NOAA KLM User's Guide

Section 3.7

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3.7 Search And Rescue Satellite (SARSAT) Instrument Package

General aviation aircraft are required to carry Emergency Locator Transmitters (ELTs), which are triggered by the impact of a crash and broadcast a signal at 121.5 and 243 MHz. These transmissions can be heard as siren like sounds on receivers of aircraft which may be flying over the transmitter. In addition, certain large ships are also required to carry 121.5 MHz Emergency Position Indicating Radio Beacons (EPIRBs) which also transmit the siren like sound. Some ocean going vessels voluntarily carry the EPIRBs.

It has been recognized for many years that receipt of ground transmissions by overflying satellites includes a Doppler shift of the transmitted frequency due to the velocity of the satellite relative to the transmitter. This Doppler shift information can be used to locate the transmitter. The Search and Rescue (SAR) Instrument Package first flown on NOAA-E and subsequent NOAA satellites carries a repeater for receiving and rebroadcasting the 121.5 and 243 MHz signals to a ground station where they can be detected and located by measuring their Doppler shift. However, these ELTs and EPIRBs were conceived prior to the satellite system and lack specifications which assure reliable detection through the satellite. Even so, by the launch of NOAA-H, over 1,000 people had been saved by SAR forces making use of satellite-derived alerts and locations.

A 406 MHz SAR system has been designed specifically to work with the satellites. The NOAA satellite SAR Instrument Package carries a 406 MHz processor which receives transmissions from 406 MHz ELTs and EPIRBs, recovers their digital message, measures the Doppler shift, and both stores the data for later transmission and also transmits it in real time. A receiver for the 406 MHz band is also included in the repeater. This new system utilizes distress transmitters (ELTs and EPIRBs) designed to be compatible with the satellite and the system provides full global coverage. The 406 MHz system was demonstrated, evaluated and declared ready for operational use by the time of the NOAA-H launch. Beginning with NOAA-H, the 406 MHz processor began utilizing a solid state memory for storage of the global data. Prior to NOAA-H, the processors used the spacecraft tape recorder for global storage.

The NOAA KLM satellites carry an improved 406 MHz processor, designated SARP-2. This unit has improved performance in system capacity, bandwidth, and protection against interference.

3.7.1 Instrument Operation

The SARSAT Instrument Package for the NOAA-K spacecraft consists of antennas/diplexer/filters, a search and rescue repeater (SARR) and a search and rescue processor (SARP-2) as shown in Figure 3.7-1.

  SRA receive antennas
      121.5/243 MHz
      406 MHz 
  UDA receive antenna 
      401.65/406.050 MHz
  SLA transmitter antenna 
      1544.5 MHz
Figure showing SARSAT instrument package

The SARR operates as a "bent-pipe" repeater for the 121.5 MHz, 243 MHz, and 406.05 MHz bands and relays these transmissions from the ground-based emergency transmitters to Local User Terminals (LUTs) in real time. The SARR also transmits the SARP-2 digital data stream. The 406.05 MHz band of the SARR is for test and interference monitoring purposes, the normal processing is performed by the SARP-2.

After frequency translation, the base band spectra of the three signals phase modulate a 1544.5-MHz L-band transmitter for data relay to the ground. This technique maintains the Doppler information contained in the received signal from the three repeated bands. By processing the Doppler information at the LUT, the location of the emergency transmitter can be determined.

SARP-2 406.05-MHz signal data are both relayed in real time to the LUTs (if in sight) and stored for later transmission over a LUT when it comes in view of the satellite. SARP-2 signals are received on the Ultra High Frequency (UHF) Data Collection System Antenna (UDA), which the SARP-2 shares with the DCS-2. A frequency diplexer is used to separate the SARP-2 signals from DCS-2 signals. The SARP-2 measures the frequency and level of the incoming signal. In its data stream are a time tag, frequency and level information, a synchronization code indicating the start of each received message and the original message contained in the incoming signal. The data are stored in the SARP-2 last-in-and first-out circular memory, which continuously transmits its contents via the SARR L-band link at 2.4 kbps. When the memory is full, new data overwrites the oldest message. The memory readout pauses at the conclusion of a message block to allow transmission of new data and then continues its cyclic transmission with the new message, having erased and replaced the oldest message.

The SARP-2 and SARR data are received by the CDA stations via either the HRPT downlink or the TIP beacons.

The SARP-2, which is flown on NOAA KLM, provides the stored data interleaved with the real-time data in the 2.4 kbps data stream sent through the 1544.5 MHz downlink transmitter. The output of the SARSAT Instrument Package is four frequency multiplexed signals on the 1544.5 MHz downlink transmitter.

  1. 2.4 kbps data
  2. 121.5 MHz band translated
  3. 243 MHz band translated
  4. 406 MHz band translated

The SARR instrument will output SARP-2 data and data stored in the SARP memory. The SARP-2 2.4 kbps data output consists of a Doppler frequency measurement (appropriately time-tagged) which includes the SAR data message from the ELT/EPIRB.

For the regional coverage mode, 2.4 kbps real-time data are transmitted to a Local User Terminal (LUT) via the SAR Repeater downlink. Global area coverage is provided by storing global 2.4 kbps data in a 406 Processor memory; these data are also transmitted to a LUT via the SAR Repeater downlink, but only during times when there is no regional coverage 2.4 kbps data to be transmitted.

