PROPOSAL SUMMARY

INSTRUMENT DEFINITION

1 Introduction

2 Measurement Requirements & Instrument Characteristics

3 Instrument Description

TABLES

Table 1 TOPAS/W Instrument Expertise

Table 2 TOPAS/W Instrument Operational Characteristics

Table 3 TOPAS/W Waveforms and Associated SignalProcessing Gains

Table 4 TOPAS/W Measurement Modes

Table 5 TOPAS/W Mass Properties

Table 6 TOPAS/W Subassembly Power Details

It is proposed to design and build a topside ionospheric sounder to provide real-time ionospheric specifications. The design of the sounder will be based on the Magnetospheric Radio Plasma Imager (RPI), currently under construction for NASA’s IMAGE satellite mission scheduled for launch on 1 January 2000, and the prototype development of the Topside Automated Sounder (TOPAS/N) for the US NPOESS program. The techniques implemented in these instruments have their heritage in groundbased ionospheric sounders, the “Digisondes”, developed and built at the University of Massachusetts Lowell and now deployed and operated by 20 different commercial and governmental organizations at 60 operational sites around the world.

The TOPside Automated Sounder for the Ukrainian WARNING mission (TOPAS/W) will probe plasma densities from the altitude of the spacecraft at 600 km down to the peak of the F2-layer using only 30 W peak transmitter power. Three orthogonal 20m dipole antennas will be used allowing the measurement of wave polarization and the angle of arrival in addition to amplitude, phase and Doppler information as a function of range. Advanced waveform coding and processing techniques will provide coherent integration of the received echoes while filtering out interfering signals and noise. Doppler and angle-of arrival information will ensure correct measurement of plasma density-versus-altitude at the subsatellite point in the presence of ionospheric irregularities that produce off-vertical echoes. Routine swept-frequency measurements will have a plasma density resolution of nominally 5% and height resolution of 5 Km, and special 1% measurements can be programmed. Automated ionogram scaling and profile inversion algorithms will provide real-time electron density profiles specifying the height and density of the F2 layer peak and the shape of the topside profile. The TOPAS/W instrument will be able to scan from 0.5 to 30 MHz so that local resonance frequencies as well as ground reflections can be measured.

A variety of sounding modes can be selected through sets of program parameters that are preprogrammed before launch and that can be modified via telemetry if the subsatellite has an uplink capability. This feature can optimize the sounding mode for different regions or for different measurement objectives. Data formats and telemetry stream will be similar to the RPI instrument on IMAGE.

Background: Digital ionospheric radio sounding from space will provide remote density measurements of unprecedented precision and coverage, from which the structure, inter-relationship, and variations of different plasma regions can be determined to give a real-time global model of the state of the ionosphere. In conjunction with other Space Environment Sensors (e.g. real-time magnetospheric remote sensing from a compliment of instruments like NASA’s IMAGE satellite) prediction of major changes to the space environment will be possible.

Objectives and Strategy: The objective of the proposed effort is to develop and build the spaceborne instrumentation together with the real time software for sounding at low power to minimize weight, volume, and the strain on components and to reduce the requirements for electric power. The transmitter power and the antenna configuration must be appropriately dimensioned to achieve signal-to-noise ratios that are adequate for reliable automatic scaling of the topside ionograms.

Heritage: The proposed effort follows naturally on UML’s 30 years experience in groundbased ionospheric sounding, especially the development of the Digisonde Portable Sounder “DPS” [Reinisch, 1996; Reinisch et al., 1997]; the Radio Plasma Imager (RPI) instrument [Benson et al., 1997; Calvert et al., 1995] for the IMAGE satellite mission which is scheduled for launch in January 2000; and the prototype of a TOPside Automated Sounder (TOPAS/N) for the United States National Polar-Orbit Operational Environmental Satellite System (NPOESS). Table 1 summarizes the relevant UML experience.

