ship gps multipath detection experiments

Annual Meeting, The Institute of Navigation, Albuquerque, N.M., June 23-25, 2003

Ship GPS Multipath Detection Experiments

Gérard Lachapelle, Olivier Julien, Glenn MacGougan, M. Elizabeth Cannon Department of Geomatics Engineering, University of Calgary

Sam Ryan Canadian Coast Guard

BIOGRAPHIES

Dr. Gérard Lachapelle holds a CRC/iCORE Chair in Wireless Location in the Department of Geomatics Engineering. He has been involved with GPS developments and applications since 1980 and has authored/co-authored numerous related publications and software. More information is available on www.geomatics.ucalgary.ca/faculty/lachap/lachap.html

Olivier Julien is a Ph.D. candidate at the Department of Geomatics Engineering of the University of Calgary, Canada, where he is a member of the Position, Location and Navigation research group. In 2001, he graduated from ENAC (French University for Civil Aviation), Toulouse, as an electrical engineer, majoring in signal processing. His research includes GPS/GALILEO interoperability as well as GNSS receiver design.

Glenn MacGougan is a research associate in the Positioning Location and Navigation Research Group at the Department of Geomatics Engineering, University of Calgary. He completed a BSc. And an MScin Geomatics Engineering at the University of Calgary in 2000 and 2003, respectively.

Dr. M. Elizabeth 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 positioning. She is a Past President of the ION and winner of the 2001 Johannes Kepler Award from the ION Satellite Division.

Sam Ryan holds a B.Eng. in Electrical Engineering (1992) from Memorial University of Newfoundland and a Ph.D. in Geomatics Engineering (2002) from the University of Calgary. He is the Manager of Engineering and Maintenance Services for Electronic systems in the Integrated Technical Support Directorate of the Canadian Coast Guard.

ABSTRACT

Ship multipath caused by the surrounding ship super-structure and water is a significant error source that can severely limit the reliability of GPS-derived navigation solutions. This is especially important given the low reliability of many current standard marine receivers (MacGougan and Liu, 2002).

The magnitude of code multipath aboard a Canadian Coast Guard vessel was assessed using a series of on-board measurements with three receivers using different levels of correlator technology and two different antennas. The antennas were successively located on the upper mast of the ship. The three receivers tested consisted of a "standard" marine receiver, a high quality receiver set to use wide correlator methods and a high grade receiver using an advanced correlator technology. A fixed base station with known coordinates was used to accurately determine the reference position of the ship during the tests using differential carrier-phase measurements. Then, a residual analysis from a single differenced position-constrained least-squares solution was performed in order to isolate pseudorange error whose main component is multipath. The data was collected over several days while the ship was in port in a static position. This enabled the detection and analysis of repeated day-to-day multipath. Two kinematic experiments were also conducted to study the differences with the static case.


INTRODUCTION

The Canadian Coast Guard (CCG) is concerned with the reliability of DGPS-based navigation for its fleet. Some operations such as buoy tendering especially require a high level of accuracy and reliability. The main concern is the presence of multipath induced errors. Multipath is caused by reflected signals entering the RF front end of fa receiver and mixing with the direct signal. The sources of reflected signal on a vessel are multiple and can come from surrounding water, the vessel structure, buildings on the shore, etc. As a result, the location of the antenna on the ship is critical. In addition, the choice of multipath mitigation technologies should not be overlooked.

Multipath mitigation can be performed at the antenna level, as well as at the receiver level. This paper presents the result of a study of multipath impact on both measurements and positions. First, the detailed field experiments are described, followed by the analysis of static and kinematic tests. A frequency domain analysis of is then performed. Three different receivers were used, namely a NovAtel OEM4 set to operate in wide correlator mode (referred to as WC), a NovAtel OEM4 operating in the Pulse Aperture Correlation™ (PAC) mode, and a standard marine grade receiver, the Trimble NT300D (referred to as NT300D). A comparison of the shape of the multipath envelopes associated with each correlation technology is shown in Figure 1.

