International Association of Geodesy SSG
QUALITY ISSUES IN REAL-TIME GPS POSITIONING
Chairman: Chris Rizos
IUGG Congress, Birmingham U.K.
18-29 July 1999
At the 1995 IUGG congress in Boulder, Colorado, USA, the IAG established
several Special Study Groups (SSG). One was to deal with what was perceived
as the critical issue with regards to precise static and kinematic positioning,
that is, the "quality" of the observations and, by extension, that of the
coordinate results as well. In order to narrow down such a broad topic
area it was decided to focus on "real-time GPS positioning". In the course
of the life of SSG 1.154, several other decisions were made in order to
set a more realistic research and scholarship agenda. In this report the
basic issues are discussed, and the outcomes and achievements of the SSG
are noted. Finally, the quality control technical issues for kinematic
GPS positioning are again raised within the context of an overall scheme
that includes the rover (user) GPS receiver and base station (reference)
receiver, the data link between the two, as well as the data processing
for high precision carrier phase-based positioning.
1.1 Terms of Reference
Concerns about GPS positioning quality are shared by all users, from
those engaged in the most precise geodetic applications through to the
casual navigator. The quality of GPS positioning, however, is dependent
on a number of factors. Experience with precise geodetic applications of
GPS has shown that sophisticated mathematical modelling, careful field
procedures and top-of-the-line GPS hardware are all necessary prerequisites.
Nevertheless great care still has to be applied to ensure that data quality
is uniformly high. The procedure of data screening, position computations,
result evaluation and quality assurance has generally been an off-line
(as well as iterative) process. With the development of precise "on-the-fly"
(OTF) GPS positioning techniques it is no longer possible to process (and
re-process) GPS data in post-mission mode until the positioning quality
is assured. The challenge therefore is to develop quality control and quality
assurance procedures that can be applied in "real-time" (or "near-real-time")
The work of the SSG 1.154 on "Quality Issues in Real-Time GPS Positioning"
will focus on identifying practical procedures, as well as mathematical
techniques, that can be applied to assure the quality of positioning results
obtained from this distinct class of GPS applications. The objectives of
the SSG therefore were identified as being:
In hindsight it would appear that this was a very ambitious set of objectives
for the SSG. In this report, in subsequent sections, this theme of noble
and wide-ranging objectives not being matched by outcomes will be raised
again and again.
(a) IDENTIFY the main issues impacting on the "quality" of real-time
GPS positioning -- including those due to instrumental effects, environmental
sources, site-dependent effects, communcations-dependent, etc.
(b) COMPILE a set of procedures, algorithms and guidelines that can
be implemented within real-time GPS positioning software -- this is
the practical outcome.
(c) DEFINE areas for further research and development -- as derived
from both practical experience, and a study of the literature and research
trends in the development of mathematical and/or empirical tools for "quality
1.2 Tasks to be Undertaken
The abovementioned objectives were translated into a series of tasks:
1.3 SSG Membership
1) Compile and document the QC procedures and algorithms as implemented
in scientific GPS geodesy software.
2) Investigate which of these procedures are adaptable for "real-time"
operation -- for example, for the detection of faulty navigation messages,
data spikes, etc.
3) Compile a bibliography of D-I-A literature specifically applicable
to precise real-time kinematic GPS positioning.
4) Research fault detection algorithms for real-time GPS applications.
5) Encourage discussion and critical evaluation of such algorithms.
6) Monitor the activity taking place in the development of quality control
(QC) and quality assurance (QA) for standard pseudo-range based DGPS.
7) Determine the appropriate "mix" of QC/QA procedures that can be recommended
for real-time precise GPS positioning -- as it is was felt that a "cocktail"
of procedures will be necessary to give greatest assurance on quality.
8) Prepare a report on the SSG's activities and recommendations.
As of June 1999, the SSG membership was (see §A.1):
C. Rizos (Australia) President
H. Abidin (Indonesia)
J. Behr (USA)
E. Cannon (Canada)
P. Collins (Canada)
R. Galas (Germany)
S. Han (Australia)
Y. Hatanaka (Japan)
X. Jin (Switzerland)
H. Kutterer (Germany)
Y. Li (Canada)
S. Mertikas (Greece)
P. Morgan (Australia)
S. Oszczak (Poland)
W. Roberts (United Kingdom)
G. Seeber (Germany)
M. Stewart (Australia)
L. Wanninger (Germany)
Was the composition of the membership ideal to undertake this task?
It could be argued that voluntary membership of an IAG SSG will never bring
together the most appropriate expertise and interests. It is one of the
roles of the Chairman of the SSG to select (or recruit) the membership
according to criteria that ensure the "best" people are coopted. If the
focus of the SSG were narrower, then the membership could "select itself",
by simply inviting the handful of "experts" and "active researchers" to
the SSG. It would then be expected that this mix of theoreticians and experimeters
(or "number crunchers") would generate the environment in which active
discourse and detailed studies could be promoted. (With email contact,
certainly the geographic separation of the SSG members could no longer
be held to be a constraint on scholarly activity.) In reality, the selection
of members was not a careful and scientific process.
The membership list contains mostly academics, graduate students and
government employees. Yet the only real-time, carrier phase-based GPS positioning
systems are commercial products. The small number (just two) of members
drawn from private industry could be construed as a glaring shortcoming.
However, it is debatable whether scientists employed by GPS manufacturers
would be able to freely contribute their expertise and knowledge to the
SSG. Another group that, in hindsight, is underrepresented (again by just
two members) are staff from the geodetic departments that deal, on an everyday
basis, with the processing of data from permanent GPS networks such as
SCIGN (USA) and GEONET (Japan). Although they would not be dealing with
"real-time" processing, the issue of "quality control" would certainly
be addressed on an institutional basis.
