L3Harris Narda SignalShark User Manual Download

Page: 181 / 231

Automatic Direction Finding

181

Config –> DF Squelch

The first and second conditions are very hard to meet if the signal is transmitted in bursts.

The bearings should only be taken during the burst period. The SignalShark can use

omnidirectional power measurements at the end of each bearing cycle to achieve automatic

burst detection. The “DF Squelch” parameter determines whether the actual bearing cycle is

used for the AoA calculation. The covariance vector of the actual bearing cycle is calculated

and used for the AoA calculation only if the omnidirectional power of the previous and the

actual bearing cycle are greater than the squelch value.
The first and second conditions are very hard to meet if the signal is transmitted in bursts.

The bearings should only be taken during the burst period. The SignalShark can use

omnidirectional power measurements at the end of each bearing cycle to achieve automatic

burst detection. The “DF Squelch” parameter determines whether the actual bearing cycle is

used for the AoA calculation. The covariance vector of the actual bearing cycle is calculated

and used for the AoA calculation only if the omnidirectional power of the previous and the

actual bearing cycle are greater than the squelch value.

Config –> min. Stability

The “min. Stability” parameter exists for the same purpose. The covariance vector of the

actual bearing cycle is calculated and used for the AoA calculation only if the magnitude of

the level difference between the omnidirectional power of the previous and the actual bearing

cycle is less than the specified stability value.
For DF of burst transmissions, the DF squelch should be set to a value of at least 10 dB

above the noise floor, and the required level stability value should be set to about 1 dB.
Using these settings it can be proven that SignalShark only uses bearing cycles for DF that

are entirely within a burst transmission when the following two conditions are met:
• The minimum burst length is greater than two bearing cycle times plus one measurement

time
• The minimum pause length between bursts is greater than one bearing cycle time plus one

measurement time.
The SignalShark automatically discards bearing cycles that contain level transitions or

transmission pauses under these conditions.

Post Avg.

The desired bearing cycle times for signals with short minimum burst and pause lengths will

also be very short. This means that the measurement time will also be very short, so the

bearing rate will probably be much higher than necessary. The short measurement time may

result in less accurate bearings, because the power fluctuation due to the signal modulation

and the additive noise may not be sufficiently reduced. The SignalShark can overcome this

problem by averaging a number of covariance vectors before the AoA is calculated. This is

accomplished using the post averaging time parameter, which can be set to integer multiples

of four times the bearing cycle time. A new bearing is calculated every quarter of the selected

post averaging time, using the average of all valid covariance vectors that are not older than

the post averaging time. Good bearing accuracy can be achieved even with very short

bearing cycle times due to this additional averaging of covariance vectors.
If the direction finder or the transmitter are moving, the third condition can be met if the

bearing cycle time is set short enough so that the AoA does not change significantly during a

bearing cycle.