GPS DISCIPLINED OSCILLATOR FOR GEOLOCATION WITH...

5th Federated and Fractionated Satellite Systems Workshop November 2-3, 2017, ISAE SUPAERO – Toulouse, France

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GPS DISCIPLINED OSCILLATOR FOR GEOLOCATION WITH THE SAMSON

SATELLITE CLUSTER

Avner Kaidar, Hovhannes. Agalarian Asher Space Research Institute, Technion – Israel Institute of Technology, Haifa, Israel

[email protected] ; [email protected]

and

Yogev Dotan, Vladimir Balabanov, Eviatar Edlerman, Pini Gurfil Asher Space Research Institute, Technion – Israeli Institute of Technology, Haifa, Israel

[email protected] ; [email protected] ; [email protected] ; [email protected]

Moshe Namer Electrical Engineering Dept., Technion – Israel Institute of Technology, Haifa, Israel

[email protected]

Abstract Space Autonomous Mission for Swarming and Geo-locating Nanosatellites (SAMSON) is a satellite mission led by the Asher Space Research Institute (ASRI) at the Technion and supported by the Adelis Foundation. SAMSON includes three nano-satellites, designed by ASRI, that fit into a 6U CubeSat structure and weighs about 8kg. The three satellites will be launched together to form a cluster with relative distances ranging from 1km up to 250 km and perform autonomous relative orbital element corrections to satisfy the relative distance constraints. Mission is planned for at least one year, and has two main goals: (1) Demonstrate long-term autonomous cluster flight of multiple satellites, and (2) determine the position of a cooperative terrestrial emitter based on time difference of arrival (TDOA) and frequency difference of arrival (FDOA). The time-synchronized sensors in each member of the cluster measure independently the emitter’s received signal, and record the respective TDOA and FDOA, resulting in a combined estimation of the position of the detected transmitter. In this paper, we outline the design of the GPS-Disciplined Oscillators used for generating precise time synchronization among the satellite clusters, enabling the geolocation processing. Each satellite carries a GPS receiver that calculates its own position and generates a highly accurate 1PPS signal, which is used as a major synchronization trigger between the satellites. An ultra-high stability OCXO serves as an added input for enhanced synchronized timing between the satellites, aiming at a higher geolocation accuracies. The combination of an OCXO whose output is controlled to agree with signals broadcasted by GPS and/or GNSS satellites is referred to as a GPS Disciplined Oscillator (GPSDO). The synchronization requirements of SAMSON’s geolocation mission call for accurately measuring the received TDOA and FDOA of the emitter signal at each of the three nano-satellites of the cluster. All GPSDO parts: AD9548 synchronization circuitry, NovAtel, OEM615 compact GPS receiver and the ultra-high stability OCXO, fit into a single 1U size PCB. The GPSDO was tested for initial prformance evaluation and flight model boards are in production and will follow intensive performance evaluation.

Keywords

GPS, GPSDO, SAMSON, Geolocation TDOA, FDOA, OCXO, Phase Noise,


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1. INTRODUCTION

Space Autonomous Mission for Swarming and Geo-locating Nanosatellites (SAMSON) is a satellite mission led by the Asher Space Research Institute (ASRI) at the Technion and supported by the Adelis Foundation. SAMSON includes three nano-satellites, designed by ASRI, and built in collaboration with Israeli industries (IAI/MBT, IAI/Elta and Rafael). The satellites fit into a 6U CubeSat structure and weighs about 8kg. The mission is planned to be launched in 2018. The three satellites will be launched together with approximately the same semi-major axis, eccentricity and inclination and separated in orbit to form a cluster with relative distances ranging from 1000 m for the closest two, to 250 km for the farthest two. The satellites will perform autonomous relative orbital element corrections to satisfy the relative distance constraints, using a cold-gas propulsion system. SAMSON mission is planned for at least one year, and has two main goals: Demonstrate long-term autonomous cluster flight of multiple satellites, while implementing algorithms developed at the Technion, and determine the position of a cooperative terrestrial emitter based on time difference of arrival (TDOA) and frequency difference of arrival (FDOA). The time-synchronized sensors in each member of the cluster measure independently the received signal, and record the respective TDOA and FDOA, resulting in a combined estimation of the position of the detected transmitter. In this paper, we outline the design of the GPS-Disciplined Oscillators used for generating precise time synchronization among the satellite clusters, enabling the geolocation processing. 1.1 GEOLOCATION