1.4 Comments to the SSG Research Agenda
Before dealing with explicit outcomes, it is appropriate to make some
comments on the rationale behind the SSG's original research agenda, and
to comment on its shortcomings:
1.5 Administrative Issues
1) At the time of the establishment of the SSG the Chairman had suggested
the following characteristics that distinguish those real-time positioning
techniques that would be the focus of study from those that would not:
the communication of data from GPS receivers to a computing site
where it is processed with no, or minimum, delay,
2) One of the first tasks of the SSG (§1.2) was to compile and document
the QC procedures and algorithms as implemented in scientific GPS geodesy
software. It was reasoned that because this type of data processing is
at the most sophisticated level, it would be expected that they would be
the most highly developed. The comment was made that it was likely that
these procedures were largely "empirical" and based on extensive experience
gained working with GPS data, and that they were unlikely to be documented
in the available literature. Members of the SSG did have experience with
a range of scientific GPS geodesy softwre packages. After several prompts,
some members did come forth and volunteered their knowledge of QC procedures,
and it was clear that these were indeed in the category of empirical "rules-of-thumb"
and were not founded on a well developed mathematical basis. What correspondence
the Chairman did receive was compiled into email SSG memos for the benefit
of the other SSG members, and can be inspected by all through a visit to
the SSG's website (§2.2). Given that several IGS Processing Centres
now have many years of experience in the automatic processing of large
volumes of GPS data on a daily basis, it is disappointing that the QC procedures
are not well defined or documented.
make use of carrier phase data,
rely on data processing on an epoch-by-epoch basis, or at the very
least small "batches" of GPS data,
do not permit extensive data "pre-processing" or the review of data
and results in iterative procedures, and
may involve kinematic or static positioning.
3) The next step was intended to be the investigation into which of
these procedures are adaptable for "real-time" operation. This would "bridge"
the pragmatic procedures based on such criteria as size and distribution
of "data gaps", signal-to-noise ratio values, faulty navigation messages,
data spans for which both L1 and L2 data were available, ionospheric activity
(as indicated by L1/L2 combinations), etc., with the highly developed "fault
detection" algorithms associated with digital signal processing in general,
and the D-I-A procedures implemented in navigation software. This would
be at the "heart" of the SSG's activities. Unfortunately very few members
had the requisite background in signal processing, reliability theory,
D-I-A algorithms and statistical testing to make contributions. Several
members did indeed make significant contributions, and the Chairman acknowledges
their contribution (§2.1). One of the important outcomes of the SSG
was to be the compilation of a bibliography of D-I-A literature specifically
applicable to kinematic GPS applications. One member of the SSG did compile
a list of general references to fundamental literature on "fault detection",
statistical testing, and the like (see §2.3 and §A.2). However,
the Chairman was remiss in not updating this list with literature that
specifically dealt with its application to kinematic GPS positioning.
4) It was recognised from the start by the Chairman that the current
"best practice" was that applicable to pseudo-range-based differential
GPS (DGPS). The first real-time applications addressed by GPS were those
for differential positioning using transmitted pseudo-range corrections
(generated by a stationary reference receiver located on a known site).
The SSG was fortunate that one member was closely involved in the development
of QC practices for DGPS for the offshore industry. It was reasonable to
conclude that the SSG could learn from that experience, and in many respects
to emulate the process, but in relation to carrier phase-based positioning.
Emails were exchanged between SSG members, and the relevant correspondence
was summarised and place on the SSG's website (§2.2). The following
comments were made at the time (and are still valid today):
While not within the direct interest of this SSG, "quality control"
for real-time DGPS is a useful starting point because of the extensive
activity in this area over the last few years.
5) From the very beginning the SSG Chairman held the view that a "mix"
of Quality Control and Quality Assurance procedures would be needed for
real-time precise GPS positioning. However, unfortunately one of the outcomes
of the SSG's work is a rather incomplete list of QC issues that need to
be addressed if fast, reliable, real-time, centimetre accuracy GPS positioning
results are to be assured close to 100% of the time (§3). It became
obvious with time that the expertise of most SSG members was in improving
the efficiency and reliability of "on-the-fly" (OTF) ambiguity resolution
(AR), a critical step in making high precision kinematic GPS positioning.
Hence the technical issues associated with OTF-AR became the focus as
these were "quality control" issues in the classic mathematical/statistical
sense. However, the holistic QC approach had to be shelved.
A recent study of "quality measures" for DGPS has identified many
factors impacting on quality that cannot be overcome by mere recourse to
new mathematical algorithms. "Quality" is something that must be viewed
as being somehow a holistic concept that requires "quality management"
to be at its core. Hence attention must also be paid to many other non-mathematical
issues such as equipment quality (certification? cabling and antenna quality),
communications reliability and integrity, site-specific disturbing effects,
hardware/software calibration procedures and maintenance schedules, and
even operator training.
A holistic study of GPS quality is likely to be well beyond
the expertise of this SSG. However, the partitioning of error sources into
distinct categories, permitting a "targetted" effort to address each error
sources using appropriate tools, should be attempted. Hence recommendations
on improving real-time GPS positioning quality are likely to include the
definition of a "cocktail" of tools, some with the rigorous mathematical
basis (e.g. "data-snooping" techniques), while others may be essentially
derived from empirical analysis.