In addition, two different antennas, namely the NovAtel 501 600 models, were used to demonstrate the impact of different antenna technologies. The right hand circular polarization (RHCP) and left hand circular polarization, (LHCP) gain patterns for the 501 and 600 antennas are shown in Figure 2. The 501 has more gain at elevation angles less than 15º and below the horizon when compared to the 600 model, making it more suitable for marine applications where roll and pitch requires the need for such a gain for continuous tracking of low elevation satellites. However, the 600 model is well suited to mitigate reflected GPS signals at low elevation angles. This is especially evident when comparing the LHCP gain patterns for the two antennas. In fact, 600 model typically has 10 dB more attenuation for LHCP signals. This likely translates into 10 dB more multipath attenuation since reflected GPS signals are often LCHP. This trade off between maintaining low elevation signal tracking in heavy seas or mitigating the associated multipath signals must be considered in the design and implementation of a marine navigation system.

FIELD EXPERIMENT

The tests took place in Trois-Rivières, Québec, onboard the CCGS George R. Pearkes (shown in Figure 3) during February 2003. Two sets of equipment were installed: one on the ship and one at a nearby CCG DPGS station that was used as a reference station.

Three receivers were installed according to the set up shown in Figure 4: one NovAtel OEM4 receiver with PAC configuration (NovAtel OEM4 PAC), one NovAtel OEM4 receiver with Wide Correlator configuration (NovAtel OEM4 WC) and one Trimble NT300D marine grade receiver (Trimble NT300D).

Figure 1 – Comparison of Multipath Envelopes Using Different Correlation Technologies (NovAtel, 2003[1])

Figure 2 – NovAtel 501 (NovAtel, 2003[2]) and 600 (NovAtel, 2003[3]) Antenna Gain Patterns for L1 on the left and right side respectively (Note the differences in scale)

The NovAtel receivers perform carrier phase smoothing on their raw pseudorange measurements while the Trimble receiver does not. This will be reflected by higher noise on the pseudorange measurements in the comparative analysis. This will also reduce some of the multipath effects for the NovAtel receivers.

The Trimble NT300D is a 12 channel L1-only receiver. However, when configured to output raw data, the user has access only to eight satellite measurements maximum. This does not have a significant impact on the test metrics


used to assess multipath. However, in position domain analysis the limitation of using only eight satellites can adversely affects geometry at a few epochs and is reflected by poorer positioning accuracy.

One-Hz data was obtained with all the receivers for 2 to 3 days with each antenna successively using receivers operating in parallel as shown in Figure 4. Concurrent reference station data was collected using a NovAtel OEM4 receiver. Antenna location is shown in Figure 4 and is representative of an operational situation. Static data was available using both antennas for successive sidereal days with the same position and about 5 to 6 hours of overlapping data with repeated GPS satellite geometry that were used for analysis. Note that testing with different antennas occurred on different days and that the position of the ship changed between tests with different antennas. Thus, comparison between static datasets with different antennas does not refer to data collected with exactly similar satellite and reflection geometry.

A kinematic test performed with the NovAtel 501 antenna lasted approximately 1.7 hours. The kinematic data performed with the 600 Antenna lasted approximately 1.4 hours.

The reference station was located at a DGPS station situated in Sainte-Marthe-du-Cap, QC. The distance between the reference station and the ship was approximately 9 km in static mode and reached 30 and 10 km during kinematic testing with the 501 and 600 antenna, respectively. The kinematic trajecctories are shown in Figure 5. The static and kinematic periods selected for the analysis are described in Table 1.

Figure 3 - The CCGS George R. Pearkes Arriving at Trois-Rivières, Québec

The differential positions of the test antennas over the course of the static and kinematic tests were also determined using batch carrier phase differential methods using the DGPS station as a reference station and are

accurate to better than 50 cm, which is sufficient for code multipath detection.

