Lunar Global Navigation System analysis and design

Welcome to the International Lunar Waystation (ILWS) working group! Please relate all posts in this thread to discussions regarding the design of a Lunar Global Navigation System. While the concept of a Lunar Global Navigation System includes both orbital systems (e.g. a GPS satellite constellation) and surface-based systems (e.g. a lunar demarcation array of beacons), due to the fact that the dynamic lunar gravity model is incomplete, the objective and scope of the work considered here should center on surface systems.

Ad Luna!

Radio Navigation

There are various radio navigation systems available, although all are based on the use of radio waves, the method by which the radio waves are used in order to determine direction, vary from system to system. Below I have introduced the various radio navigation systems that I could find through wikipedia and other sources.

RDF (Radio Direction Finder) and ADF (Automatic Direction Finder) are among the earlier radio navigation devices, they work by tuning into a broadcasting signal and then using a directional antenna to find the direction to the broadcasting antenna, after which triangulation is used to plot two measurements on a map with the intersection of the points being the location of the radio source. The ADF uses a more directional solenoid rotating on a motor instead of a loop antenna rotated by hand and also uses electronics to calculate the angle.

The Lorenz system developed in the 1930’s broadcasts two signals on the same frequency from highly directional antennas with beams that are a few degrees wide. One signal points slightly to the left of the other, there is a small angle in the middle where they overlap. The signals are dots and dashes that are timed such that when an aircraft is in the small overlapping area the sound is continuous. Planes can fly into the beams by listening to the signal, identifying which side of the middle they are on and then correcting their flight path until they are in the center. Although this system was designed as a night and bad-weather landing system, it should be possible to adapt it to for surface use.

The (VOR) VHF Omni –directional Radio Range navigation system uses two radio signals that vary in phase instead of sound. A single master signal is sent out continuously from a station and a highly directional second signal is sent out that varies in phase 30 times a second compared to the master. This signal is timed so that the phase varies as the secondary antenna spins, such that when the antenna is 90 degrees from north, the signal is 90 degrees out of phase of the master. By comparing the phase of the secondary signal to the master, the angle can be determined without any physical motion in the receiver.

Due to the use of VHF, VOR stations have a maximum propagation distance of ‘line of sight’. This limits the VOR range to the horizon, and even less if used in mountainous terrain. VOR is extensively used in civil aviation.

The problem with using VOR on the moon is that the Moon has no Magnetic field and the VOR receiver (usually on aircraft) derives the magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the Earth’s magnetic North).
This line of position is known as the radial, and the intersection of two radials from different VOR stations give a fix on the aircraft...Need to find out if VOR can still be effective without magnetic bearing.

The (NDB) Non-Directional Beacon is a radio broadcast station in a known location, and is operated on frequencies between 190 kHz and 1750 kHz. It is used as an aviation and marine navigation aid, so can be used at high or low altitudes.
Navigation by NDB works by using ADF equipment on the receiving craft and the NDB transmitter. The ADF is also capable of locating transmitters in the standard AM mediumwave broadcast band. ADF equipment can determine the direction the NDB station relative to the aircraft, which is then displayed on a relative bearing indicator (RBI). These readings are then correlated with a compass heading, usually in the form of a (RMI) Radio Magnetic Indicator. This brings us to the same problem as the VOR- no magnetic field on the moon. I am not sure how dependent the NDB & VOR systems are on magnetic readings, and if they can operate without this. I am inclined to believe that the NDB’s use of ADF equipment makes it a system that can function just on radio data.
But I need to do more research before I can be sure of this.
ADF receivers can also hear instructions from the broadcasting beacon in an emergency, such as a communications failure on regular channels.

Apart from the fact that VOR is more expensive, NDB has one major advantage over VOR: NDB signals follow the curvature of the earth, so they can be received at much greater distances at lower altitudes, but the NDB signal is affected more by mountainous terrain.

Hyperbolic systems are based upon the measurement of the difference between signal arrival times from two or more locations. These include GEE, LORAN, Decca, Omega/ Alpha navigation systems…….

