Week of September 9, 1996
Early this week, for the final time, we reinstalled the ISA (Integrated Structure Assembly) - that's the white thermal and structural box that surrounds the lander electronics and has the red "JPL" letters on it. We then calibrated the stop positions on the HGA (High Gain Antenna) - that's the lollipop-shaped articulated antenna that sits nest to the camera on the ISA. We used special theodolites (similar to those used by construction surveyors) to verify that the HGA mechanically points in the direction we want it to point with respect to the lander's base petal. We then reinstalled and checked out the pyro switching electronics and installed the lander thermal batteries. We use "thermal" batteries to provide power (current) to ignite explosive initiators in the EDL pyrotechnic devices (things like the parachute mortar, separation nuts, cable cutters and rocket ignitors). The batteries are called "thermal" because they get their electrical energy from self-generated chemical heat. Similar to the pyrotechnic initiators to which they provide, these batteries themselves need to be "lit" seconds before they are used on the spacecraft during EDL. Once "lit", these batteries will operate for only a few minutes - plenty of time to do their jobs.
Once the ISA was installed, we performed some radio communication tests between the rover and the lander. We had been uncertain whether or not we needed to launch with an RF (radio frequency) attenuator in series between the lander's rover antenna and the lander's RFD modem used to talk with the rover. This attenuator was thought some time ago to be needed to allow communication with the lander at close distances. These tests and some others coming up have nearly convinced us that we can live without it. We think that it would be good if we did not use it because the attenuator might reduce the communication range if we ever decided to drive the rover a long way away from the lander in its "extended" mission. Either way, the primary mission is unaffected.
We successfully performed other radio tests as well. Until this week, we had not yet tried to uplink the large software patch files using the real X-band radio and a ground station. The patch files are used in the unlikely event we have to reload large portions of the flight software into the EEPROM memory during the mission. Using the MIL-71 ground station at the Kennedy Space Center, we found that the process works fine.
We also took a few last verification images from each eye of the IMP camera on the lander. This is the last time the IMP camera will be taking interesting pictures until we land on Mars.
There has also been much work on the cruise stage. The HRS (Heat Rejection System) freon pumps have been installed and checked out. The HRS is the system used to flow freon inside the lander and around the perimeter of the cruise stage to keep the lander electronics cool. We need to keep the battery, the digital electronics, the rover, and the big X-band radio transmitter we call the SSPA (Solid State Power Amplifier) cool during the especially warm early part of the "cruise" phase of the mission as we leave Earth. We had to replace the pumps that had been installed during this summer's thermal tests because we think that it may have been damaged during one of our electrical tests. Because it is so hard to take apart, we can't tell for sure that it is broken. So just in case it was, we decided to replace it with the flight spare unit.
Later this week we will reattach the petals (the Sojourner Rover is already mounted on its "Y" petal). Next week we begin the long process of installing the flight airbag as well as the many pyrotechnic devices on the lander.