Methodology for Determining Compatibility of GPS L5 With ...



Post-Modernization GPS Performance Capabilities

Keith D. McDonald

Sat Tech Systems, Inc., Alexandria, VA

, VA

Christopher Hegarty

The MITRE Corporation, McLean, VA

ver. k

BIOGRAPHIES

Keith McDonald is the President of Sat Tech Systems and Technical Director of Navtech Seminars. He was Scientific Director of the DoD Navigation Satellite Program during the formative stages of the Navstar GPS program. Later, with the FAA, he directed the Aeronautical Satellite Division and managed the satellite applications and technology program. He has also been active in RTCA, preparing guidelines for using satellite systems in aviation, and he received the 1989 RTCA Citation for Outstanding Service. Mr. McDonald also received the 1988 Institute of Navigation's Norman P. Hayes Award for outstanding contributions to the advancement of navigation. He served as President of the ION in 1990-91 and President of the International Association of Institutes of Navigation during 1997-2000.

Dr. Christopher Hegarty received his B.S. and M.S. from Worcester Polytechnic Institute, and his D. Sc. from The George Washington University. He has been with The MITRE Corporation since 1992, most recently as a Project Team Manager. In August 1999, he began a one-year assignment as Civil GPS Modernization Project Lead with the FAA through the Intergovernmental Personnel Act. He was a recipient of the 1998 ION Early Achievement Award, and currently serves as Editor of Navigation: Journal of the Institute of Navigation and as Co-chair of RTCA SC159 Working Group 1 addressing theaddressing the 3nd3rd Civil GPS Frequency signal structure.

ABSTRACT

For nearly a decade, recommendations for the moderniza-tion of GPS have been put forth by various panels, committees, organizations and individuals. At this time, the definition of the principal elements and characteristics of the Modernization modernization program is nearing completion. Institutional and funding arrangements for implementation of the modernization initiatives also appear to be on track.

It is now possible and it appears appropriate to address in some detail the performance of GPS as it evolves from its current state into basic the end-state of present modernization plans., as well as the evolutionary improvements, for the various operational modes of a modernized GPS. Much improved signal observables are planned for both the civil and the defense communities.

This paper attempts to accomplish this goal.

For example, the civil community will have signals at multiple frequencies, increased code rates, improved ephemeris information and more advanced receivers. Increased use of carrier phase measurements and simplified instantaneous resolution of integer cycle ambiguities through multiple frequency processing will provide substantial performance improvements.

Real time users as well as extremely precise post-processing users will see new applications and dramatic improvements in performance capabilities, including accuracy, integrity, availability and continuity. A discussion is given of the expected changes in measurement techniques to be employed by future users for various applications. The physical implications and limitations on performance of the various modernization elements are addressed. The impact of these factors on the capabilities provided to the various categories of GPS users is then evaluated. Realistic estimates of the modernized GPS performance capabilities and their evolution is analyzed, plotted and summarized.

INTRODUCTION

The current GPS modernization program promises to deliver both the civil and military GPS communities numerous improvements to the core GPS services that have already enabled so many positioning, navigation, and timing applications in many unexpected ways. Civil GPS users, now enjoying a more accurate Standard Positioning Service (SPS) since Selective Availability (SA) has been discontinued, have two new civil signals to look forward to. Military users will soon have new signals as well.

Although a great amount of attention has been paid to the modernization program components, few researchers have yet focused on the performance levels that may be expected in the next two decades as the successive stages of the current modernization program are implemented. This paper attempts to address these incremental performance improvements in sufficient detail to reveal dominant components in error budgets. The paper begins by providing a brief overview of the GPS modernization program.

GPS Modernization Overview

An overview of principal activities and planned schedule for the current GPS modernization program is shown in Figure 1 [1][1]. Currently, the Department of Defense (DoD) is awaiting authorization to proceed (ATP) from one of four Congressional committees (the Senate Appropriations Committee [SAC]) that are required to authorize DoD’s recent proposed changes to the GPS program. Figure 1 and the following short GPS program summary (divided into space and control segment discussions) assume SAC approval.

Space Segment

In 1989, Lockheed-Martin (at that time, the General Electric Astro Space Division) was awarded a contract to build 21 “replenishment” GPS satellites (Block IIR). The current GPS program includes retrofitting the last 12 IIR satellites (referred to as “IIR Mod” in Figure 1) to include the capability to broadcast the new military signal on L1 and L2, and also a C/A-code on L2.

satellites (referred to as “IIR Mod” in Figure 1) to include the capability to broadcast the new military signal on L1 and L2, and also a C/A-code on L2.

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Figure 1. GPS Modernization Schedule

satellites (referred to as “IIR Mod” in Figure 1) to include the capability to broadcast the new military signal on L1 and L2, and also a C/A-code on L2.

satellites (referred to as “IIR Mod” in Figure 1) to include the capability to broadcast the new military signal on L1 and L2, and also a C/A-code on L2.

