Civil GPS/WAAS Signal Design and Interference Environment at 1176.45 MHz: Results of RTCA

SC159 WG1 Activities Christopher Hegarty

Federal Aviation Administration, Washington, D.C.

A.J. Van DierendonckAJ Systems, Los Altos, CA

BIOGRAPHIES

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 (3nd Civil GPS Frequency).

Dr. A.J. Van Dierendonck received a BSEE from South Dakota State University and MSEE and Ph.D. from Iowa State University. Currently, he is self-employed under the name of AJ Systems and is a general partner of GPS Silicon Valley. In 1993, Dr. Van Dierendonck was awarded the Johannes Kepler Award by the Institute of Navigation Satellite Navigation Division for outstanding contributions to satellite navigation. For 1997, he was awarded the ION Thurlow award for outstanding contributions to the science of navigation. A.J. has 25 years of GPS experience and is a Fellow Member of the IEEE. Recently, he was inducted into the GPS Joint Program Office’s GPS Hall of Fame. He currently serves as working group co-chairman of the RTCA SC159 Working Group (WG1) for the 3nd Civil GPS Frequency.

ABSTRACT

This paper describes the contributions of RTCA Special Committee 159 Working Group 1 (3rd Civil GPS Frequency) towards the implementation of a third civil GPS signal at 1176.45 MHz (L5). These contributions came primarily in two areas. First, Working Group 1 (WG1) was relied upon by the Interagency GPS Executive Board (IGEB) to develop consensus on the

design of the third civil signal. Second, WG1 developed a set of receiver assumptions, and participated in the development of a methodology, for analyzing the compatibility of the third civil signal with existing systems operating at or near L5.

INTRODUCTION

In July 1998, to promote the development of a second civil GPS signal, an ad hoc working group in RTCA Special Committee 159 (SC159) was established. In October 1998, this ad hoc working group became a full-fledged SC159 working group, known as Working Group 1 (WG1). Initial activities of WG1 included establishing requirements for the new signal and developing an RTCA position on the location of the carrier frequency of this signal (RTCA was one of several organizations that recommended that the second civil signal be located within the 960 – 1215 MHz aeronautical band).

On January 25, 1999, U.S. Vice President Al Gore announced that not just one, but two new civil GPS signals will be added to future GPS satellites [1]. The second civil signal will use the C/A codes currently used at GPS L1 (1575.42 MHz) and will be located at GPS L2 (1227.6 MHz). The third civil signal, which is intended to meet the needs of critical safety-of-life applications, will be located at 1176.45 MHz (L5). The second and third civil signals are anticipated to be included on GPS satellites that will be launched beginning in 2005 and 2007, respectively.

Shortly after Vice President Gore’s announcement, SC159 WG1 was provided an expanded role in the implementation of the L5 signal. This expanded role came about in February 1999 through the establishment of a Third Civil Signal Implementation Steering Group within the Interagency GPS Executive Board (IGEB). The Steering Group developed a plan [2] for the implementation of the L5 signal that included a strong tie (see Figure 1) between SC159 WG1 and an ad hoc

working group of the IGEB (IGEB WG3) that was chartered to define the technical characteristics of the third civil GPS signal. As will be discussed later in this paper, SC159 WG1 also had significant interaction with another ad hoc IGEB working group (IGEB WG1), that was chartered to “…address the feasibility of providing a protected safety-of-life GPS signal suitable for civil aviation use while allowing existing, authorized systems to coexist…” [2].

This paper summarizes the accomplishments of SC159 WG1 towards: Developing consensus on the signal design for GPS

and Wide Area Augmentation System (WAAS) signals to be broadcast at L5.

Defining receiver characteristics to be used in analyzing the compatibility of the L5 signals with existing systems operating at or near L5.