Figure 4 - Equipment Set Up on the CCGS George R. Pearkes and Location of the GPS Antenna on the Mast

Figure 5 – Kinematic Test Trajectories Using the NovAtel 501 and NovAtel 600 Antennas

Table 1 – Kinematic Tests - Periods User for Analysis

Dataset

Start GPS Time of Week

(s)

End GPS Time of Week

(s)

Duration (hours)

Day1 with 501 Antenna 339236 357236 5 Day2 with 501 Antenna 425400 443400 5 Day1 with 600 Antenna 500400 518400 5 Day2 with 600 Antenna 586563 604563 5 501 Antenna 415800 422100 1.7 600 Antenna 91190 96250 1.4


METHODOLOGY

Measurement Domain Analysis

To determine the extent to which multipath degrades the pseudorange measurements at the test location, the receiver’s raw data was post-processed using parametric least squares with the test position fixed to its known value based on the carrier phase differential solution. In addition, single difference corrections from the base station receiver were used to effectively eliminate spatially correlated errors. The residuals of this solution thus provide a very good epoch-by-epoch measure of the errors left in the measurements, specifically multipath and noise. This test statistic will be referred to as EPE for Estimated Pseudorange Error. This was accomplished using C3NavG2™, a software package developed at the University of Calgary. Carrier-to-noise density ratio (C/N0) is the best measurable value of the signal quality present at the input of a GPS receiver. C/N0 is an instantaneous measure of the ratio of carrier power present to noise power density. The nominal noise floor is front-end dependant, but a typical value for the spectral density is -204 dBW/Hz. With a minimum guaranteed line-of-sight GPS signal power of -160 dBW, the nominal C/N0 level is 44 dB-Hz. Theoretically the C/N0 is independent of the receiver used; however, each receiver must compute its value based on the measured signal. For this reason, the reported C/N0 values for different receivers are sometimes biased. Since the same antenna is used for all receivers under test, and the line losses to each receiver are very similar, the C/N0 conditions for each receiver are nearly identical. Variation in C/N0 often reflects interference due to multipath. In addition, the local shadowing effects due to the surrounding environment, namely in this case the ship’s mast, are reflected in the C/N0 measurements.

Position Domain Analysis

Position solutions were computed using the raw pseudorange measurements from each receiver. Three positioning methods were used to demonstrate the impact of the pseudorange measurement degradation and to show more optimal position solutions.

Firstly, the epoch-by-epoch raw positions are computed using all available measurements above 5° in elevation and no measurement error detection and rejection (EDR) scheme implemented. This is referred to as the “Raw 3D Solution”.

Secondly, a position solution using a 5° elevation mask, a PDOP mask of 5.0 and a HDOP mask of 2.5, and with an EDR scheme based on statistical testing of the least squares residuals, is used to compute a position solution. The EDR method used is discussed in (MacGougan and Liu, 2002). This type of solution will hence be referred to as the “3D Position Solution”.

Lastly, since the height of the test location is known (this is very reasonable for the marine case since height is often well known), a height constrained position solution was computed utilizing the same elevation and DOP masks and EDR as the 3D Position Solution case. Note that this is not a completely height fixed solution as the height is constrained with a variance of 2 m2. This constraint offers a greater redundancy resulting in more robust fault detection as well as improved geometry. This is referred to as the “Height Constrained Solution”.

The EDR method used is simply a 3.28 σ test of the standardized residuals of the least squares solution. If a residual fails the test, the measurement is rejected and the solution is recomputed.

Analysis Procedure

Concerning the static tests, consecutive sidereal days are presented as the GPS constellation repeats every 24 hours. Multipath is highly correlated with satellite geometry and the local reflection environment and is thus repeatable as long as the antenna position and the position of the reflector sources between consecutive days do not vary significantly as in the case of these tests. Both measurement and position domain results are presented for the 5-hour datasets described previously.

The figures shown in the following analysis are a representative subset of the complete analysis performed.

RESULTS FOR STATIC TESTS

Measurements Domain

The errors observed were clearly repeatable on a day-by-day basis. As this characteristic is particular to multipath for static tests, the dominant errors were due to multipath. In addition, the behaviour of the EPE values in time was also consistent between receivers and further verifies the presence of multipath. As a representative example of the performance of each receiver in time, the EPE and C/N0 values for each receiver along with the satellite azimuth and elevation for SV 15 using the 501 antenna are shown in Figure 6 for one of the test days.