At the time the SSG was established the Chairman had some thoughts on
how the SSG could function. It is worth mentioning them here, and to indicate
how some of the reasons for the SSG not fulfilling its ambitious agenda
can be traced to a failure to provide the appropriate leadership at crucial
In an ideal world a SSG consists of a small group of dedicated researchers,
drawn together by their interest in, expertise of and everyday involvement
with the topic under study. Once the Chairman defines the scope of the
topic and deals with the procedural matters concerning membership, etc.,
the SSG would then provide a forum for the exchange of ideas and the reporting
of the results of new investigations. The members would be able to collectively
push forward the frontier of knowledge, and the Chairman's role would be
to report progress to the wider IAG community.
In reality the members of the SSG come from different backgrounds, and
have varying levels of interest, experise and involvement in the topic
area. A small core may be identified as being particularly active, but
the others may only contribute occasionally, if at all. The core participants
may communicate regularly (particularly by email), and often without involving
the Chairman. Although it could be reported that "progress was made", it
would be difficult to disprove the claim that such progress would have
been made even without the formation of the SSG. So what can an SSG realistically
Clearly how well the SSG functions (the SSG could be deemed satisfactory
if at least some of these functions are fulfilled) is very much dependent
on the efforts and talents of the Chairman and individual members of the
SSG. So what can an SSG NOT do?
The appointment of an SSG in an area sends a "signal" that the IAG
recognises the importance of this area of work.
The SSG can focus light on the disparate activities that may be taking
place and organise specialised session at conferences where the work of
investigators can be presented to the wider geodetic community.
The SSG may initially introduce an investigator in one country to
colleagues in other countries who are undertaking similar activities (less
likely these days given the volume of published literature and the ubiquitous
use of the Internet).
The SSG report can be a valuable resource for system developers who
wish to implement the recommendations concerning algorithms and procedures,
or for investigators to continue the research work.
For those within the SSG, there is the possibility of obtaining direct
access to up-to-date reports and results (only if the SSG contains active
researchers in the area of interest).
The Chairman's task is in many ways a thankless one. For many SSGs, no
matter how well meaning and idealistic everyone is in the first months
after the formation of an SSG, enthusiasm often wanes alarmingly. If this
is not checked, then by the end of the four year life of the SSG the only
person contributing is the Chairman, and his contribution is an insipid
report to the IAG. He (or she) may have grown disillusioned as his/her
repeated exhortions to SSG members to contribute literature, recommendations
or even opinions for inclusion in the final report are largely ignored.
How does one guard against this? One suggestion is to keep the work of
the SSG "focussed" on a single well-defined issue. How does one define
a "focussed" topic for the SSG? Already, even a cursory study of the "quality
issues in real-time GPS positioning" would raise an alarm. A suggestion
may be to:
It cannot commission studies in the conventional, prescriptive sense
(afterall, the members are volunteers).
It cannot function as an "advisory board", dispensing advice and "remedies"
to individuals or organisations.
It cannot force the SSG members to collaborate when they chose not
to, or for various reasons are unable to.
It cannot insist that SSG members divulge sensitive information and
data to other members before the normal processes of publication or patent
It cannot force SSG members to attend conferences and meetings.
The first requires that the Chairman take a lead, and that members of the
SSG contribute: (a) ideas and advice on the structure and content of the
report, and (b) bibliographical lists. At this level the report can be
viewed as a well researched "plan-of-action". In the context of this SSG,
even if it just contains a clear definition of terminologies, a comprehensive
catalogue of error sources, a summary of mathematical techniques (with
their assumptions clearly stated), quality guidelines of a "non-mathematical"
nature, supported by a comprehensive bibliography, then one would be satisfied
that the SSG had succeeded in fulfilling one of its objectives.
Pepare a report that reflects the diversity of the topic "quality
issues in real-time GPS positioning".
Report on specific algorithms or procedural developments that are
practical and immediately useful.
The second is the more traditional function of an SSG, i.e. the reporting
of work of the members of the SSG as well as of others known to the members.
This is done through conference presentations, SSG internal discussion
channels, and remaining alert to developments appearing in the literature.
However, preference must be given to practical developments, i.e. those
that can be tested, implemented and used unambiguously by others beyond
The members of the SSG are drawn from many backgrounds, and from as
wide a geographic spread as possible. What this SSG does not have in significant
numbers are members from the GPS industry who are responsible for the development
current real-time kinematic systems. It could be argued that our SSG (in
fact the IAG) suffers by not having such members. While the SSG Chairman
can picture himself/herself as an orchestra"conductor", the SSG members
cannot be considered to be at his/her "beck and call". The SSG members
could be expected to contribute in the following manner:
Even a cursory glance at the above comments, and a study of the outcomes
and achievements in the next section, would indicate that this SSG has
failed to achieve the (admittedly unrealistic) objectives that it set itself.
It could be argued that this failure is partly due to the SSG Chairman
not completing the tasks that he set himself, as well as a membership that
does not have sufficient experience in developing real-time positioning
Thoughts, suggestions and advice (both solicited and unsolicited)
on the topic of QC and QA.
Submitting to the Chairman, and other SSG members, copies of relevant
reports and publications that they have prepared.
Drawing attention to the SSG relevant articles or research activity
that they may have become aware of.
Participating in sessions at conferences that this SSG may chose to
Respond to specific requests by the Chairman.
2. OUTCOMES AND ACHIEVEMENTS
Communications is what makes possible the SSG's activities. The Chairman
has periodically written memos that have been sent to members by email.
