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Mars Journal - Mars Exploration Rover Mission
Journal for Michael Watkins

January 24, 2004 - Going for it Again

Landing day number two. We've been watching the navigation solutions all night to make sure we don't need to do the last Trajectory Correction Maneuver (TCM-B6). We don't, because everything is right on course again. The rover mission has seen an amazing performance by the Navigation Team. We planned for up to 6 maneuvers on each spacecraft to get them within the 80 km (48 mile) long elliptically shaped landing sites, and we only needed 4 maneuvers on Spirit to get within 10 km (6 miles) for Spirit, and only 3 to get almost as good for Opportunity. We joked on the team that we got a birdie and an eagle, respectively. But as always, and like everyone involved in Navigation and Entry, Descent and Landing (EDL), we're tense about everything working tonight for a safe landing.

Opportunity has just landed. I'm there in the control room, surrounded by jubilation, and again, simultaneous feelings of elation and relief. Two for Two! As I look around the room at the rover team, I am as proud to be part of them as anything I've done in my life.

But we're also still working, and for Opportunity, the Navigation Team wants to get our first solution for the position of Opportunity as quickly as possible to help assess the condition of the rover.

Ten minutes after landing we get it - it's about 10 km further downtrack (to the East on Mars) from the last targeted point (which was about 10 km from the original target), due entirely to low atmospheric density on Mars. This is similar to Spirit, and well within specifications, and it looks from our maps to be an awesome landing site for the scientists. But we'll have to wait and see the first pictures'


First pictures have arrived'we're inside a small crater, and we see rock outcroppings'and it's simply amazing'and at the first press conference my friend Richard Cook (we've known each other since graduate school) describes the outstanding accomplishment of the MER Navigation Team'.and all is right with the world tonight.


January 22, 2004 - Final EDL Preparation for Opportunity

I think all of us involved in Opportunity navigation and Entry, Descent, and Landing (EDL) are feeling some unspoken pressure due to concern about the health of Spirit, which is malfunctioning on the surface of Mars right now. We all hope and believe that Spirit will be returned to perfect health, but we can't help but feel some extra pressure to get Opportunity safely down now.

A concern that has been building since the Spirit landing is what we call the 'Entry Descent and Landing Timeline', which is just a way of talking about how much time we have to slow down before we hit the ground ' and a lot of events must take place during that time ' parachute deploy, radar altimeter solutions, heat shield separation, a descent image, airbag inflation, and so on. Because the Mars atmosphere was a little thinner than expected for Spirit, everything happened a little late, and we want to give Opportunity more time. I've already talked about one way to do that (shallowing the entry flight path angle, which we decided not to do) in this journal a couple of weeks ago . We did decide, just like Spirit, to open the chute a little earlier (we refer to that as having a higher dynamic pressure target). But just in the last day or so, another way to run out of time has apparently gotten more serious. A device called the Descent Rate Limiter (DRL), which is just a brake acting on a cable to slow the lander descent from the parachute and backshell, applied more braking than expected which slowed everything down. The EDL team have been concerned that it might take longer for Opportunity, and so many tests of the braking were performed and a new test just came out indicating that it could be even slower than Spirit. So we had a long meeting this evening to decide whether to try to open the parachute even earlier to guard against that extra delay. After a lot of discussion, we decided not to, partly because you incur some additional risk of problems with the parachute opening, and we think the DRL is unlikely to be the worst possible case. We're ok in almost every imaginable case, and frequently in space exploration it's better to leave things as we designed them and not try to outsmart ourselves.


January 21, 2004 - Mars Weather too Unpredictable

After doing our best to assess the statistics on the Martian weather and looking at our best guess where Opportunity will land, we've decided that we cannot with confidence tweak the landing site anywhere 'better' for science, so we're going to go with the current trajectory and not do trajectory correction maneuver 5 (TCM-B5). Part of the reason is also that it's always a little risky to do any maneuver, since you have to open the latch valves on the spacecraft propellant tank and burn the engines, and there's always a chance something will go wrong. Given the unpredictability in the Martian atmosphere, it just didn't seem worth it to us on the project to try it. We know we will come down in a great location, rich in hematite, and beyond that, it's kind of a roll of the dice.


