Several possible Mars surface architectures for position determination and communication were examined, based on Earth-like counterparts. The following sections describe these options [Hobbs, 1990].
6.1.2.1 Loran-C Based Navigation System
The Loran-C system relies on the propagation of ground waves. It broadcasts a pulsed waveform at a frequency between 90 and 100 kHz with a transmission power on the order of 1500 kW.
The baseline between stations is 1600 to 2400 km with an accuracy of 200 to 600 m.
Differential techniques can be used to increase the accuracy (see below).
Assuming that ground waves on Mars propagate as they do on Earth, and with a baseline of 2000 km between stations, a Mars Loran system would require eleven stations along
the equator. This chain would provide navigation to latitudes up to 30°, with the highest accuracy at the equator (~200 m) and the lowest accuracy at 30o latitude (~600 m).
While it is technically feasible to build a Loran-C based navigation system on Mars, the associated risks and costs are high. For example, a Mars-based Loran system would require a self-deploying antenna structure, as well as a large source of electric power such as a solar array farm or nuclear power.
6.1.2.2 Omega Based Navigation System
The Omega system uses a Very Low Frequency (VLF) between 10 to 14 kHz at a transmit power on the order of 10 kW. On Earth, the surface and the bottom of the ionosphere act as a waveguide to carry the waves long distances over the horizon.
The baseline between stations is 8000 to 9600 km with an accuracy of 1.6 to 3.2 km.
Differential techniques can be used to increase the accuracy (see below).
Assuming that the ionosphere and surface on Mars propagate VLF waves as they do on Earth, five stations could provide global position determination for Mars with an accuracy of 1 to 2 km. Two of the stations could be at the poles, with 3 stations equally spaced along the equator. The range from the pole stations to the equator would be 5300 km, while the distance between equatorial stations would be 7100 km.
Although it is technically feasible to build an Omega based system on Mars, it would be very risky and cost prohibitive. The antenna structure of an Omega system is highly complex and requires an antenna roughly 400 m in diameter. Furthermore, the system would require an accurate model of the Martian ionosphere; none is available at this time.
The risks and costs associated with this system are similar to those for the Loran type system.
6.1.2.3 Differential Radio Navigation Techniques
In this method, a reference receiver is placed at an accurately known position. Then, the reference position given by the navigation system is compared to the actual position. The difference between the actual position and the measured position is then broadcast as a correction. This method can produce accuracies on the order of 1 to 2 m. It is limited by how far the correction data can be broadcast, and by how accurately the reference position is known.
Differential techniques are complementary to an existing navigation system and require the installation of ground references. The position of the ground references would have to be determined using DSN, which has an accuracy on the order of 100 km for objects on the surface of Mars.
Therefore differential techniques cannot be used for the primary position determination system.
6.1.2.4 Communication Array
The primary problem with ground-based communication systems is the coverage.
Without communication relay satellites, future missions would have to be deployed within line-of-sight of a Mars-to-Earth communication antenna. To meet the coverage requirements, a large number of ground-based antennas would be required.
A ground-based communication system could meet the data rate requirements, but a large number of ground stations would be necessary to meet the coverage requirements.
6.1.2.5 Location of Mars Ground Stations
The following figure describes the layout of some potential Mars ground-based position determination and communication systems. For the Loran-C type navigation system, eleven ground stations are needed. For the Omega-type navigation system, five stations are needed. These stations are equidistantly spaced around the equator, and at the poles for total coverage.
0° 30° 60° 90° 120° 150°
30°
60°
90°
120°
150°
30o 60o
30o
60o
Omega Type System (5 Stations) Line-of-Sight Type System
Loran-C
Loran-C Type System (11 Stations)
Figure 6.1: Location of Martian Ground Stations.
For communication, it is possible to build large antennas to receive and transmit high power signals between Earth and Mars. The key issue is how to get MSE data to and from the stations. While it may be possible to transmit low data rate commands through the navigation beacons to an MSE, it would be difficult for a power and mass limited MSE to transmit a high data rate stream through the navigation beacons to a transmitter station. This means that a large number of communication relays would be required. Six line-of-sight communication beacons could be located at the tops of the high volcanoes listed in Table 6.1.
Table 6.1: Communication Beacons
Name Height Position
Iecates Tholes 10 km 22° N 150° W Elysium Mons 10 km 25° N 146° W Olympus Mons 27 km 18° N 133° W Arsia Mons 27 km 10° S 120° W Pavonis Mons 27 km 0° N 113° W Ascraeus Mons 27 km 11° N 105° W
At a height of 10 km, the sight range is 260 km. At a height of 27 km, the line-of-sight range is 426 km. While ground waves could extend to this range over the horizon, the real issue is with the transmit range of an MSE. It would require over fifty 27-km high ground stations to cover just the equator with line-of-sight coverage.