GPS Receiver Test Plan for Week Roll Over and
Y2K Critical Dates Compliance
M.G. Petovello, G. Fotopoulos, M.E. Cannon and, G. Lachapelle,
Department of Geomatics Engineering
The University of Calgary
S. Ryan,
Canadian Coast Guard
BIOGRAPHIES
M. Petovello is a M.Sc. student in the Department of
Geomatics Engineering, The University of Calgary. He
obtained a B.Sc. in Geomatics Engineering from the same
department in Spring 1998.
G. Fotopoulos is a M.Sc. student in the Department of
Geomatics Engineering, The University of Calgary. She
obtained her B.Sc. from the same department in Spring
1998.
Dr. M.E. Cannon is a professor in Geomatics Engineering
at The University of Calgary. She has been involved with
GPS research since 1984 and has published numerous
papers on static and kinematic GPS positioning. She is
also the author of several GPS related software programs.
Dr. G. Lachapelle is a professor in the Department of
Geomatics Engineering where he is responsible for
teaching and research related to positioning, navigation
and hydrography. He has been involved with GPS
developments and applications since 1980 and is the
author of numerous papers on the subject.
S. Ryan has a B.Eng. in Electrical Engineering from
Memorial University, St. John’s, Newfoundland, and is an
Electronic Systems Engineer in the Technical and
Operational Services Directorate of the Canadian Coast
Guard. He is currently on educational leave at the
University of Calgary where he is pursuing a M.Sc.
degree in Geomatics Engineering.
ABSTRACT
Proper GPS positioning, navigation and time transfer
capabilities are essential to many industries in terms of
economic benefits and public safety. With the new
millennium nearly upon us, the urgency to confirm proper
operation of various GPS receivers during all Year 2000
(Y2K) epochs is increasing.
This urgency is the
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
motivation behind a series of tests designed to identify if a
GPS receiver is Y2K compliant. Several critical epochs
apply to GPS including; (i) GPS Week Roll Over 1024
(WRO1024) in August, 1999, (ii) century roll over from
1999 to 2000 (Y2K), (iii) leap year day of February 29,
2000 (Y2KLY), (iv) year roll over from 2000 to 2001
(new millennium) and, (v) various end-of-record (EOR)
dates. A test plan designed for evaluating a receiver’s use
of almanac and ephemeris data before, during and after all
of the critical dates described above using a GPS signal
simulator is presented. Finally, a procedure for assessing
receiver compliance in various domains is included.
INTRODUCTION
The anticipation and excitement surrounding the new
millennium is partially shrouded by the fear that many
electronic devices, including GPS receivers, will begin
malfunctioning causing widespread problems. The belief
is that devices performing time and date computations
will incorrectly interpret the new year as 1900 instead of
2000 thus producing unexpected results or even failure.
This stems from the fact that older devices only store the
last two digits of the year (e.g. 95 to represent 1995) in
order to save memory. Questions also arise as to the
ability of such devices to correctly interpret the year 2000
as a leap year. The problem is compounded for GPS
since the number of weeks since January 6, 1980
(necessary for proper time and date determination) will
rollover from 1023 to 0 in the navigation message in
August, 1999.
The heavy reliance on GPS for safety of navigation by
many users, including those in the marine industry, has
brought the issue of receiver compliance to the forefront.
This was the motivation behind a joint effort by the
Canadian Coast Guard (CCG) and the Department of
Geomatics Engineering, the University of Calgary to
develop a thorough test plan for assessing receiver
compliance during various critical dates through the use
of a GPS signal simulator. Tests are designed for two
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purposes, namely to assess the compliance of the CCG’s
DGPS Stations and a variety of marine user receivers
under single point and differential conditions.
The test plan was developed by critically analyzing the
various scenarios that could cause problems at each
critical epoch. Other test plans developed by agencies
such as the US Coast Guard, the US Air Force and
various receiver manufacturers were also consulted.
Discussion begins with a summary of critical dates tested
and why they are considered important. Specific receiver
issues addressed by the test plan are presented. A
description of the test plan proper is included, both for the
DGPS Station and the individual marine user receivers.