January 20, 2004 - What's the Weather on Mars'

The navigation solutions for Opportunity are now very consistent and look great, and it appears to be less than 10 km from the center of the target, given a number of changes in the atmospheric model and a change in the time the parachute will be deployed. But the science team made us an interesting challenge, which is to help them get to the most interesting site inside the landing ellipse. That would mean a very tiny correction to the trajectory, and so it's come down mostly to how well we can predict how the atmosphere will affect Opportunity on its way down to the surface. So in other words, the navigation has come down to how well we can predict the weather on Mars!


January 18, 2004 - Where exactly did Trajectory Correction Maneuver-B4 send Opportunity'

Yesterday was Saturday, but the Navigation team was here working to solve out Trajectory Correction Maneuver-B4 to see where Opportunity is exactly going. With some assumptions about the maneuver, it looked yesterday to be about 13 km from the target point.

Today, though, after looking very carefully at our 1000 Orbit Determination solutions, we made a change to the way we are trying to solve for the maneuver, and it helps things considerably. That's one of the tricky but fun parts about Navigation - the fact that we have to model what's going on with the spacecraft, and the tracking data have to change almost every day in response to what we see happening.

We now feel we have a good idea where Opportunity is going, and so we'll brief the rest of the project and the scientists today so that they can help us decide if we need to tweak it again with Trajectory Correction Maneuver-B5.


January 16, 2004 - Trajectory Correction Maneuver-B4 is over

Trajectory Correction Maneuver-B4 took place tonight, and our first analysis is that it was a good, but not perfect, maneuver by Opportunity. Looks like it overburned by about 3% or so, which pushed us about 13 km from the exact center of the target (not counting changes in the atmosphere). That should still be excellent though, since the atmospheric dispersions can move the actual landing location by up 10's of kms. Just like for Spirit, we need a couple of days for the tracking data to tell us where exactly it is going. We're hoping everything is close enough to the target that no Trajectory Corrction Maneuver-B5 is needed for fine tuning.


January 15, 2004 - Last Update for Trajectory Correction Maneuver-B4

We had the final meetings between the Navigation Team and the Spacecraft Team to set up tomorrow's Trajectory Correction Maneuver-B4, with the latest atmosphere models and Orbit Determination. It's always a little nerve-wracking, but we feel fairly certain that we've made the right decisions and everything looks go to move us over to the target in Meridiani.


January 12, 2004 - Putting Opportunity on Target

On Opportunity, we've been carefully analyzing the Orbit Determination solutions to prepare for the normal TCM-B4 (the fourth trajectory correction for Opportunity) on Friday. Since we didn't do TCM-B3 several months ago in order to allow the Mars Exploration Rover project teams to concentrate on Spirit operations, this maneuver is about 4 times larger than TCM-A4 (the fourth trajectory correction maneuver for Spirit). That matters because the bigger a maneuver is, the bigger the errors the spacecraft makes while trying to execute it. But it's still a pretty small maneuver, and so we don't expect the errors to be too big. Probably the biggest uncertainty in landing position is what the atmosphere will do, and the scientists are still trying hard to get their best estimate for landing day. We've seen changes in atmosphere models move the landing position by up to 20 kilometers (about 12.5 miles) in the worst case.


January 10, 2004 - Homing in on Spirit

Joe Guinn of the Nav Team has a new surface position for Spirit with all our data and it should be accurate to about 20 meters (about 66 feet) or so. The only problem is that the maps of Mars are only accurate to a few hundred meters, meaning we don't know exactly where all the hills and craters are to compare our navigation position with what Spirit can see in it cameras yet.


