Olympus Mons Corporation
ECE 6390 - Fall 2008

Spacecraft Systems

Overview

Building a fleet of 21 positioning satellites and delivering them to Mars is not a trivial task. Several key aspects must be taken into consideration just to get the satellites into position, including:

In addition, the design and operation of each satellite must be considered before exact communications specs can be created. Such items include:

Launch Vehicle

NASA has successfully used the Delta-II rocket to launch numerous Mars missions, such as Global Surveyor, Pathfinder, Climate Orbiter, Polar Lander, Odyssey, the twin Mars Exploration Rovers, and the Phoenix lander. While it has worked extremely well, the launch system is being phased out, with the final launch due in 2011.

There are several options available with enough power to deliver a satellite to Mars orbit. The Mars Science Laboratory will be launched by NASA in 2011 using an Atlas V. Other vehicles include the new Delta-IV line or the upcoming Falcon 9. The launch vehicles will be compared below, using payload capacity to "escape orbit" (Mars) as a comparison point.

Vehicle Payload Capacity to escape orbit (kg) Cost per kg Notes
Delta-II 1,500 $33,000 Retiring in 2011, 75 consecutive successes, numerous Mars missions
Delta-IV Medium 3,000 $30,000 Only 7 successful launches
Delta-IV Heavy 9,306 $27,300
Atlas V 551 6,400 $30,000 12/12 launch success rate, including Mars Reconnaissance Orbiter
Falcon 9 3,800 $11,800 In development

The Falcon 9 would be the cheapest option for launching, but SpaceX is still developing the rocket. They have successfully tested the much smaller Falcon 1, which does provide some hope for the future of Falcon 9. However, committment to this system would be unreasonable given the uncertainty. Instead, we are targetting the Atlas V for our Mars launches.

Our Mars Positioning System satellites will be based on the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft, which reached Mars in 2001 and 2006, respectively. In addition to scientific experiments, one of the Mars Odyssey's primary capabilities is acting as a communications relay between Earth and the Mars Exploration Rovers on the surface. That will serve as a minimum bound on the satellite's size. The Mars Reconnaissance Orbiter includes much more powerful communications and observation equipment, and will serve as an upper bound.

An Atlas V 401 launch vehicle successfully launched the Mars Reconnaissance Orbiter on August 12, 2005. That orbiter has a mass, including its fuel, of 2,180 kg. Since our satellites will be between the 725kg mass of Odyssey and the 2180kg mass of MRO, we plan to launch three satellites at a time aboard the heavier Atlas V 551. Based on a per-launch cost of $192m for the rocket, each satellite will cost approximately $64m to launch. Launching 21 satellites will cost an estimated $1.35b.

Launch Window

Because Earth and Mars orbit the Sun at different speed and radii, there are limited opportunies to launch from Earth to Mars that occur approximately every two years. More information can be found in the Orbital section.

Trajectory and Orbital Insertion

The classical trajectory to Mars uses a Hohmann Transfer Orbit. An engine burn accelerates the satellite ahead of Earth, allowing it to drift into a higher orbital plane and eventually intersect Mars' orbit.

Upon reaching Mars, the satellite would perform an orbital insertion burn to enter into a large, eccentric orbit. Typical Mars observation satellites use aerobraking to slow down and circularize their orbits. Given the high altitude of our positioning satellites, aerobraking is not possible. The satellite will need to perform a second burn at apogee to enter into a circular orbit. This will require more fuel (approximately twice as much) than previous NASA missions.

Satellite Construction

The Mars Positioning System satellites will be based loosely on the Mars Odyssey and MRO spacecraft. We can assume that our satellites will be similar to the larger MRO with much of the science instrumentation stripped. A mass of 1400kg represents a reasonable estimate (halfway in-between Odyssey and MRO). Typical Mars satellites are 50% fuel by mass, or 700kg. Additionally, data indicates that we will need twice as much fuel to enter orbit without the advantages of aerobraking. That increases total mass to 2100kg. Three satellites will still be launchable with an Atlas V 551.

Power Source

Solar radiation in Mars orbit is approximately half that in Earth orbit. The Mars Odyssey spacecraft uses solar panels and Nickel-Hydrogen batteries to generate 700W of power, while the MRO generates 2000W. Both generate approximately 1W per kg. Since our Earth to Mars transmitter uses only 63W, we expect a ~1400W solar power system to be more than adequate.

Note: Any type of nuclear power source is unlikely to be approved, given that the large number of launches increases the likelihood of a mishap and solar panels can perform the job well enough.

Attitude Control

Mars Reconnaissance Orbiter uses sixteen sun sensors and two star trackers to obtain a precise, three-dimensional location in space. It also features two inertial measurement units to detect any changes to attitude. This system is designed for precision because the high-zoom cameras onboard must be carefully aimed. For maneuvering, the MRO has twenty thrusters which use hydrazine propellant. Large thrusters are fired for orbital insertion, while smaller thrusters are used for attitude control and station keeping. The Mars Positioning System satellites will implement the same features, but with a larger fuel tank to compensate for the lack of aerobraking. The satellite will maintain its attitude with a system of spinning gyroscopes as well.

Satellite Communications Equipment

The MRO was the first Mars probe to use the Ka-band that our satellites will use. It includes a large 3m dish for high speed communications back to Earth (better than 6 Mb/sec maximum). While it used Ka-band only as a demonstration, the Martian Positioning System will use Ka-band as the primary link from Earth. A smaller 49cm dish serves as the 2.23 GHz transmitter for the positioning signal itself.

Radiation Effects

Space is a harsh environment, with galactic cosmic rays, solar wind, and trapped particles bombarding spacecraft at a heavy rate. While satellites in low Earth orbit are protected somewhat by Earth's magnetic field, those in Mars orbit have no protection, and are subject to the full power of solar wind and cosmic rays. A few quick simulations with CREME96 can yield an estimate of the total ionizing dose our satellites will receive.

On an average year, a satellite in Mars orbit will receive between 19 and 34 rad/yr in silicon just from cosmic rays and solar wind (NOT accounting for the effects of shielding). Certain events, such as coronal mass ejections from the sun, can produce enormous "flares", creating a rare "worst-week" dose of 168 krad (Si). For our Martian Positioning System, all satellites need to be hardened to several hundred krad, because a large solar event would affect most of the satellites in orbit -- including the spares. This will necessitate using certified rad-hard circuitry, radiation testing, and investigation of current mitigation techniques used in Mars missions.

Additionally, the exposure to radiation will present a significant risk of single event effects (SEE), including transients and bit-flips. As long as suitable precautions are taken, such events such should be catastrophic, as the GPS signal can sustain rare bit errors. Upsets to the configuration of the satellite itself can be corrected with uplink data from Earth. It will be critically important to test the satellite components for single event effects prior to launch, because a large solar flare (CME) could affect the entire constellation and hamper performance. Shutting down in high-flux environments, as some Earth-bound satellites do, is unacceptable because the entire positioning system would thus be unavailable.