Geolocation refers to the localization of a terrestrial signal from the Earth's surface. It relies on both the received time difference and frequency difference of the arriving signals at each of the three nano-satellites in the cluster. Space-based geolocation offers global functionality in various weather conditions, and is less susceptible to interference or environmental conditions. In a typical scenario, an emitter located at an a priori unknown location, emits a distress signal. The swarm of closely-flying satellites receives the signals. Each swarm member receives the signal concurrently. Swarm members record the data, along with auxiliary information, such as precise satellite location. When the satellites have access to the ground station, the time tagged data and the added auxiliary information are downloaded. The ground station computes the location of the transmitted signal by performing further processing on the downloaded data. Each satellite in the cluster carries a GPS receiver that calculates its own position and generates a highly accurate 1PPS signal, which is used as a major synchronization trigger between the satellites. In addition, each satellite carries an ultra-high stability accurate clock that serves as an additional input for a better synchronized timing. Better synchronization between the satellites results in higher geolocation accuracies. The combination of an OCXO with ultra-high stability whose output is controlled to agree with signals broadcasted by GPS and/or GNSS satellites is referred to as a GPS Disciplined Oscillator (GPSDO). 1.2 GPSDO


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GPSDOs that are based on a highly accurate OCXO, take advantage of the GPS signals to in order to align the own time scale and provide accurate synchronization, serve as an alternative to cesium-based reference and other variants of atomic clocks.

1.2.1 GPSDO Principles of Operation

The basic function of a GPSDO is to receive signals from the GPS satellites, and use the information contained in these signals to control the frequency of a local quartz oscillator. The GPS satellite signals can be trusted as a reference for two reasons:

• Signals originate from atomic oscillators, and • Signals must be accurate in order for the GPS to meet its specifications as a positioning and

navigation system. The goal of the GPSDO designer is to transfer the inherent accuracy and stability of the GPS satellite signals to the signals generated by the OCXO. Modern direct digital synthesizers (DDS) have excellent resolution and allow very small frequency corrections to be made. The GPSDO design does not correct the frequency of the local oscillator. Instead, the output of a free running local oscillator is sent to a frequency synthesizer. Most GPSDOs produce sine wave signals for use as a frequency reference, and also produce 1 PPS signals for use as a time interval reference and for time synchronization to UTC. GPSDO’s are able of generating frequency accuracies and stabilities on the order of parts per billion, and even better, within minutes after power-on, and are thus one of the highest-accuracy physically-derived reference standards available. 2. GPSDO IN SAMSON

The synchronization requirements and the geolocation mission of SAMSON called for accurately measuring the received time difference of arrival (TDOA) and the frequency difference of arrival (FDOA) of signals at each of the three nano-satellites in the cluster. To meet these demands, the following major building blocks were selected.

• The AD9548 serves as synchronization engine for SAMSON’s GPS Disciplined Oscillator o Generates an output clock synchronized to one of up to several external input references o The digital PLL allows for reduction of input time jitter or phase noise associated with the

external references. o Continuously generates low jitter, clean and valid output clock, even when all references

have failed by means of a digitally controlled loop and holdover circuitry. o Operates over an industrial temperature range of −40°C to +85°C.

• NovAtel, compact GPS receiver, OEM615 [6]

o High precision compact size GPS receiver o Offers precise positioning for space constrained applications o Time accuracy of 20 ns RMS

• An Ultra High Stability OCXO by CTS Electronic Components [4]

o 10 MHz High stability ± 0.2 ppb over -10°C to +60°C o Low aging rate ± 0.2 ppb/day, ± 0.03 ppm/year o Swept quartz resonator for radiation resistance o Rubidium standard replacement o GPS Disciplined frequency standards

2.1 AD9548 AS A GPS DISCIPLINED CLOCK


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Block diagram of SAMSON’s GPSDO with the AD9548 as a major disciplining and synchronization engine, is depicted in Figure 1.