In 1990, Boeing North America was awarded a contract to build up to 33 “follow-on” GPS satellites (Block IIF). The initial contract provided for a purchase of 6 satellites, with options for the remaining 27. In accordance with the current GPS program, the DoD will only exercise options for a total purchase of 12 satellites. A contract change will be negotiated to modify these 12 IIF satellites to include the new military (M-code) signals on L1 and L2 (transmitted by an Earth-coverage antenna), C/A on L2, and the third GPS civil signal at 1176.45 MHz (L5). The modified IIFs are referred to as “IIF lites.”

Beyond the IIR and IIF spacecraft, the current GPS program calls for the procurement of GPS III satellites, which will include all capabilities discussed so far for the Block IIFs and additionally will increase the power levels of the M-code signals to increase their anti-jam capability.

As shown in Figure 1, the nominal schedule would result in an initial operating capability (IOC) for the Earth-coverage M-code and L2 C/A code in 2008 (IOC is defined as 18 operating satellites with the new capabilities). Full operational capability (FOC) for these new signals will nominally occur in 2010 (FOC is defined as 24 satellites). IOC and FOC for L5 will nominally occur in 2012 and 2014, respectively. High-power M-code will reach IOC and FOC in 2016 and 2017, respectively. It should be noted that these nominal IOC and FOC dates are based on specified mean mission durations (MMDs) for the Block IIR and IIF spacecraft. .

specified mean mission durations (MMDs) for the Block IIR and Block IIF’s. As will be discussed later, Aactual IOCs and FOCs may occur much later if experienced MMDs exceed those specified and used in the current planning process. This occurrence is very likely, as has already been experienced with the growth of the MMDs for the Block II/IIA spacecraft.

Control Segment

The GPS operational control segment (OCS) determines the quality of the spacecraft orbital elements and timing data. These are periodically uploaded to the GPS spacecraft memory and then periodically continually transmitted broadcast to the users in the GPS data message. This spacecraft position and other data directly affect user accuracy. Moreover, the data is influenced by the update rate (or latency) of the uploads to the GPS space vehicles (SVs) since the data it degrades with time relative to the true S/Cspace vehicle (SV) position, the data is influenced by the update rate (or latency) of the uploads to the GPS spacecraftSVs. Recent improvements in the OCS have been reported to provide root-mean-square (rms) spacecraft ephemeris accuracysignal-in-space range errors (SISREs) for Precise Positioning Service (PPS) users at the 1-2.5 meter.5-meter level or better.

As shown in Figure 1, the GPS modernization program includes an incremental set of improvements to the OCS, to be led by a single prime contractor under the Single Prime Initiative (SPI). Each incremental step adds a new capability, such as will be necessary to operate each new class of satellite (e.g., IIR-M, IIF, and III).

The planned addition of the six (or more) ground stations of the National Imagery and Mapping Administration Agency (NIMA) to the GPS tracking network will substantially improve the quality and timeliness of the GPS tracking measurements of the Operational Control SystemCS as well as the related computed parameters. More frequent uploads to the GPS spacecraft are also planned. In the 2000-2010 period, it is expected that the near term sub-meter ephemeris accuracy for the GPS tracking network will improve to the decimeter range.

Autonav Operation

The GPS constellation may be required to operate without the GPS ground segment for an extended period. By accurately ranging to other spacecraft, the Block IIR and IIF spacecraft will have the capability to operate in an autonomous navigation (autonav) mode. The autonav ranging data obtained from UHF transmissions between spacecraft provides the GPS spacecraft with continuous on-board information that is used to compute accurate new ephemeris data. This new ephemeris data, by incorporating the measured GPS spacecraft orbital perturbations, can achieve excellent system accuracy over an extended period (several months). The autonav capability will not be fully useful until the GPS constellation consists of spacecraft that are all (or a minimum of 18) equipped with the autonav system.

STAND-ALONE GPS PERFORMANCE EVOLU-TION

Present Standard Positioning Service Performance

Until recently, users of the GPS Standard Positioning Service (SPS) were subject to performance limitations due to Selective Availability (SA). U.S. policy, from June 28, 1983 [2] to May 1, 2000 [3], has been to specify a limitlimitation on GPS accuracy of of 100 meters horizontal (95 percent). As SA was actually implemented, most users experienced a 95 percent horizontal accuracy closer to 670-80 m [4].

¶Although SA could have been realized as a combination of perturbations of the satellite clock (dither) and broadcast satellite positions (epsilon), apparently only dither was normally implemented. Ranging errors due to SA have been well characterized statistically with zero-mean and a root-mean-square (rms) value of 23 m [5], making SA the dominant error source for SPS users. An error budget for SPS with SA is shown in Table 1 (using input parameters from [6, 7]).