These have also been placed on the SSG's web site (§2.2). While this
communication is necessary the more "productive" communications is between
SSG members (to which the Chairman cannot comment on) and feedback from
individual SSG members to the Chairman. There are members who have corresponded
with the Chairman, and who have contributed to discussions, and there are
members who have played almost no part in the SSG communications. Only
6 memos were written by the Chairman, however many more emails were sent
to individual members asking for their comments and advice. Some correspondence
was also had with non-members. The last Chairman's memo was written
in early 1998.
The following is a summary of the correspondences:
M. Stewart & J. Wang -- papers and draft documents on "empirical"
QC procedures, mathematical techniques for GPS+Glonass AR and validation
procedures, modification of stochastic models, system modelling in Kalman
2.2 Web Site
W. Roberts -- definition of "quality", UKOOA QC guidelines, shortcomings
of DGPS QC, stochastic modelling, statistical testing, ideas for carrier
Y. Hatanaka -- orbit issues, broadcast vs IGS precise ephemerides,
scientific softwre data screening procedures.
L. Wanninger -- ionospheric disturbances, mitigation of biases using
multiple reference station techniques.
S. Mertikas -- definition of "quality", development of fault detection
algorithms, quality "measures", co-authored several papers with Chairman.
H. Kutterer -- terms of reference of the SSG, QC procedures within
the Bernese scientific GPS software, experiences with RTK with the Trimble
receiver, GPS signals through forest foliage.
S. Han, L.S. Lin -- ambiguity resolution and QC & validation procedures,
strategies for mitigating residual biases for short, medium and long-range
GPS positioning, multipath, ionospheric studies, data communication latency
F.K. Brunner -- literature related to data quality and modelling.
One of the most important outcomes was the establishment of a web site
by the Chairman. The URL is: www.gmat.unsw.edu.au/ssg_RTQC. The site contains
the terms of reference, the text of the Chairman's memos, membership details,
as well as email messages received from members that might be of interest
to the general community. The SSG bibliography is a HTML document on the
web site. This report can be downloaded as a PDF file from the web site.
Development of a bibliography of relevant literature was identified
as an important objective of the SSG. One member, S. Metikas, compiled
a bibliography of general references to statistical testing, quality control,
fault detection, etc., that forms a valuable resource. However, the compilation
of a similarly detailed bibliography focussing on the literature dealing
with the techniques of carrier phase-based, kinematic GPS positioning has
not been carried out. This remains, in the Chairman's opinion, one of the
greatest failings of the SSG.
As stated elsewhere, the objectives of the SSG were ambitious. By attempting
to go for the all-encompassing approach of "quality control", more achievable
(though perhaps minor) objectives could not be systematically addressed.
Yet it is conceded by most investigators that "quality control" is an ever
more important issue as high precision GPS kinematic strives to do "more
with less". That is, less data to achieve similar levels of performance
(measured in terms of accuracy and reliability). Hence the traditional
fields of investigation of on-the-fly (OTF) ambiguity resolution (AR) have
in fact contributed more to QC studies than would have been expected. The
literature on "QC issues for real-time kinematic positioning" is dominated
by papers dealing with OTF-AR techniques, and in particular the validation
procedures that must guard against wrong AR. These validation procedures
have forced a closer study of "data quality" in general. Hence, QC not
only deals with the results, but also identifying the conditions at the
data capture stage that assure "good quality data". Unfortunately no objectives
"quality measures" have been defined for "data quality", although some,
such as signal-to-noise ratios, have been proposed.
Site specific influences on data quality are important, particularly
the multipath disturbance. However, other SSGs in fact deal with topics
that overlap the terms of reference of this SSG (e.g. SSG 1.156 Advanced
GPS Analysis for Precise Positioning, SSG 1.157 GPS Ambiguity Resolution
and Validation, SSG 1.1.58 GPS Antenna and Site Effects). In the opinion
of the Chairman, the "blueprint" for a quality "audit" is the UKOOA study
on QC guidelines for DGPS in the offshore industry.
Finally, although big strides have been made in improving AR and validation,
there is little experience of real-time implementation of QC procedures.
In the Chairman's opinion this is a serious failing of the SSG. No member
had intimate knowledge of how to establish, "from the ground up", a real-time,
kinematic GPS positioning system. Hence the work that can be acknowledged
is piecemeal and generally relates to QC for OTF-AR at the data processing
level. What is ignored are the various components and algorithms for signal
tracking (including signal processing within the GPS receiver), site location
and its influence on data quality, data link issues, base (or static reference)
receiver issues. In order to go someway towards redressing this failing,
the Chairman has described the issues that had to be addressed in the development
of two real-time or "near"-real-time systems capable of supporting carrier
phase-based, kinematic GPS positioning.
3. QC ISSUES IDENTIFIED AS BEING CRUCIAL FOR FURTHER STUDY
Real-time, carrier phase-based GPS positioning techniques are now increasingly
used for many surveying and precise navigation applications on land, at
sea and in the air. Quality control (QC) issues have to be addressed at
different stages of the GPS positioning process, for example, data collection,
data processing and data display. In this section, the quality control
procedures or methodologies for the following critical operations are discussed:
Measurement quality control for single receiver data, concerned
with issues such as failured satellites, ionospheric scintillation, multipath
and cycle slips.
As examples, the proposed quality control procedures to be implemented
in: (a) the Singaporean multi-base station network, and (b) the GPS-based
volcano deformation monitoring system in Indonesia are described, and future
or unresolved considerations are outlined. The aim of this is to indicate
the range of issues that would need to be considered by the SSG, by using
specific examples of systems under development at the Chairman's institution,
the University of New South Wales, Sydney, Australia. The Chairman acknowledges
the valuable work done in this area by his colleague, and SSG member, Dr.