The Trimble NT300D clearly used some form of correlator technology superior to wide correlator technology and it is likely a Narrow Correlator™ technique. PAC technology outperformed the above with a RMS EPE value of 0.2 to 0.8 m better than the Trimble receiver for either antenna. However, this value slightly over-estimates the difference between both receivers due to the carrier phase smoothing of the OEM4 PAC pseudoranges.

Some satellite measurements were clearly affected by signal shadowing due to the ship’s mast. During all static tests, the ship’s mast and associated accessory hardware (see Figure 4) were located at an azimuth of about 220°.


This is reflected in the C/N0 measurements as well as in the EPE values at around 21:53 local time. When the satellite elevation crosses an azimuth of 220°, a significant decrease in the C/N0 of about 15 dB occurred. A local increase in the EPE magnitude, which is receiver dependent, also occurred.

The improvement in the EPE statistics of the 600 antenna over the 501 antenna is significant and apparent for all EPE error distributions. It is reflected by a greater number of measurements with small residual values as well as a lower occurrence of large residuals. This phenomenon is shown in Figure 7 where the histograms of the EPE values binned into 15° elevation groups for the NovAtel OEM4 WC receiver are given. N represents the number of samples used for each elevation group.

The relative performances of the receiver/antenna pairs are also well described by the analysis of the maximum EPE values. They reach 48 m, 18 m, and 35 m for the WC, PAC, and NT300D receivers, respectively, during testing with the 501 antenna. The corresponding values with the 600 antenna are 40 m, 21 m, and 20 m. This underlines the impact of both receiver technology and antenna gain on multipath.

The cumulative distributions deduced from the histograms shown offer an alternative way to see the relative performance of the receivers and antennas. The 95 percentile values derived from these cumulative distributions are shown in Table 2. They give a very good overview of the relative receiver and antenna performance.

Figure 6 – EPE Values and C/N0 for each Receiver along with Azimuth and Elevation Using the 501 Antenna for SV 15

Figure 7 – EPE Histogram for the NovAtel OEM4 WC Receiver during Using the 501 (Top) and 600 Antenna (Bottom)

Table 2 – 95 Percentile EPE Cumulative Distributions for the 501 (Green) and 600 (Blue) Antenna Testing for the NovAtel OEM4 WC, NovAtel OEM4 PAC and Trimble NT300D Elevation

(°) 00-15°

(m) 15-30°

(m) 30-45°

(m) 45-60°

(m) 60-75°

(m) 75-90°

(m) WC 10 8 6 4 6 6 PAC 6 3 2 2 1 1 NT300D 7 5 4 3 3 2 WC 8 5 4 3 4 3 PAC 5 2 2 1 1 1 NT300D 5 4 3 2 2 2 Position Domain

Figures showing the horizontal and vertical error distributions of each positioning method for the static tests during one of the test days associated with the 501 antenna testing and one of the test days associated with the 600 antenna testing are shown in Appendix A. It is important to remember that these days are not the same days of the week, as shown in Table 1. Figures 8, 9, and 10 show time series data including horizontal position errors, HDOP, vertical position errors, VDOP, number of satellites used and the number of satellites rejected for the


Raw 3D Solution, the 3D Solution, and the Height Constrained Solution, respectively, for one of the tests with the 501 antenna.

The Raw 3D Solutions are clearly corrupted by multipath induced errors in the case of the WC results, with maximum horizontal errors exceeding 40 m in some cases. The NT300D and PAC receivers are much less affected by multipath error. The PAC provides the best level of performance. This can be shown through the RMS and absolute maximum errorrs for both the horizontal and vertical case, as well as through the 50th and 95th percentiles given in Table 3 and Table 4 obtained from cumulative error distributions. The use of the 600 antenna versus the 501 antenna limits the amplitude and the number of the error peaks for both the vertical and horizontal positioning results. This is reflected by smaller RMS and 95th percentile errors values for both horizontal and vertical errors.