Finally, a brief discussion on how to analyze the data in
the time/date, observation, residual, position and
differential correction domains is described.
DESCRIPTION OF CRITICAL DATES
A variety of critical dates arise when considering GPS
receiver compliance. These dates can be broken down
into three main categories; (i) those dealing specifically
with GPS, (ii) those dealing with the new millennium and,
(iii) end-of-record dates. Each of these categories are
discussed below. A time line of all critical dates tested is
given in Figure 1.
GPS Specific Dates
The only date that is truly GPS specific involves the roll
over of the number of weeks since the start of GPS time
(January 6, 1980) from 1023 to 1024. The week number
is stored as a ten bit integer in the satellite and thus can
take a maximum value of (210-1=) 1023. Consequently,
when the week rolls over from 1023 to 1024 on August
22, 1999 the satellites will again transmit a value of zero
and “users must account for the previous 1024 weeks”
(ICD-GPS-200, 1993). Failure to correctly account for
the previous 1024 weeks may result in time/date errors,
position errors or failure of the receiver.
New Millennium Dates
Four critical dates are associated with the new
millennium. Each is listed below with a brief description
of why they are important:
1. Year 2000 roll over (Y2K): December 31, 1999 to
January 1, 2000. Since receivers may still utilize two
digits to represent the year, the receiver may
incorrectly determine the year to be 1900.
2. Year 2000 leap year logic (Y2KLY): February 29,
2000. Since the year 2000 is evenly divisible by four
and four hundred, it is a leap year. However, it is
believed some software programs implement an
algorithm that checks if a year is divisible by one
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
3.
4.
hundred in which case it is not considered a leap
year.
Year 2001 roll over (New Millennium - NM):
December 31, 2000 to January 1, 2001. Since the
Roman calendar originally began with year one (and
not year zero) the actual beginning of the new
millennium is January 1, 2001. It is believed that
some receivers may experience difficulties with date
computations at this epoch.
Year 2001 leap year logic: February 28 to March 1,
2001. Although the year 2001 is not a leap year,
some believe that receivers may assume all years
after 1999 are leap years.
End-of-Record Dates
End-of-record (EOR) dates are those dates which form a
series of numbers (typically 9999) that are believed to be
used by programmers to indicate the end of a record.
Consequently, when the date is computed, the receiver
may incorrectly interpret it as an EOR thus causing
malfunctions. The EOR dates addressed in this plan
include:
1. April 9, 1999. This could be interpreted as an EOR if
receivers count the number of days into the year since
April 9, 1999 is the 99th day of the 99th year.
2. September 9, 1999. This could be interpreted as an
EOR if receivers use the Roman calendar for
computation purposes since September 9 can be
represented as 9/9/99.
From a GPS perspective, WRO1024 appears the most
critical epoch since the determination of the GPS week
affects most of the important receiver functions including
satellite prediction, orbit computation, and time and date
calculation. Furthermore, it is the only epoch that
involves a “discontinuity” in time because the week
number transmitted by the satellites will actually revert to
a previous value. All other critical epochs are incremental
in nature and, in theory, should be more easily accounted
for.
RECEIVER ISSUES
Proper functioning of a GPS receiver requires that the
information transmitted by each satellite be correctly
interpreted and utilized. Ephemeris and almanac data are
typically most important for a GPS receiver. Tests must
therefore be designed to test the various ways in which
the receiver utilizes this information. A brief explanation
of how ephemeris and almanac data are used and what
problems may occur is given below.
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Figure 1 – Time Line of GPS Critical Dates
Ephemeris
Ephemeris data contains information for precisely
computing the position of a single GPS satellite. The
ephemeris data contains a set of parameters describing a
particular satellite’s orbit over time and a reference time
(TOE) to which these parameters apply.
These
parameters are typically updated every two hours and are
valid for two hours on either side of the reference time,
however deviations from this schedule are possible.
Although the GPS week number is not explicitly included
in the ephemeris message, the receiver must be able to
account for a change in week number in the event that the
reference time falls in a week different from the current
week. Failure to do so may result in a receiver error or
failure.
