Basic Radar / Pulse Radar

Objectives Explain how range is determined with the basic radar system. Apply relationships between Duty Cycle, PW, PRF, PRT, average and peak power. Calculate Signal to Noise ratio (dB and dimensionless). Apply the relationship between S/N, NEP and Minimum Signal for Detection. Explain Threshold Level (TL) and distinguish setting the receiver above and below the minimum signal for detection. Identify the components of the Basic Pulsed Radar system

Introduction Heinrich Hertz – German Physicist proved in late 1880’s that electromagnetic waves were generated by ‘sparks’ from an oscillator. Then noted “It’s of no use, what so ever”. Guglielmo Marconi – Italian Electrical Engineer. In 1895, sent EM signal to a point several kilometers away. 1899 sends communication between Britain and France. 1909 wins Nobel Prize in Physics. RADAR is yet another one of many acronyms you need to remember meaning Radio Detection And Ranging. Hertz first demonstrated the basic principles in 1886 (radio waves reflect off metallic surfaces)

Radar At War World War I was a war fought with eyes and ears – no such thing as RADAR. Pre-WWII, Germany had numerous Funkmessgeraets, or ‘radio measuring devices’. Numbers outpaced the allies along with sophistication. By late 1940, Germany ceased development of radar systems, predicting a short war. British and US research and development caught up to and exceeded Germans. Chain Home used 30 MHz range. The British were the first to use radar tactically during the final phases of WW I to help locate surfaced German submarines. Its overall mission was and still is to extend detection capabilities beyond the LOS. 1904 – Hulsmeyer patents obstacle detector/ship navigation device 1922 – NRL engineers detect wooden ship w/CW radar 1930 – NRL makes first detection of aircraft 1935 – Pulse radar successfully demonstrated (occurred in UK) 1937 – Shipboard RADAR tested Freya

Pulsed Radar Extend the environment beyond our own eyes and ears. Extends detection capability. Advantages Can see through conditions that impair visual detection, with a longer visual LOS range. Gives an accurate measurement of range and relative motion (more so than with just eyes). Gives accurate measurement of azimuth and elevation. Accurate information fed into fire control systems much more efficiently than human capable of doing. Disadvantages Poor target resolution. Poor ID capability. i)         Advantages: (1)     Enables you to see farther through conditions that impair visual detection. Not affected by night, fog, clouds or other visual obstacles. (2)     Gives you an accurate measurement of range and can indicate something about target motion. ii)       Disadvantages: (1)     Poor target resolution (2)     Poor target identification when compared to abilities of eye.

Radio Frequency (RF) The working/operating frequency Modulated to form pulse train Ranges between 400MHz and 20 GHz Most commonly radars use 1000 MHz – 17GHz a)       Radio Frequency = RF = Carrier Frequency = f - the working / operating frequency. The frequency of the carrier wave being modulated to form the pulse train; Units: Hz (GHz or MHz range)

Pulse Diagram PRF = 1/PRT Pulse Repetition Frequency = PRF - the number of times pulse is transmitted per second. Units: Hz. Pulse Width = PW - the active transmit time or duration of the EM pulse; Units: m sec   Rest Time = RT - non-transmit time or the interval between EM pulses; Units: m sec Pulse Repetition Time = PRT - the total time for one transmission cycle. PRT = PW + RT or PRT = 1 / PRF ; Units: msec PRF = 1/PRT

Pulse Repetition Time (PRT) PRT has units of time (milliseconds). Is interval between start of one pulse and start of another. Total time for one transmission (send/receive). PRT = PW + RT = one pulse / # pulses per second Also known as Pulse Repetition Interval (PRI).

Peak Power /Average Power Peak Power (PPK) Power in each pulse. Affects maximum radar range. Usually expressed in kilowatts or megawatts. Average Power (PAVG) Power in each pulse spread over entire PRT. Higher average power requires greater heat removal. Usually expressed in kilowatts. Duty Cycle Convenient way to express loading on transmitter. a)       Peak Power = Ppeak - max output power level. The higher the peak power, the greater the range resolution (and also the better the probability of detection against small targets).   b)       Average Power = Pavg - Average power is a measurement of the transmitted power divided by the entire cycle. Because the Pulsed Radar system is not transmitting for the entire transmission cycle, the average power is quite low when compared to the peak power which is measured during only the transmit time. Therefore, a)       Duty Cycle = DC - The fraction of each transmission cycle that the radar is actually transmitting. Duty Cycle = (PW)(PRF) = Duty Cycle = (PW) (PRF) = PW/PRT = PAVE/PPEAK

