Payload Analyses

Introduction

The payload is divided into 2 modules, the antenna and payload electronics. Antenna simulation was carried out using HFSS and CST while the electronics simulation was done using Keysight Genesys. The results of the simulation were compared to the pre-defined design requirements. 

1- Antenna

• The payload antenna is the Offset Cylindrical Reflector chosen for its ease of deployment with high performance capabilities. The system comprises a reflector and a feed, illustrated in figure below.

• The design relies on the geometric relationship of the parabola, which correlates the feed location, F= 91 mm, with the radiating aperture diameter, D= 144 mm; F/D= 0.63.

• Due to its cylindrical shape, the antenna has focal line that necessitates a line source (patch array feed) rather than a point source to provide proper illumination.

• The feed is offset and tilted by 45° to illuminate the reflector, chosen in this configuration to prevent feed blockage caused by center feed.

Reflector feed

• To illuminate the offset reflector, a linear array of microstrip patches was utilized. The selected setup consists of a two-row 2 ×8 element patch array as shown in figure below; L= 72 mm and W= 29 mm.

•  The Rogers RT5880 substrate was chosen to reduce the weight of the feed array while maintaining an adequate bandwidth for the intended mission.

• The feed operates at 18.7 GHz with a bandwidth ranging from 18.4 GHz to 18.9 GHz, as depicted in figure below.

Reflector gain

•  The maximum calculated directivity is 27.4 dBi. However, the simulated directivity is 24.7 dBi, resulting in an efficiency of 54%. The discrepancy of 2.7 dBi is attributed to taper and spillover losses, along with feed and mismatch losses. The figure below shows the maximum 3D gain.

• The co- and cross-polarization radiation patterns in both cuts are presented in the figure below.

Deployment mechanism

• The antenna is designed to fit within a 1U volume in its stowed configuration. 

•  The reflector can be easily deployed by popping it out of the 1U. The top wall and one side wall remain open to facilitate deployment.

2 - Radiometer

The current PLD radiometer design is in its preliminary phase, hence this page may not contain all details on the design. This page will be updated as more information is available.

The preliminary specifications of the radiometer derived from simulation and the breadboard design are listed below 

Simulation Results

Keysight Genesys was used to perform simulation on the entire radiometer system. It was used to compute the cascaded noise figure and gain. The atmospheric path loss for a satellite at 550km was calculated to be 172.7 dB. Using the atmospheric path loss and the minimum input power level required for the square law detector, an estimated system gain was specified.

Radiometer Sensitivity Analysis

A sensitivity calculation was performed for the payload radiometer to determine the Noise Equivalent Delta Temperature. This was calculated from the simulated cascaded noise figure using the formula below: 

• Tsys = system noise temperature (determined from noise figure)

• B = radiometer bandwidth

• T = integration time

• Tn = noise temperature (calculated from the noise diode ENR)

From this, the computed noise equivalent delta temperature is 0.31 K. This means the radiometer can differentiate between targets with a 0.31 K temperature difference.

Payload Footprint Analysis

For the science component of the payload design, the payload footprint was determined. The footprint is dependent of the satellite altitude, incidence angle, and payload antenna HPBW. 

Radiometer Breadboard Model

To validate the simulated results before fabricating the PCB-based flight model, a prototype was developed using easy-to-access microwave components. This prototype was designed as part of the FlatSat with easy-to-access test ports as shown in image below. Evaluation boards of the same proposed MMIC components for the EM were used to speed up the development process. The frequency response of each component was measured to ensure a match with the simulated values. 

Since the total power available to the payload is limited, the FlatSat underwent an inrush current test to determine its power consumption at startup and during normal operation. The current consumption during normal operation was 2.38 W, while at startup, it was 2.43 W, and these are both within the range of the maximum available power. To validate the radiometer's performance, it was tested under different measurement conditions in laboratory conditions. As the prototype design is still in its verification phase, the radiometer's calibration has not been completed.

The breadboard radiometer filter frequency response was measured and shown below.  Another characterization test performed was measuring the response of the radiometer due to the introduction of a target in the lab. For the measurement, the system was placed in a lab with a physical temperature of (22.9 C ). The test was carried out over 2 hours with the radiometer measuring the room for the first hour. Then a wood block that had been chilled to 4 C was introduced in view of the antenna for the next hour.