Decca Navigator system is a hyperbolic low frequency radio navigation system first used during WWII, primarily used for ship navigation but also aircraft. The Decca system consists of a number of land based stations organised into chains. Each chain consists of a Master station and two or three Slave stations. The slave stations are ideally placed at the vertices of an equilateral triangle with the master at the centre. The Baseline length (Master-Slave distance) is typically 60~120 nautical miles. Each Station transmits a continuous wave signal.A set of hyperbolic lines of position called a pattern are obtained by comparing the phase difference of the signals from the master and one of the slaves. When two stations transmit at the same phase-locked frequency, the difference in phase between the two signals is constant along a hyperbolic path. If two stations transmit on the same frequency, it is virtually impossible to separate them at the receiver. Each chain is allocated a nominal frequency, and each station in the chain transmits at a harmonic of the base frequency.
In order to identify a unique location; the point of departure needs to be known. The pattern is grouped in to lanes and zones. Earlier systems used colours to code for the different slaves and colours and numbers to code for lanes (which were grouped into zones). Later receivers incorporated a microprocessor and display a position in latitude and longitude. These systems are known to have reduced accuracy at long distances.

GEE

Like the American Decca, the British GEE was developed during WWII. The GEE uses a series of transmitters sending out precisely timed signals, at the receiver end the time of arrival is examined on an oscilloscope. If the signal from two stations arrives at the same time, the aircraft must be an equal distance from both transmitters, allowing the navigator to determine a line of position on their chart of all the positions at that distance from both stations. Additional lines of position are produced by making similar measurements with other stations. When in use the GEE was accurate to about 150m at short ranges, and up to 1.6km at longer ranges.

LORAN/ CHAYKA

The LORAN (Long Range Navigation) was an American development of the GEE system. While the GEE had a range of about 644 km, early LORAN systems had a range of 1,930 km. The current version of LORAN in common use is LORAN-C and operates in LF from 90 to 110 kHz. The Russian equivalent is called CHAYKA. Like other hyperbolic systems LORAN uses master and slave stations. Given two slave stations, the time difference between the master and the first slave identifies one hyperbolic curve and the time difference between the master and the second slave station identifies another curve. The intersections of the curves determine a geographic point in relation to the position of the three stations.

OMEGA/ ALPHA

Omega was originally developed by the United States and has been described as the first truly global radio navigation system for aircraft. They are no longer used, although the Russian version (Alpha) still is in use. Each Omega station transmitted a very low frequency signal, consisting of a pattern of four tones unique to the station and was repeated every ten seconds. OMEGA uses hyperbolic radio navigation techniques. The chain operates at VLF between 10 to 14 kHz. By receiving signals from three stations, an OMEGA receiver can locate a position to within 4 nautical miles using signal phase comparison. Extensive antennas were used in order to transmit at low frequencies, some OMEGA antennas were huge!

Before the advent of GPS, the above systems where used as the prime means of terrestrial navigation. Some of these systems are still in use. A few of these systems could be tested for use in lunar surface navigation.

NDB could be a prime candidate for use on the moon, with the exception of mountainous terrain, NDB navigation should be adequate for use in EVA equipment and beacon arrays.
Further research must still be carried out as some adverse effects associated with ADF/NDB navigation could be very dangerous on the moon. These include terrain, magnetic and electrical effects. In terms of wave propagation, I’ve assumed that we can use line of sight and horizontally aligned transmitters/ receivers, but this means that propagation distance will be minimal as there is no ionosphere to bounce signals off and thus increase propagation distance. The most accurate signals should be ground waves that follow the lunar surface, but I think that this will only be suitable in flat regions. A hyperbolic system (possibly LORAN) could also be placed to surround areas of exploration. Use of the LORAN system could be sufficient for long distance navigation needs. Adaptation of equipment may be necessary before any of these systems can be used for none aviation/marine situations.

Radio location.

Both Radio location and Radio Navigation are types of radio determination. Radio location is the process of passively finding a distant object rather than one’s own position.
In Radio location the angle at which the signal returns, as well as the time taken for it to return, determine an object’s location.

A good example of radio location is cellular phone technology using base stations. The base stations which are radio towers use a process called trilateration. The location of a caller or handset can be determined via:

Angle of arrival (AOA), this requires at least two towers and the point at which the lines along the angles from each tower intersect determine the location of the caller.

Time difference of arrival (TDOA) using multilateration, except it is the networks that determine the time difference and distance from each tower.

Location signature using ‘finger printing’ to store and recall patterns, that mobile signals are known to exhibit at different locations in each cell.

AOA & TDOA depend on line of sight, which can be difficult or impossible in mountainous terrain or around sky scrapers. But location signatures work better in mountainous terrain.

Size and construction of the radio towers needed for both radio navigation and radio location could be an issue in inhospitable terrain on the moon, however once constructed the use of radio location base stations and location signatures may be a good solution for navigation in mountainous terrain.

Any constructive criticism or additions are welcome. This is just meant to be a starting point for the purpose of selecting or designing a navigation system that could actually function on the lunar surface.

Satinder