International Coordination

Technical Feasibility of Coexistence

Interagency GPS Executive Board(IGEB)

Cost Issues/Operational Impacts

Signal Requirements

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DOD’sFutureUse of

JTIDS/MIDS

Information Flow

Third GPS SignalImplementationSteering Group

Figure 1. IGEB Third Civil Signal Implementation Steering Group Organization

SIGNAL DESIGN

In August 1999, the IGEB endorsed a proposed design [3] for the third civil signal. The GPS L5 signal will be comprised of a pair of carriers (in-phase and quadrature) at 1176.45 MHz that are Binary Phase Shift Keying (BPSK) modulated by: 10.23 Mchip/s spreading sequences – The in-phase and quadrature carrier channels are BPSK modulated by different sequences (see [3] for details of the spreading sequences). Length 10230 sequences will be employed, resulting in a 1 ms repetition period. The in-phase signal is BPSK modulated by data. The data rate is 50 bps. A rate ½ forward error correction (FEC) code is employed, yielding 100 symbols per second (sps). The subframe structure is described in [4]. The in-phase signal is BPSK modulated by a 1 kHz Neumann-Hoffmann sequence to improve symbol synchronization and cross-correlation properties.

An overview of the generation of the GPS L5 signal is provided in Figure 2.

GPS L1Data Stream

Insert InformationPertinent to L5 Add CRC50 bps 50 bps

10 - symbolNeuman-Hoffman

Code50 bps

Code Generator10.23 MHzCode Clock

1 ms epochs

XI(t)

1 kbaud

QPSK Modulator

XQ(t)

CompositeSignal

IEncodewith FEC 100 sps

Carrier

Figure 2. GPS L5 Signal Generation

The Federal Aviation Administration (FAA) also plans to eventually implement a WAAS signal at L5. A design for the WAAS L5 signal structure has been proposed [4] that is similar to the GPS L5 signal, except that only a single carrier is used, and the data rate is 250 bps. A rate ½ FEC code is employed yielding 500 sps.

The GPS L5 minimum received signal power into a 0 dBic antenna at or near the surface of the earth will be –154 dBW. This level is 6 dB higher than the specified minimum power for the C/A code on L1 and facilitates coexistence of the L5 signal with existing systems. For GPS, one-half the power (-157 dBW) is provided to each carrier component (in-phase and quadrature). The WAAS L5 signal minimum received signal power will also likely be specified at –154 dBW. All WAAS power is devoted to the in-phase carrier.

The signal design summarized above and described in detail in [3,4] has benefited from many inputs, notably the proposal for the dataless channel or “coherent carrier” component of the signal originally made in [5]. Numerous earlier iterations of the signal design were presented at WG1 meetings to solicit feedback. In return, WG1 participants analyzed virtually every aspect of the signal design and provided feedback for improvement of the design. The following subsections summarize some of the main design features that were analyzed.

COHERENT CARRIER

The dataless channel was originally proposed to allow carrier phase tracking of the signal in low signal-to-noise conditions. Figure 3 [6] illustrates the cycle slip performance of the GPS L5 signal as compared to the standard BPSK signal (e.g., the C/A code). The figure plots the probability of a cycle slip in a 1-second interval versus post-correlation signal-to-noise density, S/N0. Note that to achieve a cycle slip probability of 10-5 per second, roughly 3.5 dB less signal-to-noise is required for the GPS L5 signal as compared to the standard BPSK signal. A 10 Hz Phase Locked Loop (PLL) tracking the dataless channel is assumed for the GPS L5 signal and a 10 Hz Costas loop is assumed for the standard BPSK signal. This relative comparison does not include consideration of dynamics and oscillator jitter. Adding consideration of these important effects would widen the cycle slip performance difference between the GPS L5

signal and the standard BPSK signal, since the optimal bandwidth for a PLL, considering noise, dynamics, and oscillator jitter, will be lower than that for the Costas loop.