Figure 1 Block diagram of SAMSON’s GPSDO with the AD9548 serving as the main disciplining and synchronization engine

The simplified block diagram of the AD9548 is enclosed within the dashed frame. Additional external components that support the AD9548 are the GPS receiver (NovAtel, OEM615 compact GPS receiver) and the 10MHz, Ultra High Stability OCXO. An external Analog reconstruction filter that produces the Analog sinusoidal signal at the output. The serial port interface allows to control the functionality of the unit via an external processor. The following sections describe the major building blocks in the implementation of SAMSON’s GPSDO.

2.1.1 Functional Operation of the AD9548

The AD9548 requires a system clock (SYSCLK terminal) to drive the internal timing functions and to clock the embedded Direct Digital Synthesizer (DDS) and the digital-to-analog converter (DAC). The DDS/DAC pair can operate with a clock rate between 500MHz and 1GHz. In the current implementation, the clock input SYSCLK is 10MHz and the internal system clock generator synthesizes a 900 MHz to 1 GHz internal clock. The digital PLL is the heart of AD9548. It has a Phase-Frequency Detector (PFD) with a numerical output that drives the digital loop filter and delivers numerical frequency tuning words to the DDS. The DDS, drives the DAC that produces a sinusoidal analog signal at a frequency determined by the frequency tuning word at the DDS input. The output of the DDS serves as the feedback path of the digital PLL. It is important to note that the output frequency of the digital PLL depends only on the value of a numerical frequency tuning word. In the AD9548, a 48-bit frequency tuning word (M) that appears at the DDS input will generate a DDS output frequency given by

482DDS S

Mf f=

where Sf is the frequency supplied by the system clock generator. The tuning precision, relative to Sf , is

3.5527×10−6 ppb. The PFD can measure time offsets between the feedback and reference clock edges to within 0.5ps. The digital loop filter relies on numerical coefficients to establish the loop bandwidth of the

Digital PLL

Input

Reference

Section

Ultra High Stability OCXO

System Clock Generator

GPS Receiver

Phase-Frequency

Detector (PFD)

External Analog

Reconstruction Filter

DDSDigital Loop

Filter

Tuning

Word

Processor

Feedback Divider

DAC

Internal Control Unit

10MHzSYSCLK

AD9548

Se

ria

l P

ort

Clock

Distribution

Output

Section

OUT


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digital PLL. These are programmable coefficients and the user can adjust the loop bandwidth over a range from 0.001Hz to 100 kHz. The DAC output following the digital PLL drives the clock distribution section that produces the desired output signal at the OUT terminal. The system has actually four pairs of independently selectable output signals. Each output pair has a dedicated optional frequency divider. The current implementation uses only one output signal. The serial port allows the user to control the functionality of the AD9548. An external processor can be connected to the serial port to allow the user to control the functionality of the AD9548. The serial port gives the user access to the DDS input. The user can read the currently active DDS frequency tuning word or write a specific DDS frequency tuning word (thereby forcing a desired output frequency). In closed-loop operation, the digital PLL locks to the reference clock signal (1PPS in our case) that is applied at the IN terminal. It does so by digitally disciplining the output frequency of the DDS until the frequency of the feedback signal matches the frequency of the reference signal and the edges of both signals are time aligned. The IN terminal actually represents up to eight independent selectable input signals. The current implementation uses only one input signal. 2.2 FEATURES OF THE ULTRA-HIGH STABILITY OCXO

The key features of the CTS OCXO are summarized in Table 1. The size is 20.2x20.2x12.9 mm3 and its maximum steady state power consumption is 1.2W. The output frequency is accurately aligned to 10MHz. The OCXO serves as a SYSCLK for the AD9548 and can be disciplined to the 1PPS reference of GPS. In its free running mode, it demonstrates a ∆F/F frequency stability of ±0.2 ppb, vs. operating temperature, supply voltage variation and aging/day. The Allan variance is 5x10-12 at 1s averaging time.

