Srini H. Raghavan

A summary of the new GPS IIR-M and IIF modernization signals

This paper discusses the planned modernization of the block IIR-M and IIF GPS satellite signals, including the addition of the new military signal. In particular a brief review of the legacy signals is presented, followed by a description of the new military signal, the M code signal (with its data and data-less components). Next the new civil signal which is the replacement C/A code for L2, is discussed. Interplex combining is then presented. Interplex modulation combines three signals and provides a constant envelope to the power amplifier by producing an intermodulation term. Finally, the new L5 civilian signal is elucidated.

Communication system performance - detailed modeling of a power amplifier with two modulated input signals

The performance of multicarrier frequency division multiple access (FDMA) wireless communication systems is impacted by amplifier nonlinearities. It is vital to precisely model limiters and high power amplifiers (HPAs) to determine the spectrum distortion and other types of degradation associated with multicarrier communication systems. Contemporary simulation tools can be employed to precisely ascertain intermodulation (IM) distortion and its effect and impact on both in-band and out-of-band IM performance in a computationally timely manner. This paper illustrates analysis and simulation results in a parameterized form when the amplifier input consists of two 8-PSK modulated signals with raised cosine filter shaping. One of the significant parameters in these analyses and simulated results is the HPA operational back-off (OBO) power level, which is studied over a range of about 6 dB. The simulation illustrates the intermodulation distortion both within the signal band and outside the signal band. A novel approach is used to represent an HPA in terms of a power series expansion, which converges very rapidly and gives significant insight and provides a useful tool in predicting the spectral content of the HPA output. The results contained in this submission were generated in whole, or in part, through work supporting the MILSATCOM joint program office (MJPO). The authors are very appreciative of the support provided by the SMC/MC program office of the Space and Missile Center in this effort

Frequency Band Sharing between NRZ and Split Spectrum Signals- Analysis and Simulation Results

Srini Raghavan has over 25 years of experience in design, simulation and analysis of satellite communication systems, spread spectrum systems, and signal processing. Currently, he is supporting several communication systems and Global Positioning System (GPS)-related activities at The Aerospace Corporation, where he is a Senior Engineering Specialist. He has a B.S. and M.Tech. from India, and an M.S. and Ph.D. in Electrical Engineering from the University of MissouriRolla, and is a Senior Member of IEEE and a member of ION, Eta Kappa Nu and Sigma Xi. Jack K. Holmes is a Distinguished Engineer at The Aerospace Corporation in the Communication Systems Subdivision. He has over 40 year’s of experience in analysis, simulation, and design of communication and spread spectrum systems. He is author of the book entitled “Coherent Spread Spectrum Systems” and has published approximately 45 papers in the area of communications, synchronization, and GPS-related subjects. He has been heavily involved in the military side of the GPS Modernization effort since February 1997. He is currently the GPS Code Acquisition design risk assessment subteam leader for the GPS Modernization risk assessment team. Jack received his B.S., M.S., and Ph.D. degrees at UCLA in Electrical Engineering, and is a senior member of the IEEE and a member of ION, Tau Beta Pi, and Sigma Xi.

A NEW SIGNAL PLAN FOR GPS

GPS has become a unique satellite based service for a number of reasons. Originally a military only system, GPS civilian use has far overtaken the original user base. The limitations of the current civil service have resulted in a reevaluation of the fundamental signal structure of GPS. In this paper a proposed new GPS signal scheme is described that provides for the simultaneous modulation of the carriers with the C/A-code, the P(Y)-code, and a new military code (v-code), on the existing GPS frequencies within the allocated bands. The biggest advantage of this method is that it provides sufficient spectral isolation between the civil and military signals, allowing both user groups to co-exist in the same frequency band. Another advantage of this concept is that it allows for backward compatibility with the existing military and commercial receivers while providing an additional C/A-code on the L2 carrier. This paper outlines the assumptions and criteria used to satisfy the Presidential Decision Directive for the future of GPS using this new signal plan within the currently registered frequencies. Proof of concept results obtained through laboratory tests are presented. In addition, signal generation and receiver design are discussed.

Turnaround command effects on USB and SGLS satellite downlinks

Many satellites using commercial off-the-shelf USB and SGLS transponders such as the L3-Com CXS 2000 do not have turnaround command (TAC) suppression in their downlinks. Without TAC suppression, these satellites exhibit greater downlink service modulation losses for carrier, ranging, and telemetry. Depending on the selected uplink command modulation index and the turnaround ratio, these additional modulation losses could vary from 0.1 dB to 3 dB (for command mod indices less than 1 radian). They are due to the allocation of downlink power to the TAC, and partly to an increase in intermodulation (IM) power. For an uplink command modulation index of 0.3 radians, our calculations for both USB and SGLS signals show that the loss of downlink power to TAC and IM is less than 10%. However, when the uplink command modulation index is increased to a nominal operating value of 1.0 radian, the loss of downlink service power to TAC and IM becomes 40%. This large loss of downlink power to the TAC and IM increases the modulation losses for other downlink services, which could result in denial of services such as telemetry and ranging to ground users with small antennas. This paper shows that suppression of turnaround command will result in lower TAC and IM losses, which will in turn improve downlink services with higher link margins. These more robust downlink margins permit the use of a smaller and cheaper high-power amplifier (HPA) in the satellite transmitter12.

