Radar is a technology that in recent years has advanced in both what it can do and in becoming a lot more user friendly and intuitive. Like any other electronic device, it works as advertised only if it is installed, maintained and used properly.
John Barry is an experienced marine electronics dealer who instructs technical training courses offered by the National Marine Electronics Assoc. He wrote the article below for technicians, and as he says, "Radar troubleshooting is best left to the pros.” True enough and yet boaters will benefit from knowing what some symptoms and on-screen messages might indicate and what to expect when they call in a pro to make the repair.
By John Barry
Radar is essential equipment aboard many vessels. They’re required on commercial vessels, even two radars. Requirements vary and most pleasure boats are generally exempt from radar carriage requirements. Still, a working radar (and a skilled operator) is the primary collision avoidance device on board. Nothing replaces a good watchman-eyes through glass-but in low visibility, darkness or fog, the watchman becomes crippled and the radar can be a lifesaver. As you may know, radar is an acronym for Radio Detection And Ranging.
Reading the symptoms
When a radar fails, several symptoms are common. These days, with digital radar, the connection to the display is through Ethernet, so "No Radar Detected” may show up on the screen if a network problem occurs or if the radar goes completely dead. Troubleshooting Ethernet is beyond the scope of this article, so instead let’s look at radar symptoms that occur when the radar is not dead.
"Radar Rotates But No Targets” is one of those symptoms. When everything looks normal, the screen turns on and counts down, the transmit engages, but you have no targets. This can be a symptom of no transmit of no receive. To determine which is the problem, put the radar on the longest range, defeat the clutter filters (Sea and Rain Clutter OFF) and turn up the gain. If the screen remains completely blank, there is likely no receive. If there is "grass” (scattered clutter, evenly distributed) on the screen, then the receive side is likely OK and there is no transmit.
On analogue radar, there is a composite video signal on a coax within the interconnect cable and this can fail, causing no targets, no grass but in this case only a broken coax is the problem. "No Targets” is a common complaint and is usually a transmit problem, either magnetron, modulator board or power supply. Repairing a radar requires an FCC GROL (General Radiotelephone Operator License) with Marine Radar Endorsement.
Sometimes the complaint is "Weak Targets.” On pleasure boats this problem is usually operator error. Here’s an example: A customer told me the other day that he had radar, even though I could clearly see no radar on the boat. He explained that the display on the dash was his radar, while in fact it was a chartplotter. Once we educated him about what a radar is, we added one. Now he needs to practice operating it in fair conditions, so that when he needs the radar he can operate it without the book.
Weak targets can be caused by weak transmit or out-of spec-receive. It is usually caused by the operator setting the Sea/Rain Clutter filters too high, the Gain too low or the Tune maladjusted. If you can not improve the targets by tuning, check the magnetron current to confirm strong transmit.
Another symptom that I have seen is "Targets Blank Out” or "Targets Move Around.” This is a symptom of synchronization. When pulsed radar transmits, it measures time to determine distance and it uses bearing pulses to determine direction. If the orientation of the targets on the screen change, the bearing pulses or the heading pulse may be missing or erroneous.
Which way is up?
The heading pulse is generated when the array passes the 12 o’clock position and then the bearing pulses are counted to determine direction. If the heading pulse or bearing pulses are missing, the radar will usually show an error code. When these signals are intermittent or noisy, the targets become disoriented to the bow and "jump around” the screen. The heading pulse is generated by a magnet passing a reed switch. Failure of the head-up switch means the radar does not know where the bow is. Bearing pulses are typically generated by the motor or an optical interupter. Failure of the bearing pulses means that the radar does not know how far past the bow the radar has rotated.
The symptom "Missing a Pie-Shaped Piece of Targets” where the screen goes blank at a certain bearing is almost certainly caused by the bearing pulse generator. The symptom "Paints Targets in Wrong Direction” is more likely the heading pulse reed switch. Optical bearing pulse generators are susceptible to debris or moisture blocking some of the holes in the interrupter card. This causes a one-shaped blank spot.
Manufacturers are pretty good at troubleshooting radars. When seeking assistance, make sure you have tried the easy stuff. "No Radar Detected” on the screen means a network problem and the manufacturers do this differently. Sometimes a call to tech support can resolve this, but always read the installation instruction carefully if a radar does not show up on the network. Sometimes a restart, a software upgrade or a menu setting is all you need to do. If the problems persist, use some of the tried-and-true troubleshooting methods: What does work? Did it ever work? Simplify the system to just a display and radar. Make close and repeatable observations. Understanding how something works is the key to understanding why it does not work.
Radar troubleshooting is best left to the pros. Understanding how it works helps eliminate the easy ones like weak targets that just need operator training to resolve. This topic is deep and whole books can be written about it.
Hopefully this discussion helps the curious understand what is happening when you run a radar. Repeating what I said earlier, always tell your customers to use their radars in clear conditions and often so that when (not if) the radar becomes an essential safety device, the operator can harness it. Stay safe with radar!