Power (Peak vs. Ave) Duty Cycle = (PW) (PRF) = PW/PRT = PAVE/PPEAK a)       Peak Power = Ppeak - max output power level. The higher the peak power, the greater the range resolution (and also the better the probability of detection against small targets).   b)       Average Power = Pavg - Average power is a measurement of the transmitted power divided by the entire cycle. Because the Pulsed Radar system is not transmitting for the entire transmission cycle, the average power is quite low when compared to the peak power which is measured during only the transmit time. Therefore, a)       Duty Cycle = DC - The fraction of each transmission cycle that the radar is actually transmitting. Duty Cycle = (PW)(PRF) Duty Cycle = (PW) (PRF) = PW/PRT = PAVE/PPEAK

(2x10-6sec)(5000 Hz) =( 20x103W)/PPeak Example 1 A 50 MHz RADAR has a PRF of 5,000 Hz, Average power of 20 kW and a pulse width of 2 μSec. Calculate the peak power of this radar. Duty Cycle = (PW) (PRF) = PW/PRT = PAVE/PPEAK (2x10-6sec)(5000 Hz) =( 20x103W)/PPeak Ppeak = 2000kW

Pulsed Radars Most widely used technique, the ‘conventional’ radar. Based on electromagnetic pulse that is sent out into the environment and waited on for ‘echo’. Utilizes speed of light to measure range of echo. A close-up view of the front of the mast of the USNS HIDDENSEE (185NS9201) showing the High Pole-B EW antenna and the Square Head IFF receiver on the back side. On the front is the Plank Shave missile targeting radar antenna. Atop the bridge is a Kivach 3 surface search radar antenna. The dome unit above that is the Bass Tilt gunfire control radar antenna which controls both the AK-176 and the AK-630 gun systems. The ship is moored at the Naval Sea Systems Command facility at Solomons Annex. R = ct / 2

Range Calculations Range = c Dt 2 Example: If you have a radar signal that is transmitted and returned from a contact in 33 msecs, what is the range to the contact ? R = 3X108 m/s X 33X10-6 s = 9873 m = 4.93 km 2 2

Pulse Radar Components Coordinator of entire operation. Tells each component when to do their thing The RF oscillator that generates and|amplifies the signal. Acts as gate for transmitted and received signals. Prevents receiver from being blasted Supplies AC & DC power for radar operation 1)       PULSED RADAR COMPONENTS (Memory Reminder: stadrip)   S - Synchronizer: determines timing and coordinates action among other circuits. Regulates the rate at which pulses are sent (sets PRF) and resets timing clock T - Transmitter: generates and amplifies signal at appropriate frequency. A - Antenna: radiates and receives the signal. D - Duplexer: directs the transmitted and received signals to the appropriate path; also protects receiver from being blasted by the high power of the transmitted signal. R - Receiver: receives, amplifies and processes the return signal. Sets Threshold Level. (1)     CFAR - Constant False Alarm Rate - some receivers monitor background and adjust the SNR to maintain a constant false alarm rate. (2)     Pulse Integration - receiver takes average return strength over many pulses (3)     STC - reduces return / clutter caused by sea state. (4)     FTC - reduces return / clutter caused by rain. I - Indicator or Display section: the scope - let’s you “see” the radar return. (1)     A-scan - return amplitude strength (vertical axis) Vs time delay (horizontal axis). (2)     PPI = Plan Position Indicator - provides “top down” view; most common. P - Power Supply: provides AC and DC power for radar operation. Antenna – radiates and receives the signal. Display section, or scope. Let’s you see the radar scope. Receives, amplifies and processes the return signal,

Radar Demo Basic Pulse Radar 1 plane Long and short pulse

Signal Reception Only a minute portion of the RF is reflected off the target. Only a fraction of that returns to the antenna. The weaker the signal the receiver can process, the greater the effective range. Explain why only portion of the signal gets to the target and only a fraction of that signal gets back to the receiver.