The 3D Solutions for static testing did not statistically improve positioning accuracy compared with the Raw 3D Solutions. The rejection of multipath reflects the previous analysis of EPE as the WC unit has many rejected measurements, while the PAC unit rejects the fewest measurements and the NT300D performs somewhere in the middle. The rejection of bad satellites measurements results in the reduction of error peaks in most of the cases. However, the rejection of satellites sometimes results in an increase of the HDOP and VDOP values, degrading the position accuracy. This phenomenon is visible when comparing HDOP and VDOP values between Figures 8 and 9. Moreover, it happens that several measurements are poor at the same epoch. In such a case, if only one measurement is rejected when a low number of measurements is available, the final position computed will still use noisy or corrupted measurements but with a degraded geometry. That phenomenon happens at 21:50 local time in Figure 9 for the WC, PAC and NT300D receivers (8 satellites available, 1 measurement affected by mast shadowing and 3 very low satellites). It is also the main reason why the statistics are not significantly improved between the Raw 3D and the 3D Solution cases, especially for the WC receiver. It is important to mention that due to the availability of only a maximum of 8 satellites for the NT300D receiver, the rejection of one or two satellites can have a significant impact on the geometry and therefore on the position accuracy.

The Height Constrained Solutions improve horizontal positioning accuracy and reliability, as more redundancy is available for detection of measurement faults. This results in the rejection of more corrupted measurements, and in an improved geometry in terms of HDOP, easily identified in Figure 10. Its consequence is the mitigation of most of the peak error values present in the Raw 3D and 3D Solutions. As a result, the statistics for horizontal errors are improved, as shown in Table 3. This can be

best seen in the distributions shown in Appendix A. The 95th percentile is however corrupted by the localized bad positioning accuracy at 21:50 local time as already mentioned. The improvement when using a height constraint is greater when the 600 antenna is used as the number of corrupted measurement is smaller than when using the 500 antenna.

It is important to note that the improvement brought by the 600 antenna over the 501 antenna is consistent and significant for all three processing methods, as shown in Tables 3 and 4.

Table 3 – RMS Horizontal Errors and 50th and 95th Percentile Values from Cumulative Horizontal Error Distributions using the 501 (Green) and 600 Antenna (Blue) during Static Testing

Solution Type

Receiver 50th percentile

(m)

RMS (m)

95th percent

ile (m)

|Max| (m)

WC 2.6 3.7 6.9 39.4 NT300D 1.8 2.5 4.7 10.1 PAC 1.1 1.6 3.2 8.9 WC 2.3 3.6 7.2 20.0 NT300D 1.6 2.2 4.1 10.2 R

aw 3

D

PAC 1.0 1.4 2.8 9.0 WC 2.7 3.7 7.0 29.7 NT300D 1.9 2.7 4.9 21.0 PAC 1.1 1.6 3.3 8.9 WC 2.3 3.5 7.1 26.2 NT300D 1.6 2.3 4.2 16.2 3D

Sol

utio

n

PAC 1.0 1.5 3.0 7.1 WC 2.6 3.6 6.8 21.1 NT300D 1.8 2.6 4.9 15.6 PAC 1.1 1.3 3.2 8.1 WC 2.2 3.3 6.4 25.9 NT300D 1.6 2.3 4.2 11.7

Hei

ght

Con

stra

ined

So

lutio

n

PAC 1.0 1.5 3.0 6.9

Table 4 – RMS Vertical Errors and the 50th and 95th Percentile Values from Cumulative Vertical Error Distributions using the 501 (Green) and 600 Antenna (Blue) during Static Testing

Solution Type


(m)

RMS (m)

95th percent

ile (m)

|Max| (m)

WC 3.0 5.1 10.0 68.9 NT300D 2.1 3.3 6.4 17.9 PAC 1.3 2.3 4.6 18.1 WC 2.4 3.9 7.5 22.6 NT300D 1.6 2.6 5.1 16.9 R

aw 3

D


Sol

utio

n

PAC 1.1 1.8 3.6 8.4


Figure 8 – Raw 3D Solution Using the 501 Antenna

Figure 9 – 3D Solution with Measurement Error Detection and Rejection, 5° Elevation Mask, PDOP Mask of 5.0, and HDOP Mask of 2.5 Using the 501 Antenna

Figure 10 – Height Constrained Solution with Measurement Error Detection and Rejection, 5° Elevation Mask, and a HDOP mask of 2.5 Using the 501 Antenna

RESULTS FOR KINEMATIC TESTS

Measurement Domain

The kinematic testing for each of the two antennas was performed on different days with different test trajectories including different levels of dynamics (see Figures 5 and 11) and different satellite geometry. As a consequence, a performance comparison between the tests is not totally consistent. However a cursory analysis of the results provides an interesting insight.