Table 1. SPS Horizontal Accuracy Model with Selective Availability

|Parameter |Value (m) |

|Signal-in-space ranging error (rms) |3.1 |

|Residual ionospheric errors (rms) |7.3 |

|Selective availability (rms) |23.0 |

|Residual tropospheric errors (rms) |0.2 |

|User equipment errors due to noise and |0.7 |

|multipath (rms) | |

|TOTAL UERE* (rms) |24.3 |

|Typical HDOP** |1.25 |

|Horizontal Accuracy (95%) |58.373.0 |

*UERE = User Equivalent Range Error

**HDOP = Horizontal Dilution of Precision

SPS Performance since SA Discontinuance

On May 1, 2000, the United States announced that SA would be discontinued and removed it.. With SA discontinued, the dominant error source of the SPS is the residual ionospheric error after application of the single-frequency correction algorithm [4, 8]. The residual errors of the single-frequency correction algorithm have been well characterized in terms of their marginal distributions. The 95 percent ranging error for a satellite directly overhead (vertical residual delay) varies greatly, depending on the total electron content (TEC) along the signal path through the ionosphere, which in turn varies depending on factors including time of day, phase of the roughly 11-year solar cycle, and level of geomagnetic activity [8].

Assuming the residual ionospheric errors are independent from satellite to satellite (an assumption that will be analyzed), a typical horizontal error budget for the SPS is presented in the left- hand “values” column of values in Table 2. Although the resultant 95 percent horizontal accuracy value of 19.1 m compares well with similar tables presented in [4, 7], it does not compare well with the accuracies reported by various organizations since SA has been discontinued. Reported 95 percent values are more accurateless by about a factor of three.

Contrasting the 19.1 m 95 percent horizontal positioning value from Table 2 with the sub-ten-meter 95 percent errors routinely observed by many SPS users in the past few months, it is apparent that treating the residual ionospheric errors as independent from satellite to satellite is not a very good assumption. The fact that 2 × HDOP × UERE is a poor estimate of the 95 percent horizontal positioning accuracy for the SPS without SA (due to correlated residual ionospheric errors) was previously noted in [9-11]. The use of a 3.1 m rms signal-in-space ranging error in Table 2 (from [6]) is also pessimistic since it is based on low-level specifications from [12] that are exceeded in reality.

The right- hand “values” column of Table 2 uses a less pessimistic values for SISRE and HDOP, and assumes that the correlation of the ionospheric errors provides an effective reduction to about 42.0 m. for this component. As shown, this results in a horizontal accuracy of about 6.37 m., a value more consistent with observations.

To further explore the correlation of residual ionospheric errors between satellites, a limited set of data from the National Geodetic Survey (NGS)’s Continuously Operating Reference Station (CORS) system was examined. Figure 3 illustrates the ionospheric contribution to delay error for a representative day (January 1, 1999) at Point Loma, California (each satellite is represented with a different color). During the evening hours, the delay is in the 5-10 m range rising during the daytime hours to the 10-20 m level, with a relatively small number of measurements extending above 20 m. It is unknown as to the extent the receiver’s L1/L2 group delay bias influenced this dual-frequency “truth” source.

Table 2. Typical SPS Horizontal Accuracy Model with Selective Availability Off

|Parameter |Value (m) |

|Signal-in-space ranging error (rms) | 3.1 2.0 |

|Residual ionospheric errors (rms) | 7.3 24.0|

|Selective Availability (rms)Selective | 0.0 0.0 |

|availability (rms) | |

|Residual tropospheric errors (rms) | 0.2 0.2 |

|User equipment errors due to noise and | 0.7 0.7 |

|multipath (rms) | |

|TOTAL UERE (rms) | 8.0 |

| |4.52.9 |

|Typical horizontal DOP | 1.25 |

| |1.42 |

|Horizontal Accuracy (95%) | 19.1 |

| |6.37.0 |

To further explore the correlation of residual ionospheric errors between satellites, a limited set of data from the National Geodetic Survey (NGS)’s Continuously Operating Reference Station (CORS) system was examined. Figure 3 illustrates the ionospheric contribution to delay error for a representative day (January 1, 1999) at Point Loma, California (each satellite is represented with a different color). During the evening hours, the delay is in the 5-10 m range rising during the daytime hours to the 10-20 m level, with a relatively small number of measurements extending above 20 m. It is unknown as to the extent the receiver’s L1/L2 group delay bias influenced this dual-frequency “truth” source.

As shown in Figure 4, using the standard ionospheric model algorithm, the various SV residual error values vary with time after correction. The residual delay errors appear to have a 2( distribution around their mean of about 1-2 m (after considering that most of the dispersion visible in Figure 4 is due to measurement noise of the unsmoothed L1/L2 pseudorange measurements that were used). The high correlation of the gross delays for all SV moderates considerably the overall SV error contribution at any given time.

