MBTA PTC data on 217 MHz

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wb1hbu
Posts: 989
Joined: 08 Oct 2005 21:38
Location: Lynn, Essex Co.

MBTA PTC data on 217 MHz

Post by wb1hbu »

Hello,

I was tuning around the AMTS (217 MHz) band and found strong GE MDS TD220 data transmissions. They are similar to AMTRAK's ACSES II system that I noticed along the Northeast Corridor (NEC) (Boston to Providence and beyond) on 217 MHz. I knew the MBTA was planning on using ACSES for Positive Train Control (PTC) a couple of years ago, but did not see any new signals when I last checked. I did a FCC ULS search and found the MBTA license, WRAN721. They are licensed in the same 217.5 to 218 segment as AMTRAK. This makes sense as MBTA runs commuter trains on the NEC.

The GE MDS TD220 radios use GMSK modulation in a timeslot format. There is usually a data transmission once per second. It you look at the spectrum you may see a brief spike lower than the center frequency, then the typical hump of a data transmission centered on the channel. The transmissions are synchronized to GPS, so you see the signals come up at the same time, once per second.

If you look at the MBTA tracks on Google Earth, or the satellite view on Google maps, you may spot two square boxes in the middle of the track. These are balises the locomotive reads to get location and permanent speed restriction information.

Amtrak seems to use a four frequency rotation for the NEC.
217.54375, 217.55625, 217.56875, and 217.58125

MBTA ones I receive at my QTH. I managed to confirm a couple of them via zero antenna patrol. These frequencies are likely used in other areas.
217.60625 S3 Likely Swampscott near Essex St Bridge
217.63125 S9 Likely West Lynn near Saugus River Drawbridge
217.65625 S5
217.66875 S3
217.68125 S9 West Lynn near Shepard St Bridge
217.69375 S5
217.70625 S1
217.74375 S5 East Lynn near Fayette St Bridge

I suspect given the band plan, 217.61875, 217.64375, 217.71875, and 217.73125 are likely in use.

73 Eric

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