Quality assurance for data communication and data transmission delay.
Quality control of the position modelling procedure, related to systematic
error mitigation and stochastic modelling.
Ambiguity resolution and validation procedures
3.1 Quality Control for Data Logging
Quality Control Issues at a Single Receiver Site
Although GPS satellites are quite reliable, the failure of satellites is
not an uncommon occurrence. The GLONASS satellites fail more regularly.
Satellite failure could be indicated by the broadcast navigation message,
signal being missing, measurement signal-to-noise ratio too small, or measurement
"quality" (an admittedly vague term). However, the pseudo-range measurement
quality could be judged by a point positioning procedure if more than four
satellites are available. A procedure for scanning the raw observation
data based on using a Kalman filter to model the behaviour of phase and
phase-rate measurements (and their changes) in discrete time is described
in, for example, Mertikas & Rizos (1997). This procedure can be applied
to each of the data types separately. The information from different receivers
provides the external check for the satellite failure.
The following are some of the QC issues that impact on data logging
from a single receiver:
Detection of failed satellites
Cycle slip detection and repair using one-way data
Ionospheric delay could be estimated by pseudo-range and carrier phase
data. If a single-frequency receiver is used, the difference between pseudo-range
and carrier phase could be used to estimate the ionosphere delay (Qiu,
et al, 1995). However, the dual-frequency data offers especially rich opportunities
to construct combinations of observables (phase-only, pseudo-range-only,
as well as phase and pseudo-range) (e.g., Rizos, 1997) which may
be screened using a number of procedures based on Kalman filters of various
types, trend-following polynomials, digital filters, and so on. Ionospheric
disturbances, which can occur suddenly and can be very severe, affect the
amplitude and phase of GPS signals (Wanninger, 1993; Knight & Finn,
1996). One of the phenomena responsible for these are "travelling ionospheric
disturbances", another is due to irregularities in the ionosphere causing
"scintillations" (especially in the tropical and auroral zones). Under
such conditions the ionosphere is so perturbed that single-frequency operations
may become impossible because the GPS receiver loses lock on the satellite
signals. Where tracking is possible, the likelihood of cycle slips and
interrupted tracking is increased, both of which, for example, make ambiguity
resolution a more difficult and unreliable task. Knight & Finn (1996)
describe an algorithm for determining the so-called S4 "scintillation index".
Empirical filtering techniques will need to be developed to cope with such
effects in real-time, particularly when the next solar cycle maximum occurs
at the turn of the century.
Multipath is a signal disturbance arising from the fact that
the signal entering the GPS antenna, in addition to containing the direct
satellite-receiver component, also includes reflections from buildings,
water surfaces and the ground. Multipath and diffraction effects cannot
be easily accounted for during data processing. Fortunately the multipath
error on carrier phase observations is significantly less than that experienced
on pseudo-range data (of the order of several centimetres, compared with
metre level disturbance on pseudo-ranges). Furthermore, its effect tends
to average out for static baseline determinations with observation sessions
of the order of an hour or more. Nevertheless, for the highest precision
static and kinematic applications the effect of multipath disturbance must
be addressed. The multipath component in the L1 and L2 pseudo-range data
can be estimated (Rizos, 1997). The multipath error in carrier phase cannot
be estimated from the raw measurements on a single receiver basis, but
may be estimable on a baseline basis from the double-differenced residuals
after baseline processing. In the case of a reference receiver,
the geometry of the satellites, relative to the receiver and surrounding
reflective objects, is almost exactly the same after one sidereal day.
Hence the multipath disturbance tends to exhibit a daily signature, both
in the raw measurements and in the baseline residuals. So-called "multipath-templates"
can be constructed to correct pseudo-range measurements, from an analysis
of the past one or more day's data, or to correct the double-differenced
carrier phase measurement (Lin & Rizos, 1997; Han & Rizos, 1997a).
The SIGMA-model was suggested by Brunner et al. (1999) and Hartinger &
Brunner (1999), which using the measured signal-to-noise ratio (S/N) data
and a template technique to derive a proper variance for all phase data
in order to improve the positioning results. However, apart from mathematical
procedures there are several strategies for overcoming the problem of multipath
in the observations at permanent GPS receivers:
Careful selection of site in order to minimise the multipath environment.
Cycle slips are discontinuities of an integer number of cycles in the measured
(integrated) carrier phase resulting from a temporary loss-of-lock in the
carrier tracking loop of a GPS receiver. This corrupts the carrier phase
measurement, causing the unknown ambiguity value to be different after
the cycle slip compared with its value before the slip. It must be "repaired"
before the phase data is processed as double-differenced observables for
GPS surveying techniques. GPS manufacturers have used different techniques
to repair cycle slips before the carrier phase measurements are processed.
A procedure for scanning the raw observation data based on using a Kalman
filter to model the behaviour of phase and phase-rate measurements (and
their changes) in discrete time developed by Mertikas & Rizos (1997)
could be used for this purpose. Ambiguity recovery techniques used for
long-range GPS kinematic positioning by Han (1997a) could also be used
for cycle slip detection and repair using one-way data.
Use of multipath resistant antennas.
Use special receivers that contain "multipath elimination tracking
If it is possible, multipath should be corrected at each receiver.
Implementation of QC Procedures at a Single Site
Different software packages to implement QC procedure and to indicate
the quality of data were developed by GPS manufactures and others, e.g.