As described in [10], positioning errors do not arise from rms residual ionospheric delay errors, but rather from the variation of the residual errors around their mean value. As illustrated in Figure 4, this variation typically has a relatively modest value (of 1-2 m), even during the ionosphere’s highly active daylight hours. The net result is a considerably better 95 percent user accuracy than would be predicted from conventional analysis based on the product of the rms ranging error and twice the applicable constellation dilution of precision (DOP). The ionospheric error correlation may also explain the surprisingly good performance (2-6 m) of military single frequency (L1) Precise Lightweight GPS Rreceivers (PLGRs) observed during the past several years.

Based on the above considerations, an accuracy of 107 m (or better) accuracy for the SPS may be valid and reasonable. Again, it is not the gross value of the ionospheric error that degrades accuracy but the variation of the delay errors around a common mean value (the decorrelation effects). Unfortunately, a study that has considered a statistically significant quantity of residual ionospheric error data has yet to be completed. Until this is accomplished, a high-fidelity analytic model for SPS accuracy is not possible.

SPS Performance with the New Civil Signals

Dual-frequency L1/L2 C/A code users, once a sufficient number of L2 C/A-code capable satellites are in orbit, will obtain nearly as good accuracy as current PPS users. There are two reasons why L1/L2 C/A code service will remain less accurate, however. The first is that the C/A-code’s multipath performance is inferior to the P(Y)-code, due to their difference in chipping rate. The second, and not so well known reason, is that C/A-code users will experience signal timing errors due to the fact that GPS time is maintained using L1/L2 P(Y)-code measurements by the OCS.

[pic]Figure 3. Slant Ionospheric Delay Errors Seen at Point Loma, California on January 1, 1999.

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Figure 4. Residual Slant Ionospheric Delay Errors After Ionospheric Model Correction Algorithm

Based on the above considerations, a 10 m (or better) accuracy for the SPS may be valid and reasonable. Again, it is not the gross value of the ionospheric error that degrades accuracy but the variation of the delay errors around a common mean value (the decorrelation effects). Unfortunately, a study that has considered a statistically significant quantity of residual ionospheric error data has yet to be completed. Until this is accomplished, a high-fidelity analytic model for SPS accuracy is not possible.

The group delay between the SPS Performance with the New Civil Signals

Dual-frequency L1/L2 C/A code users, once a sufficient number of L2 C/A-code capable satellites are in orbit, will obtain nearly as good accuracy as current PPS users. There are two reasons why L1/L2 C/A code service will remain less accurate, however. The first is that the C/A-code’s multipath performance is inferior to the P(Y)-code, due to their difference in chipping rate. The second, and not so well-known reason, is that C/A-code users will experience signal timing errors due to the fact that GPS time is maintained using L1/L2 P(Y)-code measurements by the OCS. The group delay between the C/A-code and P(Y)-code on L1 is specified to be less than 10 ns, two-sigma [12]. It is unclear from [12] whether the L2 C/A to P(Y)-code transitions have the same specification, and furthermore whether L1 and L2 C/A-to-P(Y) delays are independent or highly correlated.

Table 3 presents a pair of estimates for the horizontal accuracy of the SPS when the C/A-codes on L2 are available. The left -hand column of values is a conservative estimate based on an SISRE of 3.7 meters [6], a value that is expected to be reduced to 1.5 m. or better in the next few yearswhich is dominated by group delay uncertainties (as described above). The right- hand column is a more optimistic (or realistic) estimate based upon thean improved values for SISRE (assuming measures are taken to control group delay uncertainties) and other error sources, and a more realistic value for HDOP. Considering that the civil code will not be available for use until about 2010, these values are believed reasonable.

Table 3. Typical SPS Horizontal Accuracy Model for L1/L2 C/A-Code

|Parameter |Value (m) |

|Signal-in-space ranging error (rms) | 3.7 1.2 |

|Residual ionospheric errors (rms) | 0.4 0.21 |

|Selective availability (rms) | 0.0 0.0 |

|Residual tropospheric errors (rms) | 0.2 0.2 |

|User equipment/multipath (rms) | 0.7 0.5 |

|TOTAL UERE (rms) | 3.8 1.3 |

|Typical horizontal DOP | 1.25 1.2 |

|Horizontal Accuracy (95%) | 9.1 3.2 |

L1 C/A-L5 and Precise Positioning Service Performance

Precise Positioning Service (PPS) users have never suffered from SA. Dual-frequency PPS users also have the ability to directly measure ionospheric delays. PPS accuracy is approximately equivalent to what is expected for L1 C/A-L5 dual-frequency users (see Table 4). The L5 signals are not to be available until about 2012-2014, and it is assumed that the values in the right hand column of Table 4 are reasonable. These values are also consistent with the reported capabilities of the military L1-L2 P/Y-code receivers today.