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Figure 3. Cycle Slip Performance of L5 with PLL Compared to Cycle Slip Performance of L1 C/A Code

DATA DEMODULATION

Although the cycle slip performance is better for the new signal structure than the L1 C/A code, rms carrier phase tracking errors are actually larger over the range of S/N0’s of interest [6]. This observation initially led WG1 to be concerned over the performance of coherent data demodulation when the coherent reference is the noisy carrier phase measurement. For instance, with a 10 Hz PLL tracking the dataless channel, phase errors vary with a time constant of 0.1 s, or ten channel symbols. WG1 considered the provision of additional power to the data channel (beyond one half of the available signal power). However, simulation of the data demodulation performance revealed that the rate ½ FEC scheme selected is very robust to losses in recovered symbol energy over strings of symbols.

Figure 4 [7] is a very interesting graphical derivation of the optimal division of power between the data and dataless channels for the L5 signal. Two curves are plotted on the figure, based on simulation results. The first is the required S/N0 to provide a word error rate (WER) of 0.001 versus the fraction of total signal power devoted to the dataless channel. The second curve is the required S/N0 to provide a cycle slip rate of 10-5/second. Note that, given these assumed performance requirements, the optimal division of power between the data and dataless channels is close to one-half to each.

Figure 4. Determination of Optimal Division of Power Between the Data and Dataless Channels

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Figure 5. Acquisition Performance

ACQUISITION

Importantly, although the coherent carrier significantly improves tracking thresholds for the GPS L5 signal, it does not facilitate acquisition, which is inherently a non-coherent function. As compared to the L1 C/A code, acquisition of the GPS L5 signal requires a search through ten times as many code phase possibilities. However, the lower cross-correlation values between the L5 signal codes reduces the dwell time required to search each code phase cell with a fixed probability of missed detection by around a factor of four [8]. Figure 5 [8] plots the probability of correct detection versus post correlation signal-to-noise density, C/N0, with the dwell times set so as to achieve a missed detection probability of 0.001. Note that the required C/N0 for a 0.9 probability of detection is approximately the same for the GPS L5 signal and the L1 C/A code.

RANGING PRECISION

The GPS L5 signal structure will provide improved ranging precision in the presence of multipath (see Figure 6 [4]), as compared to the L1 C/A code. The higher chipping rate also improves ranging precision in the presence of noise and interference. In the presence of additive white Gaussian noise, the GPS L5 signal structure provides an approximate factor of three improvement in root mean square (rms) tracking errors over the L1 C/A code (assuming both signals are limited to 24 MHz). This improvement is due to the roughly three-fold larger rms bandwidth of the GPS L5 signal, as compared to the L1 C/A code.

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ALPHA = 0.01

0.1 C/A Chip Spacing

Figure 6. Multipath Performance Comparisons

INTERFERENCE ENVIRONMENT FOR L5

Major existing systems operating at or near 1176.45 MHz include [9]:

1. Aeronautical systems operating between 960 and 1215 MHz – including Distance Measuring Equipment (DME) and Tactical Air Navigation (TACAN) systems, that operate throughout the band, and the systems that operate at 1030 and 1090 MHz, including Secondary Surveillance Radars (SSR), Traffic Collision and Avoidance System (TCAS), Identify Friend or Foe (IFF) and planned Automatic Dependent Surveillance – Broadcast (ADS-B). The Federal Aviation Administration (FAA) operates approximately 1000 DME and TACAN beacons. The U.S. Department of Defense (DoD) operates an additional 173 TACAN beacons [10].

2. Joint Tactical Information Distribution System/Multi-functional Information Distribution System (JTIDS/MIDS) operating between 969 and 1206 MHz – a spread-spectrum digital communications system used by the DoD and other nations to exchange data among military platforms. JTIDS is currently deployed on about 600 platforms in its Class 2 form. They are installed on F-14 fighters, a squadron of F-15 fighters, Navy E-2Cs, ships, E-3 airborne early warning platforms,

Modular Control Equipment, ABCCC, Rivet Joint, and Joint STARS platforms. MIDS is under development as the next-generation version of JTIDS and will eventually be deployed on about 2400 U.S. platforms by the year 2005.