Almanac
Almanac data consists of information describing the
approximate locations of all satellites as a function of
time. The reference time for the almanac parameters
(TOA) currently being transmitted can differ by up to 3.5
days (in the past or future) from the current time.
However, the parameters are valid for much longer
periods, namely up to six months. The main purpose of
the almanac is to assist the receiver in acquiring signal
lock at the start of and/or during a data collection
campaign. By knowing an approximate user location,
periods of satellite visibility and the Doppler shift for an
individual satellite can be computed, thus limiting the
signal search to code bins only. If satellites are not
visible, the receiver may power down some tracking
channels in order to conserve power. When a satellite is
deemed to be visible, the tracking channels can then be
powered up and the new satellite can be tracked.
If the reference week number for the almanac differs from
the current week the receiver must be able to correctly
interpret the difference. Failure to do so may result in
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
newly visible satellites (i.e. satellites rising over the
horizon) not being tracked causing degraded position
accuracy and reliability. Note that since the TOA can be
up to 3.5 days into the future, problems may arise as early
as noon on Wednesday, August 18, 1999 when an
almanac with a reference week of 1024 begins
broadcasting.
Information Stored in Receiver at Power-up
Since receiver behaviour may differ depending on which
information (i.e. almanac and ephemeris) is available at
power-up, this is the last major receiver issue addressed
herein. For example, certain information may indicate
that certain critical dates have already elapsed (e.g.
WRO1024) thus aiding the receiver in performing various
computations. By varying the reference time of the
previously downloaded almanac parameters (and to a
lesser extent, the ephemeris parameters), receiver
performance can be evaluated as function of how much
time has elapsed since the receiver was last used.
TESTING OBJECTIVES
To realistically evaluate a receiver’s performance all
components, starting with the RF front end, should be
subject to test. This can only be done using a GPS signal
simulator, which can be programmed to simulate any
period of time in the past and/or future. Simulators
further allow for the simulation of atypical occurrences
(e.g. late uploads to various satellites) thus providing the
flexibility to thoroughly evaluate a receiver.
The development of the test plan focused on addressing
the three receiver issues presented in the previous section.
Three corresponding testing objectives have been
identified and are described below.
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The first objective involves ensuring receivers properly
interpret ephemeris information referenced before and/or
after a critical epoch. Although cutovers to data sets
referenced after the critical epoch typically occur two
hours prior to that epoch, this is not mandated by ICDGPS-200 (1993). Therefore, other combinations of
ephemeris data cutover schedules are investigated.
Second, tests must ensure receivers properly track
satellites rising above the horizon before and after the
critical epoch through the use of its almanac, regardless of
when the almanac parameters are referenced (e.g. before
or after the critical epoch). If the receiver is correctly
using the downloaded almanac, new satellites should be
easily tracked. However, if rising satellites are not
tracked, position accuracy and reliability will suffer.
Simulating satellites to rise above the horizon both before
and after the critical epoch allows for an assessment of
whether the receiver is correctly using its almanac data.
The final objective is to ensure receivers will correctly
transit the critical epoch regardless of what information, if
any, is stored in memory at power-up. By varying the age
of the previously downloaded almanac parameters, any
period elapsed since the last time the receiver was used
can be simulated. This may be important if one or more
critical dates have elapsed since the last time the receiver
was used. Under such a situation, the receiver may fail to
function properly because it was forced to handle several
critical dates at once despite being capable of handling
each critical date in succession. Completely erasing all
memory before power-up simulates the scenario where a
full reset of the receiver has occurred, a typical
occurrence if problems are encountered.
TEST DESCRIPTION
This section begins with a description of the general setup
of all tests, followed by a more in-depth discussion of the
specific tests to be performed for each of the critical
dates. GPS WRO1024 tests are discussed first, as they
are the most complicated from a testing perspective and
are most relevant to GPS. The remaining tests (new
millennium tests and EOR tests) will follow. The section
concludes with a discussion of what parameters should be
considered when performing each test.
