1.

INTRODUCTION

The switchover from the traditional RF to optical frequencies was brought on by the increasing requirement for higher bandwidth in telecommunication. Over the years various optical links were tested, with different wavelengths or transmission spectral windows, including high-speed fiber optic technology over short distances and free-space optical links between two ground stations as well as for the ‘last mile’ communication. The satellite-based free-space optical communication has been studied in recent years both for the orbital communication and for the application in deep space environment, e.g. in projects like NASA’s Laser Lunar Communication Demonstration (LLCD)⁹, OPALS¹⁰, MESSENGER spacecraft¹¹ and ESA’s European Data Relay System (EDRS)¹². Our study focuses on developing a breadboard to test the performance of the satellite optical ranging protocol based on optical modem utilized for different orbits in the situation when the optical channel is affected by atmospheric turbulence causing temporal fading.

2.

TELEMETRY RANGING SYSTEM DESIGN

Satellite ranging is a procedure to determine the distance between a ground station and a spacecraft or between two spacecrafts. It is typically done by exchanging electromagnetic signals between the two objects and measuring their travel time. Depending on the specific protocol it can be either one-way, ground station to satellite and vice versa, or two-way measurement, round trip from ground station to satellite and back, with some variations in details. The optical pulses generated by the ground station’s transmitter are either detected by the satellite or reflected towards the ground station, where they are measured by the receiver. In the former (latter) of these cases the distance d between the ground station and the spacecraft can be calculated as d = cτ (d = cτ/2), where τ is the measured travel time of the signal.

There exist various ranging methods designed for optical frequencies, including satellite laser ranging in one-and two-way synchronous and asynchronous variants, interferometry laser ranging and the techniques based on bit clock or carrier phase measurements^13–19. In this work we focus on the optical telemetry ranging⁵ in two-way synchronous and asynchronous variants. Telemetry ranging is the technique in which electromagnetic signals are used for simultaneous optical ranging measurement and data sharing. In it’s basic asynchronous variant, illustrated in Figure 1, the ground station sends to the satellite a stream of data and registers the time t_T at the ground station’s clock at which a particular uplink frame was initiated. Simultaneously, the satellite sends to the ground station a separate stream of data, which is not synchronized with the uplink transmission. Whenever a new downlink frame begins, the satellite’s receiver measures the current phase of the uplink signal. For example if the measured bit is the n-th bit belonging to the current uplink frame and there are N bits in each frame, then the current phase of the uplink signal, measured with the accuracy of 2π/N, is ϕ = 2πn/N. The information on ϕ is included in the downlink telemetry data, typically in the subsequent frame after the one that initiated the particular measurement. Then, when the first bit of a new downlink frame is detected by the ground station’s receiver, the information of the current local time t_R at the ground station’s clock is taken and the information included in the downlink data stream is used to calculate the round-trip time of the signal τ. If T_u (T_d) is the time duration of a single (uplink) downlink frame, then τ can be calculated as

Figure 1.

Telemetry ranging system concept

where the third term is due to the assumed one-frame delay in relaying of the ranging data at the satellite. In the synchronous version of the protocol uplink and downlink telemetry signals are synchronized with each other, making the phase, ϕ, measurement unnecessary. However, such synchronization may be difficult to obtain, especially if the distance between the spacecraft and the ground station changes rapidly e.g. in the case of LEO satellites.

3.

PARAMETERS OF OPTICAL LINK

Over the years, there have been different studies on modelling and simulation of atmospheric turbulence^20–23. The references^{20, 22} uses Kolmogorov atmospheric turbulence model for simulation while the reference²³ uses the von Karmann model as basis for modelling of time-varying turbulence signal as a function of receiver aperture diameter and rms wind speed. In our case, we focus mainly on optical scintillation effect observed on ranging signal due to atmospheric turbulence. The optical communication link is affected by the atmospheric turbulence as well as the motion of satellite leading to fluctuations in optical frequency and signal strength over the duration of communication. Optical scintillation originates from temperature and pressure variations in the atmosphere, causing continuous fluctuations in the value of refraction index along the propagation path. It leads to the time varying fluctuations in the intensity of the optical signal and variation in propagation time of the signal. The fluctuations of refractive index are quantified with the scintillation index, . Statistical models such as Kolmogorov turbulence model for weak turbulence, non-Kolmogorov and von-Karmann model for moderate to strong turbulence, describe the power spectral density of refractive index fluctuations. They are characterized by the refractive/turbulence structure parameter . The scintillation index, , quantifies the normalized intensity variance caused by atmospheric turbulence. For a plane wave it is given by Rytov variance based on Kolmogorov model as:

where, k = 2π/λ is the wavenumber, L is the path length between transmitter and receiver. The Rytov variance represents scintillation index of an unbounded plane wave in weak fluctuations based on Kolmogorov spectrum^{5, 8}. The wavelengths considered in this project are λ = 1530 nm for uplink and 1550 nm for downlink. For the vertical and slant optical path between satellite and ground stations, the equation of scintillation index is further modified with respect to the satellite’s co-ordinates with respect to ground station⁸. The scintillation index is form of log-irradiance, N (Np²) function²⁴:

where, Φ is zenith angle, h₀ is the height of the ground station measured from sea level, H is the effective height of optical turbulence path, z is height above ground level.

