In the Bearing view, you configure the direction finding settings like Ftune, CBW, Attenuator, Measurement Time / Cycle Time, North Reference etc. It also displays the bearing results as a polar chart and as absolute values.
Ftune of the Bearing view and Fcent of the Spectrum view are coupled to each other.
Tap the Atten. button.
The attenuator of the SignalShark should be set to the lowest value possible for no overload to occur. This is often the 0 dB setting, due to the huge dynamic range of the SignalShark.
Tap the Ftune and CBW buttons.
The SignalShark uses a channel filter in the Auto DF task to separate the signal under investigation from other signals during the DF process. The center frequency of the channel filter is set by the Ftune parameter, and its bandwidth is set by the CBW parameter.
The CBW denotes the 6-dB bandwidth of a digital Parks-McClellan filter. The channel filter must be positioned such that the ratio of the received signal power to the received power of the noise floor or adjacent signals is maximized. A setting very close to the optimum is achieved by setting Ftune to the center frequency Fcent of the occupied spectrum and CBW to the occupied bandwidth Occ. BW.
Tap the Meas. Time button.
The three necessary conditions mentioned under “Automatic DF measurement and localization” must be kept in mind for the remaining parameter settings. Clearly, signals that are received at constant and sufficient power and with a constant AoA fulfill all three conditions. Care must be taken to ensure that all three conditions are met if the received power or AoA varies with time.
The measurement time spent on each power measurement in a bearing cycle is an adjustable parameter on the SignalShark. Users can change the bearing cycle time indirectly and minimize the negative effects of a variable signal power or AoA by means of a proper setting of the measurement time.
If the received power varies periodically over time, the measurement time should be set to the period duration or an integer multiple of it. This ensures that the average power received during each measurement time is constant, and the first condition is met exactly. If the received signal contains a frame structure, the measurement time should be set to the frame length or an integer multiple of it. The average power received during each measurement time will then be more or less constant, and the first condition will be met approximately. All modern mobile communications networks are frame based.
• The GSM frame length is 4.6154 ms.
• The frame length for UMTS, LTE, and 5G NR is 10 ms.
If the received signal power varies randomly, the measurement time should be set as high as is feasible when other factors are considered. The average power received during each measurement time will then be more or less constant, and the first condition will be met approximately.
Tap the Config button, then tap the DF Squelch button.
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.
Tap the Config button, then tap the min Stability button.
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.
Tap the Post Avg. button.
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.
Tap the min. DF Quality button.
With SignalShark a bearing measurement or bearing cycle consist of several level measurements for each antenna element in combination with phase shifters. SignalShark than calculates so called covariance vectors out of these values for every bearing cycle. This covariance vector is correlated to stored reference vectors to calculate the azimuth and elevation angle of the bearing.
The DF quality parameter indicates how good this correlation fits. A DF quality of 100% means, that the currently measured covariance vector fits perfectly to a stored reference value. A value of 100 % can only be reached under ideal conditions. The DF quality value will be less at low signal to noise ratios, or with multipath propagation. The SignalShark has a parameter called “min. DF Quality” that can be used to discard bearings with low DF quality.
Tap the Config button, the tap the corresponding button.
The position and orientation of the direction finder must be known for a bearing from it to be meaningful. In practice, the geo location of the direction finder is determined with a GNSS (global navigation satellite system) receiver located in the antenna array or the receiver. The GNSS position uncertainty is about 15 m RMS and thus precise enough for most applications.
All DF antenna arrays have a reference direction, which is also marked on the array construction. Only AoA values relative to this reference direction are used during calibration of the antenna array. When the DF system is used in practice, it indicates the AoA of the suspicious transmitter relative to its reference direction. However, it is desirable to determine the AoA relative to geographic north for localization purposes. There are several ways to achieve this.
The SignalShark has three north reference settings for bearings:
• Ref. Mark. Dir.
• Compass
• GNSS Velocity
Use of the reference mark direction is the most precise method. This is somewhat time-consuming, so it should be employed only if the DF antenna is to be used at a fixed location over a period of longer than one day. The DF antenna reference direction is marked roughly by an arrow and precisely by optical bearing marks. The antenna should be adjusted so that the bearing marks are aligned with a landmark. The azimuth angle of this landmark relative to geographic north and the fixed location of the DF antenna must be determined from the geographic positions of the landmark and the DF antenna. The “Ref Mark Dir.” parameter is set to the azimuth angle of the landmark. The AoA indicated by the SignalShark is the sum of the “Ref. Mark. Dir.” and the internal AoA value, which is relative to the antenna reference direction. The extensive adjustment procedure may make the “Ref. Mark Dir.” method too time-consuming when the DF antenna is only stationary during the DF process but otherwise changes its position frequently.
All NARDA DF antennas have a built-in electronic compass with an azimuth uncertainty of typically 1.5° RMS when the DF antenna is located in the undisturbed magnetic field of the earth. This can be used as the north reference if the DF antenna is mounted such that no ferromagnetic materials and no DC currents are present in its vicinity. A good example of this is to mount the antenna on a wooden tripod. It is important that the correct declination of the earth’s magnetic field at the current location is also entered when the “Compass” setting is used for the north reference.
If the DF antenna is mounted on a vehicle, the earth’s magnetic field in the vicinity will normally be heavily disturbed, especially if mounting is by means of a magnetic plate, which is very convenient and often used. If the vehicle is also moving during the DF process, as is often the case, only one option remains for the north reference: GNSS Velocity.
This method assumes that the DF antenna reference direction is the same as the movement direction of the vehicle. The position of the DF antenna on the vehicle must therefore be carefully adjusted so that the antenna reference direction is parallel to the vehicle’s normal direction of travel. Note that the DF antenna bearing marks may also be helpful for this adjustment. The accuracy of the movement direction measured by the DF antenna GNSS receiver is proportional to the magnitude of the vehicle velocity. The typical uncertainty is about 0.3° RMS at a velocity of 100 km/h. This method therefore works only well when the vehicle is moving faster than about 10 km/h. The GNSS receivers in NARDA DF antennas will try to recall the last valid direction estimate even when the vehicle stops. However, this direction is only a useful estimate of the true direction if the vehicle did not change direction considerably while moving very slowly. Bearings taken when the vehicle is parked are thus only useful if the vehicle did not change direction abruptly during the period of slow movement before reaching the parked position. If this is not the case, the north reference will need to be changed to “Compass” when the vehicle is parked. The DF antenna must then also be unmounted from the vehicle if low bearing uncertainty is required in the fixed position.
If there is a known reference direction mismatch, it can be corrected in every “North Reference” settings by a parameter called Azimuth Corr.. The value of this parameter is just added to the uncorrected azimuth result. It is worth noting that a perfect north reference is always assumed for the specification of the DF uncertainty. The uncertainty in the north reference is an additional source of uncertainty that must be taken into account when calculating the overall DF accuracy.
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