Simrad EK500 Power to Sv and TS

The following algorithms apply to the Simrad EK500 echosounders only. They are documented under Theory of Operation in the Simrad EK500 Operator Manual and their validity has been confirmed by Simrad technical support.

In Echoview two raw variables derived from the power data are available: Sv and TS. They are listed among the other raw variables as:

Sv Wn telegrams (power) Tt
TS Wn telegrams (power) Tt

where: t is an integer and refers to the transducer number.

The Sv and TS values are calculated from the raw power data according to formulas provided by Simrad. These formulas are supplied below.

Note: TVG is applied to samples with ranges greater than 1 meter.

Calculation of Sv and TS

Sv and TS are obtained for range and power according to:

where:

R the corrected range (m) - see TVG Range Correction

Pr is the received power (dB re 1 W) - see Simrad EK numbers to Power

Pt is transmitted power (W)

α is the absorption coefficient (dB/m)

G0 is the transducer peak gain (non-dimensional, Sv or TS gain as appropriate)

Echoview calculates G0 as 10^(G'0/10) where:

G'0 is the Sv gain or TS gain respectively (dB re 1) displayed as Ek5SvGain or Ek5TSGain on the Calibration Page of the Variable Properties dialog box.

λ is the wavelength (m)

Simrad calculate λ as 1470/f where f is the nominal frequency (Hz).
Echoview takes λ as displayed on the Calibration Page of the Variable Properties dialog box.

c is the speed of sound (m/s)

τ is the transmit pulse duration (s) - also known as the pulse length

ψ is the Equivalent Two-way beam angle (Steradians)

Echoview calculates ψ as 10^(Ψ/10), where:

Ψ is the Two-way beam angle (dB re 1 steradian) displayed on the Calibration Page of the Variable Properties dialog box.

FSv and FTS are filter corrections as defined by Simrad as follows (see history below):

FSv (pulse length, bandwidth):

Bandwidth\Pulse length	Long	Medium	Short
Wide	FSv = 0.955	FSv = 0.858	FSv = 0.576
Narrow	FSv = 0.737	FSv = 0.407	FSv = 0.157

FTS (pulse length, bandwidth):

Bandwidth\Pulse length	Long	Medium	Short
Wide	FTS = 1.000	FTS = 1.000	FTS = 0.816
Narrow	FTS = 0.971	FTS = 0.651	FTS = 0.264

If the bandwidth is set to auto, the following rules are applied:

- - - If the pulse length is short or medium, the bandwidth is taken to be wide
    - If the pulse length is long, the bandwidth is taken to be narrow
    - If either or both bandwidth or pulse length is set to unknown in Echoview, the filter correction will not be applied
  Note: 10 log (FSv) and 10 log (FTS) are used as transducer constants in Sv to TS and TS to Sv conversion equations.

Differences between Echoview Sv and Simrad Sv:

The values for Sv and TS as calculated here (from Simrad W telegrams with the Simrad range correction is applied) are expected to be identical to the Sv and TS values as given by the EK500 firmware v 5.30 and higher (Q or V and P telegrams respectively).

Echoview support has examined EK500 files and observed that the Sv calculated from power (W telegrams), and Sv directly (V telegrams), matched very closely. A difference of at most 0.08dB was typical over a ping.

Small varying differences (of 0.01dB or less) may be observed as well, but could easily be ignored. They are caused by truncating and rounding in the calculations with the 16-bit EK numbers. The highest accuracy that can be obtained in principle is 1 EK number, corresponding to 0.012 dB.

Warning: In some data files (EK500 firmware v5.30) we have observed a logarithmic difference, decreasing with depth. The difference was about 2dB at 2m, 0.6dB at 5m and 0.25dB at 10m.

Simrad has confirmed (personal communication, February 2001) that the formulas implemented in Echoview are correct. Therefore, whenever a difference is observed, the power telegrams should be regarded as the correct ones.

Advice

We advise calibrating using the raw variable (based on either W or Q/V/P telegrams) that will be used for echo integration later.
We advise you to construct a virtual variable as the difference between the Sv-from-power and the Sv raw variables, and the same for TS. This will give a better insight in the differences in a particular case. Note that you may have to apply the ping-time-shift operator first on the Sv variable to align it with the power Sv data. It tends to be off by one ping typically.

History of Filter corrections

The application of the filter corrections has changed during the development of Echoview.

The following table lists the Echoview version numbers and corresponding corrections.

Echoview versions	Filter corrections
1.51.xx	N/A
2.00.xx	not applied
2.10.00 to 2.10.09 (not released)	not applied
2.10.10 to 2.10.37	applied
from 2.10.38	applied

Simrad EK500 Power to Sv and TS

Calculation of Sv and TS

Differences between Echoview Sv and Simrad Sv:

Advice

History of Filter corrections

See also