Laurel A. Watts - Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder
Steven J. Ciciora - National Oceanic and Atmospheric Administration
Troy Thornberry - Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder
David W. Fahey - National Oceanic and Atmospheric Administration
Ru-Shan Gao - National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory in Boulder, Colorado, works to discover and understand the processes that govern the chemical reactions of the earth's atmosphere and improve the NOAA's capability to understand long-term climate changes and predict atmospheric behavior.
The Atmospheric Composition and Chemical Processes group measures atmospheric soot, ozone, nitric acid, chlorine nitrate, and water vapor during field campaigns. We fly our instruments, along with those from other groups all over the United States, on aircraft at different altitudes, positions, and times of the year. Using gathered data, we can understand the composition and reactions in the atmosphere and create and test behavior models.
Updating the Ozone Instrument
The ozone instrument previously used was built more than 22 years ago, weighs 57 lbs, and has DOS with the application programmed in Microsoft QuickBASIC. We needed to update to a new, lighter, and more Internet-capable instrument.
Our new UAS O3 instrument is a dual-beam ultraviolet (UV) absorption photometer. One beam measures the ozone in air sampled from the atmosphere and the other beam passes through the same air that has had the ozone removed by a scrubber. This provides a reference for the measurement. The entire instrument weighs only 37 lbs (including 3.2 lb for the CompactRIO system), runs on 28 V DC power, and has a variable sample frequency of 0.5 to 10 Hz. The precision is 2 by 1010 O3 molecules/cm3 with an accuracy of ±5 percent.
Selecting a Rugged, High-Performance Controller
In a previous case study, the NOAA conducted bell-jar testing of a CompactRIO chassis that operated to 0.53 mbar or 173,000 ft for eight hours without failing. Compact RIO systems can operate over a wide temperature range (-40 to 70 °C), have low power consumption (less than 20 W), and are rated to 50 g shock and 5 g vibration. The controllers have Internet connections, a USB port for USB data products, and various sizes of field-programmable gate array (FPGA) backplanes for various levels of programming.
We tested a PC/104 stack running either Windows CE or a Linux and LabVIEW OS for this application. However, the processor's power consumption was too high making it difficult to implement the programming to run the instrument. We programmed the low-power PC/104 computer with QuickBASIC, but this made Internet communications difficult. Additionally, the low-power version would not support an OS capable of running LabVIEW. Thus, we chose CompactRIO for the instrument controller based on its low-pressure performance, low weight, communications capability, available analog and digital I/O modules, and expansion capability.
We measured temperature throughout the instrument enclosure using thermistors. To provide the proper signal conditioning to acquire accurate temperature readings with a thermistor, we created our own custom module for CompactRIO using the blank module casing provided in the CompactRIO Module Development Kit.
The Software Architecture
The LabVIEW program for the instrument consists of three parts: the FPGA code compiled on the chassis backplane to interface between the modules and CompactRIO, the LabVIEW Real-Time code on the controller to run the instrument and serve the data to the ground, and the ground-based data display and LabVIEW software running on a Windows OS laptop.
The controller provides an ozone measurement using two specially designed UV absorption photometers with an SPI digital interface to low-noise 24-bit analog-to-digital converters. Temperature, pressure, and voltage of other components need to be monitored, and valves need to be controlled as part of the instrument function. This program is interrupt-based, and the controller is in charge of the timing.
With all of the different instruments on the aircraft, we required a timestamp to synchronize the data and correlate the measurements. NASA requires synchronization to the same time source so our time is synced with the aircraft using the Simple Network Time Protocol (SNTP). This feature was new to the CompactRIO platform in LabVIEW 8.6 and is required for our instrument. The main application is a state machine that performs the following tasks:
• Initialize – Read the configuration files, start the FPGA, and set up the data serving and file saving
• Run – A producer-consumer architecture with a timed loop for data acquisition, control, some analysis and error checking, and consumer loop to save data to files and queue it for serving to the ground via UDP and TCP/IP connections
• Configure – Reconfigure the data acquisition conditions
• Shutdown – Clear program elements in preparation for exiting the program or entering the Initialize or Configure states
• Exit – Stop the FPGA, close files and ports, and release queues
In addition, a server VI sends and receives data with the aircraft via UDP and with the ground via UDP and TCP/IP communications. The data is passed with referenced queues.
The laptop computer has control and display client applications written for UDP or TCP/IP communications. These programs receive data from the instrument and send commands if needed. The data communications are passed between the ground and the aircraft using iridium phone lines for UDP communications or Ku band satellite connections. There may be a considerable time lag in command transmission, so a handshaking protocol is implemented.
Successful Operations at Extreme Altitudes
The CompactRIO system was rugged and provided the interface. The available modules delivered the analog and digital I/O required for our instrument to function. We have test flown the new UAS O3 instrument unpressurized to 64,000 ft on the NASA WB-57 aircraft during the NOVICE field campaign in Houston, Texas, and will fly it in a partially pressurized bay on the first NASA Global Hawk Pacific mission (GloPac) from Dryden Flight Research Center at Edwards AFB in California in 2010.
Laurel A. Watts
Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder