GPS goes martian: nav/com for a red planet; Researchers assess the performance of a proposed Martian satellite positioning system, calculate inter-satellite and satellite-ground ranges and range rates, and finally predict the ability of Mars exploratory vehicles—the Netlanders—to position themselves by observing passing satellites

GPS World, June, 2004 by Kyle O'Keefe, Gerard Lachapelle, Susan Skone

Renewed interest in Mars exploration since the mid-1990s has brought forward the need for more reliable communication and navigation to and at Mars. In the past, Earth-based radio transceivers performed all these functions, limiting both the communication bandwidth and the navigation accuracy.

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In 1999, National Aeronautics and Space Administration (NASA) Jet Propulsion Lab (JPL) researchers led by R.J. Cesarone, T.A. Ely, and R.C. Hastrup presented a plan for the Mars Network, a six-microsatellite navigation and communication constellation, in circular orbits at an altitude of 800 kilometers. Each satellite would carry a shortrange UHF transceiver to communicate with landers and orbiters near Mars and a longrange radio to relay information back to Earth.

Such a communications network would make it unnecessary for future landers to be equipped with longrange radios for communication with Earth. These six communication relay satellites would provide navigation services for Mars by making range and Doppler measurements on the communications signals. Recent publications indicate that JPL still has the constellation under study but will not likely launch it soon. However, the development of the multi-purpose UHF transceiver continues with prototypes onboard Mars Odyssey and the Mars Exploration Rovers, and a preliminary version on the Mars Reconnaissance Orbiter due to launch in 2005.

In the plan, two satellites are in near-equatorial orbits, while the other four are in near polar orbits. All six are retrograde orbits with inclinations greater than 90 degrees. This design balances the communication and navigation requirements of the dual use constellation. For example, orbital radius was optimized between higher orbiting satellites that would provide better navigation services, but would be less useful for communication due to power limitations. Two subconstellations were chosen to provide enhanced equatorial coverage (using the two equatorial satellites) while maintaining global coverage using the four near-polar satellites. This configuration also provides varied pass geometry to equatorial users that would not be available with only polar satellites.

Initially, Mars Network satellites would be positioned by tracking from the NASA Deep Space Network, however, the principal investigators suggest that the eventual goal is to have an autonomous global navigation satellite system for Mars. However, there is no mention of establishing any ground control on Mars itself.

Netlanders

A European consortium led by the Centre Nationale des Etudes Spatiales (CNES) plans to deploy the Netlanders, a network of four identical landers, on the Mars surface. Each lander will carry seven science payloads including a weather station, an electric field sensor, a magnetometer, a ground penetrating radar, a stereoscopic multispectral camera, a seismometer, and most importantly for this work, a UHF transceiver capable of making range and Doppler measurements. Because the Netlanders plan to land at four locations around the planet, they might be useful as ground reference stations for tracking the Mars Network satellites. Figure 1 shows the proposed lander locations and sample satellite ground tracks.

Mars Network Geometry

To compare the Mars Network with other existing and proposed global navigation satellite systems, it is useful to evaluate its positioning geometry using the same measures often applied to other systems, particularly, availability, accuracy, and reliability. For the Mars Network, these measures are not as easily applied due to the relatively small number of low elevation satellites in the system. Availability, which in the context of GPS is often defined as either the number of satellites in view, or the percentage of the time that four or more satellite are in view, may be used to evaluate the Mars network, except that the number of satellite in view is often zero and never more than three. Likewise, accuracy, often represented by instantaneous dilution of precision (DOP) coupled with a user equivalent range error, becomes difficult to use since there are rarely enough satellites in view to compute an instantaneous position solution. This same problem affects reliability measures, since redundant observations, which are now almost always available to GPS, would almost never be available to Mars Network users.

We used four figures of merit to assess the geometry of the Mars Network constellation. The first, and simplest, is satellite availability, or the number of satellites in view for a particular user at a particular time. The second and third are additional measures of availability: the number of satellite passes per sol (Mars solar day, 24 hours and 39 minutes) and the number of observations per sol.

Geometry. Finally, position dilution of precision (PDOP), is used to assess the geometric quality of the satellite pass geometry. Unfortunately, the standard definition of PDOP cannot be used in this case since instantaneous position solutions are only briefly available near the equator when three satellites are visible, and then only when using two-way observations or a height constraint. As an alternative, cumulative position dilution of precision may be computed by considering the contributions to positioning geometry of observations made at successive epochs. Since most surface users of the Mars Network would likely be landers or slowly moving rovers, cumulative PDOP, or time required to obtain a certain cumulative PDOP, are applicable measures of positioning accuracy when combined with a user equivalent range error.