The TOPAS/W instrument will be built using UML’s extensive groundbased and space-based experience. The proposed miniaturized digital sounder will be able to step through the frequency range from 0.5 to 30 MHz, driving a short tubular dipole antenna with nominally 30 W of RF power. Digital signal processing techniques, including coherent Doppler integration and pulse compression, will be adapted from the highly successful Digisonde technology developed during the last 25 years for groundbased sounding. This technology has also been incorporated into the design of the Radio Plasma Imager instrument for IMAGE.

The Topas/W transmitter will have two independent power amplifiers (thus providing redundancy) each driving a maximum of 15 W into each monopole of a 20 m dipole. All three dipoles are used for reception. Tuneable series L-C antenna couplers will be used for the transmit antenna, and a variable-voltage power supply will limit the maximum radiated power to 10 W. The transmitter antenna couplers and power amplifiers for the two monopoles will be mounted at the satellite skin directly driving the monopoles. The maximum RF voltage will be 4.5 kV at 0.5 MHz, eliminating the risk of arcs and component damage.

Based on the available preliminary specifications for the subsatellite Active Monitoring spacecraft orbit, the instrument characteristics for the WARNING mission have been developed and are summarized in Table 2. A 600 km circular orbit was assumed with an inclination of 73.5o. It was further assumed that the active subsatellite can support three orthogonal dipole antennas with 20 m tip-to-tip length.

Digital signal processing tradeoffs: The proposed digital processing techniques require long transmitter pulses with a width of Mx33 ms where M is the number of phase chips that compose the transmit pulse. An 8 chip complemetary phase code [Haines, 1994] with orthogonal pulse pairs (resulting in a total of 16 chips) provides 12 dB processing gain. The resulting long transmitter pulse allows reception only after 267 ms, i.e. the minimum range is 40 km. For the observation of the F2 layer peak which will generally be at least 200 km below the satellite, a longer 16 chip code can be used providing an additional 3 dB of processing gain. The different waveforms in Table 3 provide the same range resolution of 5 km, but the sensitivity increases with the length of the pulses as shown in the Gain column. To measure the plasma density in the vicinity of the satellite a short 33.3 ms pulse can be used. This “relaxation” mode does not require high processing gain because the received resonance signals are strong.

(*an 8 point Fourier transform is assumed)

A 2N point Fourier transform for each range provides additional signal processing gain of Nx3 dB, e.g. 9 dB gain for an 8 point transform [Bibl and Reinisch, 1978]. Each of the 2N time-samples used in the Fourier transform is taken at the same time delay after each of the transmitted pulses making the total integration time 2N times the inter-pulse period T. For the complemetary phase code two pulses are transmitted for each time-sample, and therefore T=10 ms for a pulse repetion rate of 200 Hz. The coherent integration time for an 8 point Fourier transform is therefore 80 ms, and the total time to scan from 2 to 18 MHz with 2.5% frequency steps becomes about 7 s. Given a satellite velocity of 7.6 km/s this corresponds to a spatial resolution of 53 km. The spatial resolution can be improved to 27 km if 5% frequency steps (10% density resolution) are selected.

This section provides a brief technical summary of the proposed TOPAS/W subsystem. Referring to Figure 6, the TOPAS/W creates the transmitted signals in the Exciter block using coherent local oscillators (sine-wave sources provided by the Oscillator and Synthesizer blocks) and gated by signals from the Timing block. Sequencing of frequencies and measurement functions is controlled by the CPU. The signals received at the three orthogonal antennas are amplified in low-noise preamplifiers and fed into three sensitive phase coherent receivers. Sample records containing the desired echoes are made at each receiver output by the multi-channel Digitizer and processed independently in the CPU. At the end of each frequency step the three (one for each antenna) processed records are placed in a buffer to be downlinked after thresholding and compression.

The TOPAS/W instrument is comprised of the TOPAS/W Electronics unit, six 10 m stem antennas, four antenna Pre-Amps, and two antenna Pre-Amp/Couplers (for reception/transmission).

