NavCom SF-3050 A Computationally Efficient Ambiguity Resolution - Manual

A COMPUTATIONALLY EFFICIENT AMBIGUITY
RESOLUTION TECHNIQUE
Ron Hatch, Tenny Sharpe, NavCom Technology, Inc.
BIOGRAPHY
Ron Hatch is the Director of Navigation Systems at
NavCom Technology, a company of which he was a co-
founder. He has developed a number of innovative
techniques for processing GPS measurements and has
obtained more than a dozen patents related to GPS. Ron is
currently the president of the Institute of Navigation, is a
Fellow of the ION and has received the Kepler and
Thurlow awards from the ION.
Mr. Tenny Sharpe is Director of Advanced Development
at NavCom Technology Inc. Mr. Sharpe received a B.S.
in Physics from Case Institute of Technology and a M.S.
in Computer Science from the University of California,
Los Angeles. Mr. Sharpe has over 30 years experience in
the development of aerospace and industrial electronics.
His specializations are software and systems design for
GPS navigation systems.
ABSTRACT
A new method of carrier-phase ambiguity resolution is
described. The new technique is a variation of the least-
squares residual search technique in the ambiguity
domain. It uses a very efficient algorithm to compute the
residuals associated with each potential combination of
ambiguities to be tested. Several other techniques are
employed to simplify the calculations and to enhance the
probability of identifying the correct ambiguity vector.
The intent is to minimize the number of data epochs
required to correctly identify the integer ambiguity values.
The capability of the technique to rapidly identify the
correct ambiguity vector is illustrated by plotting the
results of scoring runs which exercise the search
algorithms using recorded field data taken over different
rover-to-base separation distances. To maximize the
number of searches exercised in these runs, as soon as the
ambiguity vector is identified, the results are cleared and a
new search is initiated with all navigation parameters re-
initialized. Some navigation results are also shown which
are typical of RTK carrier-phase navigation results.
INTRODUCTION
The RTK ambiguity resolution technique implemented
within the NavCom dual-frequency receiver has several
unique features designed to minimize the computational
task while ensuring that a minimal data collection interval
is required. The fundamental approach is similar to
several least-squares residual search techniques which
have been employed by others. [1-4]
The first unique feature is that the base station transmits
corrections rather than the raw data, which most RTK
implementations transmit. This has several advantages:
1) it offloads part of the computation from the user
receiver to the base station receiver; 2) it allows code
smoothing of the base station data to occur even before
the user receiver is turned on, which ensures a more
accurate initial code solution; 3) it simplifies the
processing algorithms because no differencing across
receivers is required.
The second unique feature is a simplified computational
technique to generate the residuals resulting from
candidate ambiguity vectors. This technique is the main
subject of the paper and will be explored in some detail.
The search process is a two-stage process which depends
upon the availability of dual-frequency measurements at
both the base-station receiver and the user receiver. First a
wide-lane search is performed and up to 10 ambiguity
vectors, which meet specific requirements (e.g. residuals
less than a threshold value) are saved for further
processing. In the second stage, each of the ambiguity
vectors determined in the first stage are tested by
searching each satellite (except the first to avoid
redundant clock solutions) across the two narrow-lane
ambiguity values which result in a narrow-lane measured
range closest to the corresponding wide-lane measured
range. Those narrow-lane combinations, which meet a
number of specific criteria, are scored and if more than
one are acceptable their relative score is used to determine
whether or not one of them can be declared the correct
ambiguity vector.