The SoCal ATCS Sysops Group is an informal alliance of ATCS Monitor site builders, layout designers, and equipment installers, focused on Southern California and Arizona, a very busy region for railroading, dominated by the Class-1 freights BNSF and UP, and home to many heavy-rail transits including SoCal's Metrolink (SCRRA). As a volunteer group, everything is on our own time and most everything we do we pay for out of our own pockets, but we truly appreciate those can help us to "light up" more of the region through donations to the cause.

Equipment is rarely free, though sometimes good deals can be found on eBay, and the cost of acquiring the necessary quality equipment and installing a single site can easily approach or exceed $1000. Consider participating in our activities with a donation of $5, $10, $20 or more by clicking on the PayPal "Donate" button above. Donations will go toward site maintenance expenses and the potential for new server locations in the Southern California area. Please note that your donations are not tax-deductible. All donations go to Jon Adams, who acts as treasurer for the funds.

Active Sites

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Putting up an antenna at a convenient location, no matter how good the equipment, may net little but a local control point or two. The first trick is to know where the control points are, map them into a tool like Google Earth, and see what the terrain looks like. Once this information is available, see where there's people or businesses that might be suitable for covering the area desired. Sometimes the specific location for a control point is available in the ATCS Monitor group's file area, but just as often it's found by visiting the location and recording latitude and longitude from a GPS. Occasionally, it's even visible on Google Earth, so long as you know what you're looking for.

Modeling Radio Coverage for Your Territory

With any radio receive site, it's important first to have an estimate of what kind of coverage might be expected - it's one thing to eyeball it, but radio waves don't necessarily behave like light. Sometimes better, sometimes worse! Some people drive an area, using a mobile setup in their vehicle, to try to understand where coverage is best. At the cost of fuel nowadays, that can be an expensive and (still) incomplete exercise. However, some territories are pretty straightforward, with little topographic challenge.

Radio Mobile is a powerful and sophisticated RF coverage modeling tool. It's available at http://www.cplus.org/rmw/english1.html. Roger Coudé VE2DBE is an amateur radio operator who developed this tool, and provides it as freeware to the community. There is excellent support via the Yahoo Groups Radio_Mobile_Deluxe group at http://groups.yahoo.com/group/Radio_Mobile_Deluxe/. There are over 8000 members there, and lots of people who can help the newcomer climb the sometimes-steep learning curve for Radio Mobile.

Finding a Home for Your Monitoring Site

Once a general idea of where good coverage can be had via Radio Mobile, it's time to start looking for a practical place to put the neccesary equipment. While a commercial radio site may have excellent coverage, it often comes at a steep monthly cost for site rental, tower space, electricity, internet, etc. That's almost always out of the question for a volunteer activity. The next best thing is to find neighborhoods where people live or there are businesses. Check hobby shops, smaller stores that are likely locally owned, tourist attractions, etc. Since I'm a ham, I look for ham radio operators. They understand antennas, might have a tower or other antenna support structure, sometimes appreciate railfanning, and can be very generous. Going back to the Radio Mobile tool, try different hams' locations and see if any of them fit your needs. Sometimes you can even see if they have towers or other structures on Google Earth! Ask nicely, politely, respecting that they may not share your interests, and every once in a while you might score a great location.

In General, How ATCS Works

ATCS is somewhat like the digital version of the voice messages that you might hear on the scanner. On many railroads, the engineer calls out on the voice radio information like train ID, direction, signals and locations. You might hear something like "Metrolink 321 South diverging approach CP Crest" if you're listening to the Lancaster (CA) Railroad Radio feed. That tells you there's a Metrolink train #321, headed south from Palmdale, within visual range of the southbound signal immediately before control point Crest, and the track switch at Crest is set for the siding (diverging route). The signal aspect that the engineer sees is probably a red over yellow (GCOR 9.59). Here's a link to a photo of a train before a diverging approach signal. The voice radio has generally more information than you can get from monitoring the ATCS data messages.

In the ATCS data world, that control point Crest is sending out a regular message indicating the occupancy (or not) of the approach block to the north, the two approach blocks to the south, the terminal section between the signals, the position of the track switch (whether normal or reverse). The signal aspects are not visible, only whether a signal is displaying a clear or stop indication. Without a train nearby, these bits don't generally change.

CP Crest Approach Blocks

CP Crest Signals

CP Crest Track Switch Normal Position

CP Crest Track Switch Reverse Position

Southbound Train Approaching CP Crest, Track Switch Reversed

From this last diagram, a fair assumption is that the red block represents occupancy by a train, and that this train is moving south toward CP Crest, but that's all an assumption based upon what happened before. It's also possible that the red block represents a problem with the track circuit, or a train moving away from CP Crest with CP Crest's southbound signal and switch already set for a future train that will be moving southbound, or something else.

The Truth about Error Rate

A typical control point using ATCS communications transmits an indication message once a minute even when there is no activity. That's 1440 messages per day. And if a train passes through that control point, that generates at least another 3 to 8 messages during its passage. Some control points, like those on SCRRA's Metrolink, appear to transmit an indication once every two minutes by default. There is sometimes a large difference in the absolute number of indication messages per day per CP. ATCS Monitor attempts to estimate the error rate for indications through the use of the message sequence number, but this doesn't tell the whole story. Good reception for a given control point requires that a very large percentage of the total messages sent per day from a given control point are captured, and that will generally drive down the indication error rate as well.