DOMINANT ERROR SOURCES

Based on the discussion in the previous section, the following error sources are expected to be of the greatest concern for certain periods within the GPS modernization program:

Table 4. SPS L1 C/A-L5 and PPS Horizontal Accuracy Model

|Parameter | Value (m) |

|Signal-in-space ranging error (rms) | 1.5 0.86 |

|Residual ionospheric errors (rms) | 0.4 0.1 |

|Selective availability (rms) | 0.0 0.0 |

|Residual tropospheric errors (rms) | 0.2 0.2 |

|User equipment/multipath (rms) | 0.7 0.4 |

|TOTAL UERE (rms) | 1.8 0.9 |

|Typical horizontal DOP | 1.25 1.2 |

|Horizontal Accuracy (95%) | 4.3 2.2 |

DOMINANT ERROR SOURCES

Based on the discussion in the previous section, the following error sources are expected to be of the greatest concern for certain periods within the GPS modernization program:

• Current SPS (single-frequency) – Residual errors due to the ionosphere after application of the single-frequency ionospheric correction algorithm are the dominant error source. As pointed out earlier, although the distribution of the residual errors for a single-satellite are well understood, there is still a great deal to learn about the level of correlation of the residual errors between satellites. Only with a statistical characterization of the joint distribution of the residual errors can analytic models be developed that accurately predict actual performance levels.

• Dual-frequency SPS (L1/L2 C/A) – Theoretically, the dominant error source for L1/L2 C/A users will be due to group delay uncertainties (e.g., C/A-to-P(Y), P(Y)-to-GPS time) [6]. These uncertainties result because GPS time is based on dual-frequency P(Y)-code measurements made by the OCS. The C/A code is not at present monitored, and furthermore, even if it were, the GPS navigation message does not include fields for C/A-to-P(Y) biases. Civil GPS users and advocates are encouraged to further study group delay error sources. Possible solutions include: tighter specifications for the group delay uncertainties, and OCS C/A code monitoring combined with use of reserved fields in the navigation message (if there really are any spare bits that are not used by the military) to broadcast corrections.

1.5 0.4 0.0 0.2 0.7 1.7 1.4 4.0

• 0.6 0.30 0 0.0 0.2 0.4 0.651 1 1.4 1.8 The L5 signals are not to be available until about 2012-2014, and it is assumed that the values in the right hand column of Table 4 are reasonable. These values are also consistent with the reported capabilities of the military L1-L2 P/Y-code receivers today.

• Dual-frequency SPS (L1 C/A-L5) or PPS – The dominant error source for these users will likely be due to signal-in-space ranging errors. Given that SISREs for some satellites have already been observed below 80 cm [11], there is an excellent possibility that the 1.5 m value shown in the left hand column of Table 4 may be greatly diminished. The right hand column assumes a diminished value and indicates the improvement in performance.

DIFFERENTIAL USERS

The previous sections focused on stand-alone GPS use, i.e., GPS without any augmentations. In stand-alone GPS applications, the user equipment receives and uses only the signals received from the constellation of spacecraft to determine user position, velocity, time (PVT) and related parameters. This section looks briefly at the implications of modernization on the differential user..

Over short baselines, GPS modernization improvements will hardly affect code differential GPS (DGPS) accuracy

CLASSES OFDIFFERENTIAL USERS

GPS has a number of different modes of operation, each with its own set of performance capabilities. FirstThe previous sections focused on stand-alone GPS use, GPS can be used autonomously (on a stand-alone basis), i.e., GPS without any augmentations. In this caseIn stand-alone GPS applications, the user equipment receives and uses only the signals received from the constellation of spacecraft to determine user position, velocity, time (PVT) and related parameters. This section looks briefly at the implications of GPS modernization on the differential user.

Over short baselines, GPS modernization improvements will hardly affect code differential GPS (DGPS) accuracy at all. Code DGPS, over short baselines, is currently limited in performance by user equipment and multipath errors, not GPS SISRE errors which are mostly common to the DGPS reference station and user. Short-baseline DGPS users will mostly benefit from the robustness provided by the new civil signals, or for military DGPS users, by the anti-jam capability provided by high-powered M-code signals.

Over longer baselines, DGPS users will benefit from the additional civil signals in that they will be able to compensate for ionospheric errors that are not common between their location and the reference station. Wide-area DGPS users will obtain similar benefits from being able to directly measure ionospheric delays. Although it has been stated in [11] that stand-alone GPS SPS without SA can meet all requirements for aviation through non-precision approach, this view is not shared by aviation experts (see, e.g., [13-14]).

