Tx “bad” lags and range data gaps Pasha Ponomarenko 10/10/2014STELab discussion1

Sampling rate problem Problem: for the single-pulse technique, the sampling rate should satisfy two mutually exclusive requirements: |V|  2000 m/s: f  200 Hz d  3500 km: f  40 Hz Range/time targets pulses 10/10/2014STELab discussion2

Solution Solution: measuring autocorrelation function (ACF) with sampling rate > 200 Hz and cancelling unwanted echoes from other ranges through coherent averaging. It can be measured using a sequence of unevenly spaced pulses which provides a set of time lags. 10/10/2014STELab discussion3

SD pulse sequence Blanking pulses 10/10/2014STELab discussion4

Sampling ACF Each pulse in the sequence generates its own echo profile, e.g. like this: Power Time/group range 10/10/2014STELab discussion5

ACF sampling and Tx overlap1 10/10/2014STELab discussion6

Sampling ACF (cont.) For different pulses, these profiles are very similar but shifted in time domain according to the pulse separation in the sequence. As a result, the sampled echoes represent superposition of the echoes from different samples. 10/10/2014STELab discussion7

ACF sampling and Tx overlap /10/2014STELab discussion8

Sampling ACF (cont.) For any given range, we can calculate when the return from a given pulse should arrive by shifting the “mask” (pulse sequence) along the receiver sample time series. The sampled range gate is determined by a position of the first pulse in the sequence. 10/10/2014STELab discussion9

Sampling ACF (cont.) /10/2014STELab discussion10

Sampling ACF (cont.) Then we combine pairs of the receiver samples (pulses) to generate complex ACF values at different time lags. The lag values are determined by the separation of the pulses in each pair. 10/10/2014STELab discussion11

010 2020 3030 10/10/2014STELab discussion12

Averaging Together with the “wanted” signals coming from the desired ranges, we also receive echoes from “unwanted” ranges generated by other pulses of the sequence. In order to minimise this interference, we need to average ACFs. In this case the coherent components from desired ranges will remain unchanged while the incoherent cross-range interference will be suppressed due to its random phase relation with the desired signal. The signal itself would also become less coherent with increasing  so that the averaging also allows to measure ACF power decay time (spectral width), which is impossible with just a single pulse sequence. AB c  / 2 cc 10/10/2014STELab discussion13

Tx overlap and power gaps At certain locations along the receiver sample sequence some of the “pulses” in the mask coincide with the Tx emission times so that the receiver is blanked and the respective sample has very low (ideally zero) power. 10/10/2014STELab discussion14

Tx overlap and power gaps /10/2014STELab discussion15

Tx overlap and power gaps If one of the pulses is “blanked” (i.e. it has very low power) then the respective ACF lags should also have low power. 10/10/2014STELab discussion16

If we know which pulse is a low-power one, it is easy to calculate which lags of the respective ACF will have low power too. This combination is unique for each “lost” pulse For example, in the old pulse sequence, for pulse #2 these will be lags 3, 9, 11, 13, 17 and Low-power lags 10/10/2014STELab discussion17

It can be predicted The dostribution of low-power lags vs range gate is also unique for each combination of pulse sequence, basic ACF lag (mpinc), spatial resolution (rsep) and initial range (lagfr) so that it can be predicted for the given set of the above parameters. 10/10/2014STELab discussion18

On-line VT: automatic “bad” lag calculator 10/10/2014STELab discussion19

On-line VT 10/10/2014STELab discussion20

On-line VT: Predicted and observed gaps 10/10/2014STELab discussion21

Range periodicity The affected range gates appear periodically, every mpinc/rsep gates. old sequence: 2400 µs /300 µs = 8 gates katscan: 1500 µs/300 µs = 5 gates high-resolution katscan: 1500 µs/100 µs = 15 gates 10/10/2014STELab discussion22

Range offset estimate Matching observed and predicted range/lag patterns of low-power lags can be used for finding range offsets which are greater or equal to the spatial length of the blanking pulse (1-2 gates). Using the observed position of the Tx-overlap lags as a time-of-flight reference is possible because these blanking pulses are accurately synchronised with the emission regime. Otherwise, the input circuits of the receiver would have been burned, or at least we would observe power peaks instead of the power gaps. 10/10/2014STELab discussion23

Range offset estimate (cont.) The “blanking” occurs at the input circuits of the receiver so that any following filtering could shift or smooth the whole pattern but it would not change its temporal structure, i.e. relative positions of the power gaps both in range and ACF lag. 10/10/2014STELab discussion24

Another on-line VT tool: ACF plotter 10/10/2014STELab discussion25

Example: No offset Tx overlap 10/10/2014STELab discussion26

Example: No offset Tx overlap 10/10/2014STELab discussion27

Example: No offset Tx overlap 10/10/2014STELab discussion28

Example: No offset Tx overlap 10/10/2014STELab discussion29

Example: No offset Tx overlap 10/10/2014STELab discussion30

Example: No offset Tx overlap 10/10/2014STELab discussion31

Example: No offset Tx overlap 10/10/2014STELab discussion32

Example with offset 10/10/2014STELab discussion33

Example: 3-gate delay Tx overlap 10/10/2014STELab discussion34

10/10/2014STELab discussion35 Example: 3-gate delay Tx overlap

10/10/2014STELab discussion36 Example: 3-gate delay Tx overlap

10/10/2014STELab discussion37 Example: 3-gate delay Tx overlap

10/10/2014STELab discussion38 Example: 3-gate delay Tx overlap

10/10/2014STELab discussion39 Example: 3-gate delay Tx overlap

10/10/2014STELab discussion40 Example: 3-gate delay Tx overlap

10/10/2014STELab discussion41 Example: 3-gate delay Tx overlap

Yet another on-line tool: offset estimator 10/10/2014STELab discussion42

Range offset Automatic way to estimate the offset value and sign is implemented at the VT website under Data Diagnostic / Badlag Finder Just enter the suspect radar, data and time and press “Find”. Wait until the calculation is finished and then click on the image to open a PDF file. Scroll to the second page. 10/10/2014STELab discussion43

10/10/2014STELab discussion44

Range offset (cont.) The peak on all plots should coincide with the vertical dashed line. If this is not the case, then there is a range offset. Its magnitude and sign are determined by the shift from the dashed line. In the above case we have a 3-gate negative shift so that the sampling starts earlier than expected. In this case the first range gate has to be shifted by 3 gates closer to the radar. i.e. from 120 km to (120 km-3*15 km)=75 km. 10/10/2014STELab discussion45

“Normal” offset (time delay) 10/10/2014STELab discussion46

Reverse offset (late sampling) 10/10/2014STELab discussion47

Possible causes of the time offset Sampling start earlier than required (“normal” offset): – Time delay of the signal inside the receiver (e.g. due to narrow-band filtering). Sampling starts later than expected (reverse offset): – Overcorrection of the “normal” offset – Something else (?) 10/10/2014STELab discussion48

Enough for today? 10/10/2014STELab discussion49