TEQC by UNAVCO, EVALUATE by Ashtech. The University of New South Wales,
in conjunction with Australian Defence Science and Technology Organisation,
is developing a QC system which could be used for data logging and indicating
data quality. The cycle slip detection and repair procedures based on the
algorithms of Mertikas & Rizos (1997) and Han (1997a) are implemented.
The multipath-template for multipath is also generated. Another feature
is that the system is able to detect, track and indicate ionospheric scintillation
in real-time. A fuzzy expression for scintillation intensity has been used
to overcome the ambiguity existing in the numerical and linguistic definition
and to provide an indication of scintillation intensity.
The data flowchart is illustrated in Figure 1. The input can be from
GPS receivers directly, referred to as the "Real-Time Mode" or from the
RINEX files, referred to as the "Post-Processing Mode". The raw data are
then scanned to detect all jump outliers, such as cycle slips in carrier
phase measurements. Two algorithms proposed by Mertikas & Rizos (1997)
and Han (1997a) are used. In order to give the measurement standard deviations,
multipath significance, and ionospheric scintillation status, three components,
which are called data quality assessment, multipath monitoring and ionospheric
scintillation, are then introduced. For continuous GPS reference stations,
the computer facility, e.g. harddisk, power, etc. should be also considered
for quality assessment and QC.
Figure 1. Implementation of QC Procedure on a Single Site
3.2 Quality Assurance for Data Communication
Communication Format, Data Rate, and Latency
The United States body, the Radio Technical Commission for Maritime
(RTCM) Services, is a group concerned with the communication issues as
they pertain to the maritime industry. Special Committee 104 was formed
to draft a standard format for the correction messages necessary to ensure
an open real-time DGPS system (Langley, 1994). The format has become known
as RTCM 104, and has recently been updated to version 2.2.
The RTCM SC-104 message types 18 to 21 provide for RTK service, however
the awkwardness of the format and their message frame "overhead" make them
relatively inefficient for RTK. For example, to satisfy once per second
data transmission for RTK, a baud rate of 4800-9600 would be required (the
higher baud rate would be required if DGPS correction messages are also
sent), quite a technical challenge, and even more so if radio repeaters
have to be used (for each repeater employed, the data rate must be doubled).
As a consequence, GPS-RTK manufacturers have designed their own proprietary
data transmission standards to overcome the RTCM problems. One which had
been used by the Trimble RTK systems for several years, has been proposed
as an "industry standard" (Talbot, 1996). This format is referred to as
the Compact Measurement Record format. It uses an efficient compression/decompression
algorithm which makes it suitable for communications links that run at
2400 baud, and still deliver once per second GPS solutions. The latest
version of RTCM-104 may make redundant such a need for an "industry standard"
that is based on a single manufacturers format.
Different countries have different regulations governing the use of
radios, their frequency and power, hence there is considerable opportunity
for confusion. In Australia, the Spectrum Management Agency is responsible
for issuing permission on the use of selected radio frequency bands for
data communication. In general, the UHF and VHF bands are favoured for
RTK applications, in particular the "land mobile" band, 450-470 MHz. The
maximum power is dependent upon the type of licence issued to the user,
and may range from about 5 W for roving users, to 50 W for fixed local
sites. There is a complex relation between: height of transmitting antenna,
the type of antenna used (Yagi or omnidirectional), transmitting power,
cable length, tree cover and other intermediate objects; and the range
of the radio. For test/demonstration purposes up to a few kilometres, a
1 W transmitter operating within the UHF "land mobile" band, should be
adequate if the site conditions are ideal.
Data latency problems for RTK can be resolved in either of the
following two ways: (a) synchronise reference receiver data and mobile
receiver data (which gives the maximum precision but a substantial delay),
or (b) use the latest reference receiver data and extrapolate them to the
time of the mobile receiver data (which will cause some additional error).
The former is better for the carrier phase ambiguity resolution process,
as all errors have to be minimised for maximum reliability and performance.
However, the kinematic position will suffer due to a time delay of up to
1-2 seconds (which may be crucial for some real-time applications). The
latter solution will introduce additional errors due to observation extrapolation.
Experimental results show that the linear extrapolation model will introduce
an additional double-differenced error of about 2cm for a 1 second delay
and about 8cm for a 2 second delay. A quadratic extrapolation model will
introduce an additional double-differenced error of about 4cm for a 2 second
delay (Landau, et al., 1995; Lapucha, et al., 1995).
Communication Link Considerations
The following considerations must be addressed by DGPS/RTK communication
Coverage: This is generally dependent on the frequency of the radio
transmission that is used, the distribution and spacing of transmitters,
the transmission power, susceptibility to fade, interference, etc.
3.3 Refinements of Functional Model & Stochastic Model
Type of Service: For example, whether the real-time DGPS/RTK service
is a "closed" one available only to selected users, whether it is a subscriber
service, or an open broadcast service.
Functionality: This includes such link characteristics as whether
it is a one-way or two-way communications link, the duty period, whether
it is continuous or intermittent, whether other data is also transmitted,
Reliability: Does the communications link provide a "reasonable" service?
For example, what are the temporal coverage characteristics? Is there gradual
degradation of the link? What about short term interruptions?
Integrity: This is an important consideration for critical applications,
hence any errors in transmitted messages need to be detected with a high
probability, and users alerted accordingly.
Cost: This includes the capital as well as ongoing expenses, for both
the DGPS/RTK service provider as well as users.
Data rate: In general the faster the data rate, the higher the update
rate for range corrections, and hence better positioning accuracy. Typically
a set of correction messages every few seconds is acceptable.
Latency: Refers to the time lag between computation of correction
messages and the reception of message at the rover receiver. Obviously
this should be kept as short as possible, and typically a latency of less
than 5 seconds is suggested.