Radiation hardening: Since the proposed orbit passes through the lower Van Allen belts and stays in this region for a large percentage of the orbit a highly radiation hardened system will be built. As on the IMAGE satellite, a radiation hardened processor board (Loral Federal Systems, RAD6000) based on Motorola’s Power PC RISC chip will be used. A Rad-Hard digital synthesizer chip identified for the RPI synthesizer board is well suited to the WARNING mission. Also, digital interface logic will be implemented in Rad-Hard FPGA’s (field programmable gate-arrays) and Rad-Hard CMOS circuits to complete the Rad-Hard design approach. As for the analog parts, radiation hardness is inherent in many of these materials, however, only those that have been successfully radiation tested by the manufacturer will be selected for the final flight hardware. The entire payload will be designed using components rated for 100 krad total dose or more.

Software Description: The measurement sequences are derived from stored measurement parameter tables, specifying the desired measurement in terms of frequency steps, number of pulses to be integrated at each frequency step, pulse repetition rate, transmitted waveform, range resolution and number of range bins to be observed.

A schedule table is also stored which describes the start times of each measurement sequence and which parameter table is to be used in the measurement. A set of schedule tables is available and can be selected, based on orbital position automatically switching as orbital position is updated. Several of these parameter and schedule tables can be stored (in EEPROM) and updates can be made from the ground.

The software also controls the data acquisition functions of sampling the received signals, processing the sampled echoes, formatting output records and transferring data for downlinking via telemetry through the S/C system. These output data records will vary in length according to the measurement parameters selected, but they will always begin with a 60 byte header, the data “preface”, which contains the time and all measurement parameters applicable during the measurement. The typical ionogram record following this header will consist of 256 amplitudes for each frequency step (representing the echo profile) and 256 status characters representing Doppler and angle of arrival. Therefore, the data volume produced is about 6500 bytes per second. If TOPAS is operated continuously this >50 kbps data rate could be difficult to provide. If it is, standard data compression techniques can reduce this to 10kbps and a parametric output, possibly with a compressed ionogram image could get the datarate down to about 2 kbps. Note that a raw data dump which includes the entire Doppler spectrum at each height and on each antenna, requires roughly 16 times more telemetry capacity, and would probably only be requested at very infrequent intervals.

Upon power-up of the TOPAS/W, the TOPAS/W will load a bootstrap program from on-board EPROM, then load the operating software and the measurement and schedule tables from on-board EEPROM. The EEPROM would allow new operating software to be uploaded from the ground.

Operational Modes: The TOPAS/W can operate in a variety of modes, depending on the parameters selected from the measurement parameter tables; however, all measurement modes result in a range profile with range, amplitude phase, and Doppler for observed echoes at each range on each antenna and at each frequency step. So-called fixed-frequency modes on one or several selected frequencies can be programmed. By attaching a data preface (a header) to each data record, all parameters pertaining to that measurement can be associated with the signal data, allowing data to be output as packets for transmission to the ground without concern about loss of tagging data. The extensive source code for mode control and data packaging will be adapted from the DPS and RPI instrument development.

The variety of measurement modes listed in Table 3 mainly entail different transmitted waveforms, but all are resulting in the standard output data format. A fourth mode provides a passive operational mode which allows useful science data to be gathered during periods of low S/C power; a fifth mode is a standby mode which applies power to the TOPAS/W CPU only, no measurements are made in this mode with the CPU awaiting a future command for activation of the rest of the instrument. A summary of these five modes is presented in Table 4.

Performance Requirements:

Frequency Range: The frequency range for TOPAS/W shall be limited from 0.5 to 30 MHz.

Transmitter Power: There shall be 10W peak power radiated from one dipole antenna.

Pulse Repetition Rates: The pulse repetition rates shall be 50, 100, or 200 Hz.

Pulse Widths: The pulse widths shall be 33.33 µs, 267 µs or 533 µs.

Minimum Detection Range: Minimum detection range shall be 10 km, 40 km or 80 km.