All tests are performed on a GSS 4760 DGPS signal
simulator using software version 6.30. Each test is
broken down into three stages, namely a loading period,
an initialization period and a testing period. Prior to the
loading period, the receiver has all previous almanac data,
ephemeris data, and date information nulled/erased
(herein referred to as performing a full reset). Testing has
showed that depending on the GPS receiver manufacturer,
this may be necessary since some receivers do not operate
correctly if consecutive tests are not sequential in time
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
(i.e. appear to go back in time) and therefore is performed
as a safeguard.
The loading period is used to download desired almanac
and ephemeris parameters into the receiver. These
parameters are those to be stored in memory at the
beginning of the test (i.e. at power-up). Once the receiver
is tracking its first satellite the loading period is 15
minutes in length, 12.5 minutes to download an almanac
plus 2.5 minutes of contingency. After loading the
necessary data, the receiver is power cycled before the
initialization period begins. Note that the loading period
is omitted if the receiver’s memory is to remain nulled at
the beginning of the test.
The initialization period is a 15-minute period prior to the
test, which is used to allow the receiver to acquire the
almanac parameters currently being transmitted. This is
needed because the almanac parameters stored in the
receiver from the loading period do not necessarily match
those currently transmitted, thus simulating an extended
period when the receiver was not used.
The testing period is a 30-minute period beginning 15
minutes before the critical epoch and continuing until 15
minutes after the critical epoch. Note that this 30 minutes
follows immediately after the initialization period, see
Figure 2, and constitutes the period of data to be analyzed
to determine receiver compliance.
Most tests begin with six visible satellites with a seventh
satellite rising five minutes before and an eighth satellite
rising five minutes after the critical epoch. Again, this is
used to evaluate the receiver’s use of its almanac. A
diagram depicting satellite visibility as a function of time
is given in Figure 2. Because the normal satellite
constellation has to be modified to obtain satellites rising
at the desired times, care is taken to ensure that a good
Position Dilution of Precision (PDOP) is maintained
throughout the test. Otherwise, if the PDOP becomes too
large it may become difficult to separate receiver
malfunctions from the effects of poor geometry.
Consequently, a PDOP lower than or equal to six
(PDOP≤6) is maintained throughout the test period.
Some tests are also performed using only four satellites in
order to evaluate receiver performance under reduced
satellite geometry.
GPS WRO1024 Tests
As previously discussed, the GPS WRO1024 epoch is the
most difficult from a testing perspective because of the
various combinations of almanac and ephemeris reference
times that could be simulated. Consequently, some
“modification” of the navigation data transmitted by each
satellite must occur, relative to the typical case. Since this
is a time consuming process, a balance between test
reliability, that is the ability to identify if a receiver has
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passed/failed, and testing time needs to be achieved.
Therefore, only a representative subset of all possible
tests is selected that still allows for a confident assessment
of receiver performance. A list of all WRO1024 tests
performed is given in Table 1 along with a brief
description of each. In the table, tests occurring before,
during and after WRO1024 are identified by shading of
the corresponding rows.
Figure 2 – Satellite Visibility as a Function of Time for
Tests Using Eight Satellites
For WRO1024, three separate sets of tests were devised,
namely those before, during and after the critical epoch.
Tests before the critical epoch serve two purposes. The
first is to obtain baseline behaviour of the receiver under
test during a “normal” week roll over (e.g. week 1022 to
1023). These results are to be compared to those obtained
during WRO1024 to aid in the assessment of compliance.
Second, it allows for the investigation of receiver
performance when an almanac with a reference week of
1024 is downloaded prior to the actual week roll over
(recall this could happen as early as noon on August 18,
1999).
Tests performed during WRO1024 are designed to
evaluate receiver compliance under various scenarios.
The primary variable between all of the tests during
WRO1024 is the ephemeris information transmitted by
the satellites.
Several combinations of ephemeris
parameters referenced to week 1023 and/or 1024 are
included. Although tests B2 and B5 represent the typical
cases, the other tests could possibly be realized under the
specifications of ICD-GPS-200 (1993).
Finally, tests simulating post-WRO1024 time periods
serve two purposes. The first is to ensure that the next
“normal” week roll over, and presumably all subsequent
week roll overs, will be properly handled by the receiver.