Quality assurance for data communication and data transmission delay.
The double-differenced observable is normally used in GPS positioning
because of the elimination or reduction of many error sources through differencing.
The notion of "short-range" is generally accepted as the distance that
distance-dependent errors (or "residual biases") could be ignored in the
functional model and the coordinate and integer ambiguity parameters can
be estimated. The maximum distance is dependent on the tropospheric delay,
ionosphere activity and orbit bias level, and a typical value is 10-15km.
However, with the increase in distance between two receivers, the "residual
biases" become larger and the fidelity of the functional model will be
reduced. If the distance between two GPS receivers is beyond this distance,
the distance-dependent errors must be considered in someway if the integer
ambiguity needs to be determined. The different modelling methodologies
have been developed using multiple reference receivers, e.g. Han &
Rizos (1997b), Wanninger (1995), Webster & Kleusberg (1992), Wu (1994),
Wübbena et al. (1996). The notion of "medium-range" is then defined
for carrier phase-based GPS positioning. The limit of medium-range is between
the minimum distance at which the functional model cannot ignore the distance-dependent
biases and the maximum distance at which the distance-dependent errors
could be modelled accurately enough to fix integer ambiguities. The range
is highly environment-related and typically would be between 10km and 100km.
If the distance is beyond this medium-range, the integer ambiguity must
be fixed using a special procedure, e.g. initialisation at the beginning
and then maintain GPS signal tracking during the campaign (Colombo &
Rizos, 1996; Han, 1997a; Blewitt, 1989; Dong & Bock, 1989).
The stochastic model is used to describe the error of the measurement
apart from its functional model. Although the accuracy of the GPS carrier
phase measurement is better than 1% of the cycle and almost independent
between epochs and different satellites, the stochastic model cannot be
determined based on the measurement noise alone. The stochastic model must
consider the misclosure of the functional model and it then becomes environmentally
dependent. The importance of the stochastic model can be experimentally
demonstrated. For a set of data on a static baseline, the stochastic model
could be determined using the residuals after data reduction. The single
epoch solution using the estimated stochastic model is much better than
the solution using a simple (or conventional) stochastic model (Cross,
1999). However, the stochastic model could only be obtained after intensive
data analysis in post-mission mode. For real-time applications, GPS data
may be separated into different segments, and the previous data segment
can be used to estimate the stochastic model for the current segment (Han,
1997b). The segment length may be assumed to be just a few minutes in length.
Real-time stochastic modelling is therefore still a challenging research
The data quality could be judged using the classic data snooping theory
(e.g. Baarda, 1977; Förstner, 1983). However, the statistic testing
and reliability analysis can only be efficient if the stochastic models
are correctly known or well approximated.
3.4 Ambiguity Resolution & Validation Procedures
Ambiguity Resolution Techniques
Ambiguity resolution (AR) strategy is dependent on the distance between
GPS receivers. For short-range applications a number of instantaneous AR
techniques have been reported (Han, 1997b; Al-Haifi, et al, 1997). Developments
in fast ambiguity resolution algorithms and validation criteria procedures,
together with improvements in stochastic modelling and the application
of careful quality control procedures, have generally been responsible
for this increased level of performance. Ambiguity searching procedures
have been developed to significantly reduce the computation load, for example,
LSAST (Hatch, 1990); FARA (Frei & Beutler, 1990), Cholesky decomposition
method (Euler & Landau, 1992) and LAMBDA (Teunissen, 1994).
Carrier phase-based medium-range GPS kinematic positioning has been
reported for baselines several tens of kilometres in length (Wanninger,
1995; Wübbena et al., 1996). The instantaneous AR has also been reported
for medium-range GPS kinematic positioning (Han & Rizos, 1997b). Such
medium-range performance requires the use of multiple reference
stations in order to mitigate the orbit bias, as well as the ionospheric
and tropospheric biases. These are exciting developments that will require
testing and implementation in operational positioning systems. A joint
project between UNSW and the Nanyang Technological University (Singapore)
is concerned with establishing a multiple reference system in support of
various real-time applications (Rizos et al., 1998). It has been demonstrated
that the multiple reference technique could be used to improve the medium-range
GPS kinematic positioning and also improve the accuracy of short-range
GPS kinematic positioning.
In the case of long-range kinematic positioning several innovative concepts
have been reported. Colombo & Rizos (1996) report results of decimetre
accuracy navigation over baselines up to a thousand kilometres in length.
Although it is not yet possible to resolve ambiguities OTF for baselines
of several hundreds of kilometres in length, ambiguity re-initialisation
or ambiguity recovery is achievable (Han, 1997a; Han & Rizos,
1995). In other words, if loss-of-lock occurs, the AR algorithm can recover
the ambiguities as long as any data "gap" is less than a minute or so.
Initial AR must be carried out using traditional techniques, including
static initialisation. The sea surface determination using long-range airborne
GPS kinematic positioning and Laser Airborne Depth Sounder (LADS) system
is discussed by Han et al. (1998).
Validation Criteria and Adaptive Procedure
Using the above mentioned model, the real-valued ambiguities can be
estimated and the integer ambiguity search procedure then used to determine
the correct integer ambiguity set (that which generates the minimum quadratic
form of the residuals). The ratio test of the second minimum and the minimum
quadratic form of the residuals is normally used to validate the correct
integer ambiguity set (Frei & Beutler, 1990). Euler & Schaffrin
(1990) have derived another ratio test, but the critical value is still
too conservative and is often experimentally specified as being the value
2 (Wei & Schwarz, 1995), or 1.5 (Han & Rizos, 1996). The testing
of the difference between the minimum and second minimum quadratic form
of the residuals has been suggested (Tiberius & de Jonge, 1995; Wang,
et al., 1998). The other validation criteria based on reliability theory
were derived by Han (1997b).