Carrier-phase DGPS users interested in achieving centimeter-level accuracies or better through ambiguity resolution will be greatly assisted by the new civil signals, which will improve wide-laning performance over that currently provided using semi-codeless receivers, and enable tri-laning. Benefits will accrue mostly in terms of minimizing the time required for ambiguity resolution (including reacquisition) and maximizing the probability of correct resolution over short spans (e.g., single-epoch), especially over long baselines.

GPS can also be used in a differential mode in which known navigation data at a reference point and time are compared (or differenced) with GPS measured data at the same point and time. The corrections from this process are then applied at the same time to the GPS measured data taken at a remote point. For real-time operation, a data link between the reference receiver and the remote receiver is normally used to communicate the corrections. This process has the great advantage of canceling the fixed, or slowly varying, (bias) errors in measurement that have the same effect at both locations. The differential correction technique, which many other navigation systems have used to improve performance, performs well with GPS.

These differential corrections can be applied directly in real time or they can be stored and employed later using post processing methods. The corrections are usually accomplished at the reference location. These corrections can be based on either the GPS differential code measurements, or for greater precision, on the differential measurements of the GPS carrier phase.

The various modes of operation for GPS user equipment and the corresponding nominal current performance capabilities are summarized in Figure 2. The change in GPS performance characteristics for various GPS stand-alone and differential modes of operation for 2000 and estimates for 2010 are discussed later. Performance accuracy values in position, velocity, time and angle measurement (attitude) are provided.

The new signals that will become available create many more classes of users employing various combinations of civil and military combinations of signal typessignals. These include C/A-, P/Y-, M-, and F/I5-, and Q5-codes and frequencies L1 and L2 for C/A-, P/Y- and M-codes, and the new civil L5 for the F(I,Q)I5- and Q5-codes. The principal user classes will be given later with their performance characteristics.

All civil users will benefit from the robustness against inteference provided by having multiple frequencies available to them. If interference causes the loss of a single frequency, applications may still continue with only a loss in performance, not a loss in service. For example, a usable ionospheric correction is possible using L2/L5, albeit with less accuracy.

PROJECTIONS FOR THE FUTURE

Figure 5 illustrates the performance that can be expected from the various implementations of GPS receivers representing three principal categories of equipment. The top group of performance curves are for the conventional current civil SPS user equipment. This includes receivers that operate as stand-alone or differential receivers at the L1 frequency using only the C/A-code. The minor exception to this is the use of the “L2 carrier phase” capability for primarily static survey receivers. The sharp improvement in accuracy shown in the figure for the stand-alone receivers indicates the removal on May 1, 2000 of the Selective AvailabilitySA degradation from the signals.

The performance values for the stand-alone SPS receiver are in the 7 meter m range at this time and should improve somewhat with the decline of the solar cycle during the first half of the decade. Typical DGPS receivers using the C/A-codes are typically performing at the 1-2 meterm level now. Kinematic receivers using carrier phase measurement methods improve the currently provide accuracy atto the 20-30 cm. range now and clearly will continue to improve. in the future. GPS survey equipment currently provides impressive results in the cm. range and also will continue to improve substantially from this in the future. Both kinematic and static survey equipment have improved their capabilities by more than an order of magnitude in the past decade.

The center group of performance curves indicates the future capabilities of the second civil signals placed at L2 and at L5. The use of two separated frequency signals essentially eliminates the effect of the ionospheric group delay on the receiver. Since this is now the largest error component in the GPS error budget, its removal much improves the performance capabilities of the system, as shown. The higher chipping rate ((X10) of the L5 codes provides a reduction in code noise and better measurement precision. When combined with the use of the excellent precision of carrier phase measurements, the performance easily reaches the cm and mm ranges.

The lower group of curves in Figure 5 represents the expected capabilities of military receivers. Except for the single frequency (L1) PLGR receiver, current military units all use the P/Y-codes on L1 and L2, and the C/A-codes on L1 for acquisition. The new military signals (the M-codes) employ a new split spectrum signal structure that provides improved measurement precision and simultaneously displaces the signal energy away from the C/A-codes and the P/Y-codes. The new M-codes also allow military users direct access to their secure signals, a significant advantage over their current arrangement. This combination of features provides substantial performance improvements for the military signals resulting in the estimated capabilities shown.

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substantial performance improvements for the military signals resulting in the estimated capabilities shown.

In a few cases, it may appear that an optimistic view is being taken for future GPS capabilities. However, in view of the technological advancements that continue to occur in the GPS development area, it is likely that these estimates may, in fact, be conservative. “It is difficult to make predictions, especially into the future”, as Yogi Berra has observed, but the position accuracy estimates

In a few cases, it may appear that an optimistic view is being taken for future GPS capabilities. However, in view of the technological advancements that continue to occur in the GPS development area, it is likely that these estimates may, in fact, be conservative. “It is difficult to make predictions, especially into the future”, as Yogi Berra has observed, but the position accuracy estimates

presented are believed to be generally reasonable and based on a sound rationale.