The second purpose is to ensure the receiver will operate
properly regardless of which information is stored in
memory at power-up. In particular, can the receiver
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
properly interpret the week number if no information is
available at power-up? Proper computation of the date
may occur if the receiver has apriori knowledge that the
week number has already rolled over but the receiver may
fail if such information is unavailable. This scenario is
important since memory resets are commonly used in
practice when difficulties are encountered.
New Millennium and EOR Tests
The remaining critical dates are less concerned with what
is being transmitted by the satellites than with:
1. The general performance of the receiver during the
transition of the critical epoch. This is of minor
importance since the conversion from GPS time to a
UTC/local time and date is relatively trivial.
Consequently, these transitions are only tested with a
nulled memory (presumably the most challenging
scenario for a receiver).
2. The performance of the receiver after the critical
epoch as a function of the information available in
receiver memory at power-up. The concern is that
performance after the critical epoch may be
dependent on whether the receiver is aware that
various other critical dates have elapsed since the last
power-up. For example, the receiver may fail if an
attempt to handle more than one critical epoch at a
time is made. The first week roll over (WRO) after
the critical epoch is used since WROs presumably
pose the largest problems for receivers.
A list of all remaining critical epochs tested is given in
Table 2 along with a brief description of each. Again,
row shading is used to discriminate between critical
epochs. Those epochs believed to be of less significance
(e.g. EOR and “Leap Year” 2001) are given less attention
accordingly but are included for completeness.
All of the above tests can be performed in single point or
differential mode. This decision was based on how the
receiver is most likely to be used in practice. For typical
marine requirements, differential GPS is most commonly
used and therefore is tested more thoroughly.
Simulation Parameters
One of the major benefits of using a GPS signal simulator
lies in the flexibility of selecting which errors are to be
generated.
For example, should tropospheric and
ionospheric delays be included? Should a Selective
Availability-like (SA-like) ranging error be superimposed
on the satellite ranges? Testing has shown that some
receivers automatically apply some form of atmospheric
correction to the measured ranges that cannot be disabled.
Consequently, simulated signals always contain the
normal atmospheric delays/advances as this most closely
simulates operational conditions.
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However, the corruption of satellite signals with an SAlike error is only performed if testing the receiver in
differential mode. The reason for this is SA-like signals
will contaminate the measured ranges and computed
single point positions to such a large extent that any small
discontinuity resulting from receiver non-compliance may
pass unnoticed.
Test
A1
Date
14/08/99
to 15/08/99
Almanac
Rx: week 1022
Tx: week 1023
A2
14/08/99
to 15/08/99
Rx: week 1022
Tx: week 1023
A3
20/08/99
to 21/08/99
Rx: week 1023
Tx: week 1024
B1
21/08/99
to 22/08/99
Rx: week 1023
Tx: week 1023
B2
21/08/99
to 22/08/99
Rx: week 1023
Tx: week 1024
B3
21/08/99
to 22/08/99
Rx: week 1023
Tx: week 1024
B4
21/08/99
to 22/08/99
Rx: week 1023
Tx: week 1024
B5
21/08/99
to 22/08/99
Rx: week 1024
Tx: week 1024
C1
28/08/99
to 29/08/99
Rx: Nulled
Tx: week 1025
C2
28/08/99
to 29/08/99
Rx: week 1023
Tx: week 1025
C3
28/08/99
to 29/08/99
Rx: week 1024
Tx: week 1025
Evaluation of a DGPS reference station always uses SAlike corrupted signals. The accuracy of differential
corrections generated by a base station are more reliably
determined when using SA-like corrupted signals because
the corrections have to track larger range and range rate
errors. Without the corrupted ranges, errors in the
differential corrections may be difficult to separate from
receiver noise.
Table 1 – GPS WRO1024 Tests
Ephemeris
Test Description
Rx: week 1022
Four satellites only. Obtain baseline behaviour of
Tx: week 1023
receiver WRO performance during a normal week
roll over under reduced satellite visibility.
Rx: week 1022
Obtain baseline behaviour of receiver WRO
Tx: week 1023
performance during a normal week roll over under
good satellite visibility.
