Mission Statement

The mission of the ground station is to close the link between the spacecraft and the ground in order to receive telemetry and tracking and provide control of the spacecraft.

Mission Requirements

Antenna transmit gain of 32.29 dB, receive gain of 34.20dB.
Slew rate less than or equal to 6.55 deg/sec.

Antenna gain requirements are derived from the link budget. The antenna gain requirements are met by the design choice of a 3-m parabolic dish antenna. Slew rate is determined by the relative azimuth/elevation rates as seen at USNA SGS for an object in 494-km circular orbit at 46-degree inclination.

System Requirements

Equipment must use 110 VDC and 30 amps.
High speed internet connection.
Software/hardware is TCP/IP compatible.
120 foot cable run from antenna to ground station.

The physical infrastructure of the USNA SGS (Rickover Hall, USNA Annapolis MD) dictates the stated current and voltage, as well as the need for the 120-ft cable run.

Bus Requirements

1 Mbps date rate, Binary Phase Shift Key (BPSK) modulation.
100 Kbps transmit data rate, 64 Kbps receive date rate, GMSK modulation.
Uplink frequency of 1.767 GHz and downlink frequency of 2.2022 GHz
Bit error rate of our communications must not exceed 2*10^-5.

ICSat requires a 1 Mbps data rate and BPSK modulation scheme for its support, as well as a high speed internet connection and TCP/IP protocols. The SpaceQuest main communications equipment chosen uses 100 Kbps and 64 Kbps GMSK modulation. CFTP requires a bit error rate of less than 2* 10^-5 and the use of high speed internet in order to transfer the data to the NPS. The uplink and downlink frequencies are a design decision made at the concept level in order to provide interoperability between USNA and NPS.

Design Decisions

3-meter parabolic dish antenna on steerable az/el mount

Derived Requirements

+/- 0.5 degree azimuth and elevation pointing accuracy.

The pointing accuracy is determined by the half-angle of the antenna acceptance cone.

Description

The ground-side of the communications system uses a 3-m parabolic dish antenna with an electrically driven azimuth-elevation (Az-El) mount (AZEL1000) purchased from M2 Antenna Systems, Inc.

The mount is controlled by an M2 Antenna Systems RPC2800PRKX rotator under the direction of the Nova for Windows software package from Northern Lights Software Associates .

Uplink frequency is 1.76700 GHZ; downlink frequency is 2.220226 GHz. Communications with the spacecraft is full duplex, necessitating the placement of a diplexer (Microwave Circuits, Inc. 1750-2120/2200-2300 Part No. D0421601) immediately behind the S-band feed. The downlink signal passes through a low noise amplifier (LNA) (Miteq AMF-3F-02200230-04-13P), two Bias-Ts which provide 12VDC to the LNA, a switching relay and then approximately 200 feet of heliax (931B00-01) cable to the receiver. The receiver sends the signal to a bit synchronizer, which sends it to a Cisco router, which sends it to a PC.

The transmitting side starts with the PC and continues to the router. The Cisco router sends the signal through a RS 422 level converter and then to the signal generator. The signal generator sends the signal to the power amplifier and then through to the diplexer. From the diplexer, the signal is sent to the feed horn of the antenna to be transmitted to MidSTAR.

Power is supplied from the first deck of Rickover and run to the roof.

Table 1: Equipment Specifications