ERROR CONSIDERATIONS

The space and control segment errors have become small for the more capable receivers, especially those that provide two frequency ionospheric correction and utilize low noise front ends and aiding, or other special processing techniques to enhance their measurement

precision. Also, there are some factors that have not been adequately considered in the past that appear to provide improved performance for even the single frequency, C/A-code civil receivers. This relates to some interesting properties of the ionospheric error.

Tropospheric Errors

It has been demonstrated by a number of investigators that tropospheric errors can be modeled accurately if some basic estimates are available for the observer’s atmospheric pressure, or altitude. For S/C elevation angles of about 15 degrees and greater, the tropospheric refractive error can be normally modeled to account for 90-95% of the gross error, or to reduce the contribution from the troposphere to ten cm or less.

Ionospheric Errors

For single frequency users, several references indicate that a 5-10 m (95%, or 2 sigma) residual iono delay error after applying Klobuchar’s iono correction model, using the coefficients provided in the GPS data message. Figure 3 illustrates the ionospheric contribution to delay error for a representative day (January 1, 1999) at Point Loma, CA. During the evening hours, the delay is in the 5-10 m. range rising during the daytime hours to the 10-20 m. level, with a relatively small number of measurements extending into the 20 -30 m. region.

[pic]

Figure 3. Slant Ionospheric Delay Errors Seen at Point Loma, California on January 1, 1999.

Importantly, the ionospheric residual errors after correction using the ionospheric model are typically highly correlated as shown in Figure 4. This results in considerably better user accuracy than would be predicted from the conventional product of the rms ranging error and the constellation DOP.

As shown in Figure 4, using the standard ionospheric model algorithm, the various S/C residual error values vary with time after correction but they are well contained during the entire day in the 1-7 m. range. Further, the residual delay errors appear to have a 2( distribution around their mean of about 2-4 m. The high correlation of the gross delays for all S/C moderates considerably the overall S/C error contribution at any given time.

The actual error experienced by the receiver is primarily from the variation of the S/C errors around their mean value. This variation has a relatively modest value (of 2-4 m.), even during the ionosphere’s highly active daylight hours. Applying normal DOP values for the GPS constellation to these errors results in a model corrected SPS accuracy of about 5-10 meters for GPS on a 2drms basis, or at a 95% confidence level.

[pic]

Figure 4. Residual Slant Ionospheric Delay Errors After Ionospheric Model Correction Algorithm

When the Selective Availability (SA) intentional degradation of GPS was removed by the DoD on May 1, 2000, many immediately observed surprisingly good accuracies from the GPS Standard Positioning Service (SPS). Most of those familiar with the expected levels of ionospheric propagation delay error found the accuracies achieved better than expected and difficult to explain. Many knowledgeable observers generally attributed these surprisingly accurate results to anomalous solar activity (unusually low TEC [total electron content] values, even near solar max) or doubted the validity of the receiver measured accuracy claims. Reliable sources, including the US Department of Transportation and the GPS Joint Program Office, had predicted a performance level for the SPS of about 15-30 m. (2() after SA removal. Using S/C single path ionospheric delay error data, this would have appeared appropriate.

In fact, the 5-10 m. (or better) accuracies for SPS may be valid and reasonable. It is not the gross value of the ionospheric error that degrades accuracy but the variation of the delay errors around a common mean value (the decorrelation effects). For example, if the errors were all large and of exactly the same value (say, 30 m.), the effect on accuracy would be negligible.

The high correlation of the ionospheric delays of all S/C provides effective values of iono error that are more consistent with the variation of the error around the temporal mean than the absolute error. This results in the correspondingly improved accuracy results. The ionospheric error correlation described may also explain the surprisingly good performance (2-6 m.) of the military single frequency (L1) PLGR receivers during the past several years.

PERFORMANCE CAPABILITIES

Figure 5, entitled Position Accuracy Estimates for Civil and Military GPS Receivers in Various Modes of Operation for the 2000 to 2010 Period illustrates the anticipated system performance improvements in GPS receivers operating in their various modes. This chart addresses position accuracy only, however there are many other measurements that will be affected as well.

SUMMARY

This paper has reviewed the current GPS Modernization program and evaluated the accuracies anticipated for successive stages towards “full-modernization.” Current observed SPS accuracies, better than 7 m 95 percent horizontal, 95 percent, can only be understood by recognizing that residual ionospheric errors after application of the single-frequency correction algorithm are highly correlated. Civil users can look forward to abetter than pproximately 2-33 meters 95 percent horizontal positioning accuracy with the advent of L5, as good as PPS users currently enjoy. L1/L2 C/A code users cannot expect quite as good a

level of performance, unless group delay uncertainties between

performance, unless group delay uncertainties between L1/L2 P(Y)-code and L1/L2 C/A code are addressed, either through a tightening of satellite specifications, or through additional corrections provided to the user via the GPS navigation message.