Rx: week 1023
Obtain baseline behaviour of receiver WRO
Tx: week 1023
performance when the satellites transmit an
almanac referenced to week 1024.
Rx: week 1023
Investigate whether the receiver can properly
Tx: week 1023
interpret ephemeris information referenced before
the critical epoch during the epoch transition.
Rx: week 1023
This is the typical scenario to be expected. It tests
Tx: week 1024
that the receiver can properly cross the critical
epoch when old almanac parameters are stored in
memory.
Rx: week 1023
Investigate how the receiver handles ephemeris
Tx: week 1023 and information reference both before and after the
1024
critical epoch under good satellite coverage.
Rx: week 1023
Four satellites only. Investigate how the receiver
Tx: week 1023 and handles ephemeris information reference both
1024
before and after the critical epoch under reduced
satellite coverage.
Rx: week 1024
This is the typical scenario to be expected. It tests
Tx: week 1024
that having an almanac referenced to week 1024 in
memory does not adversely affect receiver
performance.
Rx: Nulled
Investigate receiver WRO performance after
Tx: week 1025
WRO1024 without any information stored in
memory. This simulates a full reset of the receiver
after WRO1024.
Rx: week 1023
Investigate receiver WRO performance after
Tx: week 1025
WRO1024 if it has stored week 1023 almanac
parameters.
Rx: week 1024
Investigate receiver WRO performance after
Tx: week 1025
WRO1024 if it has stored week 1024 almanac
parameters.
Rx – Stored in the receiver
Tx – Transmitted by the satellites
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
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Table 2 – New Millennium and EOR Tests
Almanac
Ephemeris
Test Description
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
Tx: week 1005
Tx: week 1004
transition into the potential EOR indicator date of
April 9, 1999.
D2
09/04/99
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
to 10/04/99 Tx: week 1005
Tx: week 1004
transition out of the potential EOR indicator date of
April 9 to April 10, 1999.
E1
08/09/99
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
to 09/09/99 Tx: week 1027
Tx: week 1026
transition into the potential EOR indicator date of
September 9, 1999.
E2
09/09/99
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
to 10/09/99 Tx: week 1027
Tx: week 1026
transition out of the potential EOR indicator date of
September 9 to September 10, 1999.
F1
31/12/99
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
to 01/01/00 Tx: week 1043
Tx: week 1042
transition of Y2K when memory has been nulled.
F2
01/01/00
Rx: Nulled
Rx: Nulled
Investigate receiver WRO performance after Y2K
to 02/01/00 Tx: week 1043
Tx: week 1043
when memory has been nulled.
F3
01/01/00
Rx: week 1023
Rx: week 1023
Investigate receiver WRO performance after Y2K
to 02/01/00 Tx: week 1043
Tx: week 1043
when almanac data is referenced before WRO1024.
F4
01/01/00
Rx: week 1042
Rx: week 1042
Investigate receiver WRO performance after Y2K
to 02/01/00 Tx: week 1043
Tx: week 1043
when almanac data is referenced between
WRO1024 and Y2K.
F5
01/01/00
Rx: week 1043
Rx: week 1043
Investigate receiver WRO performance after Y2K
to 02/01/00 Tx: week 1043
Tx: week 1043
when almanac data is referenced after Y2K but
before Y2KLY.
G1
28/02/00
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
to 29/02/00 Tx: week 1051
Tx: week 1051
transition of Y2KLY when memory has been
nulled.
G2
29/02/00
Rx: Nulled
Rx: Nulled
Investigate receiver performance during transition
to 01/03/00 Tx: week 1051
Tx: week 1051
from February 29 to March 1, 2000 when memory
has been nulled.
G3
04/03/00
Rx: Nulled
Rx: Nulled
Investigate receiver WRO performance after
to 05/03/00 Tx: week 1052
Tx: week 1052
Y2KLY when memory has been nulled.
G4
04/03/00
Rx: week 1051
Rx: week 1051
Investigate receiver WRO performance after
to 05/03/00 Tx: week 1052
Tx: week 1052
Y2KLY when almanac data is referenced between
Y2K and Y2KLY.