Range Resolution: The range resolution of TOPAS/W shall be 5 km

Receiver Bandwidth: The receiver bandwidth shall be 30 kHz.

Frequency Accuracy: The Frequency Accuracy shall be .01 %.

Frequency Stability: The Frequency Stability shall be 1 x 10E-9 over 1 s interval

Angle of Arrival Accuracy: The Angle-of-Arrival Accuracy shall be 1 deg. for echoes with SNR >40dB.

Receiver Sensitivity: The Receiver Sensitivity shall be 500 nV rms. at receiver input across 100MW (i.e. at the antenna input to the preamplifier).

Signal Processing Gain: The Signal Processing Gain shall be nominally 21 dB (e.g. from pulse compression and/or spectral coherent integration).

Amplitude Accuracy of Receiver Output: The Amplitude Accuracy of Receiver Output shall be +/-0.375 dB.

Phase Accuracy of Receiver Output: The Phase Accuracy of Receiver Output shall be ±1.5 deg.

Dynamic Range of Receiver: The Dynamic Range of Receiver without Gain Control shall be 80 dB, or 120 dB with Gain Control (signal levels of 500 nV to 0.5 V rms.).

Receiver Gain Accuracy: Gain steps of 9 dB shall be applied under computer control immediately before each range profile integration in order to avoid saturation of any stage in the receiver. These steps shall be accurate to within 1.5 dB from the nominal 9 dB with a total cumulative deviation of less than 3.0 dB.

Out-of-Band Interference Rejection: The system shall be able to operate with less than 10 dB degradation of receiver sensitivity when signals more than 25% away from the received frequency are 120 dB larger than the in-band signal.

3^rd Order Intermodulation Rejection: The system shall be able to operate with less than 10 dB degradation of receiver sensitivity when the sum of two out-of-band signals is 100 dB greater than the echo signal, and occur at frequencies which produce a 3^rd Order Intermodulation Product at the operating frequency.

Receiver Recovery Time: The receiver shall recover to within 10 dB of maximum sensitivity after being non-destructively overloaded (such as occurs during the TOPAS/W transmission) within 100 msec after the interfering signal is removed.

Antenna Lengths: Each stem antenna shall be 10m in length.

E-field sensitivity: The E-field sensitivity shall be 50 nV/m.

Mechanical Dimensions: The physical outline drawing of the TOPAS/W electronics is (29.07 x 20.65 x 22.65) cm. The proposed housing for the TOPAS/W is similar to the housing designed for the RPI/IMAGE instrument.

Mass Properties: The total mass of TOPAS/W shall not exceed an estimated 32 kg. The mass properties associated with the TOPAS/W are presented in Table 5.

Field-of-View: Not applicable.

Power: TOPAS/W shall not consume more than 27 W (including 20% margin) of power (averaged over any orbit); however, its peak power can be as high as 55 W. Table 6 is a detailed power consumption breakdown by TOPAS/W subassembly, showing peak and average power. NOTE: These power estimates are for the TOPAS/W instrument only, they do not include spacecraft systems such as telemetry or altitude control systems.

Grounding: All conducting surfaces within the TOPAS/W chassis will be treated as an equipotential ground plane. Signals will be referenced to ground at the closest proximity to a conducting surface. Power distributed from the S/C power system will be isolated from the TOPAS/W chassis by DC/DC converters. The output of these converters will be filtered (common-mode chokes and bypass capacitors) and grounded to the equipotential plane as close as possible to the DC/DC converter module.

Thermal Design Requirements: The TOPAS/W electronics cards shall dissipate their rated power through ground planes to thermally conductive card guides.

Dr. Bodo W. Reinisch is the Director of the Center for Atmospheric Research at the University of Massachusetts Lowell, and Professor in the Department of Atmospheric Sciences.

Dr. D. Mark Haines is the Project Engineer for RPI. He has developed HF sounding and digital processing techniques since almost twenty years, first at US Air Force research laboratories, and the last ten years at the University of Massachusetts Lowell Center for Atmospheric Research.