January 8, 2004 - No Extra Maneuver

The Mars Exploration Rover project had a big late night meeting reviewing the whole Spirit Entry, Descent, and Landing (EDL) and what it means for Opportunity, and we all agreed NOT to change the flight path angle for Opportunity. It looks like it isn't necessary from an EDL safety point of view, and since it would require extra maneuvers on Opportunity and the Mars Global Surveyor orbiter, we all decided it wasn't worth it. Although we could all have done it, I think it's a relief for everyone to cancel it.


January 6, 2004 - Can Spirit Help us with Opportunity'

For Spirit, the job of the Nav team is now to continue refining its position on the surface. We've got great data from the surface of Mars to the Earth which gets us within 100 meters (about 109 yards), and we're awaiting some data from Spirit to the Mars Odyssey orbiter. We can use the Mars Odyssey tracking of Spirit kind of like GPS on the Earth to help us pinpoint its position.

For Opportunity, members of the Nav team are part of the Entry, Descent, and Landing (EDL) reconstruction team for Spirit, and we're trying to learn what happened and whether we should do anything differently for Opportunity EDL. There's still some concern about the atmosphere, and whether how to make sure we have enough time to slow Opportunity down enough before landing. One suggestion that would really affect the Nav team is to change what we call the 'Entry Flight Path Angle' to be shallower, so that Opportunity would be going more horizontally than originally planned to give it more time to slow down. That would mean doing an extra maneuver this Friday, followed by the regular TCM-4 (fourth trajectory correction maneuver) on Jan 16 to further clean up the targeting. So we've been very quickly analyzing the effects of these maneuvers to make sure we can do them and still maintain all the accuracy we need for landing.


January 3, 2004 - The Big Day (for Spirit)

Well, the big day has finally arrived after over 3 years of waiting. We have one last chance to change the landing site with a sixth trajectory correction maneuver (TCM-6), but everything is still right on target and we're canceling that.

So our attention is now turning to getting things set up to use our tracking data to monitor the Entry, Descent, and Landing (EDL) events in real-time. Our tracking data basically measures the speed of Spirit toward or away from us, and so when Spirit turns to get ready for entry into the Mars atmosphere, and when it vents the Heat Rejection System (HRS), and starts getting decelerated by the atmosphere, we can see all that. We'll probably lose our signal when the parachute deploys, and then a different communications system (The EDA) will tell us what we need to know about the progress in EDL.

I was standing in the control room when we first got the signal from the bounce'.then a few painful minutes'and then we got the signal again from the surface. For me there was a simultaneous feeling of joy and immense relief. It might seem funny to say relief, but when you get so emotionally invested in something you tend to worry so much about what can go wrong that you're first just glad none of them did!

The next job for the nav team now that we've landed is to find out where exactly we landed.

We can process the same radio signals I was talking about above (technically we call it 'Differenced one-way Doppler') to get the approximate lander position. I say approximate because we only have data up to parachute deploy, so we don't know exactly how far the wind and the bouncing on the airbags moved the lander. Brian Portock of our team was able to fit this data within a couple of hours and get the position to within a couple of kilometers (about a mile or so) and will show that at tomorrow's press conference.

Time for us all to try to get some sleep'if we can.


January 2, 2004 - Stardust Does it!

Stardust had a successful encounter with Comet Wild-2 today, and the navigation was great, and the autonomous navigation to track the comet for picture taking also went almost perfectly. These were almost certainly the best pictures ever taken of a comet. What an amazing couple of days this is for the Nav section and all of JPL, with Stardust one day and Spirit the next. We hope Spirit does as well as Stardust!


January 1, 2004 - Still on course

It's New Years Day and indeed, everything continued to be fine and we didn't do the fifth trajectory correction maneuver (TCM-5). We're assuming it will stay that way for TCM-6! Our main attention now is ensuring that we have the entry, descent, and landing parameters set right for a successful landing given our best understanding of the atmospheric conditions on Mars.


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