Figures 12 and 13 show the EPE, C/N0, and azimuth and elevation values for a representative satellite with the 501 and 600 antenna, respectively.

As shown in Table 5, the maximum EPE values are significantly greater when using the 501 antenna. Even if the satellite geometry is different, the use of the 600 antenna corresponds to lower peak error values, as it was already shown in the static testing. In general, the magnitude of multipath is smaller during the kinematic testing than during static testing, as anticipated, due to lower low frequency effects.


Figure 11 – Vessel Speeds During Kinematic Tests Using the 501 and the 600 Antennas

Table 5 – Maximum EPE Values -Kinematic Tests NovAtel

OEM4 WC NovAtel

OEM4 PAC Trimble NT300D

501 Antenna 39 m 20 m 18 m 600 Antenna 18 m 13 m 14 m The relative performance between receivers for each kinematic test is clear. The PAC unit has the best performance, followed by the NT300D, and the WC units, although the performance difference between the NT300D and the WC unit is small in terms of RMS error. This phenomenon is mainly due to the carrier phase smoothing of the WC pseudorange measurements. Multipath is more noise-like when in kinematic mode, as the environment is changing more quickly. Noise and multipath are effectively reduced through carrier phase smoothing. The latter also reduces the RMS error values for the PAC and WC receivers. This is visible when examining the EPE values in Figure 13. However, the WC still has more frequent large multipath errors than the other receivers tested.

The impact of the mast shadowing on the C/N0 is once again clear in Figures 12 and 13 when the azimuth of the satellite considered reaches approximately 220°. The EPE cumulative distributions binned by 15° elevation groups provide another metric to assess the receiver/antenna pair performance. Table 6 shows the 95th percentile values from the cumulative distributions for both 501 and 600 antenna. The earlier hierarchy in terms of receiver performance is maintained.

Figure 12 – EPE Values and C/N0 using the 501 Antenna for PRN10 - Kinematic Test

Figure 13 – EPE Values and C/N0 using the 600 Antenna for PRN14 - Kinematic Test

Table 6 – 95th percentile Cumulative Distributions for the 501 (Green) and 600 (Blue) Antenna - Kinematic Testing Elevation (°)

00-15° (m)

15-30° (m)

30-45° (m)

45-60° (m)

60-75° (m)

75-90° (m)

WC 6 4 4 4 3 5 PAC 6 3 2 2 1 1 NT300D 5 4 3 3 2 2 WC 8 5 3 2 3 2 PAC 7 2 2 1 1 1 NT300D 6 4 3 2 2 2


Position Domain

Figures showing the horizontal and vertical error distributions of each positioning processing method for the kinematic tests with the 501 and 600 antenna are shown in Appendix B. Figures 14, 15, and 16 show time series data including horizontal position errors, HDOP, vertical position errors, VDOP, number of satellites used and the number of satellites rejected for the Raw 3D Solution, the 3D Solution, and the Height Constrained Solution respectively for testing with the 501 antenna.

The RMS and absolute maximum values for both horizontal and vertical errors, as well as the 50th and 95th percentiles, are given in Table 7 and Table 8, respectively, as obtained from cumulative error distributions.

The Raw 3D Solutions, once again, show the corruption of the final position by multipath. Either with the 501 or the 600 antenna, the WC unit shows a comparatively high number of peak multipath induced position errors. However, these peaks have a smaller magnitude than in the case of the static mode. The performance of the PAC is the best as can be seen through the lower RMS values shown in Table 7.

