From 4fbd8814b97108b593243281c69b63401f670ae9 Mon Sep 17 00:00:00 2001
From: krakenrf <78108016+krakenrf@users.noreply.github.com>
Date: Fri, 14 Oct 2022 17:40:24 +1300
Subject: Updated 08. Passive Radar (markdown)

---
 08.-Passive-Radar.md | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/08.-Passive-Radar.md b/08.-Passive-Radar.md
index fc96e8f..7a574df 100644
--- a/08.-Passive-Radar.md
+++ b/08.-Passive-Radar.md
@@ -81,6 +81,31 @@ If you are using wideband illuminators such as HDTV or 4/5G signals, you want to
 
 If you are using DAB (or other more narrow illuminators), note that this only has a bandwidth of about 1.5 MHz. Therefore you will obtain better results if you adjust your bandwidth to match as you don't want to receive the noise floor along the sides of the illuminator. From a base of any pr_ preconfiguration, under the DAQ reconfiguration settings, change the sample rate to 1.4 MHz. And then hit the "Reconfigure & Restart DAQ chain" button. This may take 1-2 minutes before the software has restarted.
 
+# Range Doppler Units
+The range-doppler graph displays bistatic range and bistance speed. The axes are currently described in CELLS. In order to convert cells to meters or m/s use the following formulas:
+
+Cell To Meters = cell * c/fs
+
+Where c is the speed of light, and fs is the sample rate.
+
+So for example if we see an object at cell 50, and we have a sample rate of 2.4 MHz (assuming an illumination signal at least that large too) We can go 50 * 299792458 / 2400000 = 15,614m = 15.6 km (bistatic distance)
+
+For doppler:
+
+Cell to Hertz = cell * fs / (2*N)
+
+Where fs is the sampling frequency and N is the sample size of the coherent processing interval.
+
+Example cell 100, sampling rate 2.4 MHz and N = 2^22
+
+Doppler Frequency = 500 * 2400000 / (2*2^22) = 143 Hz
+
+To get to m/s we simply multiple the doppler frequency with the wavelength of the illuminator, and multiple by -1. (Positive Doppler decreases the range between you and the target so it has negative speed, it is approaching)
+
+So if we were using 560 MHz as our illuminator:
+
+Bistatic Doppler in m/s = c / f * -doppler_freq = 299792458 / 560000000 * -143 = 76.5m/s = 275 km/h
+
 # Bistatic Range
 The graph provided is a bi-static range doppler graph. Bi-static means that the measurement consists of a transmitter and receiver separated by some distance. This can get complicated, as instead of getting a simple range distance value from the receiver, we end up with an ‘constant range ellipse’ of possible range solutions that depend on some calculations based on the transmitter and receiver positions.
 
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