In a shootout between two economy-class anti-collision gadgets, the Monroy ATD-200 comes out on top.

Anti-collision equipment of some kind seems to rank high on the wants-and-needs list of most owners, yet few aircraft are equipped with these devices. Our guess is that the high cost of entry is the reason for this, since most of us agree that scanning for traffic with electronic eyes is a good idea.

The antidote to high-priced panel-mount traffic scanners comes in the form of two inexpensive portable devices, the Monroy ATD-200 and the more recently introduced TPAS RX-100 from SureCheck Aviation, a product thats stirred interest thanks to wide promotion by SureCheck. By comparison, the Monroy unit has been advertised hardly at all and buyers have to beat the bushes to find it. Both scanners work on the identical principle; theyre nothing but passive receivers which listen for the transponders in other aircraft replying to either ground-based ATC radar or airborne TCAS. Based on received signal characteristics, the traffic scanners roughly calculate range to the target but not azimuth or relative altitude.

With smallish antennas limited to the host aircrafts glareshield and rudimentary detection logic when compared to a high-dollar TCAS or Skywatch system, these devices have limited capability, thus anyone considering buying one shouldnt go into the deal expecting too much.

We recently tested both units side by side, comparing the results against a more expensive range-only model of a panel-mounted Ryan TCAD. In a nutshell, we conclude that both the ATD-200 and TPAS have value in that theyll detect some nearby traffic that you might not see otherwise.

On the other hand, its a mistake to assume that either will always see all transponder- equipped traffic, even when its replying to radar or TCAS queries. We found that both units have sharp limitations and the TPAS is given to false alerts; the distraction of scanning for traffic thats not there is of little benefit, in our view.

Monroy ATD-200
The Monroy ATD-200-for air traffic detector-appeared on the market first, about three years ago, having been developed by Monroy Aerospace, a company known for its long-range fuel tanks for Mooneys. Although not promoted nearly as heavily as SureChecks TPAS has been, the ATD-200 caused a ripple among aircraft owners, for at $789, it was a fraction of the cost of the next cheapest competitor, the Ryan TCAD 8800 at about $6000. The price of the ATD-200 has since been reduced to $695, if youre an AOPA member.

According to the manufacturers specs, the ATD-200s passive-only receiver can detect traffic at a range of at least four miles and vertical limits of 2500 feet, tapering down to 1500 feet for detected traffic thats closer.

It determines range by relative signal strength, displaying the value in miles on five yellow LEDs on the front of the instrument. It can sense vertical separation beyond the limits stated above to reduce false alerts on non-threatening targets. One LED illuminated means the traffic is distant-about 4 miles-two LEDs means its closer-3 miles and within 2500 feet and so on. When all five illuminate, the traffic is nearby and the unit also annunciates a voice warning traffic nearby. The audio is piped to headsets via a splitter cord and jack provided with the product.

Setting up the ATD-200 involves merely plugging it into ships power-it will run on 11 to 28 volts but has no internal battery option. A 3-inch rubber-ducky-style antenna plugs into the back and the audio cable is routed through to the airplanes onboard headset jack.

The ATD-200 has only two controls to worry about: a dimmer switch to set display light level and voice switch that allows the pilot to hear all the voice messages, only those which apply to traffic 1 mile or closer or to mute the voice annunciation entirely. Theres also a test button; pushing it checks the circuitry, the display and the ATD-200s voice annunciation.

To keep from driving the pilot nuts with hits from the host transponder, the ATD suppresses signal reception and annunciates this fact with a small blue light. Another blue annunciator illuminates when the nearby traffic is broadcasting a Mode-S pulse, a good clue that its hearing airline traffic. (Then again, Mode-S is finding its way into more corporate aircraft and light aircraft GA, too.)

What we find most impressive about the ATD-200 is its tiny size; it measures only 2 3/4 inches wide by 5 1/4 inches deep and is a mere 3/4 inch high. It will easily occupy the glareshield without being intrusive and is small enough to be carried from airplane to airplane by a renter.

The ATD can also be mounted under the panel lip-brackets are provided-or in-panel, in which case a standard transponder antenna mounted on the belly is recommended.

SureChecks TPAS RX-100-for traffic proximity alert system-has many of the same features as the ATD-200, in that its a passive listener with graduated threat annunciation, including audio voice alerts. Although the the TPAS can be panel-mounted, its likely market seems for glareshield/portable use only, given the battery option. It retails for $595.