3. Military and civil radars operating between 1215 and 1385 MHz – including about 250 systems in the U.S. used for long-range primary air traffic control (ATC), the North Warning System (NWS), drug interdiction, and other applications.

One common characteristic of all of these systems is that they are pulsed. This commonality facilitates the coexistence of these systems with L5, since GPS receivers can be made very robust against pulsed interference. Additional details on these systems may be found in [9].

RECEIVER CONSIDERATIONS

After being briefed on the technical characteristics of the emissions from the existing systems operating nearby in frequency to L5, WG1 participants agreed that modest receiver design changes would be highly desirable to facilitate coexistence. The two changes addressed by the group were: (1) the incorporation of some form of pulse detection and blanking circuitry (or any other technique that provides equivalent performance), and (2) better out-of-band filtering relative to current civil equipment operating at L1. The pulse detection circuitry detects when pulsed energy is being received above a certain threshold that is set relative to the nominal level of noise in a receiver’s passband. When a pulse is detected, the input signal level is zeroed (“blanked”). Blanking is equivalent to excluding a portion of the received signal from the complex correlation sums generated by the receiver. The WG1 consensus on out-of-band filtering requirements for L5 equipment is shown in Figure 7. A combination of radio frequency (RF) and Intermediate Frequency (IF) filters provides a total of 5.5 dB/MHz attenuation for interfering signals that fall beyond 10 MHz of L5. The maximum attenuation is 70 dB. WG1 participants have recommended use of a higher (5 dB) noise figure for L5 GPS/WAAS receivers than is specified in [11] for L1 (see Figure 8 [12] for an illustrative comparison of L1 and L5 front-ends). This recommended increase is due to the anticipated greater filter insertion loss resulting from the more stringent filtering requirement. Combined with a 100 K sky noise, the 5 dB noise figure results in a receiver noise floor at -200 dBW/Hz [12].

20 log|H(f) |

f (MHz)

0

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(10,0)

(22.7,-70)

Figure 7. Assumed GPS/WAAS L5 Receiver RF/IF Filter Response

Z-mismatch -0.2 dB

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BPF Lim LNA

cable Rx-Proc

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Figure 8. Illustrative Implementation of L1/L5 Receiver Front-End

COMPATIBILITY ANALYSES

The IGEB Third Civil Signal Implementation Steering Group ad hoc Working Group 1 (IGEB WG1) developed a methodology for determining compatibility of L5 with exsting systems [9]. RTCA SC159 WG1 provided assistance to IGEB WG1 in two important ways. First, SC159 WG1 provided assumptions regarding receiver characteristics (i.e., the values described previously in this paper for noise floor, selectivity, and blanking performance). Second, there was considerable interaction between the IGEB and RTCA during the development of the compatibility methodology. This methodology consists of determining the degradation to a GPS/WAAS L5 receiver’s post-correlation signal-to-noise power density ratio (S/N0) due to the presence of interfering signals. An acceptable level of degradation is defined to be one that yields 33.7 dB-Hz (for acquisition). This S/N0 is attained at L1 for WAAS under the following conditions: Minimum received signal power of –161 dBW into a

0 dBic antenna –4.5 dBic antenna gain towards a desired low-

elevation angle satellite 2 dB receiver implementation loss Maximum interference level of –122.5 dBm/MHz (-

116.5 dBm/MHz to protect all receiver functions, including acquisition, from [11] adjusted by a 6 dB safety margin [14])

In the absence of interference, a minimum S/N0 of 39.5 dB-Hz is expected (-154 dBW received power - 4.5 dBic

antenna gain - 2 dB implementation loss - (-200) dBW/Hz noise floor = 39.5 dB-Hz). Compatibility is achieved at a geographic location, from a WG1 standpoint, if signals from existing systems do not result in more than a 5.8 dB degradation to the starting L5 S/N0

(39.5 – 5.8 = 33.7).

