SPEAR Technical


Technical Overview

SPEAR is a vertical looking, HF radar system based on a distributed transmitting arrangement. The full system would consist of a 6x6 array of crossed dipoles each driven by a 4kW solid state transmitter. At the current time we are only proposing to construct a 4x6 system but hope that additional funding will become available to extend it to the full 6x6 system at some time in the future.

The information provided here is based on the full 6x6 system since this would provide the 'worst case' values for interference and safety. The initial configuration will constitute 2/3 of the full system being a 6x4 array driven by 48, 4kW transmitters i.e. a maximum power of 192 kW. The full 6x6 system will have a maximum power of 288 kW. These power levels, whilst they may appear high, are significantly less than broadcast transmitters (typically 1000-2000kW) and the existing EISCAT Tromsø heater (1200 kW).

The system is designed to operate on spot frequencies within two bands, 2 MHz to 3MHz and 4 MHz to 6MHz. A variety of modulation schemes and power levels will be required according to specific scientific requirement as described below.

The distributed arrangement is based on a new transmitter design developed specifically for this project. This solid state design has extremely low levels of spurious emissions and incorporates high power filters to minimise out of band signals. The actual levels are indicated in the attached system specifications. This level of performance will minimise potential interference problems.

Operating modes and frequency requirements

SPEAR has been designed to address three scientific interests and requires different operation modes for each. Each mode will be described in terms of the frequency, bandwidth and modulation required. The general requirement is for a set of spot frequencies evenly distributed throughout the operating range of SPEAR.

1 Active Radar Mode
2 Low Power Radar Mode
3 Magnetospheric Radar Mode

Active Modes

Depending on ionsopheric conditions this mode requires CW transmission at spot frequencies in either the high frequency (4 to 6 MHz) band or the low frequency (2 to 3 MHz) band.

These transmissions will be CW with on off keying at frequencies not greater than 1 Hz. The bandwidth requirement would be less than 100 Hz.

This mode of operation will only be carried out on an intermittent basis. We plan to operate for not more than 1000 hours per year (aprox. 3 days/month) in this mode.

Low Power Radar Mode

It will be possible to utilise SPEAR as a coherent radar with power being scattered from naturally occurring irregularities in the ionosphere. Such irregularities exist for some 30% of the time in the ionosphere above Svalbard and the scientific objective is to map the motion of the irregularities and infer the horizontal ionospheric electric field. The power required for this mode of operation is low since the scattering cross-section of the irregularities is large requiring a peak power of 10 kW. 300 microsecond pulse length and 3% duty cycle will be employe and the bandwidth requirement would be less than 3.5KHz

This mode of operation is similar to that employed in the SuperDARN radar network which includes radars in Finland, Iceland, Canada, USA, Tasmania, Kurgelan and the Antarctic. These radars operate continuously. Leicester University have operated two of these radars in Iceland and Finland for several years and have not encountered any interference problems.

SPEAR will operate in this mode at all times when not being used in other modes.

Magnetospheric Radar mode

This mode of operation is intended to detect structures in the earth's magnetosphere at ranges from 1000 to 8000 km. This requires full power pulse operation with 1 millisecond pulses and a 10% duty cycle and a bandwidth requirement of approx. 1KHz.

For this mode of operation to be successful the operating frequency must be higher than the ionospheric critical frequency since the energy must penetrate to very long ranges. It will therefore be necessary to operate ONLY when ionospheric conditions are appropriate e.g. winter nightime. The potential for interference is minimal in this mode of operation since ionospheric propagation is significantly decreased by the choice of frequency. Real time monitoring of the critical frequency with the existing Longyearbyen ionosonde operated by the University of Leicester will ensure the use of only the appropriate frequency for this mode of operation.

This mode of operation would only be carried out on an intermittent basis and would involve not more than 100 hours per year.

Specification overview for each of the operating modes

Active Mode - High Frequency Active Mode - Low Frequency Low Power Radar Mode
Magnetospheric Radar Mode
Location 78°9'N 16°4'E 78°9'N 16°4'E 78°9'N 16°4'E 78°9'N 16°4'E
Transmitter Configuration 48 off 4 kW transmitters 48 off 4 kW transmitters 48 off 4 kW transmitters 48 off 4 kW transmitters
Total Power Output 192 kW 192 kW 10 kW 192 kW
Operation Frequency Spot Frequencies between 4-6 MHz Spot Frequencies between 2-3 MHz Spot Frequencies between 4-6 MHz Spot Frequencies between 4-6 MHz
Antenna Configuration 6 x 4 array of horizontal, rhombically broadened, crossed dipole 17 m above ground 6 x 4 array of horizontal, rhombically broadened, crossed dipole 17 m above ground 6 x 4 array of horizontal, rhombically broadened, crossed dipole 17 m above ground 6 x 4 array of horizontal, rhombically broadened, crossed dipole 17 m above ground
Antenna gain 22 dBi 16 dBi 22 dBi 22 dBi
Beam Width (3db) 14° x 21° 24° x 36° 14° x 21° 14° x 21°
Pointing Direction Steerable within 20% of zenith Steerable within 20% of zenith Steerable within 20% of zenith Steerable within 20% of zenith
Power ERP 75dBw 69dBw 62dBw 75dBw
Modulation CW CW 300 micro-sec pulse 1 micro-sec pulse
Duty Cycle 100% 100% <3% <10%
Bandwidth <100 Hz <100 Hz <3.5 kHz (with pulse shaping) <1.5 kHz (with pulse shaping)
Spurious emissions
Harmonics <-60dBc <-60dBc <-60dBc <-60dBc
Other <-55dBc <-55dBc <-55dBc <-55dBc

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