combined with a
choke ring ground plane are fairly effective means of reducing multipath reception.
Internal receiver solutions for multipath elimination are achieved through various types of correlation techniques,
where the "standard correlator" is the reference by which all other techniques can be compared.
The Narrow Correlator has a two fold advantage over standard correlators: improved ranging measurements due
to a sharper, less noisy correlation peak, and reduced susceptibility to multipath due to rejection of
C/A code delays
of greater than 1.0 chip. When used with a choke ring ground plane, the Narrow Correlator provides substantial
performance gains over standard correlator receivers operating in differential mode.