An equation for computing the S/N0 degradation to an L5 receiver is:

where

is the ratio of peak non-blanked pulse power to precorrelation noise power, PDCB (pulse duty cycle – blanker) is the total duty cycle of all pulses strong enough to activate the blanker, N in the total number of low-level undesired received signals (i.e., those not strong enough to trip the blanker), Pi is the peak received power of the i-th undesired signal, and dc i is the duty cycle of the i-th low-level signal. The right-hand terms of the above equation are: 1st term – S/N0 is the absence of interference 2nd term – Lost signal power due to the blanker 3rd term – Adjustment of the noise floor due to: (1)

suppression of thermal noise by the blanker, and (2) noise floor contribution due to low-level pulses and continuous interference.

Using the assumptions and methodology described in the previous sections, interference analyses were performed for the IGEB using a software tool developed by The MITRE Corporation’s Center for Advanced Aviation System Development known as the GPS RFI Environment Evaluation Tool (GREET). Interference scenarios in the conterminous U.S. (CONUS) and other regions of the world were evaluated on a half-degree grid. Maps showing signal-to-noise ratio degradation at grid points of different flight altitudes were generated for different emitter configurations (see Figures 9-13).

Figures 9-10 [13] show signal-to-noise degradations for the world for two flight levels. Each of these two plots includes effects only from DME/TACAN ground beacons. Note that signal-to-noise degradation is very dependent on altitude. At 5,000 ft above ground level (AGL) (Figure 9), very few emitters can be seen and the environment is very clean. At 40,000 ft above mean sea level (MSL), many ground-based emitters can be seen. Figure 10 indicates an unacceptable level of degradation

at this altitude certain regions in CONUS, Europe, and Japan.

Figures 11-13 show signal-to-noise degradations in the United States for these same two flight levels. Figures 11 and 13 include the effects from all known emitters including:

DME/TACAN ground beacons JTIDS/MIDS Out-of-band emissions from radars Aeronautical emitters – A conservative allowance

for emissions from DME/TACAN interrogators, SSR interrogators and transponders, TCAS, and ADS-B was included.

The high altitude electromagnetic environment surrounding 1176.45 MHz is dominated by DME/TACAN, as can be seen by comparing Figure 11 that includes all emitters with Figure 12 that only includes the effects of DME/TACAN. A very promising solution identified by IGEB WG1 is to reassign a subset of the inband DME/TACANs. Figure 13 plots signal-to-noise degradation due to all emitters with the inband DME/TACANs reassigned to other frequencies in the 960 – 1215 MHz band. The plot indicates that, with this DME/TACAN reassignment, an acceptable environment for L5 may be achieved throughout CONUS at this flight level.

Figure 9. GPS Receiver Postcorrelation S/N0 Degradation at 5,000 ft AGL from DME/TACAN

Figure 10. GPS Receiver Postcorrelation S/N0 Degradation at 40,000 ft MSL from DME/TACAN

Figure 11. Total GPS Receiver Postcorrelation S/N0

Degradation at 40,000 ft MSL over CONUS

Figure 12. GPS Receiver Postcorrelation S/N0

Degradation from DME/TACAN at 40,000 ft MSL over CONUS

Figure 13. Total GPS Receiver Postcorrelation S/N0

Degradation at 40,000 ft MSL over CONUS, DME/TACAN Emitters at 1176.45 ±10 MHz

Reassigned

SUMMARY

This paper has described the contributions of RTCA SC159 WG1 (3rd Civil GPS Frequency) towards the implementation of a third civil GPS signal at 1176.45 MHz. These contributions came primarily in two areas. First, Working Group 1 (WG1) was relied upon by the Interagency GPS Executive Board (IGEB) to develop consensus on the design of the third civil signal. Analyses performed by WG1, described in this paper, have facilitated IGEB consensus on the signal structure described in [3].