G5
04/03/00
Rx: week 1052
Rx: week 1052
Investigate receiver WRO performance after
to 05/03/00 Tx: week 1052
Tx: week 1052
Y2KLY when almanac data is referenced between
Y2KLY and new millennium.
H1
31/12/00
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
to 01/01/01 Tx: week 1095
Tx: week 1095
transition of NM when memory has been nulled.
H2
06/01/01
Rx: Nulled
Rx: Nulled
Investigate receiver WRO performance after NM
to 07/01/01 Tx: week 1096
Tx: week 1096
when memory has been nulled.
H3
06/01/01
Rx: week 1094
Rx: week 1094
Investigate receiver WRO performance after NM
to 07/01/01 Tx: week 1096
Tx: week 1096
when almanac data is referenced between Y2KLY
and NM.
H4
06/01/01
Rx: week 1096
Rx: week 1096
Investigate receiver WRO performance after NM
to 07/01/01 Tx: week 1096
Tx: week 1096
when almanac data is referenced after NM.
I1
28/02/01
Rx: Nulled
Rx: Nulled
Investigate receiver performance during the
to 01/03/01 Tx: week 1103
Tx: week 1103
transition from February 28 to March 1, 2001 when
memory has been nulled.
Rx – Stored in the receiver
Tx – Transmitted by the satellites
Test
D1
Date
08/04/99
to 09/04/99
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
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All receivers not being tested as DGPS reference stations
are simulated in kinematic mode to check the validity of
the GPS time stamp. Since the reference position, that is
the position to which the simulated ranges apply, is
available from the simulator at every epoch a truth
trajectory can be generated. If static data is used, only
one reference position is generated. This means that
comparisons between the reference position (generated by
the simulator) and computed positions (generated by the
receiver) for a given epoch may appear correct while the
time stamp is incorrect. By contrast, in kinematic mode, a
separate reference position is generated by the simulator
for each epoch. Therefore, for a given epoch in kinematic
mode, if the reference and computed positions coincide,
the receiver is compliant in terms of time and position for
the critical epoch being tested. However, if the reference
and receiver positions do not coincide, the receiver is
deemed non-compliant.
Finally, all tests simulate the transmission of L1 and L2
data. Receivers capable of tracking the P-Y code signal
on L2 are forced (if possible) to use semi-codeless
tracking techniques despite the fact that the simulated L2
signal is not encrypted. This realistically recreates what
will be experienced by the receiver under normal
operating conditions.
EVALUATING RECEIVER COMPLIANCE
Receiver compliance is evaluated in the position,
time/date, residual, observation, and differential
correction domains. Failure of the receiver in any of
these aspects is sufficient to deem the receiver noncompliant. However, if a receiver has failed a test, the
test is redone before designating the receiver noncompliant. This helps to ensure that operator errors or
simulator inconsistencies do not artificially skew test
results. If the test fails a second time, a non-compliant
status is confidently assigned. Data analysis is performed
on the appropriate NMEA messages output from the
receiver and on results obtained in post-mission. Postmission results are computed using the University of
Calgary’s C3NAVG2™ GPS/GLONASS processing
software.
Comparison of reference and receiver positions was
discussed at the end of the last section. By analyzing
position errors versus time, especially near the critical
epoch, position domain compliance can be ascertained.
Time domain analysis was also briefly discussed above,
however verification of the time stamp alone, usually
given as seconds into the week, is insufficient. The
UTC/local time and date output by the receiver may also
be of use in certain applications.
Therefore, an
investigation of the appropriate NMEA messages is
performed.
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
Analysis of measurement residuals obtained from a leastsquares procedure typically demonstrate low frequency
trends (within the limits of SA) superimposed with high
frequency noise. Comparison of results with those
observed during the baseline test should reveal any
atypical behaviour, possibly resulting from receiver noncompliance. This may include discontinuous residuals or
data gaps.
A GPS simulator also provides access to the true ranges
that should be measured to each satellite.