This screengrab from ATCS Monitor shows the statistics over approximately a 2-hour period for a variety of control points monitored by the SoCal Aggregator. The control point names are the second column. The third column is the absolute number of indication messages received during that 2-hour period. The fourth column is the Ierr (Indication Error Rate) in percent. Note that there are control points for which there's a 0% error rate but only one indication was received in that two-hour period. That's obviously completely unusable performance even though the Ierr number is zero. This is an extreme example, but it's important to pay attention to the absolute number of indications received over a period of time, and check to see if that's consistent with other similar control points and the general rule of 1440 indication messages per day, or 60 indication messages per hour. A two-hour period should show approximately 120 indications if there's no activity. If there's activity, and control points like Daggett, East Barstow and Highgrove are generally very busy, the absolute number of indication messages per hour will be significantly greater than 60. Note CP Serra - 2% error rate but only 108 indications received in that two hours. That's still near 60 indications per hour, but shows that there was probably little or no train traffic during that time.

Building the Site

Once a site location is identified, it's time to pull together the appropriate equipment to do the monitoring. Obviously, a computer running Windows 2K/XP/Win7 and their variants is necessary to host the ATCS Monitor application. Internet is necessary, but only hundreds of bits per second for the trickle of data that is generated, even at relatively busy sites. To put that data rate into perspective, that's about 1% of an old 56 kbaud dial-up modem from the 1990's. On a modern network connection, it's barely measurable.

Good quality receivers are important for hearing the MCP signals. While a simple scanner set to a single channel can often provide reasonable performance, a commercial radio configured for receive-only operation is the best way to go. Old MDS radios, Motorola Maxtracs, EF Johnsons, Kenwoods, and many others have excellent performance. However, if all that's available is a scanner, an external preamp, with the right conditions, connected immediately after the antenna can make the receive sensitivity remarkably good.

Here's a block diagram of the site at Grand Terrace. This site uses high quality MDS and Maxtrac radios, and a rooftop high performance preamplifier.

Why use a preamp up on the roof? There's two reasons, the first is easier to explain. It's important to combat the downstream losses inherent in the coaxial cable, any splitters used to connect multiple receivers to one antenna, and other system inefficiencies. 50 feet of high quality Times Microwave LMR400 coax (nearly $1 a foot, plus connectors) has about 2 dB of loss at 900 MHz. What's that mean? That means that of the desired signal collected by the antenna, over 37% is lost sending it down that 50 feet of coax to the radio. If a splitter is used to connect that antenna to both an MCP and a BCP radio, that's another 3 to 4 dB loss. 3 dB is 50% loss, and 4 dB is 60% loss. Adding that up, 50 feet of LMR400 connected to a good-quality 2-way splitter reduces the received signal by 71%.

But there's another, more insidious problem. At the antenna, the system noise figure is the best it can be. The desired signal, received at the antenna, is combined with undesired but unavoidable natural noise from the radio environment. That 50 feet of LMR400 coax also adds about 2 dB of thermal noise power to the natural noise floor. Putting a preamplifier after that 50' of coax amplifies the now-attenuated desired signal but also 2 dB-higher natural noise level. So, first make sure to put the preamp as close to the antenna as possible. And second, use the very best preamplifiers, like ones designed to be low noise amplifiers (LNA) at the frequency of interest. In the case of 900MHz ATCS monitoring, look for quality LNAs from vendors like Advanced Receiver Research or Angle Linear. They're not cheap, but poor performance sucks worse.

In the case of the Grand Terrace site, RG6 quad-shield cable is used instead of LMR400, partially for cost reasons but also because RG6 is smaller diameter and easier to handle, and crimping connectors is a breeze. Also, the availability of high-quality 75-ohm splitters and DC power injectors is very good, with prices driven down to rock-bottom by the huge cable television industry. 44' of RG6 is about 2.8 dB of loss, compared to about 1.8 dB for the same length of LMR400. The splitter is a 4-way, with 6.5 dB of loss. The impedance mismatch from a 50 ohm to a 75 ohm system adds another 1.5 dB worst case (but almost zero best case). This all adds up to a total loss from the roof to the individual radios of 10 to 11 dB loss. Thus, 90 to 92% of the desired signal is lost unless something is done up on the roof. The preamp up on the roof has 18 dB gain, and a 0.7 dB noise figure. The preamp's noise figure keeps the ultimate system noise figure very close to the one measured at the antenna, and the transmission loss is more than made up by the preamp's gain.

Support Our Efforts by Donating

If you find our feeds and the coverage they provide useful, and that it adds to your enjoyment of railfanning, consider helping to defray our costs with a donation of $5, $10, $20 or more by clicking on the PayPal "Donate" button below. We intend the donation process to be transparent and will keep a running total published on this site. If you wish to have your donation acknowledged publicly, add a note when you send it in. Donations will go toward server expenses and the potential for new server locations in the Southern California area. Please note that your donations are not tax-deductible. The donations go to Jon Adams, who acts as treasurer for the funds.

Just think of how much gasoline and time these sites can save you by not having to guess where trains may be. Depending on the volume of the donations and our ability to find potential sites, this group may be able to continue to expand coverage through the Cajon and Beaumont Passes, into the Mojave Desert and Coachella and Imperial Valleys, and Arizona. By donating, you will be a valuable part of our effort, and allow us to procure quality, reliable equipment for future sites.

Thanks in advance for your generosity!

Jon Adams N7UV and the group.

jon (at) jonadams (dot) com