The UNSW strategy uses a series of test to assure the results. This
procedure using validation criteria suggested by (ibid, 1997b) assumes
that the integer ambiguity set generating the minimum quadratic form of
the residuals is correct but detects the outlier of the integer set generating
the second minimum quadratic form of the residuals. If this outlier can
be detected, the integer set generating the minimum quadratic form of the
residuals is considered to be the correct one. On the other hand, the sequence
generated by differencing the double-differenced ionospheric delay on L1
and L2 carrier phase can also be used as a validation criteria. If this
sequence has a slip (or "jump") at the current epoch, the wrong ambiguity
resolution can be confirmed at this epoch. If ambiguity resolution fails
and six or more satellites are observed at the current epoch, an adaptive
procedure can be applied using a satellite elimination procedure, starting
with the one with the lowest elevation, repeating the process until ambiguity
resolution is successful. If all possible sets of five or more satellites
are combined and the ambiguity test still fails, the ambiguity resolution
step is considered to have failed.
3.5 Concluding Remarks
The above discussion is intended as a demonstration of the multi-dimensional
approach to QC if the total positioning system is taken into account. It
tries to mimic the UKOOA guidelines suggested for real-time, pseudo-range-based
DGPS, by dismissing the notion that there is one "magic QC test" that can
be implemented. Instead, the different sources of "bad data" and "questionable
or unreliable results" are identified and QC/QA tests are suggested (some
mathematical in nature, others empirical). The attention to OTF-AR (and
in particular to single epoch implementations) is to acknowledge that OTF-AR
is the most challenging of GPS data processing problems and that advances
in this area will make significant contributions to carrier phase QC. However,
unlike the UKOOA guidelines the Chairman does not propose that the tests
outlined above should be the "officially" sanctioned ones. The standardisation
of QC procedures for carrier phase-based kinematic GPS positioning is still
some way off.
The focus on OTF-AR to be implemented in real-time is critical. As no
SSG members are actively involved in the development of operational systems,
the contribution that can be made in this regard is problematic. Nevertheless,
the Chairman has attempted to provide some ideas on how such a system could
be implemented. It may take a few more years to thoroughly test and evaluate
the appropriate "mix" of QC/QA procedures that would be needed. The Chairman
hopes that this report has made a humble contribution to this area of study.
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A.1 Address List of SSG 1.154 Members
Assoc. Prof. Chris RIZOS
School of Geomatic Engineering,
The University of New South Wales,
Sydney 2052 AUSTRALIA
Dr. Hasanuddin ABIDIN
Dept. of Geodetic Engineering,
Institute of Technology Bandung,
Jl. Ganesha 10,
Bandung 40132 INDONESIA
Mr. Jeff BEHR
SCIGN Operations Center,
U.S. Geological Survey,
525 S. Wilson Ave.,
Pasadena, CA 91106 U.S.A.
Assoc. Prof. Elizabeth CANNON
Department of Geomatics Engineering,
The University of Calgary,
2500 University Drive N.W.,
Calgary, Alberta T2N1N4 CANADA
Mr. Paul COLLINS
Geodetic Research Laboratory,
Department of Geodesy & Geomatics Engineering,
University of New Brunswick,
Fredericton, N.B. E3B5A3 CANADA
Dr. Roman GALAS
D-14473 Potsdam GERMANY
Dr. Shaowei HAN
School of Geomatic Engineering,
The University of New South Wales,
Sydney 2052 AUSTRALIA
Mr. Yuki HATANAKA
Geodetic Observation Center,
Geographical Survey Institute,
Ibaraki, 305 JAPAN
Ph: +81-298-641111 ext.8643
Email: firstname.lastname@example.org / email@example.com
Dr. Xinxiang JIN
Dr. Hansjoerg KUTTERER
University of Karlsruhe,
D-76128 Karlsruhe GERMANY
Mr. Yecai LI
Department of Geomatics Engineering,
The University of Calgary,
2500 University Drive N.W.,
Calgary, Alberta T2N1N4 CANADA
Assoc. Prof. Stelios MERTIKAS
Exploration & Positioning Division,
Mineral Resources Engineering Department,
Technical University of Crete,
GR-73100 Chania, Crete GREECE
Email: firstname.lastname@example.org / email@example.com
Assoc. Prof. Peter MORGAN
School of Computing,
Faculty of Information Science & Engineering,
University of Canberra,
PO Box 1 Belconnen,
ACT 2616 AUSTRALIA
Email: firstname.lastname@example.org / email@example.com
Prof. Stanislaw OSZCZAK
Institute for Geodesy & Photogrammetry,
Olsztyn University of Agriculture & Technology,
Oczapowskiego Str. 1,
10-957 Olsztyn POLAND
Dr. William ROBERTS
Quality Engineering & Survey Technology Ltd.,
St Thomas St.,
Newcastle-upon-Tyne NE14LE UNITED KINGDOM
Prof. Günter SEEBER
Institute for Geodesy,
University of Hannover,
D-3000 Hannover GERMANY
Dr. Mike STEWART
School of Spatial Sciences,
Curtin University of Technology,
GPO Box U1987,
Perth 6001 AUSTRALIA
Dr. Lambert WANNINGER
Technical University Dresden,
D-01062 Dresden GERMANY
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