As its second major contribution to the implementation of a new civil signal at L5, WG1 developed a set of receiver assumptions, and participated in the development of a methodology for analyzing the compatibility of the third civil signal with existing systems operating at or near L5.

Preliminary results from applying this methodology indicate that on the surface of the earth and at low altitudes, no modifications to known existing systems will be necessary to protect the GPS L5 service. At high altitudes, many more emitters are visible and present an unacceptable level of interference to the new signal. A promising solution to this problem is to reassign DME/TACAN frequencies that fall in-band to the new GPS signal.

ACKNOWLEDGMENTS

The authors would like to acknowledge the many contributions to this paper from the participants in RTCA SC159 WG1. Special thanks to Dr. Tom Morrissey (Zeta Associates), Bob Erlandson (Rockwell-Collins), Tae Kim and Swen Ericson (MITRE) for providing some of the figures presented in this paper. Also acknowledged are Tom Stansell, Richard Keegan and

Dr. Charles Cahn, all representing Leica, for their suggestions and support with regard to the coherent (dataless) carrier component of the L5 signal.

REFERENCES

Many of the references listed below are presentations that were made at RTCA SC159 WG1 meetings. These presentations may be obtained from the WG1 co-chairs (ajvd@aol.com or Chris.Hegarty@faa.gov).

[1] Anon., Vice President Gore Announces New Global Positioning System Modernization Initiative, Press Release, The White House, Office of the Vice President, January 25, 1999.

[2] Anon., Plan for Implementation of the Third Civil GPS Signal, Interagency GPS Executive Board, February 9, 1999.

[3] Spilker, J.J., and A.J. Van Dierendonck, “Proposed New Civil GPS Signal at 1176.45 MHz,” Proceedings of The Institute of Navigation ION GPS ’99, Nashville, Tennessee, September 1999.

[4] 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.

[5] Stansell, T., C. Cahn, and R. Keegan, Lc Signal Structure, Memorandum to A.J. Van Dierendonck, February 12, 1999.

[6] Hegarty, C., Evaluation of the Proposed Signal Structure for the New Civil GPS Signal at 1176.45 MHz, The MITRE Corporation, Working Note WN99W34, McLean, Virginia, June 1999.

[7] Morrissey, T., Forward Error Correction for GPS L5 Data, RTCA SC159 WG1, London, July 20-21, 1999.

[8] Van Dierendonck, A.J., L5 Acquisition Considerations, RTCA SC159 WG1, Washington, D.C., June 7, 1999.

[9] Hegarty, C., T. Kim, S. Ericson, P. Reddan, T. Morrissey, and A.J. Van Dierendonck, “Methodology for Determining Compatibility of GPS L5 with Existing Systems and Preliminary Results,” Proceedings of The Institute of Navigation Annual Meeting, Cambridge, Massachusetts, June 1999.

[10] Anon., 1996 Federal Radionavigation Plan, U.S. Department of Defense and Department of Transportation, DOT-VNTSC-RSPA-97-2/DOD-4650.5, July 1997.

[11] Change 3 to RTCA/DO-229B Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment, RTCA Paper No. 061-98/SCI59-781, March 31, 1998.

[12] Erlandson, R.J., L5 Receiver Implementation Aspects, RTCA SC159 WG1, London, July 20-21, 1999.

[13] Kim, T., and S. Ericson, Analyses of GPS L5 Receiver Radio Frequency Interference from DME/TACAN, JTIDS/MIDS, ARNS, and Radar, RTCA SC159 WG1, London, July 20, 1999.

[14] Anon, Modification of preliminary draft new Recommendation ITU-R M.[RNSS.CHAR], Technical Characteristics of Current and Prospective RNSS (Space-to-Earth) and ARNS Receivers to be Considered in Interference Studies in the Band 1559-1610 MHz, U.S. ITU-R Working Party 8D, Document USWP8D/13 (Rev. 5), April 12, 1999.