3.7.2 System Description

The orbiting SARSAT Instrument Package receives signals from ground-based ELTs/EPIRBs and returns them to both raw form and preprocessed form (406 MHz Experimental Units only) to one or more ground stations. Figure 3.7-1 shows the SARSAT Instrument Package which consist of three elements:

  1. SARSAT Antennas;
  2. SAR Repeater (SARR) and
  3. SAR Processor (SARP-2).

3.7.2.1 SARSAT Antennas

The antenna subsystem consists of the:

  1. SAR Receiver Antenna (SRA);
  2. Data Collection System Ultra High Frequency (UHF) Antenna referred to as DCS/UDA and
  3. SAR L-Band Transmitter Antenna (SLA).

3.7.2.1.1 SARSAT Receiver Antennas

The SRA feeding the SARR consists of two coaxial Quadrifiliar designs. The outer quadrifiliar operates at two frequencies: 121.5 MHz and 243.0 MHz. The inner quadrifiliar operates at 406.05 MHz. The DCS/UDA is a quadrifiliar design and operates at 406.050 MHz for SARP-2 and also 401.650 MHz for the Data Collection System/2 (which is not part of the SARSAT instrument package).

The Search and Rescue Receiver antennas are mounted on a boom which is deployed clear of the other spacecraft instruments, antennas and solar array. There are four receive antenna ports for the SARSAT instruments: three for the SARR, and one for the SARP-2.

The 243 MHz and 406 MHz for both SRA and UDA receiving antenna patterns are shaped partially to compensate for increased signal path losses for ELTs at increasing distances from the subsatellite point.

3.7.2.1.2 L-Band Transmitter Antenna

The L-Band transmitter antenna is a small 1544.5 MHz quadrifiliar helix located on the Earth-facing side of the spacecraft which has a single lobe pattern for full Earth coverage. There is one transmit antenna port for connection to the SLA of the SARR.

3.7.2.2 SAR Repeater (SARR)

As shown in Figure 3.7-1, the SARR subsystem receives the ELT/EPIRB signals on 121.5, 243.0, and 406.05 MHz, down converts to selected intermediate frequencies, remodulates this data, and retransmits on 1544.5 MHz (repeater data mode). The baseline concept is that each receiver is a dual conversion unit with automatic gain control (AGC) which converts the received bandwidth down to a frequency range between 35 kHz and 210 kHz. These bands are then summed with 2.4 k-bit data from the SAR 406 MHz processor and phase modulated on the 1544.5 MHz downlink carrier frequency. The modulation level of each band is independently adjustable to account for any long-term changes either in the operational procedure or the system noise environment.

Each channel includes a commandable attenuator that allows the modulation index to be varied independently, subject to the constraint that the composite rms modulation index shall not exceed 0.74 radians.

The 2.4 kHz channel is controllable in 5 dB steps, whereas the 121.5, 243 and 406 MHz channels are controllable in 1 dB steps.

3.7.2.3 SAR Processor (SARP-2)

The 406.05 MHz on-board processor (see Figure 3.7.2.3-1) receives the 406 MHz transmissions from distress beacons and translates to an Intermediate Frequency (IF) using a double conversion. The IF signal is demodulated in a phase-lock loop to recover the data; the carrier frequency is measured and the data time tagged. For the regional coverage mode, real-time data are transmitted to a LUT via the SARR downlink. Global area coverage is provided by storing global data in a 406 Processor memory; these data are also transmitted to a LUT via the SARR downlink, but only during times when there is no regional coverage data to be transmitted.

Figure showing SARP-2 block diagram

The received signal is fed first to the receiver power unit (RPU) which utilizes a double conversion process to translate the signal to a lower frequency. The receiver is a constant gain type, linear over a 23 dB dynamic range.

The signal processing unit (SPU) is a Fast-Fourier Transform which searches the full 80 kHz bandwidth. When a signal is detected, the control unit assigns the signal to a Data Recovery Unit (DRU) on the basis of an algorithm designed to optimize the multiple-access performance. The algorithm for assignment is based on the frequency and level of input to the receiver as well as the current assignment state of the DRUs.

When a signal is assigned to a DRU, the voltage controlled oscillator (VCO) is pre-steered to within the lock-in range by a voltage which is proportional to the signal frequency as determined by the search unit. After VCO lock-up, the signal is demodulated and fed to a bit synchronizer which supplies the clock for subsequent digital processing. When the clock is available, the sync word contained in the message is detected and the remaining digital message processed. Doppler frequency is determined by counting the VCO signal for a specified time base referenced to the processor ultra stable oscillator (USO).

The processor formatter outputs a composite message made up of eight 24-bit words. Messages are organized into either long or short messages depending on the total number of data bits to be transmitted. The expected message length is determined by the processor by detecting a single bit flag indicating message length contained in the received message from the experimental ELT/EPIRB. The formatted message output is sent to the main memory and to the interface unit where it is transferred to the SARR 2.4 kbps input port.

The processor is implemented physically into two units with a total weight of 24.5 kg, and a total volume of 37 liters. Power consumption is 19.9 Watts.

3.7.3 Calibration Requirements

The SARR and SARP-2 have no on-orbit calibration requirements.


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