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The UHF transmitter mainly transmits real-time telemetry data that is ..... 20-24 July 2009. [3] Telemetry Channel Coding, CCSDS 101.0-B-4, May 1999. [4] Space ...
Communication Subsystem of RASAT Dilşad İçöz, Nazlı Deniz Kahyaoğlu, Muhsin Bölücek, Emrah Öncü, Tunahan Kırılmaz, Celal Dudak, Hacer Sunay, Volkan Akan, Özlem Şen Communication Systems Group TUBITAK UZAY- Space Technologies Research Institute 06531 ODTU Ankara, TURKEY [email protected] Abstract— RASAT will be the first satellite designed and manufactured in Turkey and it is planned to be launched in 2011. RASAT is a Low Earth Orbit (LEO) observation microsatellite with a mass of around 100 kg. The main purpose of this mission is remote sensing with use of a high resolution optical imager. The communication subsystem is one of the primary systems for Earth observation satellites. It is used for telecommand/telemetry signaling besides downlink data transfer. In RASAT’s communication subsystem, there are mainly five modules. For commissioning phase and as back-up of the downlink/uplink operations, Ultra High Frequency (UHF) / Very High Frequency (VHF) modules are used. The primary uplink is provided by the S-Band receivers and the primary downlink is provided by the SBand transmitters on-board. X-Band transmitter module, which is designed and developed by engineers at TUBITAK-UZAY, is the experimental payload of RASAT communication subsystem. Keywords- RASAT, Communication System, LEO satellite

I.

INTRODUCTION

After successful completion of BILSAT-1 project in 2003, TUBITAK-UZAY (formerly TUBITAK-BILTEN) started RASAT project to develop and manufacture small satellite technologies covering all phases from design to production and in-orbit operation in 2004. For this purpose, all the infrastructure required, which includes assembly and integration rooms, laboratories for environmental and functional testing and a satellite mission control ground station, is constructed at TÜBİTAK-UZAY’s premises [1]. In RASAT, the commissioning phase will be achieved by UHF/VHF modules, since omni-directional coverage required in that phase can only be provided by UHF/VHF antennas of the system. S-Band, being the nominal operation system, has relative directionality; the primary uplink is provided by the SBand receivers and the primary downlink is provided by the SBand transmitters. UHF/VHF and S-Band modules are the core modules of RASAT and they have space heritage from SSTL’s previous missions.. On RASAT, the core receiver modules are hot redundant. On the other hand, the core transmitter modules are cold redundant on-board due to power consumption. The VHF receivers use 9k6 bps frequency shift keying (FSK) modulation scheme for uploading and four blade antennas are utilized for omni-directional coverage. The UHF transmitter comprises of two transmitters, with 3 W and 10 W RF outputs, respectively. Modulation scheme is FSK here, with 9k6, 38k4 and 76k8 data rate choices for downlink.

This work has been supported by RASAT Satellite Development Project which is financed by DPT (State Planning Organization of Turkey) under grant contract no 2007K120030.

The S-Band receivers use 9k6 bps FSK modulation in commercial space band. Two patch antennas will be used for each receiver, one on the earth-facing facet and the other on the space-facing facet of the satellite. For each S-Band transmitter, a directional Quadrifilar Helix (QFH) antenna is utilized on RASAT. S-Band transmitters are capable of 2 Mbps data rate with binary phase shift keying (BPSK) modulation and they provide at least 2 W RF output power. X-Band transmitter is one of the experimental payloads on RASAT, providing high data transfer rate for downlink of image data. This module has three different data rates (25 Mbps, 50 Mbps, 100 Mbps), two different modulation scheme options (quadrature phase shift keying - QPSK and offset– QPSK) and 7 W (38.5 dBm) output power which satisfies the link budget for a LEO satellite [2]. This module will have space heritage with successful launch of RASAT. In order to prove the space conditions compatibility of a satellite module, the engineering qualifying model of that module should be exposed to a set of tests, namely thermal vacuum test, vibration test and electromagnetic compatibility (EMC) test. These tests are realized for the X-Band transmitter, the first two at TUBİTAK UZAY facility in Ankara and the EMC test at a private facility in Gebze. As well, thermal vacuum and vibration tests are performed for the other modules which have space heritage from BILSAT-1. In the following sections, the specifications of each communication module are presented together with the results of the environmental tests. II.