Table 1 10MHz OCXO - Main Features [4]

Parameter Symbol Condition Min Typ Max Unit

Output Frequency

F 10.000000 MHz

Initial Frequency Tolerance

∆F/F At +25°C ±0.1 ppm

Frequency Stability ∆F/F

Vs. operating temperature

(Ref. to 25°C) ±0.2 ppb

Vs. Supply voltage (Vcc ±5%) ±0.2 ppb

Vs. Aging/Day Vs. Aging /Year (After 30 days)

±0.2 ±30

ppb

Operating Temperature

Range T -10 +60 °C

Allan Variance 1s 5 E-12

SSB Phase Noise

1Hz 10Hz 100Hz

-102 -135 -155

-100 -132 -151

dBc/Hz


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Parameter Symbol Condition Min Typ Max Unit

1kHz 10kHz

100kHz

-161 -167 -170

-160 -165 -168

Retrace After 30min following 24 hours off

±20 ppb

Supply Voltage

Vcc 4.75 5.0 5.25 V

Power Consumption

P Warm-up state

Steady state, 25°C 1.0

4.5 1.2

W

Warm Up Time

Tau to ∆F/F = 10 ppb,

25°C 5 min

Dimensions 20.2x20.2x12.9 mm3

2.3 MAIN FEATURES OF NOVATEL, COMPACT OEM615 GPS RECEIVER

The GPS receiver used for the GPSDO is the NovAtel OEM615, a high precision and compact-size GPS receiver. It offers precise positioning for space constrained applications with time accuracy of 20 ns RMS. In the current implementation, we plan to use only the GPS L1 signal tracking. SAMSON uses the real-time calculated position values from the GPS receiver as inputs to the Orbit Control Module (OCM) that runs in each nano-satellite to control the cluster. 3. RESULTS

All GPSDO’s parts: AD9548 synchronization circuitry, NovAtel, OEM615 compact GPS receiver and the ultra-high stability OCXO, are integrated to fit a single 1U size PCB and are pictured in Figure 2. The left side of Figure 3 shows the upper view of the enclosed AD9548 synchronization circuitry. Figure’s 3 right side, describes the board with NovAtel’s, OEM615 compact GPS receiver and the ultra-high stability OCXO.


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Figure 2 Photo of the integrated GPSDO 1U board showing the OCXO and the GPS receiver

Figure 3. Integrated GPSDO 1U board showing: Left: upper view of the enclosed AD9548 synchronization circuitry, and Right: upper side of the board with NovAtel’s, OEM615 compact GPS receiver and the ultra-high stability OCXO.

Several prototypes were built and initially tested for prformance evaluation. Flight model boards will follow intensive performance evaluation. Upon start up of each board the GPSDO uploads previous frequency stability parameters in order to enhance its performance. These parameters include: frequency stability versus operating temperature, vesus supply voltage, and vesus aging. Typical warm up time would be ~8 minutes for frequency lock and additional ~15 minutes for phase lock totaling ~23 minutes. Figures 4 shows a typical measured Allan Deviation of the GPSDO from start up time. Figure 5, describes the measured frequency stability after the warm up time.


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Figure 4 Measured Allan Deviation of the GPSDO along warmup up time

Figure 5 Typical measured Allan Deviation of the GPSDO after a the warmup time

4. ACKNOWLEDGMENTS

This work was partially funded by Adelis Foundation.

Allan D

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Allan D

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5. REFERENCES

[1] C. W. T.Nicholls and G.C. Carleton, Adaptive OCXO Drift Correction Algorithm, Frequency Control Symposium and Exposition 2004, Proceedings of the 2004 IEEE International: 509-517. [2] K. C. Ho and Y. T. Chan, Geolocation of a Known Altitude Object From TDOA and FDOA Measurements, IEEE Transactions on Aerospace and Electronic Systems Vol. 33, No. 3 July 1997. [3] K. Gentile, The AD9548 as a GPS Disciplined Stratum 2 Clock, AN-1002, Analog Devices [4] CTS Electronic Components, VF-901574-10MHz OCXO-Ultra High Stability. [5] M. A. Lombardi, The Use of GPS Disciplined Oscillators as a Primary Frequency Standard for Calibration and Metrology Laboratories, Measure, Vol.3 No. 3, September 2008, 56-65. [6] NovAtel OEM615 Compact GPS Receiver

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