Communication Link setup betwwen Earth and Moon

As per our plan the Lander will act as a base station on moon which will receive command signals from the DSN located on earth. The Lander will then instruct the Rover as per the commands received from earth. The communication link between Lander and earth is for two purposes. The earth issues command signals to the Lander which in turn can instruct the rover to carry out its task. The Lander will also be passing telemetry signals received from the rover. These telemetry signals send the health of the rover time to time which is very essential. There are good reasons for the Rover to act as slave to Lander rather than getting commands directly from the earth station. One, it is more reliable this way. Also the response time will be shorter as the distance between Rover and Lander is very small.

This way the rover need not carry huge directional antenna with accurate pointing mechanisms (The X band high antenna requires two axis pointing to earth). The rover can use a simple UHF low gain monopole antenna as it only needs to send signals to the Lander. Therefore the power requirement on the rover for signal transmission will also be considerably less.

Link Budget: ROVER to LANDER

The link between Rover and Lander must support high data rates as the rover will be sending High Quality video from its camera. The rover has limited power too. Since rover is mobile and may experience uneven surface on its way having a directional antenna is not a good idea. We therefore plan to use an omni directional UHF antenna operating at 405 MHz. This way we can eliminate any antenna pointing mechanism on the rover. The below link budget shows that for a 7.5 Mbps link we can get a link margin of 18.22 dB with a transmit power as low as 50 mW.

For the System noise temperature calculations at the Lander antenna we considered a LNA Noise figure of 3 dB, antenna feeder losses to be 1 dB. With an antenna temperature around 500K (taking into account galactic noise, physical temperature of moon), we get an overall system temperature to be around 1000 K. Considering an 1-m ominidirectional antenna with a receive gain of -2 dB(worst case) the G/T ratio comes out to be -32 dB/K.

We can see that with this link margin the maximum distance that the rover can travel is 2 Km. With the extra margin, we can either further reduce the transmit power or can increase the data rate for the link. We can also make the rover go further distance which will enable us to collect valuable science data.    
 

 

 

Link Budget: LANDER to ROVER

For issuing command signals to the rover we can either send the commands directly from the Earth station or can instruct the Lander to issue those commands. The former should not be used because the rover is moving and hence its antenna is not guaranteed to be pointing towards the earth station DSN always. To achieve reliable communication expensive pointing mechanisms will be required. Hence we thought of letting the Lander issue command signals to the Rover. For the communication between Lander and Rover we opted for UHF (450 MHz) mode with omni directional antenna. This way we can ensure a reliable way of communicating to the Rover. 

Since the Lander just needs to issue command signals to the rover a very low data of 48Kbps is more than enough. Again we used a 1-m omnidirectional antenna of gain -2 dB (worst case) for this purpose. The overall system noise temperature for the receiver antenna on rover will be similar to that of the Rover-Lander Link.

We can see from the link budget table that we can achieve a very reliable communication between the Rover and Lander with transmit power as low 1 mW. Link margin is a very high 22.25 dB. In this case too we either further reduce the transmit power or increase the data rate if at all needed.

Link Budget Downlink (Lander to Earth)

For the downlink link, we need to transmit at 7.5 Mbps data rate. We plan to transmit the data using X band phased array antenna (XPAA) [6] operating at 8.225 GHz. The XPAA has a 23.30 dBi transmit gain. We make use of the DSN antenna on the earth located at 3 different locations for receiving our signal. The downlink will encounter a huge path loss of -222.45 dB. We therefore require a very high gain receiver antenna capable of withstanding high noise temperatures in order to pick up the low strength signal. The 34 meter DSN has an extremely high G/T ratio of 50 dB/K [8]. This DSN antenna has extremely low noise receive capability. They operate with a total X band noise temperature of about 20-30 K [10]. This is primarily achieved using a cryogenically cooled ultra low noise feed. Since we use a 2/3 turbo coder the Eb/No required is only 1 dB. We were able to obtain a link margin of 10.42 dB using a transmit power of 4 W.

 

 

Link Budget Uplink (Earth to Lander)
We again use DSN network operating at an X band frequency of 8.425 GHz to send command instructions to the Lander.  Since we only need to send command signals, the data rate can be 48 Kbps. Transmit power is also not an issue here as these DSN antenna are capable of transmitting at 20 KW with a gain of around 90 dB at X band frequency. We decided to transmit the signal at 50 W. The transmit antenna gain is 63.78 dBiC at 8.425 GHz.
For receiving the command signals from the earth we use a Quadrifilar helix antenna which is extremely light weight and is space qualified. This antenna has a receive gain of 3 dB, Bandwidth of 40 MHz and weighs less than 250 gms. This type of antenna is ideally suited to be used on a Lander.
For the system noise calculation at the receiver antenna on Lander we took into account a LNA noise figure of 3 dB, NF of 1 dB due to feed loss. The antenna temperature was considered to be around 400 K. This gave an overall system noise temperature to be 900K. Hence the G/T ratio for the receiver antenna is 26.54 dB/K. The link budget shows that using a 30 W transmit power we can establish a reliable link between the Lander and Earth with a safe link margin of 7.06 dB.