SPACE FLIGHT PROJECTS

Spacecraft Navigation

Objectives:

Upon completion of this chapter you will be able to describe basic principles of spacecraft navigation, including spacecraft velocity and distance measurement, angular measurement, and orbit determination. You will be able to describe spacecraft trajectory correction maneuvers and orbit trim maneuvers.

Navigating a spacecraft involves measuring its radial distance and velocity, the angular direction to it, and its velocity in the plane-of-sky. From these data, a mathematical model may be constructed and maintained, describing the history of a spacecraft's location in three-dimensional space over time. Any necessary corrections to a spacecraft's trajectory or orbit may be identified.based on the model. The navigation history of a spacecraft is incorporated in the reconstruction of its observations of the planet it encounters; it may be applied to the construction of SAR images. Some of the basic factors involved in acquiring navigation data are described below.

Data Types

The art of spacecraft navigation draws upon tracking data, which includes measurements of the Doppler shift of the downlink carrier and the pointing angles of DSN antennas. Navigation also uses data categorized as very long baseline interferometry (VLBI), explained below. These data types differ from the telemetry data, generated by science instruments and spacecraft health sensors, which is transmitted via modulated subcarrier.

Spacecraft Velocity Measurement

In two-way coherent mode, recall a spacecraft determines its downlink frequency based upon a very highly stable uplink frequency. This permits the measurement of the induced Doppler shift to within 1 Hz, since the uplink frequency is known with great precision. The rates of movement of the Earth in its revolution about the sun and its rotation are known to a high degree of accuracy, and are removed. The resulting Doppler shift is directly proportional to the radial component of the spacecraft's velocity, and the velocity is thus computed.

Spacecraft Distance Measurement

A uniquely coded ranging pulse may be added to the uplink to a spacecraft, and its transmission time is recorded. When the spacecraft receives the ranging pulse, it returns the pulse on its downlink. The time it takes the spacecraft to turn the pulse around within its electronics is known from pre-launch testing. When the pulse is received at the DSN, its true elapsed time is determined, and the spacecraft's distance is then computed. Distance may also be determined as well as its angular position, using triangulation. This is described below.

Spacecraft Angular Measurement

The angles at which the DSN antennas point are recorded with an accuracy of thousandths of a degree. These data are useful, but even more precise angular measurements can be provided by VLBI, and by differenced Doppler. A VLBI observation of a spacecraft begins when two DSN stations on separate continents, separated by a very long baseline, track a single spacecraft simultaneously. High-rate recordings are made of the downlink's wave fronts by each station, together with precise timing data. DSN antenna pointing angles are also recorded. After a few minutes, and while still recording, both DSN antennas slew directly to the position of a quasar, which is an extragalactic object whose position is known with high accuracy. Then they slew back to the spacecraft, and end recording a few minutes later. Correlation and analysis of the recorded data yields a very precise triangulation from which both angular position and radial distance may be determined. This process requires knowledge of each station's location with respect to the location of Earth's axis with very high precision. Currently, these locations are known to within 3 cm. Their locations must be determined repeatedly, since the location of the Earth's axis varies several meters over a period of a decade.

Differenced Doppler can provide a measure of a spacecraft's changing three-dimensional position. To visualize this, consider a spacecraft orbiting a planet. If the orbit is in a vertical plane edge on to you, you would observe the downlink to take a higher frequency as it travels towards you. As it recedes away from you, and behind the planet, you notice a lower frequency. Now, imagine a second observer halfway across the Earth. Since the orbit plane is not exactly edge-on as that observer sees it, the other observer will record a slightly different Doppler signature. If you and the other observer were to compare notes and difference your data sets, you would have enough information to determine both the spacecraft's changing velocity and position in three-dimensional space. Two DSSs separated by a large baseline do exactly this. One DSS provides an uplink to the spacecraft so it can generate a stable downlink, and then it receives two-way. The other DSS receives a three-way downlink. The differenced data sets are frequently called "two-way minus three-way." High-precision knowledge of DSN Station positions, as well as a highly precise characterization of atmospheric refraction, makes it possible for DSN to measure spacecraft velocities accurate to within hundredths of a millimeter per second, and angular position to within 10 nano-radians.

Optical Navigation

Spacecraft which are equipped with imaging instruments can use them to observe the spacecraft's destination planet against a known background starfield. These images are called OPNAV images. Interpretation of them provides a very precise data set useful for refining knowledge of a spacecraft's trajectory.

Orbit Determination

The process of spacecraft orbit determination solves for a description of a spacecraft's orbit in terms of its Keplerian elements based upon the types of observations and measurements described above. If the spacecraft is enroute to a planet, the orbit is heliocentric; if it is in orbit about a planet, the orbit determination is made in reference to that planet. Orbit determination is an iterative process, building upon the results of previous solutions. Many different data inputs are selected as appropriate for input to computer software which uses the laws of Newton and Kepler. The inputs include the various types of navigation data described above, as well as data such as the mass of the sun and planets, their ephemeris and barycentric movement, the effects of the solar wind, a detailed planetary gravity field model, attitude management thruster firings, atmospheric friction, and other factors.

The highly automated process of orbit determination is fairly taken for granted today. During the effort to launch America's first artificial Earth satellite, the JPL craft Explorer 1, a room-sized IBM computer was employed to figure a new satellite's trajectory using Doppler data acquired from Cape Canaveral and a few other tracking sites. The late Caltech physics professor Richard Feynman was asked to come to the Lab and assist with difficulties encountered in processing the data. He accomplished all of the calculations by hand, revealing the fact that Explorer 2 had failed to achieve orbit, and had come down in the Atlantic ocean. The IBM mainframe was coaxed to reach the same result, hours after Professor Feynman had departed for the weekend.

Trajectory Correction Maneuvers

Once a spacecraft's solar or planetary orbital parameters are known, they may be compared to those desired by the project. To correct any discrepancy, a Trajectory Correction Maneuver (TCM) may be planned and executed. This involves computing the direction and magnitude of the vector required to correct to the desired trajectory. An opportune time is determined for making the change. For example, a smaller magnitude of change would be required immediately following a planetary flyby, than would be required after the spacecraft had flown an undesirable trajectory for many weeks or months. The spacecraft is commanded to rotate to the attitude in three-dimensional space computed for implementing the change, and its thrusters are fired for a determined amount of time. TCMs generally involve a velocity change (delta-V) on the order of meters or tens of meters per second. The velocity magnitude is necessarily small due to the limited amount of propellant typically carried.

Orbit Trim Maneuvers

Small changes in a spacecraft's orbit around a planet may be desired for the purpose of adjusting an instrument's field-of-view footprint, improving sensitivity of a gravity field survey, or preventing too much orbital decay. Orbit Trim Maneuvers (OTMs) are carried out generally in the same manner as TCMs. To make a change increasing the altitude of periapsis, an OTM would be designed to increase the spacecraft's velocity when it is at apoapsis. To decrease the apoapsis altitude, an OTM would be executed at periapsis, reducing the spacecraft's velocity. Slight changes in the orbital plane's orientation may also be made with OTMs. Again, the magnitude is necessarily small due to the limited amount of propellant typically carried.