Upper bound on C/a code spectral separation coefficient

It is well known that spectral lines of the Global Positioning System (GPS) coarse acquisition (C/A) codes increase the amount of code division multiple access (CDMA) noise generated, occasionally far exceeding what is typically expected, depending on the user-satellite geometry. 12This degrades the available effective carrier-to-noise density ratio (C/N0)effective to a GPS receiver to a greater degree than when spectral line effects are ignored. (C/N0)effective is a key metric widely used to characterize the performance of the GPS receivers in terms of code acquisition, carrier loss of lock, and data bit error rates. Because of the short duration, limited geographical extent and a limited number of signals in the GPS frequency band that existed most analysis performed several years ago ignored the spectral line effects altogether. But in the changed signal environment with the growing number of Global Navigation Satellite Systems (GNSSs) in the Radio Navigation Satellite System (RNSS) band, there is a need to account for all the interference sources accurately to make sure that the intersystem and intrasystem Radio Frequency Compatibility (RFC) is achieved. Since C/A code is very widely used currently and will be used possibly in the foreseeable future in a number of applications, some of them of a critical nature, interference into C/A code receivers must be carefully considered. Since C/A code intrasystem interference is a significant contributor to (C/N0)effective, the focus of many studies done in the past couple of years have been on C/A code spectral line issues. A number of methods based on quasi-analytical models of the correlator output, simulation models of GPS satellite constellation and the receivers, and limited laboratory measurements were employed in earlier studies. In this paper we take a different approach to provide an upper bound on the C/A code spectral separation coefficient (CASSC) based on the code spectral line properties. This upper bound is computationally much simpler to obtain. This bound is also applicable to other spreading codes, resulting in spread-spectrum signals with spectral line effects.

The mean cycle slip time for first-, second-, and third-order PLLs

This paper presents an analytic model for the mean time to lose lock for first-, second-, and third-order phase-locked loops (PLLs). The analytic model is of the same form as that of a first-order PLL with the appropriate corrections for second- and third-order PLLs. The modeling was done on SystemVue simulation.

Inter-BOC Signal Interference - Tracking Loop Performance*

Abstract-The Global Positioning System (GPS) is a spread-spectrum system that employs direct-sequence spreading of the spectrum to achieve excellent ranging accuracy. Modernized GPS will also use a directsequence spread-spectrum (DSSS) system, but unlike the current GPS signal, which uses a non-return-tozero formatting of the code symbol, it employs a code-formatting scheme known as Binary Offset Carrier (BOC) that results in split-spectrum modulation. An important consideration in the design of a spreadspectrum signal is to provide a certain level of protection against intentional and also unintentional interference that may be experienced by the spread-spectrum signal. Even without the external interference, a Code Division Multiple Access (CDMA) system such as the GPS and the Galileo must contend with the CDMA noise within the system. GPS plans to add one more BOC signal in addition to the current C/A, P and M code signals. With this new signal, additional interference potential exists, along with the normal CDMA noise. In this paper such interference due to the new BOC signal to the current M code BOC signal, we call it Inter-BOC Signal Interference, is considered. Some simulation results are also presented.

BER performance of notch filtered direct sequence spread spectrum signal

A certain level of immunity to narrowband interference is inherent in a direct sequence spread spectrum (DSSS) signal. In many applications a system using notch filters rejects any narrowband interference occurring in the spread-spectrum band. Depending upon the number of notches, the notch width, the notch depth, and the location of the notches within the spread-spectrum band, the bit-error rate (BER) performance degradation varies. The spread signal distortion and the resulting intersymbol interference (ISI) at the chip level cannot always be directly accounted for in the data BER through analysis, and it is much easier to employ Monte Carlo simulation techniques to characterize the BER degradation. In some scenarios a notch-filtered spread-spectrum signal may also result in blanking certain portions of the spread-spectrum signal to satisfy regulatory requirements. In such situations it is important to know the level of degradation a priori so that it can be accounted for in the link performance. In this paper, we describe the simulation work carried out to relate the notch-filtering effect of the spread signal on the BER followed by simulation results

Modeling satellite vehicle passive intermodulation

Passive intermodulation (PIM) can degrade the transmitted signal quality by acting as an interference signal resulting from subsystem, components, material, poor mechanical connections, and sometimes the physical layout when they fall inside the receiver bandwidth. In a multicarrier system, signals sharing the same transmission path can mix together generating PIMs [1]. This can happen in active devices as well as passive components. Passive components include antennas, cables, connectors, power dividers, and mixers. Typically, components with higher ferromagnetic content contribute to the generation of PIMs. PIM products are generated on space vehicles also, and direct measurement of PIMs is extremely difficult. In this paper an indirect method of verifying that PIM budget is not exceeded is discussed.