This
information is used to evaluate receiver performance in
the observation domain. The problem is that the offset of
the receiver clock from GPS time is large and will mask
all other errors unless it is accounted for. The difference
between the true range and the measured range is an
estimate of the receiver’s internal clock offset (all other
errors are cancelled in the difference). Since this error is
common to all satellites for a given epoch, averaging the
range errors of all satellites will effectively smooth
receiver noise thus allowing for an accurate determination
of the receiver’s internal clock error:
dt Rx ≈
1
N
∑ [P̂ − P ]
N
i
i
Eq. (1)
i =1
where:
dt Rx
N
P̂i
Pi
is the receiver clock error [m]
is the number of satellites tracked
is the measured range to the ith satellite [m]
is the true range to the ith satellite [m]
The observation error can then be obtained using:
ErrorP = Pi − P̂i − dt Rx
Eq. (2)
where:
ErrorP is the observation error [m]
Note that Pi in Eq. (1) should be interpreted as the C/A
code range. This is because an estimate of the true range
to the satellite is needed and the C/A is the most reliable.
However, once the clock offset is obtained from Eq. (1),
Pi in Eq. (2) could symbolize any type of range
measurement, including L1 and L2 carrier phases.
By analyzing the errors produced from Eq. (2), the
receiver’s performance in the observation domain can be
evaluated. If the results differ significantly from the
baseline behaviour, the receiver may be experiencing
problems.
If a receiver is to act as a reference station for generating
differential corrections, the accuracy of these corrections
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must be investigated. This is done by comparing the true
range and range rate errors to the generated corrections.
Any large or sudden deviations between the two values
may constitute receiver non-compliance.
Finally, if an entire differential system is to be evaluated,
the testing should proceed in an incremental fashion. For
example, one CCG DGPS Station contains, among other
things (Ryan et. al., 1997):
1. Two Ashtech Z12-R receivers acting as reference
stations (RS).
2. Two Ashtech Super C/A receivers acting as integrity
monitors (IM).
3. Two control station computers (CS) to monitor and
control the operation of the DGPS Station.
Testing all components simultaneously limits the ability
to identify problems and more importantly limits the
ability to isolate a problem if one occurs.
The
incremental procedure used to evaluate the CCG DGPS
Station includes:
1. Testing the RS and IM receivers in single point mode
to ensure proper standalone operation.
2. Testing one pair of RS and IM receivers operating
differentially, without any interaction from the CS
computers.
3. Testing the entire DGPS Station including the CS
software testing. Note that failures at this stage of
testing would isolate the problem to the interaction of
the CS with the RS and IM or to the CS software.
Ryan, S., F. Forbes, and S. Wee, (1997). Avoiding the
Rocks – The Canadian Coast Guard Differential GPS
System, Proceedings of the International Symposium
on Kinematic Systems in Geodesy, Geomatics and
Navigation. Department of Geomatics Engineering,
The University of Calgary. pp. 367-375.
Spilker, J.J.Jr., (1996). GPS Navigation Message, Global
Positioning System: Theory and Applications,
Volume I, American Institute of Aeronautics and
Astronautics, Inc., pp. 121-176.
Year 2000 & Week Rollover Compliance Test Description
– DGPS, United States Coast Guard.
The following Internet site was used as a reference for
information provided in this paper:
GPS Receiver Boundary Rollover Test Plan.
http://www.laafb.af.mil/SMC/CZ/homepage/y2000/
rollover.htm, September 28, 1998.
The above procedure is included to illustrate how to
easily identify problems in complex systems. Although
testing of the entire system at once may save time, this
must be balanced with the ability to confidently detect
failures within various components of the system.
CONCLUSIONS
The test plan described herein is being used to test the
Canadian Coast Guard DGPS Stations and a variety of
marine receivers used on CCG vessels.
REFERENCES
Cannon, M.E., (1998). ENGO 561 – Satellite Positioning,
Department of Geomatics Engineering, The
University of Calgary.
Global Positioning System Standard Positioning Service
Signal Specification, (1995).
ICD-GPS-200, (1993) Arinc Research Corporation, USA.
Marine Technical and Support Services Directorate
DGPS Specification - Annex A. Canadian Coast
Guard. August 2, 1995.
Proceedings of ION NTM 99, San Diego, CA, January 25-27, 1999 (pp. 891-899)
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