The 3D Solutions reduces the number of high error peak values of the three receivers for both kinematic test cases. However, high error peaks can be noted for the WC receiver for the same reasons as mentioned for the static case. In certain circumstances, the geometry is degraded when measurements are removed as reflected by the corresponding HDOP and VDOP values. Even if this problem concerns only a few epochs, it is certainly a drawback of WC receivers mainly due to the rejection of more measurements. The NT300D and PAC receivers have some similar behaviour but on a smaller scale, because the number of rejected satellites is lower. The PAC receiver shows the best position accuracy for this type of 3D positioning using EDR. The impact of EDR is visually more obvious when looking at the horizontal and vertical error distributions shown in Appendix B.

As in the case of the static mode, the Height Constrained Solutions provide better horizontal positioning accuracy and reliability as more redundancy is available for detection of measurement errors and the geometry improves in terms of HDOP. As a consequence, the peak error values are further reduced, as well as the number of their occurrences. This can also be seen in the plots in Appendix B. A direct comparison between the kinematic tests with the two antennas is not statistically robust as discussed in the measurement domain section. However, the relative improvement brought by the height constraint is greater when using the 600 antenna.

Table 7 – RMS Horizontal Errors and 50th and 95th Percentile Values from the Cumulative Horizontal Error Distributions Using the 501 (Green) and 600 Antenna (Blue) - Kinematic Tests

Solution Type


(m)

RMS (m)

95th percent

ile (m)

|Max| (m)

WC 1.5 2.2 4.1 11.8 NT300D 1.7 2.3 4.1 10.0 PAC 1.0 1.5 2.7 7.4 WC 1.8 2.5 4.7 9.7 NT300D 1.8 2.5 4.9 11.0 R

aw 3

D


Sol

utio

n

PAC 1.3 1.9 3.9 7.8 WC 1.5 2.1 3.9 9.4 NT300D 1.7 2.3 4.0 15.7 PAC 1.1 1.5 2.7 5.8 WC 1.9 2.7 5.1 11.1 NT300D 1.8 2.5 4.7 10.7

Hei

ght

Con

stra

ined

So

lutio

n

PAC 1.3 2.0 4.1 7.4

Table 8 – RMS Vertical Errors and 50th and 95th Percentile Values from the Cumulative Vertical Error Distributions Using the 501 (Green) and 600 Antenna (Blue) - Kinematic Tests

Solution Type


(m)

RMS (m)

95th percent

ile (m)

|Max| (m)

WC 2.1 3.4 6.9 16.4 NT300D 1.8 2.9 5.5 12.4 PAC 1.2 1.7 3.4 10.6 WC 2.0 3.1 6.3 18.2 NT300D 2.1 3.3 6.7 14.3 R

aw 3

D


Sol

utio

n

PAC 1.4 1.9 4.6 12.1


Figure 14 – Raw 3D Solution Using the 501 Antenna –

Kinematic Test

Figure 15 – 3D Solution with EDR, 5° Elevation Mask, PDOP Mask of 5.0, and HDOP Mask of 2.5 Using the 501 Antenna

Figure 16 – Height Constrained Solution with EDR, 5° Elevation Mask, and a HDOP mask of 2.5 Using the 501 Antenna

FREQUENCY ANALYSIS OF STATIC AND KINEMATIC MULTIPATH

To compare the spectral characteristics of multipath between static and kinematic modes, the spectral characteristics of the EPE values calculated between consecutive days were computed. Only the NT300D data was analyzed because it provides raw pseudorange observations that are not carrier phase smoothed and analysis of only one receiver is sufficient to demonstrate the spectral differences between the kinematic and static modes. The power spectrums of each satellite’s EPE values for both the static and kinematic cases were studied. Furthermore, the distributions of the satellite’s EPE spectral power with respect to frequency for both cases were analyzed. Representative power spectrum plots and distributions are given in Figure 17 and 18 for PRN13 when using the 500 antenna.

