What Is MSRS:
MSRS is a radio system for sending real-time data through an FM signal. The maximum data rate achieved over such a channel is around 1200 baud or about 60 bytes/second.
This allows only the most rudimentary of text messages to be sent. However, this is enough for many purposes: car navigation and position reporting, weather stations, earthquake detection and so on.
An MSRS receiver scans the airwaves every ten seconds or so, looking for signals that have been continuously transmitted long enough to achieve adequate signal strength (about 50kS/s). When it finds one, it decodes the ID code from the signal’s modulation and checks the ID against its list of known IDs. If it finds a match, it starts to accumulate the message.
An MSRS radio is two radio receivers in one box: a VHF receiver tuned between 138 and 174 MHz, which scans for messages, and an FM receiver tuned between 88.5 and 108.0 MHz, receives only the letters addressed it.
The received signal from the upper sideband of the FM sub-carrier is fed into a computer or microcontroller, which decodes the data by multiplying it down to baseband with a suitable local oscillator, stripping off any carrier frequency using a phase-locked loop (PLL), solving the data stream using software running on your computer or microcontroller, then re-modulating onto 38 kHz onto another VHF transmitter operating at a much lower power level than the MSRS transmitter.
The beauty of MSRS is that it can operate in the background without your intervention. You don’t have to do anything special to make it work; leave your computer or microcontroller turned on and connected to the Internet. It will automatically decode any messages it receives and stores them in a file on your computer.
Some applications which can use MSRS are as follows:
• Car navigation and position reporting
• Emergency communication (mass messaging)
• Weather stations
What is the range of MSRS?
The maximum distance at which reliable reception can be achieved for a given power output depends on the antenna height, frequency and propagation conditions. However, assuming an antenna is 150 metres high, there should not be any problem in getting line-of-sight to your transmitter from any spot within 40 km or so. This means that you can cover an area with about six radios. The presence of hills etc., will decrease this somewhat but not significantly. If you need to go farther than this, you’ll need to use higher frequencies such as 2.4 GHz or 3 GHz, where your range will increase thanks to the improved propagation characteristics significantly.
How much does an MSRS system cost?
An MSRS system is not cheap, but it’s not horrendously expensive either. The radios themselves are about £100 each, the antennas will be a few hundred pounds, and the microcontroller will be another £50 or so. This brings the total cost to around £1000 for a basic system. However, this price can be brought down considerably if you already have some equipment (such as computers and microcontrollers).
Who should use MSRS?
MSRS is aimed at anyone who needs to send real-time data over an FM signal. This could be individual enthusiasts, businesses or even large organisations. The beauty of MSRS is that it works just as well no matter how many people are using it. Suppose you need to send data over the Internet. In that case, you’re probably better off using alternative technology such as e-mail or SMS messages, which don’t suffer from reliability problems, and they also work when the Internet is down.
How is the data transmitted?
The messages are sent as a series of 38.4kbps FSK modulated packets with any number of user bytes per packet, up to a maximum of 255 bytes. Each frame starts and ends with a unique two-byte frame ID (one byte for MSRS radios that only do half-duplex transmissions, and the second byte for those that can receive and transmit simultaneously).
The frame ID is followed by four so-called “error correction” bytes which help to ensure that errors occurring during transmission can be detected and corrected before continuing with more data or ending with an acknowledgement. If there is no message data in the frame, this will end with another four error-correction bytes followed by a zero byte.
The error-correction bytes enable the message to be retransmitted up to three times, ensuring that any errors can be detected and corrected even if they occur in some data packets. If the acknowledgement byte was not received after three attempts, this indicates that some or all of the other data packets were lost or corrupted and cannot be decoded. In this case, the transmitting radio will wait for a random time before trying again; if it fails again, it will continue to resend every 30 minutes until an acknowledgement is received.