ACKNOWLEDGMENTS

The authors would like to thank Dr. Richard Greenspan of Draper Laboratories for pointing out some of the cited references regarding residual ionospheric effects on positioning accuracy.

REFERENCES

1] Fontana, R., and D. Latterman, “GPS Moderniza-tion and the Future,” Proceedings of IAIN/ION Annual Meeting, San Diego, California, June 2000.

2] Anon., Department of Defense Announces Revised NAVSTAR Global Positioning System Accuracy Policy, News Release, Office of Assistant Secretary of Defense for Public Affairs, June 28, 1983.

3] Anon., SA White House oofficial press release on Selective Availability, May 1, 2000.

4] Parkinson, B., “GPS Error Analysis,” in Global Positioning System: Theory and Applications, B. Parkinson and J. J. Spilker, Jr., Ed., Washington, D.C.: AIAA, Inc., 1996.

5] Van Dierendonck, A.J., and J.J.Spilker, “Proposed Third Civil GPS Signal at 1176.45 MHz: In-Phase/Quadrature Codes at 10.23 MHz Chip Rate,” Proceedings of The Institute of Navigation Annual Meeting, Cambridge, Massachusetts, June 1999.

6] Van Dierendonck, A. J., “GPS Receivers,” in Global Positioning System: Theory and Applications, B. Parkinson and J. J. Spilker, Jr., Ed., Washington, D.C.: AIAA, Inc., 1996.

7] van Graas, F., M. Braasch, “Selective Availability,” in Global Positioning System: Theory and Applications, B, B. Parkinson and J. J. Spilker, Jr., Ed., Washington, D.C.: AIAA, Inc., 1996.

8] Kovach, K., “New User Equivalent Range Error (UERE) Budget for the Modernized Navstar Global Positioning System (GPS),” Proceedings of The Institute of Navigation National Technical Meeting, Anaheim, California, January 2000.

9] Shaw, M., K. Sandhoo, D. Turner, “GPS Modernization,” Proceedings of The Royal Institute of Navigation GNSS-2000, Edinburgh, Scotland, May 2000.

10] Klobuchar, J., “Ionospheric Effects on GPS,” in Global Positioning System: Theory and Applications, B, B. Parkinson and J. J. Spilker, Jr., Ed., Washington, D.C.: AIAA, Inc., 1996.

11] Stephens, S., Navigation Improvement with the GPS Single Frequency Ionospheric Time-Delay Algorithm, Aerospace Technical Memorandum 88 (3476-02)-2, The Aerospace Corporation, El Segundo, California, March 3, 1988.

12] Greenspan, R., A. Tetewsky, J. Donna, and J. Klobuchar, “The Effects of Ionospheric Errors on Single-Frequency GPS Users,” Proceedings of The Institute of Navigation ION GPS-91, September 1991.

13] Conley, R., and J. Lavrakas, “The World After Selective Availability,” Proceedings of The Institute of Navigation ION GPS-99, Nashville, Tennessee, September 1999.

14] Anon., NAVSTAR GPS Space Segment/Navigation User Interfaces, Interface Control Document No. ICD-GPS-200C, Department of Defense, June 2000.

15] Van Dyke, K., “The World After SA: Benefits to GPS Integrity,” Proceedings of The Institute of Navigation ION GPS-99, Nashville, Tennessee, September 1999.

16] Corrigan, T., et al., GPS Risk Assessment Study: Final Report, Johns Hopkins University, Baltimore, Maryland, January 1999.Jorgensen, P., “An Assessment of Ionospheric Effects on the GPS User,” NAVIGATION: Journal of The Institute of Navigation, Vol. 36, No. 2, Summer 1989.

17] Greenspan, R., A. Tetewsky, J. Donna, and J. Klobuchar, “The Effects of Ionospheric Errors on Single-Frequency GPS Users,”

18] Coco, D., C. Coker, and J. Clynch, “Mitigation of Ionospheric Effects for Single Frequency GPS Users,” Proceedings of The Institute of Navigation ION GPS-90, Colorado Springs, Colorado, September 1990.

19] Kovach, K., “New User Equivalent Range Error (UERE) Budget for the Modernized Navstar Global Positioning System (GPS),” Proceedings of The Institute of Navigation National Technical Meeting, Anaheim, California, January 2000.

20] Anon., Department of Defense Announces Revised NAVSTAR Global Positioning System Accuracy Policy, News Release, Office of Assistant Secretary of Defense for Public Affairs, June 28, 1983.

Figure 5

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