Signal-to-Noise Ratio Ability to recognize target in background noise. Noise is always present. At some range, noise is greater than target’s return. Noise sets the absolute lower limit of the receiver’s sensitivity. The average amount of background noise is called Noise Equivalent Power (NEP). Threshold level used to suppresses excess noise. S/N sets a threshold for detection that determines what will be displayed and what will not. If S/N = 1 then only returns with power equal to or greater than the background noise will be displayed. Signal-to-Noise Ratio: a. Noise (always present) sets the absolute lower limit of the sensitivity of the radar sets. (At some range the noise will be greater than the echo) Example: Look at a radio. If you turn down the volume eventual you will not hear the music only the static. The static is noise. b. Noise includes atmospheric disturbances, Jamming, stray signals. Noise is inherent in electronic circuits as random electron motion through a resister causes stray noise. c. To cope with this problem, the operator can set a threshold level. If signals are below this threshold level, they will not be displayed. * If threshold level is set too low - you get many false detentions. * If set to high - could mask out the real contact. Must compromise. a)       Signal to Noise Ratio - the ability to discern a received signal from background noise. Refer to Page 3 of your Appendices.

Noise Equivalent Power (NEP) Noise is always present. internal & external / natural & intentional pulses returning from long range can be less than ambient noise level Noise sets absolute lower limit of radar receiver’s sensitivity. The average amount of background noise is called Noise Equivalent Power (NEP). NEP relates noise to a detected power level so that it may be compared to the returning radar signal. NEP usually expressed in Watts. NEP = Noise Equivalent Power - the average amount of background noise. Directly relates noise to a detected power level so it may be compared to the returned signal from a contact.

Threshold Level Targets False Alarms TL #1 Mean Noise level time a)       Threshold Level - signal level above which receiver recognizes return as valid target.   * If set too low - false targets * If set too high - missed targets Noise Spikes

Targets False Targets PPI

Threshold Level Targets False Alarms TL #2 TL #1 Mean Noise level time a)       Threshold Level - signal level above which receiver recognizes return as valid target.   * If set too low - false targets * If set too high - missed targets Noise Spikes

Targets Undetected Target PPI

Signal to Noise Ratio Review Signal to Noise quantifies the background noise intensity compared to the total signal intensity S/N is the signal to noise quotient S/N = 10 (SNR/10) SNR is S/N expressed in decibels (dB) SNR = 10 log (S/N) (SNR)dB = levelsignal(dBm) - levelnoise(dBm) 1000 Hz signal level is 15 dBm Level of noise is 5 dBm SNR is 10 dB

Signal to Noise Ratio SNRdB = 10log (S/N) S/N = 10 (SNR/10) Signal to noise ratio- the ability to discern a received signal from background noise SNRdB = 10log (S/N) S/N = 10 (SNR/10) Signal-to-Noise Ratio Ability to recognize target in random noise. Noise is always present. At some range, noise is greater that target’s return. Noise sets the absolute lower limit of the unit’s sensitivity. Threshold level used to remove excess noise.

(Pr) > (S/N) (NEP) = Smin Smin = Minimum Signal for Detection Measure of the strength of the weakest signal return received by the radar and evaluated as a valid contact To get a valid contact, Return Power (Pr) needs to be greater than Smin therefore… a)       Smin = minimum signal for detection; The measure of the strength of the weakest signal return received by the radar and evaluated as a valid contact (measured in Watts). Therefore, to ensure detection of a contact we desire the power of the return signal   (Pr) > (S/N) (NEP) = Smin (Pr) > (S/N) (NEP) = Smin

MDS MDS = Minimum Discernable Signal Decibel equivalent of Smin Calculated relative to 1 mW a)       MDS = Minimum Discernible Signal - the decibel (dB) equivalent of Smin.  

Objectives Explain how range is determined with the basic radar system. Apply relationships between Duty Cycle, PW, PRF, PRT, average and peak power. Calculate Signal to Noise ratio (dB and dimensionless). Apply the relationship between S/N, NEP and Minimum Signal for Detection. Explain Threshold Level (TL) and distinguish setting the receiver above and below the minimum signal for detection. Identify the components of the Basic Pulsed Radar system

Quiz on Friday Lessons 2 - 4 Assignment Read Chapter 3 pp 30 - 41 Do Guided Reading 5 and 6 Quiz on Friday Lessons 2 - 4