In the CP-140 Aurora, the AN/APS-506 search radar is the Canadian nomenclature for the Texas Instruments AN/APS-116/A (Jane's Information Group, 1993).The AN/APS-506 is a pulse-compression radar system operating in the X band, in the frequency range 9.5 to 10.0 GHz, with its scanner housed in a nose radome. This system reduces sea clutter by performing scan-to-scan integration. The scan converter provides optimum scan-converted ground-stabilized Plan-Position-Indicator (PPI) or a 140-degree wide bearing scan (B-scan) video. It is a multimode high-resolution system with three primary modes:
a. Periscope mode 1
b. Navigation mode 2
c. High-resolution scanning mode 3As will be further explained below, it can be used in full-scan, sector-scan or searchlight operation. Pulse-repetition frequency can vary from a minimum of 500 pulses per second (pps) for modes 2 and 3 to a maximum 2000 pps in mode 1 (CP-140, 1990). In the high resolution mode 1 or "periscope search mode", the radar operates at a scanning rate of 300 revolutions per minute (RPM) and its signal is usually detected by a submarine in a relatively short time. Its main function is therefore to provide initial positional input for sonobuoy launching and subsequent use of the MAD. Because the information it provides is of limited temporal usefulness, it is not a mode amenable to long-term fusion [1] . But it can be used to initiate sub-surface tracks within its effective range of 32 data miles (DM). Both PPI and B-scan video are available in this mode.
Note that the radar measure of 1 DM = 6000 feet = 0.987 nautical mile (NM), the latter unit of measurement being used primarily for navigation. The nautical mile is a unit of length defined as 1.852 km = 1.15078 (statute) miles. The nautical mile is used in navigation because it is approximately equal to the distance along 1 arc-minute of latitude at the Earth's equator. The regular (statute) mile is of course 1.60934 km. Hence 1 DM = 1.13582 miles = 1.82792 km.
In the low resolution mode 2, the so-called "navigation/weather mode" operating at 6 RPM, the radar is used mainly by the navigation communication (NAVCOM) operator for course charting (and thus is of limited tactical interest), mainly providing surface plots of medium-to large contacts. It can however provide an outlook of environmental conditions, mainly meteorological, in selected sectors of tactical interest, and this information should be used (fused is too strong a word for such global peripheral information) to correct for range and/or bearing inaccuracies of targets detected in those sectors. This mode is the default mode in off-line operation. The maximum range of mode 2 is 150 DM.
Mode 3 is by far the most important since it is the usual high resolution scanning mode (at 6 RPM also) for detecting surface vessels and aircraft up to 150 DM from the surveillance aircraft. It should be noted that the radar emits such strong pulses that they must be electronically suppressed through an arc of 140 degrees towards the rear to protect the crew. This leads to a biased coverage unless the CP-140 regularly banks left and right by at least half that angle. Bearing accuracy has a ±1.25° probable error and bearing resolution has a 2.5° probable error (Lockheed, 1979), mainly due to the aircraft's angular drift with respect to absolute ground coordinates.
The Aurora Aircraft Operating Instructions manual provides this specific description:
The APS-506 has three modes of operation (periscope and snorkel detection, search and navigation, and ocean search) for online operation and only mode 2 (search and navigation) can be operated offline. The Receiver-Pulse Compressor Contains a Duplexer, allowing both the Receiver and Transmitter to share a single Antenna. When processing returns, the Receiver compares a sample pulse from the Synchronizer-Exciter with the return for correlation, minimizing the potential for jamming. Both Frequency Modulated and Agile pulse returns undergo Amplification and Automatic Gain Control (AGC). The Frequency Agile return is further processed to increase Gain, ensuring it fulfills its role in the Navigation mode. Frequency Modulated returns are Pulse Compressed from 0.5 µsec to 0.003 µsec. This greatly increases the Range Resolution. Two more AGC loops are applied and the return must exceed the Operator selectable False Alarm Rate (FAR) setting to validate for further processing. Primarily these techniques enhance the radar's small target detection. All three Modes determine Range using Range Video Filtering, this process also allows integration in Modes 1 and 3, improving the Signal to Noise ratio. The resultant signal is referred to as Radar Video and is sent to the Radar Indicator Unit ( RIU) .[1] Fusion is a principle of integrating data from all sensors so a tactical solution can be derived from all the input.
Reference:
1) Extract from: www.pubs.drdc.gc.ca/PDFS/unc48/p525407.pdf
2) Extract from CP-140 Aircraft Operating Instructions.