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  • The Space Suit
  • The RFID System
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Communication

Overview

Two separate communication links are established for our lunar location service. First, a link is provided between the astronaut and the Lander. This link should provide sufficient bandwidth to support audio, video, or other informational data during the moonwalk. Additionally, the link should be fully functional up to 2 km away from the Lander. Second, a link between the astronaut and the RFIDs is provided.This link should operate at fairly low received powers in order to successfully receive information from the passive RFIDs.

Astronaut / Lander Final Specifications			Astronaut / RFID Final Specifications
Frequency	2.43 GHz		Frequency	2.43 GHz
Bandwidth	0.6 MHz		Bandwidth	20 kHz
Throughput	200 Kbps		Throughput	13.3 Kbps
Modulation scheme	BPSK		Modulation scheme	BPSK
Transmit power	250 mW		Transmit Power	10 W
Maximum distance	2 km		Maximum distance	9.79 m
Forward Error Correction	Rate ½ Convolutional Encoder (K=7)		Forward Error Correction	(127, 64) BCH

RF Chain

The RF chain was designed and built to support both communication links.  The major factor considered during the RF chain design phase was noise power.  Noise power (P_n) is given in equation 1.

Pn	= k Tsys B	(1)
Where,
k	= Boltzman’s constant (1.39 × 10−23 JK-1)
Tsys	= Physical temperature (K)
B	= Noise bandwidth (Hz)

Reducing the noise power will allow a reduced receive power while maintaining sufficient carrier-to-noise (CNR) ratios.  While noise bandwidth can be handled separately from the RF chain design, the physical temperature, T_sys, can be reduced by choosing low-noise devices in the RF chain.  A simple RF chain block diagram from Pratt [1] is shown in Figure 1.

Figure 1: RF Chain Block Diagram
For system temperature, the most important stage is the low-noise amplifier (LNA).  Choosing a high-gain LNA will nearly cancel the subsequent noise temperatures in the RF chain.  Therefore, the overall noise temperature, T_sys, can be estimated as T_in + T_LNA. Using a value of 50 K for the LNA noise temperature and 295 K for the temperature inside the astronaut suit, T_sys, is approximately 345 K.

References:

[1] T. Pratt, C. Bostian, T. Allnutt, Satellite Communications, 2nd edition, Wiley, 2002.

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