A comparison between the kinematic and static modes indicates clearly that, in static mode, the frequencies of multipath errors are primarily (90%) less than or equal to 0.05 Hz. Whereas, for the kinematic case only 60-75% of the multipath errors are less than or equal to 0.05 Hz and higher frequency effects are prevalent. This slower decrease of the power spectrum with the frequency for the kinematic case is visible in Figure 17.


Figure 17 – PRN13 EPE Power Spectrum for the Trimble NT300D Receiver Using the 501 Antenna

Figure 18 – PRN13 EPE Power Spectrum Distribution With Respect to Frequency for the Trimble NT300D Using the 501 Antenna

Similar conclusions can be drawn for the 600 antenna frequency analysis. However, the multipath errors are of lower frequencies than the corresponding values with the 501 antenna. When using the 600 antenna, 75-85% of the multipath errors are less than or equal to 0.05 Hz and 60-75% when using the 501 antenna. The frequency behaviour for the EPE values observed for the 600 antenna test indicated dominant low frequency effects in comparison with the 501 antenna. This could be due to the dominance of local reflection sources on the ship when testing with the 600 antenna. However, the ship was generally going at lower speeds during the 600 antenna tests and thus multipath decorrelates more slowly and multipath errors have lower frequencies.

CONCLUSIONS

The experimental results described herein show that multipath is a major source of concern for marine applications where a high level of accuracy and reliability performance is required. Limited choices for antenna installation, combined with the presence of numerous ship reflecting surfaces are the major cause of this error source. Direct signal obstruction caused by ship masts is another major error source. The correlation technology used was shown to be very important to minimize multipath. However, the majority of “marine” receivers are still using the wide correlator (WC) technology, which is shown to deliver the poorest level of performance.

The use of an epoch-by-epoch least-squares approach is ideal for a sensitivity analysis. Under operational conditions however, a Kalman filter, whose parameters would depend on ship specific behaviour, would improve error detection substantially, without such an adverse effect on accuracy as seen using an unconstrained least-squares approach. This would be ideal for a ship underway. However, in the case of a buoy tendering ship or a dredging vessel where an attempt is made to keep the platform from moving, the filter would be of more limited use and concerns about multipath would still be valid. Other techniques to augment the single antenna system on the ship would likely overcome the problem, such as augmentation with a self-contained sensor or the use of a dual-receiver/antenna system. REFERENCES Lachapelle G., G. MacGougan, and L. K. Siu (2002), CCG Shipboard Multipath Assessment, HMS Martha Black, Trois-Rivieres, Quebec, Project Report for the Canadian Coast Guard.

MacGougan, G. and J. Liu (2002) Fault Detection Methods and Testing. Proceedings of ION GPS 2002, pp. 2668-2678.

NovAtel, 2003[1]. OEM4 Family of Receivers, User Manual – Volume 1, Installation and Operation. [http://www.novatel.ca/Products/productmanuals.html] NovAtel, 2003[2], NovAtel L1 GPSAtenna™ Model 501 Antenna. [http://www.novatel.ca/Products/productmanuals.html] NovAtel, 2003[3], NovAtel L1 GPSAtenna™ Model 600 Antenna. [http://www.novatel.ca/Products/productmanuals.html]


APPENDIX A – Horizontal and Vertical Error Distributions - Static Tests

Figure A-1 – Horizontal Error Distribution for 3D Raw solutions, 3D Solutions and Height Constrained Solutions Using the 501 and 600 Antenna for the WC, PAC and NT300D Receivers – Day 2

Figure A-2 – Vertical Error Distribution for 3D Raw Solutions, 3D Solutions Using the 501 and 600 Antenna for the WC, PAC and NT300D Receivers – Day 2


APPENDIX B – Horizontal and Vertical Error Distributions - Kinematic Tests

Figure B-1 – Horizontal Error Distribution for 3D Raw Solutions, 3D Solutions and Height Constrained Solutions Using the 501 and 600 Antenna, Kinematic Tests, for the WC, PAC and NT300D Receivers

Figure B-2 – Vertical Error Distribution for 3D Raw

Solutions, 3D Solutions Using the 501 and 600 Antenna, Kinematic Tests, for the WC, PAC and

NT300D Receivers

ship gps multipath detection experiments

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