Fast-forward to today, and humans are going back to the moon—but this time in an overarching spirit of cooperation and perhaps even necessity. We’ve become increasingly aware of the fragile nature of our existence on Earth, making the desire to explore space all the stronger. We’ve come up against limitations imposed by our atmosphere and physics, but the perennial need to explore more and to break new ground is not diminished.

So, the first stop is the moon. But despite its relative proximity to Earth, going to the moon, putting boots on the moon, and staying on the moon comes with a daunting set of challenges. One of the foremost is positioning, navigation, and timing (PNT for short). We’ve come to depend on PNT in almost every aspect of our lives. Our communications, banking networks, energy distribution, and critical infrastructure rely on precise timing. Our vehicles and operations depend on positioning and the derived navigation. From the military to the civilian smart phone user, PNT is a cornerstone of all our existences.

Navigating to the moon, then. Navigating on the moon. Even navigating beyond the moon is one of the first questions to answer. How do we do it, and what challenges must we overcome to achieve this great enabler?

Navigating to the moon

Navigating to the moon is probably the most approachable challenge here. We’ve done it before – without the use of advanced satellite navigation – and we’ve done it more recently (see recent experiments on using GPS receivers in lunar orbit). To navigate to and from the moon with regularity, precision, and consistency, though, it makes sense for us to take advantage of the modern PNT infrastructure that we all depend on: global navigation satellite systems – or GNSS for short (the most well-known of these is GPS, but there is also Europe’s Galileo, China’s BeiDou, and Russia’s GLONASS).

The trouble with GNSS is that it was designed to aid navigation and timing on Earth. The satellites orbit Earth; their transmitting antennas are trained on Earth; our calculations to use GNSS are based on their use on Earth. However, as recently shown by NASA, GNSS can be used to navigate as far as the moon. The challenge here is in power and calculation. GNSS signals are extremely low-powered. On Earth, they are heard by our devices as low as -165 dBm, far below the noise floor. For GNSS to be used as a navigation aid on lunar transfers, we need high-gain antennas and finely tuned signal processing. Picking out the signal and applying enough gain is no trivial matter. But it is not insurmountable, and using GNSS from the start of the journey from Earth means we need to worry less about the tricky acquisition sensitivity than we do about the simpler tracking sensitivity. As for signal processing, the calculations here are known or can be made. We would primarily be using satellites that are about to disappear behind the Earth as their transmit antennas swing around and point in our direction. Not only does this mean the satellites are further away – at any time, most GNSS are orbiting around 20,000 km above the surface of the Earth, meaning the difference in the distance if heading away from the Earth could be a mere 50-55,000 km. While this is important, it falls mainly into the power consideration. Signals travel further, get weaker, and are harder to pick out.

For signal processing, the challenge lies in how far those signals travel through the ionosphere. Our phones, cars, and communications networks all make calculations based on established models to account for ionospheric delay. This is calculated by knowing how far the signals will have traveled through the ionosphere and how much this slows them down to calculate what we call the pseudorange accurately. Accurate pseudorange gives us precise positioning and exact timing. Signals coming from the other side of the Earth are passing through more ionosphere, being slowed more, and even refracting around the Earth. New calculations must be applied! But it can be done. We can navigate to the moon consistently using our existing infrastructure – particularly as the precision requirements are generally much lower. There’s less stuff to crash into in space than in, say, downtown New York or Tokyo.