According to the ITU recommendations ITU-R P1621-2 and ITU-R P1622-0^{24, 25}, for frequencies between 20 THz and 375 THz (0.799 μm and 14.98 μm), the Huffnagel-Valley 5/7 model equation can be used to estimate the value of :

where h is the height above the ground level, C₀ stands for nominal value of at the ground level, typically 1.7×10^–14m^–2/3, and v_rms is a speed along vertical path calculated using Bufton wind model^{8, 24} and it is given by:

Here, if the ground wind speed, v_g, is unknown, then v_g = 2.3m/s can be taken as an approximation, thus giving v_rms value to be 21m/s^{8, 24}. The duration of time for which satellite is visible to ground station depends on the orbit height and its velocity. The characteristic time scale for continuous fluctuations due to scintillation is of the order of milliseconds. They are caused by the pockets, eddies, of varying refractive index formed in atmosphere due to varying temperature and pressure. As the satellite passes over the ground station, the propagation path of optical signal varies continuously, so does the thickness of atmospheric layers. This systematic variation in optical path length and constitution results in a overall ‘slow fading’ of the signal. On the other hand, with respect to the former effect, scintillation is ‘fast-fading’ effect. The time-varying optical turbulence effect is simulated based on combination of these two effects and additionally Doppler effect due to satellite’s relative motion.

4.

SATELLITE ORBITING AND ATMOSPHERE EFFECTS SIMULATOR

The schematic diagram of the breadboard implementation is presented in Figure 2. Free space link between the ground station and the spacecraft will be implemented by SERDES transceivers connected to the optical system through SFP+ optical modules. The Ground station and the spacecraft FPGA along with the atmospheric turbulence simulator are interfaced by the Windows OS personal computer with restful API server. The breadboard is fiber based with no free space links.

Figure 2.

Test Setup Design

The atmosphere simulator, implemented in fiber optic technology, emulates the optical turbulence effects such as optical scintillation experienced by optical ranging signal while propagation.

In atmospheric turbulence simulator, the transmitter, receiver and the network PC for turbulence simulation are interfaced using Python platform. The time-varying turbulence simulation code based on scintillation will be developed using Python. An arbitary waveform generator (SDG 2042X, 40MHz max bandwidth and 1.2 G sampling rate, 20V pp output) with python interfacing compatibility is used to generate the turbulence signal which is then supplied to the fiber based electro-optical modulator. The experimental design is presended in Figure 3. This modulator (JDSU, 10020425 OC-192, with operational wavelength 1530-1605 nm) is used for varying the intensity of the optical signal. A fraction of the transmitted signal, directed to a photodiode and observed on oscilloscope, is used to monitor and verification of the performance of the simulator. This signal is stored and can be used for comparison with the modulated signal observed at the receiver end after successful optical ranging measurement. The specifications of both waveform generator and optical modulator is sufficient for the required operating specification as the time scale of optical scintillation is of the order of milliseconds.

Figure 3.

Optical turbulence simulator: ‘T_x/R_x’ module controlled by FPGA for optical signal/ranging, electro-optic modulator (EOM) (part. no: JDSU 10020425 OC-192, 10Gbps), FBS: Fiber beam splitters, SMF: Single mode fibers, PD: Fast photodiode.

5.

SUMMARY

In this paper we have presented the preliminary design of the test setup for the new variant of satellite optical telemetry ranging protocol. It is being developed under the contract ARTES 4.0 SPL Optical Communication – CCSDS standardized ranging for optical communication terminals with European Space Agency. The setup is currently in the detailed design phase.

As the result of detail design phase the Ground Station IP Core for O3K RS ranging implementation will be prepared. Ranging IP Core will be used in M3O optical modem, which is under preparation in Work Microwave GmbH company as a part of 4000135208/21/NL/FGL/fm - M3O optical receive only modem project from ESA.

We have also presented the design of the turbulence simulator that will be used to emulate the Earth-to-satellite free space optical link implemented in fiber-optic technology. The simulator can be used to test any satellite laser communication protocol. The tests results of the developed hardware setup will be the inputs to Consultative Committee for Space Data Systems (CCSDS) working group standardization process.