Sentinel Gets Its Start

For help in solving the problem, MODIS Rapid Response Leader Chris Justice contacted Bob Kozon, the Flight Operations Director for the Terra spacecraft. Kozon had no trouble identifying the problem. “The MODIS direct broadcast signal transmits using what is called X-band,” Kozon explains. “The main radio frequency is about 8.2 gigahertz, but it doesn’t transmit exclusively on that frequency. The signal has side lobes that cause the transmission to spill over into the 8.4-gigahertz range or higher.”

That spill over created a problem: NASA reserves the 8.4-gigahertz frequency for communicating with its deep space missions, such as the Voyager spacecrafts. NASA operates three Deep Space Network stations spaced roughly 120 degrees of longitude apart across the Earth. When Terra flew within radio “earshot” of a Deep Space Network antenna, the MODIS direct broadcast signal had the potential to cause radio frequency interference if NASA were communicating with one of its more far-flung progeny. The problem for Allan and Held and their fire mapping projects was that one of those stations was located just outside Canberra.

When NASA first identified the potential conflict between the MODIS direct broadcast and the Deep Space Network, the Terra Flight Operations Team and the Radio Frequency Spectrum Management group at NASA’s Jet Propulsion Laboratory developed a blackout schedule that would prevent interference while maximizing access to direct broadcast observations for the increasing number of users building receiving stations around the world. “The schedule we worked out took into consideration not just where Terra was in relation to the three Deep Space Network stations—in the Mojave Desert in California, near Madrid, Spain, and Canberra—but also where it was with respect to the actual location of the nine crafts presently in deep space,” says Kozon. Using this approach, they had reduced the blackouts to a few minutes, sometimes even seconds, each day.

But even these brief blackouts hampered the near-real-time effectiveness and fire tracking abilities of the Bushfires Council and the emerging Sentinel Program. So in November 2002, when the Sentinel Hotspots Project was set to begin delivering data to fire agencies, Held added his voice to those petitioning NASA for a waiver of the mandatory Deep Space Network blackout. Held was optimistic. “I sent an open letter as a concerned CSIRO team leader involved in a public-good project. I had every confidence of a positive response,” he says, “given the close relationship NASA has shared with Australia going all the way back to the Apollo moon missions.”

On behalf of the Australian direct broadcast users, Kozon asked the Deep Space Network personnel for one last concession. Kozon suggested they leave the MODIS direct broadcast on near the Canberra Deep Space Network so long as there were no scheduled contacts planned between the Canberra station and any spacecrafts. Given the possibility that unscheduled contacts might need to be made, the group was initially a little reluctant. Kozon persuaded them with a promise to immediately return to complete blackout if any suspected interference occurred. The results of the first four-week trial period passed without a single incident, and a second waiver was granted.

International Cooperation Yields Success
Both Allan and Held were thrilled with the arrangement. As the fire season in the southern part of Australia began to pick up late in 2002, it gave every indication of becoming at least as severe as the fire season of 2001-02 that initiated the Sentinel Hotspots project. According to Kozon, “Subsequent waivers were easier to get, since by the time the last one was set to expire, the fires in southeast Australia were getting pretty close to the Deep Space Network station in Canberra. Suddenly we all had an interest in keeping the MODIS direct broadcast feed going. We are into our fourth waiver at this point, and we haven’t had a single radio frequency interference incident, so it’s likely we’ll be able to continue helping out.”

The waiver ensures that Terra MODIS direct broadcast data can keep Sentinel Hotspots going even as the project expands to include data from other satellite sources. “The response to Sentinel Hotspots has been overwhelming—much larger than anticipated,” says Held. “Over the past few weeks, with the fires around Canberra and the Snowy Mountains, this prototype system has undergone a baptism of fire. We have recorded over 1.6 million [Web] hits on peak days and over 20,000 active users per day creating their own maps. The primary use of the Sentinel information seems to be in the preparation of fire status reports before the 6 a.m. planning meetings that emergency agencies have during such times, but we are also getting some feedback from the public, especially in remote rural areas, who are using our information to help them decide to evacuate their properties or not. The system has filled an important niche in public information, and the media have also been using our information as part of their regular updates on the national fire situation.”

Held feels that the new system has brought many people in closer touch with their natural surroundings and has increased awareness of potential environmental threats. “This prototype system has ignited the imagination of many Australians. In a sense Sentinel has become a ‘Webcam in the air’ for them.” In the future Held looks forward to using Sentinel to bring information to Australians about not just fires, but all kinds of threats from the natural environment, such as floods, cyclones, or oil slicks.

 Sentinel Suffers a Setback