One thing sure to surprise some buyers is the size of the TPAS. SureChecks advertising gives no relative idea of scale unless you look carefully but the thing is big. It measures 4 1/4 inches wide by 8 1/2 inches deep and is 1 3/4 inches high. Its actually closer to the size of a panel-mount radio than a portable device, in our view.

This is a problem because on a narrow glareshield, its difficult to position the TPAS so the antenna can stand vertically. SureCheck can provide a suction-cup remote antenna to alleviate this-its still an internal antenna, however-but we felt the box is just too large for comfortable glareshield positioning. At times, we had to move it to see the traffic we were trying to avoid and it definitely comes off the glareshield during landing.

The TPAS will operate from a power source between 8 and 35 volts but it can also be battery operated on six AA batteries in those aircraft in which accessory voltage isnt available. Claimed battery life is three to 10 hours, however we didnt check the battery function.

The RX-100 is shipped with the power cable plus an audio splitter jack so the audio can be piped into a headset. In our view, the jack provided is not quite as elegant as the ATDs splitter cable, but it gets the job done.

Unlike the ATD-200, the TPAS has two levels of range sensing, one for terminal flying and one for enroute flying, each of which is set with a rotary knob. In enroute mode, TPAS scans out to 10 miles and displays the distance in 1-mile increments. In TCA or terminal mode, the range is limited to 5 miles and .1 increments are displayed.

The display itself is a large, bright red digital LED thats hard to miss. We had no trouble reading it in sunlight and it has a light sensor for automatic dimming at night, so its brightness doesnt reach the nuisance level. In addition to the range selector, theres a volume control for the voice annunciation and a small indicator to tell the pilot if transponder queries are coming from ground-based radar or airborne TCAS unit.

The box has three levels of threat warning in terminal mode. Traffic at 5 miles illuminates a blue airplane icon but no other warning. At 2 to 3 miles, a yellow traffic advisory illuminates and traffic is heard. At 1 mile, a red traffic alert illuminates and traffic alert, obtain visual contact is heard.

Along with the voice, theres tone annunciation, too, a high-low short warble. Fortunately, the volume control is always at hand to silence the tone and voice if it becomes intolerably annoying, as it did for us occasionally.

To test the units side-by-side, we flew with them for several hours and did some range accuracy checking by flying formation with another aircraft equipped with an Ryan TCAD. During some of the testing, we enlisted ATC to help eyeball traffic and to assist in range determination.

In response to our previous reports on anti-collision equipment, a few owners who have this gear installed have written us to express their amazement at the volume of air traffic out there that you never see. True. But its also true that most of what you cant see doesnt represent a collision threat, either.

This relates directly to the practical range limitations of traffic nannies. During our testing, in which we measured range with both the Ryan TCAD and comparative GPS data, we found that beyond a mile or so, a small aircraft is difficult to see at all, let alone well enough to distinguish it from a cluttered or hazy background. Unless youve got the eagle eyes of a young Chuck Yeager, seeing a small aircraft at two miles is all but hopeless, although larger airplanes are often visible. This is a shortcoming for range-only anti-collision devices, which purport to tell you how far an aircraft is but not its azimuth. You therefore spend a great deal of attention scanning vast areas of sky for something that may or may not be there and may or may not represent a threat. In our view, this cry-wolf phenomenon is distracting and, at times, worse than having no scanning at all.

Inflight Testing
So how did the traffic nannies work in our side-by-side testing? In a word, randomly. We found them both to be iffy in accurate range display, although the Monroy ATD-200 seems to be the better of the two, with far fewer false alarms and no hits from the host transponder, something the TPAS had trouble with.

We flew formation with the TCAD-equipped aircraft from all points of the clock and at various distances and used GPS data to measure true range. Approaching the TCAD aircraft from behind and at a GPS derived range of 1.4 miles, the target aircraft was not visible against a milky sky. The TCAD showed 1.6 miles, the TPAS vacillated between .3 and 3 miles and the Monroy illuminated three warning lights, indicating a range of 2 miles. Both devices annunciated audio warnings.

As the range closed, the Monroy illuminated four or five LEDs at about a mile while the TPAS stayed locked on .3 mile. The Ryan TCAD continued to display relatively accurately range, within a tenth or two of the true distance as referenced by GPS.

We repeated these results from various positions around the TCAD-equipped target aircraft and found a degree of randomness to the ranging estimates of both the TPAS and the Monroy ATD-200. In general, the TPAS alerted first but tended to be wildly conservative and inconsistent in its range display.

For example, on one pass at a measured distance of 2 miles, the TPAS indicated 1.6 miles then immediately dropped to its minimum of .3 miles and issued its obtain visual contact voice warning. Yet, as noted, seeing a small aircraft at 2 miles is a prayer at best. In this situation, the Monroy showed two or three LEDs, indicating the range a bit more accurately and, we thought, prompting a more reasonable degree of vigilance.

When mounted on the glareshield with internal antennas only, we expected both TPAS and the ATD-200 to be blind from traffic approaching from the rear. In reality, both were able to detect it beyond the 1-mile range but again, the accuracy of the displayed range was doubtful, especially on the TPAS.

When scanning for traffic immediately below the aircraft, the Monroy appeared to be essentially blind, showing no warning. The TPAS did better here, indicating the presence of the target, albeit at a much shorter-than-actual range.

In head-on scenarios, the Ryan TCAD nicely detected the closing traffic at some 2 miles or more, then smoothly counted down the range as the two aircraft merged. The TPAS seemed to see the traffic at a greater distance but its wild vacillation in range made it impossible to judge the threat, leaving the pilot with no choice but to scan the entire field of vision and hope for the best. In the head-on test, the Monroy alerted one LED at about 2 miles actual range, then rapidly upped the ante by firing up all the LEDs and issuing the audio warning.

Although the Monroy ATD-200 seems less sensitive than the TPAS for distant traffic, it also doesnt false alarm as often, if at all. In numerous situations, the TPAS indicated traffic within a mile yet ATC couldnt see anything within 7 to 10 miles.

Further, the TPAS occasionally interpreted our own transponder as nearby traffic and went into continuous alert. We confirmed this by switching the transponder to standby, which immediately caused the traffic indication to vanish. SureCheck says TPAS can be calibrated to reduce host sensing so it may take some back and forth with the company to sort this out.

Given the uneven performance of the SureCheck TPAS, were hard pressed to recommend it. We found that it alarms so often that its impossible to tell when the alert is real or not and the range display is too inconsistent to be trustworthy. While an alert of any kind may be seen as a good thing, we found ourselves scanning for traffic constantly at unknown range, some of it not there at all. That gets exhausting after awhile.

Although the TPASs display and controls are well designed, we were put off by its large size. In a small aircraft such as our Mooney, it occupies too much of the glareshield to be practical. Add to this the nuisance of wiring in the power and the audio and throw in a portable GPS and it gets to be too much mess.

Last, as Lance Fisher noted in his report, SureChecks customer support needs improvement. It took us more than a year of trying to get a test unit and numerous phone calls to the company went unreturned, despite the fact that SureCheck sent us a glossy press release announcing the unit and inviting us to give it a try. We think the TPAS idea has potential, but in our view, its execution needs work.

This is not to suggest that the Monroy ATD-200 is perfect. Its ranging and display is rudimentary as best but on balance, we thought it was more effective than the TPAS. As we reported in our previous review, the ATD-200 says somethings out there, better look and it does so in a way thats not distracting or insistent. We like the fact that its ranging doesnt purport to have resolution better than a mile or so, while the TPAS tries to deliver on .1 mile accuracy, something it just isnt capable of.

The ATD-200 does appear to miss traffic now and then but it effectively suppresses the host transponder. Moreover, its range may not be much more accurate than the TPAS but it displays its warning in a way more appropriate for scanning for the relative threat. We like its small size and the fact that it can be panel or lip mounted and wired with an external antenna.

Although the ATD-200 costs $100 more than the TPAS, we think it represents the better value in low-cost traffic detection gear.

Contact- Surecheck Aviation, P.O. Box 131482 Carlsbad, CA 92013-1482; 888-340-8055; Monroy Aerospace, P.O. Box 8217 Coral Springs, FL 33075; 954-294-9006;

Also With This Article
Click here to view “Checklist.”
Click here to view “TPAS and ATD-200 Controls.”
Click here to view “Third Times Not The Charm.”