OpticalLink Analysis

The OpticalLink object models laser-based communications between spacecraft and ground assets, including calculation of received power, losses, signal-to-noise ratio (SNR), bit error rate (BER), and link margin.

Creating the OpticalLink Object

You can create and edit the OpticalLink object in FreeFlyer via the FreeForm Script Editor. The OpticalLink object includes configuration for wavelength, modulation type, transmitter/receiver models, and atmospheric loss modeling.

Using the FreeForm Script Editor

Transmitter and Receiver Child Objects

The OpticalLink.Transmitter and OpticalLink.Receiver child objects provide access to specific hardware configurations. These components support modeling of real-world systems including APDs, PINs, preamplified detectors, and coherent receivers. The OpticalLink object allows the following objects to act as the transmitter or receiver.

Once a transmitter or receiver has been set to a reference object, you can access the transmitter or receiver properties and configure it appropriately. The following script sample demonstrates both the definition of an OpticalLink object and the assignment of a number of receiver and transmitter properties.

Modulation Type

You can define whether to use On-Off Keying (OOK) or Pulse Position Modulation (PPM) schemes [1] for performing OpticalLink analysis. OOK is the simplest form of intensity modulation, where the presence or absence of a light pulse represents a binary 1 or 0. PPM, on the other hand, encodes multiple bits per symbol by varying the position of a single pulse within a series of time slots. For example, 16-PPM allows encoding 4 bits per symbol.

PPM generally offers higher sensitivity and better performance under low power or high-noise conditions but requires more precise timing and wider bandwidth. OOK is simpler and more bandwidth-efficient for lower data rates. By default, OpticalLink objects will use the OOK modulation type.

FreeFlyer supports several variants and advanced schemes:

To configure modulation behavior in FreeFlyer, set the OpticalLink.ModulationType property. Some modulation types, such as Pulse Position Modulation (PPM), have additional properties for example, LPPM defines the number of time slots per symbol in PPM (e.g., 16 for 16-PPM). Other types, like DPSK or OOK, may not require extra configuration beyond selecting the scheme.

Atmospheric Loss Modeling

The OpticalLink object's AtmosphericLossParameters property provides a configurable model for estimating the impact of various atmospheric phenomena on an optical communication link. It accounts for meteorological visibility, rain rate, snow rate [2], and scintillation-induced fading. These parameters help approximate real-world loss conditions experienced during clear and degraded weather events.

Configuring Atmospheric Parameters

Scattering Loss

Scattering loss is primarily influenced by the visibility distance, and is modeled as a function of the wavelength, elevation angle, and empirical extinction coefficients [3]. Lower visibility values result in greater attenuation. Use the OpticalLink.GetAtmosphericScatteringLoss() method to compute this term.

Scintillation Loss and Fading

Atmospheric turbulence causes fluctuations in the received signal power, referred to as scintillation. FreeFlyer models scintillation using a log-normal fading model [4]. The user can specify a target fade probability, and FreeFlyer will compute a link deficit value that accounts for the probabilistic fade at that level of confidence.

oScintillation Index: Represents the normalized variance of signal intensity due to turbulence [4].

oFade Probability: The user-defined percentile (e.g., 1%) at which fades are expected. FreeFlyer computes a corresponding margin using a log-normal model [4].

Rain and Snow Attenuation

Attenuation from precipitation can be modeled by setting the rain rate (mm/hr) and snow rate (mm/hr) independently. Optical frequencies are particularly susceptible to rain-induced attenuation due to absorption and scattering by water droplets and snow. Snow attenuation accounts for larger particles and can be combined with visibility modeling. These losses are frequency dependent, and combine with visibility-driven attenuation to model degraded weather conditions.

Performance Metrics

Signal-to-Noise Ratio (SNR)

SNR (Signal-to-Noise Ratio) represents the ratio of received signal power to noise power at the receiver. In optical communications, this includes the effects of received power, modulation type, and receiver characteristics such as responsivity and quantum efficiency. SNR is a key driver of BER performance. You can calculate a required SNR directly from a targeted BER [5], or compute it indirectly through a full link budget analysis.

Bit Error Rate (BER)

BER (Bit Error Rate) indicates the likelihood that a transmitted bit will be received incorrectly. BER depends on the selected modulation type (such as OOK or PPM), the SNR, and physical receiver characteristics. FreeFlyer uses modulation-aware analytical expressions to compute BER for a given SNR value. These expressions take into account the specific characteristics of the chosen modulation scheme - for example, On-Off Keying (OOK) assumes a binary detection threshold, while Pulse Position Modulation (PPM) accounts for the number of time slots per symbol (e.g., 16-PPM) and calculates the probability of symbol and bit error accordingly. This allows FreeFlyer to provide accurate BER estimates across a variety of modulation types and performance requirements.

SNR to BER Conversion

You can convert between SNR and BER values using FreeFlyer's built-in functions. This is useful when assessing whether a system can meet error rate thresholds at a given power and distance, or when designing to a required margin.

OpticalLink Calculations

The OpticalLink object calculates the transmitter and gain as well as a number of other values. In the following calculations, a standardized notation system has been used as follows.

Optical Gain

Transmitter gain [6] represents how effectively the optical transmitter focuses its output laser beam, typically using a diffraction-limited aperture.

where Θ represents the beam divergence.

Receiver gain [6] represents the receiver's effective area for collecting incoming photons and converting them into electrical signals. It depends on the receiver’s aperture size and optical efficiency.

where λ represents the wavelength and A represents the Area. The Area, A, is calculated using Equation 3:

where D represents the telescope diameter.

The real gain can be calculated in FreeFlyer using the following methods.

Free Space Loss

Also known as the free space path loss (FSPL), represents the attenuation of optical power as the beam propagates through space, due solely to the expansion of the beam over distance. This is a geometric loss, not caused by the medium. FSPL is a major driver for requiring high-gain optics in laser systems. The formula [3] to arrive at this value is related to the transmitter wavelength (λ) and range between the receiver and transmitter both in meters.

The FSPL can be calculated in FreeFlyer using the following method.

Received Power

Received power is the total optical power received at the aperture of the optical receiver after accounting for free space loss, atmospheric attenuation, pointing losses, etc.

where represents the transmitter pointing loss, represents the transmitter system losses, and represents the receiver system losses.

The received power can be calculated in FreeFlyer using the following methods.

Link Margin

The link margin in dB can be found in a number of ways much like the SNR above. Link margin represents the difference between the actual received power at the optical receiver and the minimum power required for reliable data reception. It provides a quantitative measure of how much excess signal strength is available above the system’s sensitivity threshold. A positive link margin indicates a viable link, while a negative margin suggests that the communication may fail under the current conditions. FreeFlyer computes link margin dynamically based on the full optical link budget, including free space loss, atmospheric effects, pointing loss, and receiver sensitivity.

where represents the received power in dBm, and represents the receiver's sensitivity in dBm. The link margin can be calculated in FreeFlyer using the following approach.

Note, for transmission through the atmosphere, this method computes the losses inclusive of precipitation effects (rain, snow)  and atmospheric scattering.

References:

See Also

•OpticalLink Properties and Methods

•Contact Method Summary