MadSci Network: Astronomy
Query:

Re: How are interplanetary space craft navigated?

Date: Tue Jan 11 23:03:46 2000
Posted By: Troy Goodson, Staff, Spacecraft Navigation, Jet Propulsion Laboratory
Area of science: Astronomy
ID: 944173064.As
Message:

Fred,

This is an excellent question and I'm very pleased that you asked it. You stated that

Newspaper accounts of the recent loss of the Mars Climate Orbiter suggest that the space craft's "handlers" had no direct measurements of its position along its course but rather were using some form of dead reckoning...

It pains me to read newspaper accounts like one you have described. There is only one spacecraft in operation that attempts to determine its own position; its name is Deep Space One or DS1. We'll get back to DS1, below; first, I'd like to describe the traditional approach.

The traditional approach is ground-based. Radio antennae of the Deep Space Network take range and Doppler measurements of the spacecraft's position and velocity. The range measurements tell us how far the spacecraft's antenna is from the antenna on the ground. Doppler measurements tell us how quickly the spacecraft's antenna is moving toward or away from the antenna on Earth.

For those of you reading this with a college physics background, note that there are six states to the spacecraft: three coordinates of position and three components of velocity. One range measurement and one Doppler measurement only add up to two pieces out of the six. We have to take many measurements over the course of many weeks and sometimes months, for trajectories to the outer planets, to have enough information to accurately determine the trajectory.

The process of computing a best-estimate of an object's trajectory using measurements is called Orbit Determination. This is a field of specialty for many engineers in the Navigation and Mission Design section at JPL. (It used to be called the Navigation and Flight Mechanics section, and it was called something else before that).

There is an excellent article in print about navigation. It far exceeds the quality and detail of any information you'll find on the web today. I have copied the reference for you, below.

Melbourne, W. G., "Navigation Between the Planets," Scientific American, Vol. 234, No. 6, pp 58--64, 68--74, June 1976. ISSN 0036-8733

A much more brief treatment is given at JPL's Basics of Spaceflight pages. Chapter 13 is about navigation and orbit determination.

Like most technologies, the navigation of interplanetary spacecraft is changing. For quite some time, the idea of on-board navigation has been studied. Some limited experiments have occured, but a much larger experiment is being conducted with the Deep Space One spacecraft. DS1 can use pictures that it takes of other bodies, e.g. comets and asteroids, to figure out it's own position and velocity. However, ground-based navigation is also being conducted; it is used both as a double-check and a back-up to the on-board system.

All that said, there is still the issue of course corrections and other perturbations to the orbit. If you don't know precisely and accurately how much a recent thruster firing changed the spacecraft's velocity, you will find that your orbit determination problem is very difficult. One solution is estimate the orbit before the thruster firing and to estimate the orbit after the thruster firing. The difference between these orbits must be due to the thruster firing. However, this is not always so simple because we don't have complete information on the spacecraft's position and velocity. In particular, it is very difficult to determine the degree to which the orbit is above or below the ecliptic. As Melbourne writes, "Of the six trajectory parameters (three of positiona and three of velocity) usually onl the component of the spaceraft's velocity perpendicular to the plane of the ecliptic remains insufficiently determined after two days."

It is precisely this component of the spacecraft's velocity which, if incorrect, could lead a spacecraft to enter Mars' atmosphere. You see, MCO was to enter into a polar orbit around Mars, requiring that it approach the south pole, then circle up towards the north pole. If the component of a spacecraft's velocity perpendicular to the ecliptic was too great in the northerly direction (that's Mars' North), it is easy to see how a spacecraft could enter Mars' atmosphere. Of course, this is a well known difficulty for interplanetary navigation and JPL's navigators are well aware of it. And, if I made this problem sound simple, then I have woefully mislead you! Finally, I must say that I was not part of the investigation board so I'm not someone to give you an official statement about the failure of MCO. For that, you should look to JPL or NASA.

Troy.


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