Interrogator

Updated: September 28, 1998

The DARTS Interrogator (ground unit) is the link between the rocketeer and the rocket. Communicating with the transponder, the Interrogator performs all signal
processing necessary to:

Control the ground antenna;
Measure position, velocity, and acceleration;
Provide telecommand and telemetry.

The Interrogator:

Provides all system timing, including forming transmit pulses and telecommand data;
Demodulates telemetry data;
Measures range by counting the time for the signal round trip;
Keeps the antenna pointed at the transponder;
Measures antenna angles and combines them with the slant range to form position estimates;
Performs processing on the position estimates to provide smoothed position, velocity, and acceleration data.

The Interrogator provides state-vector information to the host while tracking is in progress.

The Interrogator is powered from two 12-V gelled-electrolyte lead-acid batteries ("gel-cells"). No commercial power is required. With a laptop computer, this makes DARTS completely portable.

Transmit

The ranging/telecommand bitstream, generated by the controller, is modulated onto a low-level 5.8 GHz carrier which is sent to the antenna feed.

DARTS transmits a pulse at a regular interval, regardless of the reply time-of-arrival. The pulse repetition period (PRP) is chosen to accommodate the maximum expected range. The standard PRP will be 2 ms (re-configurable). The pulse width is modulated with telecommand data.

The transmitter outputs a low-level (10 dBm) modulated carrier to the antenna / transverter assembly. A power amplifier brings the carrier level to an appropriate level before being applied to the T/R switch.

Antenna Feed / Scanning

The antenna is pseudo-monopulse in elevation, and frequency-scanned in azimuth.

The interrogator obtains elevation angular information on every pulse by switching between the outputs of two subarray groups. The relative signal strength at the two beam positions are combined to produce a elevation boresight error, which is used to point the antenna in elevation.

By switching frequencies in concert every transmitted pulse, the transponder and interrogator toggle between two frequency-selectable receive beams. The angular information derived from these two beams allows the antenna to be pointed at the rocket in azimuth

Receiver RF

The receiver RF section consists of a low-noise downconverter, antenna switch and T/R switch.

The low-noise downconverter consists of a preamplifier, an image-reject double-balanced mixer, and phase-locked local oscillator.

The RF switches are GaAsfet MMIC type.

Receiver IF

The receiver IF consists of a log amp, A/D converter, and threshold detector.

The log amp is a two-stage, integrated-circuit, 80-db-dynamic-range logarithmic amplifier, which has been optimized for fast rise time response.

The log amp feeds the A/D converter and the threshold detector. The threshold detector provides a clean pulse to the controller, which is used to measure range and to generate timing for the antenna switching. The A/D converter is a "single-shot" sampling unit which can, upon command from the controller, digitize the signal strength to 10-bit resolution. There is also a test input to the A/D, used in built-in test.

Controller

The controller consists of a PIC16C76 microcontroller running at 20 MHz, and its peripheral boards. Peripheral boards include range measurement, servo control, and antenna control.

Antenna Pedestal & Control

The pedestal proper is the two-axis (AZ-EL) servo-motor-driven gimbal for the parabolic antenna. The gimbal motors are brush-type DC motors driven by a PWM controller. They have integral optical encoders for position feedback.

There are two 12V gell-cell batteries which power the interrogator. If necessary, the large gell-cells can be dispensed with and the antenna motors powered from a 12V automobile system.

System Mechanization

Signal Acquisition / Uniqueness

Provided the transponder is near enough to the interrogator, it is not necessary that the interrogator antenna beams to be pointed directly at the transponder. Since there is a possibility of multiple transponders operating in the same area, the interrogator is given the ID of a single transponder to track. The interrogator sends a GLOBAL_SHUTDOWN command, which commands all transponders with its range to turn their transmitters off. Then, the interrogator sends a TRANSMIT_ENABLE command to the transponder that is to be tracked. Also, during flight, the transponder periodically returns its ID so that the interrogator is sure it is tracking the correct object.

Interrogator Specifications

Range Measurement	Fixed threshold with gated 16-bit counter
Range Gating	Software-controlled by enabling range interrupts at appropriate time
Range Counter Rate	30 MHz
Range Resolution	±5 meters
Ultimate Measurable Range	327.68 km (203 mi)
Pulse repitition interval	2 - 50 ms
Pulse width, transmitted	12.8 uS / 19.6 uS
Pulse width, received	11-27 uS, 12.8 / 19.6 uS nominal
Transmit frequency	5650 - 5925 MHz
Transmit power, continuous	28 dBm (680 milliwatts)
Transmit power, peak pulse 12.8 uS, 2 mS interval	98.6 Watts
Antenna gain, nominal	23 dBi
Antenna configuration	88-element phased array consisting of dual 4x11-element corporate-fed traveling wave subarrays
Receiver bandwidth	20 MHz
Receiver IF frequency	70 MHz
Receiver Input frequency	5650 - 5925 MHz
Azimuth Gear Ratio	20:1
Azimuth Encoder Resolution	40,000 counts per revolution
Elevation Gear Ratio	9:1
Elevation Encoder Resolution	18,2000 counts per revolution
Motors (Az/el)	40 in-lbs torque brush-type DC

Path Parameters

at 5760 MHz, 20 km

Path	Pt (dBm)	Gt (dB)	Gr (dB)	Pr (dBm)
Uplink	28	26	0	-74.0
Downlink	13	0	26	-77.3

[DARTS Main] [Gallery][Overview][FAQ][News] [Interrogator] [Transponder]

Send comments and suggestions to: steveb@c2-tech.com
URL: http://darts.c2-tech.com/interog.htm
This document copyright Steve Bragg, KA9MVA.