UHF TRANSMITTER

The UHF transmitter mainly transmits real-time telemetry data that is processed by on-board computer (OBC) to the ground station. It synthesizes the clock signal for the OBC which uses this for telemetry data transmission. UHF transmitter, which has omni-directional coverage and a long flight heritage, is also a back-up module for S-Band and XBand transmitters. The module is cold redundant due to power constraints of RASAT. The operating temperature range of the module is -20˚C to +50˚C. UHF module is housed by a microtray and the mass is approximately 2 kg. There are two UHF transmitter chains placed on the same micro-tray in which the RF output powers are the only difference. One of the transmitter chains has 3W RF output and the other transmitter chain has 10W RF output and they are

named as Tx0 and Tx1, respectively. The carrier frequency of the transmitter is 437.4 MHz. The modulation type is continuous phase FSK (CPFSK) with square root raised cosine (RRC) filtering. The data rates of the transmitter are 9.6 kbps, 38.4 kbps and 76.8 kbps. The block diagram of the transmitter module is shown in Figure 1.

rejection of out of band signals and passes the RF signal with a minimum attenuation. The Tx switch circuit is used to select one of the two transmitter chain outputs (3W chain and 10W chain) and pass the selected chain’s RF signal to the antennas. The flight model of the module is shown in Figure 2.

CAN I/F

Modulator

Synthesiser

Driver Amplifier

Intermediate Amplifier

Directional Coupler

TX Filter TX0 Chain

DC/DC Converters

TX Switch TX1 Chain

Modulator

Synthesiser

Driver Amplifier

Intermediate Amplifier

Power Amplifier

Directional Coupler

TX Filter

Figure 2. UHF Flight Model

CAN I/F

Figure 1. Block Diagram of UHF Transmitter

UHF transmitter module has two DC/DC converters for each chain. One is 28/15V converter and the other one is 28/5V converter. They feed all the circuits in the module. The modulator circuit generates RRC shape pre-distorted pulse which is used to modulate a constant envelope FM carrier wave. This circuit synthesizes the required clock and amplitude deviation changes required for multi-rate FSK. The modulator circuit scrambles the digital data in order to minimize the number of concurrent 1’s in the transmitted data. The analog output data from the modulator is connected to the modulation input of the synthesizer and it is used to modulate the information onto the RF carrier signal. The synthesizer circuit produces the RF signal. It is based on a phase locked loop (PLL) chip and a (voltage-controlled oscillator) VCO. The RF signal has poor long term drift characteristic. This is the reason for synthesizer circuit locks the RF signal to a very stable crystal oscillator. The circuit also contains a P87C592 micro-controller. The micro-controller contains 8 A/D inputs that are used for telemetry monitoring within the module. The amplifier chains are used for increasing RF power of the modulated signal. Tx0 has two amplifier stages and Tx1 has three amplifier stages. The driver amplifier is a thermally stabilized, class AB RF amplifier. The heart of the circuit is the Motorola MRF313 RF transistor. The input and output of this device is matched to 50 ohm by using L-matching circuits. The intermediate amplifier is also a class AB RF amplifier. The main component of the circuit is NEC 2559 transistor. Lmatching circuits are used to match the input and output of the transistor. The power amplifier is built around a Motorola MRF641 transistor. The input and output matching circuits are L-matching circuits with series DC blocking capacitors. The gain of the power amplifier is 6 dB in the flight model of the module. The directional coupler board is a microstrip coupler that monitors the forward and reverse RF powers and gives an analog telemetry value by using AM detector. The transmit (Tx) filter circuit is a lumped element filter which provides

Thermal test results of the Tx0 transmitter chain for 9k6 data rate are given in Table I. TABLE I.

THERMAL TEST RESULTS OF UHF TX0 (9K6)

Temperature RF Output Power (dBm) Frequency Deviation (KHz) Carrier Drift (KHz) Spurious < -40dBc?

III.

-20 ˚C 34.6

+20 ˚C 34.8

+50 ˚C 34.4

7

7

8

3

1

4.8

YES

YES

YES

VHF RECEIVER

On RASAT there are two hot redundant VHF receivers that are switched on automatically when the satellite leaves the launcher vehicle. VHF receivers are used in commissioning phase and as back-up of the uplink operations; and for omnidirectional coverage four blade antennas are employed. The receivers were thermally tested at a range of -20 to +50˚C which is the estimated range for RASAT at an altitude of approximately 700 km. Both receivers are housed in one nano-tray and each is about 0.5 kg. VHF receiver module is shown in Figure 3.

Figure 3. VHF Receiver Module

VHF receiver module mainly consists of two sections, a digital circuitry and an RF section which is separated by screening can from the digital part. In the RF section, there exist; a low pass filter (LPF) to compete interference from the transmitters, a 20 MHz wideband pass filter with in-band insertion loss of 1.9 dB, a low noise amplifier (LNA) with 1318 dB gain and