Adam Gutterman

Project METEOR

11/02/06

Rev 2.0

RIT Project METEOR Instrumentation Platform

Data Communication Subsystem

Overview

The data communication subsystem is the primary means of ground-to–platform control and the backup telemetry downlink. Previous launches of the METEOR instrumentation platform used the Advanced Position Reporting System (APRS), a widely implemented protocol for broadcasting position data (typically directly from a GPS) over the air via Amateur Radio and reporting this data on the Internet. The open protocols used allowed Amateur Radio operators to follow our launch and assist with recovery using commonly available radio equipment. The results of previous launches showed that while the infrastructure of the tracking system generally reliable and versatile, the onboard transceiver was unreliable at high altitude. With this in mind, the communication subsystem is being redesigned to use a more modular architecture and that includes additional capabilities.

Terminal Node Controller

The main purpose of the communication subsystem is to enable successful exchange of data between the onboard controller and ground stations. A terminal node controller (TNC) must be used to interface the microprocessor controller system to the radio and, eventually, the rest of the world.

The TinyTrak4 was selected as the primary TNC. Based on the Atmel ATmega168, the TinyTrak4 is extremely powerful and capable of a number of functions. It includes DSP functionality and can be field-upgraded to support new protocols or DSP algorithms. When connected to the microprocessor controller system, the TinyTrak4 will be configured to transmit and receive 1200 baud bidirectional data. The TinyTrak uses RS232-level data inputs, so a level-shifter is required for connection to TTL-level devices.

Redundancy

The system uses a second TinyTrak4 as a backup in case of controller errors. This TinyTrak4 is operated in APRS mode. The device reads data in from a dedicated GPS module, processes it into a valid APRS packet, and sends it to the transceiver for broadcast to ground stations. The TinyTrack4 includes onboard voltage and temperature sensors, so a limited set of data can be transmitted even if there is a complete failure of the rest of the controller system. The redundant system is enabled as long as the ‘Status OK’ line from the controller is not properly asserted. Even when the system is in APRS mode, receive data from the ground is still routed to the controller, so restarts or mode changes may still be attempted.

Modulation Scheme and Data Protocols

The system will use audio frequency-shift keying (AFSK) over an FM signal. The audio protocol is for compatibility with the APRS infrastructure. The AX.25 protocol is used for the data link layer and Bell 202-compatible audio protocol is used for the physical layer. Modem functions are performed by the TinyTrak4.

Transceivers

The selected transceiver is the Icom IC-F14S two-channel VHF FM handheld transceiver. The radio is extremely simple, with a minimum of functions configurable by the user. Programming is done by connection to a PC and is not normally needed after initial setup. The radio is capable of 5W output power on throughout the Amateur band.

Channel Selection

The platform will receive data on the 2M amateur band (144-146 MHz) and will transmit on the same band. In the United States, the standard APRS frequency is 144.39 MHz.

Antenna

Previous METEOR platforms were equipped with a simple omnidirectional vertically-polarized dual-band antenna intended for automotive use. During the most recent platform launch, position reports were received by stations as far away as Philadelphia, indicating that the current design is more than sufficient with its current output power.

Other Configurations Considered

There are numerous other handheld amateur transceivers that may be used. As long as they have sufficient output power, they could be used in place of the Icom IC-F14S.

There are other TNCs available that could be substituted, but the TinyTrack4 has the best capabilities and enjoys wide support among the Amateur community.

A Kenwood TH-D7A Dual-band FM transceiver was used on previous launches. This radio is capable of 5W power output on both the 2m and 70cm amateur bands. The radio also has an integrated TNC and can be used to transmit or receive 1200bps serial RS232 data. The radio can also receive data from a GPS and automatically convert it to a properly-formatted APRS packet for rebroadcast at preset intervals. While the feature set of the TH-D7A is impressive, the radio was unreliable at high altitude. During the last launch, the radio was silent for several minutes at the peak of its flight. The reason for this failure was unknown, and could not be replicated on the ground. This unreliability has disqualified the TH-D7A from further use in the air.

The only other COTS Amateur handheld that has an internal TNC is the Icom IC-91AD. This newly-introduced radio offers an impressive feature set, and incorporates forward error correction and a higher data rate than the TinyTrak. Unfortunately, the radio is complex and communicates using Icom’s proprietary D-STAR digital protocol, which is not yet in wide use. This would limit the involvement of other Amateur Radio operators who would be forced to buy new equipment to support the standard.

Dedicated data radio modules operating in the ISM band were considered, but the benefits of operating in the Amateur bands far outweighed their many positives. A METEOR alumnus made a dedicated data radio specifically for the METEOR project as part of a graduate paper, but the radio emitted excessive spurs and harmonics. In addition, it was made form EOL components that have not been replaced.

Primary Risks and Concerns

Previous METEOR launches have been beset by mysterious failures while in the air: GPS signals stop transmitting, cameras stop working, etc. The reason for these failures is unknown, but there are several potential culprits. The temperature is a major potential risk. The temperature in the stratosphere where the balloon operates varies between -40 and -55C. Suitable mil-spec components are not always available, so insulating techniques will have to be used. Another possible risk is radiation, which is more prevalent at high altitudes. Power exhaustion is also a risk, but during the previous launch the platform operated for at least 3/12 hours, which should be sufficient. If needed, transceiver’s built-in battery can be augmented with the main platform battery to extend transmission time. Loss of communication with the primary ground station is also a concern, but the use of an omnidirectional antenna, multiple ground control stations, and more powerful ground stations mitigates that risk as much as possible